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METODOLOGIA PARA EL DISEÑO DE SISTEMAS TECNICOS AUTOMOTRICES

MAURICIO TORO RESTREPO

UNIVERSIDAD EAFIT INGENIERÍA MECÁNICA DISEÑO METODICO MEDELLÍN 2007

INTRODUCCION El diseño de automóviles es uno de los campos más amplios y lucrativos del diseño a nivel mundial. Sin embargo no es muy común que esta se haga con las metodologías adecuadas e incluso las metodologías existentes son bastante excluyentes y se centran en un solo tipo de vehiculo. Es por esto que se quiere en el presente trabajo plasmar en papel los lineamientos para el diseño al igual que información técnica que permita simplificar la tarea de diseño automotriz. Se tendrán en cuenta todos los aspectos del mismo desde la investigación de mercadeo y viabilidad hasta las evaluaciones post fabricación. Se mostrara la aplicación de las diferentes metodologías de diseño aplicadas al caso particular del automóvil y se mostrara cuales son mas importantes y porque. También se intentara mostrar que algunas veces es conveniente realizar un proceso de diseño convencional para algunos subcomponentes y que no es el objetivo de este trabajo convertir el diseño automotriz en un proceso meramente mecánico del cual emergerán vehículos estándar. Mas aun el objetivo de este trabajo es simplemente ampliar el espectro del diseñador y ayudarle a tener en cuenta los distintos factores que han entorpecido las labores de diseño en procesos anteriores. Existen básicamente dos corrientes de diseño automotriz y las dos son igualmente usadas y validas. La primera parte de una forma del vehiculo tanto exterior como interior y alrededor de esta se procede al diseño mecánico de los componentes. La segunda parte del diseño mecánico y una vez se completa este el diseñador sale a darle una forma agradable a el mismo, inclusive en algunos casos la forma no la da el diseñador sino el ingeniero experto en aerodinámica. Como se puede intuir de la primera metodología surgen

vehículos muy estilizados y de la segunda se obtienen vehículos de altísimo desempeño y/o rendimiento. En otras palabras en el diseño automotriz actual existe una clara barrera entre los vehículos diseñados con objetivos de ingeniería y los vehículos diseñados con objetivos estéticos o de diseño. También se tiene la concepción según García 1 que el diseño automotriz es un proceso lineal. Sin embargo en este documento se planteara un nuevo ángulo de diseño, muy utilizado en la industria aeronáutica, que además de acortar el tiempo que se toma el proceso de diseño busca obtener vehículos que son fuertes en ambos campos. Esta metodología busca evitar una dependencia del diseñador mecánico o en su contraparte del diseñador industrial y simplemente seguir una secuencia lógica en la cual ambos diseñadores toman decisiones relacionadas con su especialidad.

METODOLOGIA DE DISEÑO Como se puede observar en la figura 1 el proceso de diseño se aleja un poco del diseño lineal y hace que las tareas de procesos que no tienen que ser secuenciales en un brazo paralelo que básicamente se convierten en tareas del Ingeniero, y tareas del diseñador. Obviamente estos roles no indican de ninguna manera la verdadera profesión e incluso en algunos casos el oficio (por ejemplo el trabajo de un ingeniero aerodinámico en la carrocería externa) sin embargo en este documento se separaran las dos tareas ignorando estos casos “excepcionales”. Cada paso de la metodología planteada exige una serie de documentos que deben ser presentados para garantizar la debida realización d dicho proceso de diseño.

1

Garcia, Alvaro. Race Car Design. 4 Sept. 2006. 31.

Fig. 1 Esquema del flujo de actividades de diseño

Una vez se conoce el diagrama de flujo se puede comenzar a explicar cada paso del proceso con mayor profundidad y de igual importancia los resultados en lo que a documentación se refiere de cada uno de estos pasos. También antes de comenzar se considera importante hacer un énfasis en lo que el uso de una metodología de este tipo significa en un proceso de ingeniería. Hay que comenzar por insistir que todo proceso de ingeniería se resume a dos factores importantes que se llaman tiempo y dinero. Es increíble pero cierto, cuando un cliente solicita un servicio de ingeniería lo único que pregunta es ¿cuanto se demora?, ¿cuanto me vale?, ¿cuanta plata me ahorra implementarlo? etc. En todo caso utilizar una metodología de diseño que además tenga incorporada una metodología de gestión de proyectos será posible estimar de una manera mas precisa desde el comienzo del proyecto.

CONFORMACION DEL EQUIPO DE TRABAJO Y DIVISION DE TAREAS La primera tarea que se debe tener en cuenta antes de comenzar un trabajo de diseño es la conformación del equipo de trabajo, esta se puede hacer de muchas maneras pero la ideal es conformarlo por las áreas de especialidad por ejemplo, los especialistas en diseño mecánico y mecanismos serán los encargados de la suspensión, así como el especialista en ergonomía y teoría de la forma encargado del diseño de la carrocería, etc. Pero como este mundo no es ideal, algunas veces los equipos de trabajo se forman por “dedocracia” y en algunos casos cuando no hay nadie que tenga el nivel de especialidad o los deseos de cumplir con una tarea tendrá que aparecer el denominado doliente, porque en el trabajo de diseño no todas las tareas a realizar serán del total agrado del equipo de trabajo. También es importante notar que hay ciertas jerarquías y posiciones que se deben llenar en un equipo de diseño independiente del producto y estas son:

Jefe de Proyecto: El jefe del proyecto es un dictador que tiene que ser suficientemente inteligente para hacer sentir a los demás miembros del equipo que el trabajo es una democracia. Además es la persona encargada de verificar el estatus de la gestión del proyecto y más importante aun es el encargado de ser la imagen del proyecto ante las fuentes de financiamiento del mismo. Generalmente este puesto lo ocupa la persona con más experiencia y reconocimiento del equipo de trabajo.

Tesorero del Proyecto: El tesorero del proyecto como su nombre lo indica es el encargado de las finanzas del proyecto, el será el único miembro del equipo que tiene acceso a los fondos, también este miembro debe llevar un registro completo de cuanto dinero se toma un determinado componente y que tanto esto se acerca o aleja del presupuesto inicial asignado al proyecto.

Encargado de la Documentación: El encargado de la documentación deberá almacenar y ordenar todos los documentos que surjan del proceso de diseño, algunos surgirán del proceso en si y otros surgirán del acto de diseñar. Los documentos que surgirán del proceso son los documentos que se plantearan como resultado de cada una de las fases del diseño. También se deben incluir todos los documentos que surgen del acto de diseñar como lo son los cálculos realizados por todos los ingenieros participantes en el proyecto, las órdenes de trabajo, los planos de taller, de ensamble y hasta las facturas de compra del material. Estos documentos facilitaran la tarea de hallar un error en el diseño y serán útiles sobre todo en áreas como la automotriz donde es tan sencillo que algo salga mal y mas aun que los resultados de esto sean catastróficos.

Encargado de Seguridad Todo equipo de diseño automotriz debe tener un encargado de seguridad que es la persona que verifica el correcto cumplimiento de todas las normas de seguridad como que las estructuras anti-vuelco, la protección contra impactos laterales, los airbag, etc.

Una vez se ha conformado el equipo y los roles principales dentro de este se han definido se puede comenzar a formular el proyecto. Atención porque no es lo mismo formular el proyecto a definir el problema. Para formular el proyecto simplemente se hace un balance de lo que se tiene como recursos de ejecución de personal, capital y capacidades de manufactura y analizando todos estos parámetros se definen unos objetivos claros que serán cumplidos al finalizar el mismo. También pues será importante teniendo en cuenta los recursos de tiempo y dinero con los que se cuenta ir definiendo las tareas de diseño o especificación de deberán ser realizadas por el equipo de trabajo. De este paso saldrán dos documentos clave para el desarrollo del proyecto uno es un documento fijo que una vez se redacta y se fija no deberá ser cambiado una vez se comienza la ejecución del proyecto, este documento se denominara Anteproyecto (este será complementado una sola vez cuando aparezca el documento de definición del problema). El segundo documento es un documento dinámico que servirá de guía durante el resto del proceso de diseño y que será complementado durante todo el proceso de diseño. Este documento se denominara Documento de Estructuración de Tareas. Ahora se entrara con más profundidad a estos documentos.

Anteproyecto

El anteproyecto es un documento en el que se especifica a grandes trazos que es lo que se va a hacer en el proyecto. Como parte del anteproyecto se encuentra una definición del problema que simplemente indica grosso modo que es lo que se busca con el proyecto y porque. También esta puesta en

papel la información del equipo de trabajo, quienes van a ser los ejecutores del proyecto, algo sobre sus trayectorias y cuales son sus rangos en el equipo de trabajo. Además se debe tener un objetivo general del proyecto que en este caso seria pues diseñar un vehiculo o algo por el estilo. Se deben también incluir objetivos específicos que el equipo de trabajo quiere lograr. También debe enumerar de una manera clara y concisa cuales son los resultados esperados del proyecto. Por ultimo el anteproyecto deberá poseer una bibliografía inicial que desde el momento en que se redacta el anteproyecto será puesta a disposición de todo el equipo de trabajo. A continuación se mostrara un ejemplo de anteproyecto:

PLANTEAMIENTO DEL PROYECTO, DISEÑO Y FABRICACION DE UN VEHICULO MODULAR, ECONOMICO Y MULTIUSOS. Mauricio Toro R Estudiante Grupo de Investigación en Desarrollo de Productos Universidad EAFIT Medellín, Colombia PALABRAS CLAVE Diseño, Automotriz, Comercial, Bajo Costo, Modular

1. Planteamiento del Problema Al analizar el mercado automotriz colombiano se detectan dos falencias clave, Primeramente se observa un alto precio de los automóviles con respecto al exterior, Esto se debe en gran parte a los altos aranceles de importación que tienen estos vehículos para ingresar a Colombia. Esto a su vez ha causado un auge en la venta de soluciones mas económicas al transporte como lo son las motocicletas, en especial las motocicletas de fabricación china. Estas tienen dos problemas claves, el primero es su baja capacidad de carga y el segundo es la alta tasa de accidentalidad inherentes a este diseño de vehiculo. También se expresa un interés por demostrar que en Colombia tenemos las capacidades técnicas e intelectuales para diseñar y fabricar nuestras propias soluciones de transporte aptas para las condiciones económicas y fiscales de nuestro país. Además si a esto añadimos la altísima demanda que existe actualmente por los vehículos a motor recreativos y la creciente demanda resulta bastante atractivo embarcarse en un proyecto de este tipo.

2. Ejecutores del proyecto El Proyecto será ejecutado por los profesores y estudiantes miembros del grupo de investigación en desarrollo de productos de la Universidad EAFIT. Se propone además como practica del uso de la ingeniería colaborativa, dividir el equipo de trabajo en al menos dos grupos de trabajo, uno encargado del diseño del chasis, suspensión y demás componentes mecánicos y el segundo encargado del diseño de la carrocería, de los posibles módulos y de todo lo que tiene que ver con la ergonomía y la arquitectura del diseño.

3. Objetivo General El objetivo general del proyecto es diseñar, fabricar analizar la producción de un vehiculo utilitario multifuncional buscando siempre la economía tanto de producción como de operación. El vehiculo contara con un chasis y carrocería de diseño propios del equipo y un motor comercial de baja cilindrada.

4. Objetivos Específicos • • • • • •

Realizar un análisis sobre la viabilidad de diseñar y producir un vehiculo en Colombia Trabajar con el modelo internacional de ingeniería colaborativa en el cual un grupo de ingenieros trabajan en el desarrollo independiente de cada componente de un determinado componente y después se hace el ensamble del mismo. Utilizar el Know How adquirido en el desarrollo de los distintos vehículos en la universidad para evolucionar sobre estas bases. Utilizar las metodologías de diseño conocidas como ground up para optimizar el tiempo de desarrollo del vehiculo. Utilizar equipos de medición que permitan verificar los parámetros y evolucionar el diseño una vez este completado el primer prototipo. Realizar un proceso de diseño completo desde la concepción inicial hasta la evaluación y mejoras del prototipo.

5. Resultados Esperados del Proyecto Resultados de documentación 1. 2. 3. 4. 5. 6. 7. 8. 9.

Análisis de viabilidad del vehiculo, soportado en estudios de mercado y legales que regirán el resto del proceso de diseño. PDS y Cronograma de trabajo y diseño donde se deben especificar todos los parámetros de diseño y condiciones de frontera del sistema. Matriz de selección de componentes indicando los conceptos de solución para todos los sistemas del vehiculo Resultados de la evaluación de la matriz de selección de componentes Modelación detallada del sistema con análisis de resistencia de materiales y elementos finitos de los elementos que así lo requieran. Planos de taller y ensamble completos del vehiculo con revisiones realizadas durante la manufactura y las pruebas. Resultados de las pruebas y planteamiento de soluciones Cartas de proceso y de producción para todos los componentes del vehiculo Análisis financiero y costos de fabricación unitarios.

Resultados Físicos 1.

Prototipo funcional del vehiculo

6. Fuentes de Consulta Iniciales •

Engineer to Win. Smith, Carroll. 1a ed. Osceola: MBI Publishing Company, 1984. 277 p. ISBN 0 87938 186 8

•

Race Car Vehicle Dynamics. Milliken, William F.. 1a ed. Warrendale: SAE Publications, 1995. 890 p. ISBN 1 56091 526 9

•

Competition Car Composites. McBeath, Simon. 1a ed. Sparkford: Haynes Publishing, 2000. 208 p. ISBN 1 85960 624 5

•

Competition Car Suspension. Staniforth, Allan, Simon. 3a ed. Sparkford: Haynes Publishing, 2006. 232 p. ISBN 1 85960644 X

•

Tune to Win. Smith, Carroll. 1a ed. Osceola: Aero Publishers, 1978. 172 p.

7. Lineamiento de Actividades de Diseño Diseño Mecánico 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Investigación de viabilidad y Mercado. Recolección de información y especificaciones. Selección preliminar de partes Diseño preliminar. Re-diseño o re-selección de partes Diseño final. Análisis estructural completo de diseño final. Fabricación de modelo 1:1 para evaluación de dimensiones y ergonomía. Refinamiento diseño final. Construcción prototipo. Pruebas preliminares. Refinamiento de diseño

Diseño Ergonómico 1. 2. 3. 4. 5. 6. 7. 8.

Investigación de viabilidad y Mercado. Recolección de información y especificaciones. Bosquejos iniciales exterior e interior. Evaluación diferentes bosquejos. Modelación y Render Bosquejo seleccionado. Fabricación de prototipo. Pruebas preliminares. Refinamiento de diseño.

9. Herramientas Computacionales a Utilizar Modelación y análisis: Para el diseño y análisis estructural del vehiculo se utilizara la plataforma de Solidworks con su complemento Cosmos. Si se requiere modelar en otro programa se puede modelar y transferir el archivo en formato IGES. Cálculos Manuales: Estos se realizaran utilizando MATLAB y Microsoft Excel Gestión del Proyecto y Administración de Recursos Microsoft Project

Debe notarse que los documentos que aquí se presentan como ejemplo solamente tienen el fin de ilustrar al usuario la forma y el contenido de este tipo

de documento y no son restrictivos, si el usuario considera que hace falta algún elemento o sobra algún elemento esta en libertad de removerlo.

Documento de Estructuración de Tareas (DET)

Este tipo de documento es introducido en la metodología planteada gracias a la colaboración del ingeniero León Darío Castaño y es un desarrollo que el ha hecho a partir de la metodología de gestión de proyectos alemana REFA. Se ha expandido este documento para que además de la parte de gestión de tareas incluya un componente funcional y este es un buen momento para que el grupo comience una investigación. Gracias a los casi 100 años de diseño automotriz ya no es necesario realizar una síntesis funcional o en su defecto una estructura funcional, una profunda investigación de los sistemas y subsistemas de un automóvil nos haría dar cuenta que estos permanecen iguales desde los primeros vehículos y que sistemáticamente no hay ninguna diferencia clara entre un Ford modelo T y un Formula 1. Sin embargo las técnicas y tecnologías para estos sistemas han evolucionado hasta un punto en el cual pareciera que estos vehículos son completamente diferentes, esto no es así. En todo caso esto hace que la fase de diseño conceptual del vehiculo se haga de una manera extemporánea y poco ortodoxa. Se comienza realizando una síntesis o estructura sistémica. Esta se realiza de una manera similar a la síntesis o estructura funcional pero en vez de contener verbos que denotan una función tiene sustantivos que denotan un elemento o sistema.

Fig. 2 Síntesis Sistémica de un Motor con inyección

Esta técnica es bastante útil porque seria un proyecto bastante ambicioso por no decir imposible pretender diseñar un vehiculo completamente nuevo en todos los aspectos igual que fabricar un todos los componentes de un vehiculo, incluso en las grandes casas fabricantes de automóviles el diseñador de cada modelo meramente selecciona un motor estándar y lo utiliza en su diseño. Bueno en todo caso si se realiza una síntesis sistémica se pueden identificar todos los componentes de un vehiculo y la relación entre ellos que será de suma importancia en las etapas siguientes del diseño. También será útil para luego sustituir los componentes que se desea desarrollar con sus verbos o funciones, además estos sustantivos se pueden utilizar para armar las matrices de selección de componentes que deberán ser realizadas mas adelante en el proceso. Bueno en la figura se mostró la síntesis sistémica que es importante para el diseñador pero en verdad no es la parte mas importante, su principal función es mostrar las interrelaciones entre las piezas y como fluyen las cargas y las informaciones por ellos.

Ahora se mostrara un ejemplo de un DET,

nótese que como es un documento dinámico no siempre es necesario llenar todos los campos sin embargo se recomienda ser lo mas completo posible en el análisis de este documento ya que esto evitara que se cometan errores por omisión en el desarrollo del producto. Después de realizado el DET del grupo cada subgrupo, o miembro del grupo deberá realizar su propio DET, esto permitirá a cada uno organizar sus tareas y responsabilidades además permitirá asignar fechas de entrega y presupuestar el trabajo, además permitirá al ingeniero analizar cada detalle y además si

organizamos los niveles

inferiores podremos organizar una matriz morfología o de selección de componentes de acuerdo con si estamos diseñando o especificando.

112 Elementos de Rigidez

11 Chasis

113 Puntos de Soporte y Anclaje

114 Elementos de Protección 1 Kart Nacional 12 Motor

13 Transmisión

14 Sistema de Frenos

15 Dirección

121 122 123 131 132 133 141 142 143 144 145 146 151 152 153

Motor 2T/4T Sistema de Combustible Sistema de Anclaje Tren de Arrastre Sistema Embrague Sistema de Cambios Pedal Bomba Mangueras Mordazas Disco + Soporte Pastillas Transmisión de Movimiento Barras de Dirección Cabrilla

16 Habitáculo

Alaitz Izaguirre Hans Ley Savelsberg Felipe Aguirre Mauricio Toro Luis David Cardona León D. Castaño Miguel Gómez

AI HL FA MT LC LD MG

Tel 268 45 16 261 95 00 Ext 489 313 69 34

309 94 80 Ext 108

Cel 310 891 9941 [email protected] 301 419 9129 [email protected] 300 780 72 [email protected] 300 606 1333 [email protected] 313 509 4847 [email protected] 300 675 9401 [email protected] 300 659 1655 [email protected]

Correo Escala de Priooridades 1 a 5 (De menos a mas)

MT LC LD LD LD LD LD

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.

Variacion

Estimado(mil)

HL HL MT AI LC LC LC LC HL HL HL HL HL MT MT MT LD LD LD MT MT MT MT MT MT LD LD LD LD

Fecha Final

LD LD MT MT MG LC AI AI MG MG AI AI AI AI AI FA FA FA FA FA FA MG MG MG MG MG MG LC LC LC MG

Estado Actual

MT MT LD LD LD LD AI AI AI AI MT MT MT MT MT LC LC LC LC LC LC LD LD LD LD LD LD MT MT MT LD

Fecha Inicial

1 1 2 3 3 5 5 5 2 1 5 5 5 5 5 4 3 5 5 5 5 2 2 2 2 2 2 5 5 5 5

Asesora

Nivel 4 111.1 Formas de la Estructura 111.2 Material de la Estructura 112.1 Frontal 112.2 Posterior 113.1 Eje Trasero 113.2 Sistema de Dirección 113.3 Protecciones 113.4 Motor 113.5 Sistema de Frenos 113.6 Sistema de Acelerador 114.1 Cadena 114.2 Carenaje Lateral 114.3 Frontal Carro 114.4 Posterior 114.5 Frontal Piloto

Controla

Nivel 3 111 Estructura

Ejecuta

Nivel 2

Dirige

Nivel 1 07 de mAYo de 2007 Grupo Kart Nacional Hans Ley

Prioridad

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Real(mil)

Costo

PROYECTO DE DISEÑO Y CONSTRUCCION PROTOTIPO KART NACIONAL

EVALUACION DE CONOCIMIENTOS

En el diseño automotriz no hay ningún sustituto para la experiencia, sin embargo un equipo bien documentado y con buenos conocimientos teóricos es la segunda mejor opción. Por esto antes de proceder con las tareas que se han dividido y asumiendo que se de suficiente tiempo para

investigar el

coordinador del equipo deberá evaluar los conocimientos de los miembros del equipo. En este momento cada uno de los equipos debe ser puesto a prueba sobre las regulaciones de seguridad y los aspectos técnicos que debieron haber sido investigados

para la etapa anterior. Esta fase del diseño es

opcional ya que si el equipo considera que tienen las bases suficientes pues no tiene necesidad de hacerlo pero se recomienda que cada equipo de trabajo conozca su área y para esto se recomendara una bibliografía que cada equipo deberá tener en cuenta a la hora de realizar su desarrollo.

Bibliografía Recomendada

•

Aird, Forbes. Automotive Math Handbook. Motorbooks International

•

Aird, Forbes. Race Car Chassis Design and Construction. Motorbooks International.

•

Adams, Herb. Chassis Engineering. HP Books.

•

Stanifoth, Allan. Competition Car Suspension. Haynes.

•

McBeath, Simon. Competition Car Aerodynamics. Haynes.

•

Smith, Carroll. Tune to Win. Aero Books.

•

Smith, Carroll. Engineer to Win. Aero Books.

•

Milliken, William F., and Douglas L. Milliken. Race Car Vehicle Dynamics. SAE.

En estos libros esta incluida mucha de la información técnica que se debe conocer antes de aventurarse a diseñar un carro, especialmente la parte estructural que es tan critica para la seguridad de los pasajeros del vehiculo, se puede ver que muchos de los libros están dirigidos hacia los deportes a motor, sin embargo esto se hace porque es en este área donde esta la máxima tecnología y sorprende la semejanza de los sistemas técnicos, además los diseñadores de los vehículos de calle nos harían un favor a todos si analizan algunas de las medidas de seguridad que se utilizan en los vehículos de competición.

ANALISIS DE VIABILIDAD, MERCADEO Y ESTADO DEL ARTE

Antes de comenzar a pensar en como va a ser el carro el equipo debe hacerse y responderse de manera satisfactoria las siguientes preguntas. 1. ¿Porque esta diseñando un carro en vez de comprarlo o importarlo? 2. ¿Qué hace que su vehiculo sea mejor que los que actualmente están en el mercado? 3. ¿Qué gama ocupara el vehiculo que se esta diseñando? 4. ¿Cuál será la autonomía del vehiculo? 5. ¿Cuáles son las condiciones de carga a las que el carro estará sometido? 6. ¿Cada cuanto se harán actualizaciones o rediseños al vehiculo?

7. ¿Cuáles son las especificaciones de los vehículos de la gama de la que quiero hacer parte? 8. ¿Con que recursos cuento para el proyecto?

La respuesta a estas preguntas será determinante para orientar el proceso de diseño y definir la especificaciones del vehiculo, para esto serán necesarias tanto reflexión como investigación, La investigación se hará básicamente en tres frentes, la primera es una investigación de mercadeo con los potenciales clientes, a estos se les preguntara sobre las características de uso, formas, tamaños, desempeño, etc. Al cliente en las encuestas se le pregunta básicamente todo lo que se quiere saber y a donde se va a orientar el diseño, todo excepto una cosa ¿Cuánto pagaría usted por este vehiculo? O cualquier pregunta por ese estilo, la regla numero uno del diseño no se le pregunta al cliente cuanto quiere pagar. Aunque sonaría tentador ya que uno aseguraría que habría un mercado para el proyecto, en el mundo actual el precio esta dado por el mercado y la utilidad esta dada por los costos, por ende la mejor manera de determinar el precio de venta de un vehiculo es realizar un análisis de estado del arte de vehículos similares. Además hay que tener en cuenta que va a ser muy poco probable que el vehiculo que se vaya a diseñar sea completamente novedoso, es mas es muy probable que lo que se vaya a realizar sea simplemente una innovación en uno de los sistemas del vehiculo, por esto también resulta muy importante estudiar y analizar que componentes de cada vehiculo son comerciales abiertamente en el mercado y realizar trabajos de ingeniería inversa. Es importante orientar el mercadeo hacia las

cuatro p’s del mercadeo que son: Producto, Precio, Plaza y Promoción. Antes de comenzar el proyecto se debe tener muy claro que es lo que quiere, cuanto me va a valer, por cuanto lo puedo vender, como lo voy a promocionar, y a que segmento del mercado le estoy apuntando. Es importante apuntarle a un segmento determinado del mercado ya que el tiempo del modelo T ha cambiado y el cliente quiere un vehiculo que se adapte específicamente a sus necesidades. Esto es sumamente importante porque determinara el éxito o fracaso de nuestro proyecto y recordemos que solo un 10% de los vehículos concepto se materializan y por esto es importante para asegurar que el nuestro se materializa contar con la aprobación del mercado. Como reporte del estado del arte y ayuda en la especificación se deberá anexar al proceso un documento de estado del arte similar a este: ESTADO DEL ARTE PROYECTO VEHICULO COMPACTO Mauricio Toro R Nombre: X-T002 Trocha: D/Ejes: Alto:1535mm Ancho:1440mm Largo: 2805mm Nombre: Crossline Super Luxe Trocha (D/T): 1286/1265mm D/Ejes: 1960mm Alto: Ancho:1474mm Largo: 3050mm

Fabricante: Ligier Potencia: 5.36 hp Cilindrada: 505cc Pasajeros: 2 Velocidad Máxima: 45km/h Frenos (D/T):

Modelo: 2008

Fabricante: Aixam

Modelo: 2008

Potencia: Cilindrada: 400cc Pasajeros: 2 Velocidad Máxima: Frenos (D/T): Disco/Tambor

Nombre: Scouty R Trocha (D/T): 1337/1314mm D/Ejes: 1744mm Alto: Ancho:1508mm Largo: 2729mm

Fabricante: Aixam Potencia: 5.36hp Cilindrada: 400cc Pasajeros: 2 Velocidad Máxima: 45km/h Frenos (D/T): Disco/Tambor

Nombre: A721 Trocha (D/T):

Fabricante: Aixam Potencia:

D/Ejes:

Cilindrada:

Alto: Ancho:

Pasajeros: Velocidad Máxima: 45km/h Frenos (D/T): Disco/Tambor

Largo:

Nombre: M72 Trocha (D/T): 1231mm

Fabricante: Matra Potencia: 50hp

D/Ejes:

Cilindrada:

Alto: Ancho:

Pasajeros: 2 Velocidad Máxima: 100km/h Frenos (D/T): Disco/Disco

Largo: 3241mm

Nombre: Fortwo Trocha (D/T): 1275/1354mm

Fabricante: Smart Potencia: 51/61

D/Ejes: 1812mm

Cilindrada: 698cc

Alto: 1815mm Ancho: 1518

Pasajeros: 2 Velocidad Máxima: 135km/h Frenos (D/T): Disco/Tambor

Largo: 2500mm

Modelo: 2008

Modelo: 2008

Modelo: 2000

Modelo: 2008

Toda esta información será de utilidad a la hora de realizar el paso siguiente donde se deberá dimensionar el sistema para poder comenzar la bifurcación de las tareas de diseño.

ESPECIFICACION Y DIMENSIONADO BASICO DEL SISTEMA

Toda la información recopilada anteriormente nos permitirá realizar una especificación del sistema orientada tanto a lo que el cliente quiere como los factores que diferenciaran claramente al vehiculo de la competencia. Para esta etapa se realizara el PDS o un documento similar en el cual se anotan los requerimientos del cliente, Se puede realizar un PDS para el vehiculo completo pero este seria muy masivo ya que cada sistema del vehiculo tendrá sus propios requerimientos, por esto se recomienda que el equipo o persona encargada de cada uno de los sistemas realice una especificación de su sistema y después en reunión del equipo de trabajo se discuta cada una de estas y la coherencia que debe existir entre las diferentes de cada uno de los sistemas. Es la labor del coordinador del grupo de trabajo decidir, cuando deba existir un compromiso entre dos aspectos, cual sistema tiene prioridad sobre el otro. Este análisis debe ser muy minucioso ya que si existe alguna incompatibilidad en los PDS a partir de este proceso del diseño y no se identifica puede causar incompatibilidades en el modelo físico, lo cual traería costos adicionales y retrasaría el proceso de diseño. A continuación se presentara un ejemplo de un PDS realizado para un componente aerodinámico de un vehiculo de competición.

Necesidad

Que agarre

Interpretación

Métrica

genere Que genere una fuerza de Fuerza elevación invertida (downforce) a una velocidad menor a 160 Km/h Que no pierda Que no genere mucho arrastre Fuerza mucha velocidad Que no pese Que el material seleccionado y Masa mucho el espesor utilizados permitan minimizar el peso Que cumpla con Que la envergadura del ala no Longitud el reglamento sea superior al máximo ancho permitido Que cumpla con Que la altura del alerón permita Altura el reglamento la visibilidad del tercer stop. Que sea barato Que los costos de materiales y Precio mano de obra sean bajos Que se pueda Que permita variar el ángulo del Angulo ajustar ataque del alerón Que sea Que la relación entre downforce Eficiencia eficiente y arrastre sea alta Que sea fácil de Que la manufactura o Tiempo fabricar y reparación del alerón se puedan reparar realizar en un corto tiempo. Que sea Que estéticamente haga ver el Aprobación deportiva carro mas deportivo

Unidad

Valor

Deseo/Demanda

Newton

Mayor a 1000

Demanda

Peso en Decisión (110) 10

Newton

Menor a 200

Demanda

5

Kg

Menor a 8

Demanda

8

mm

1637

Demanda

10

mm

Mayor a 50

Demanda

10

Dolares

Menor a 250

Demanda

9

Grados (°)

Entre 0° y 20°

Deseo

6

Adimensional Mayor que 3

Deseo

7

Semanas

Menor a 2

Deseo

5

Personas

50% de Deseo encuestados

5

Una vez se complete la especificación del vehiculo tanto global, como sistemáticamente se puede proceder a realizar otra actividad extemporánea, la actividad del diseño de la arquitectura, esta actividad se debe realizar en una reunión de todo el equipo y para esta se deben considerar parámetros como: •

Ergonomía del conductor y los pasajeros.

•

Tamaño de las ruedas, y espacio requerido para que la suspensión pueda tener todo su recorrido

•

Tamaño y ubicación del compartimiento del motor

•

Tamaño y ubicación de la transmisión

•

Volumen, forma y ubicación del tanque de combustible

•

Volumen, forma y ubicación del espacio para el portaequipajes

Una vez se fijan estos parámetros se puede proceder a realizar la división de tareas.

Fig. 3 Esquema de Arquitectura de un vehiculo diseñado en la Universidad de Missouri, (USA)

TAREAS ESPECIALIZADAS

No esta en el alcance de este documento enseñarle a los diseñadores a diseñar o a los ingenieros a calcular y las fases del diseño establecidas en la primera sección son prácticamente auto explicativas, sin embargo hay dos cosas en las que se debe hacer hincapié. La primera es la lógica con la que se deben realizar las tareas de diseño, esta se basa en una filosofía que es aceptada como “ground up” cuya traducción significa desde el suelo hacia arriba, esta lógica esta basada en el flujo de las fuerzas y parte de la premisa que las únicas dos fuentes de fuerzas externas a un carro son las fuerzas aerodinámicas y mas importante las fuerzas de las ruedas y por ende desde aquí hacia arriba deberá realizarse el diseño del vehiculo, además resulta ser el mas lógico desde el punto de vista de los mecanismos porque evita la existencia de cualquier interferencia. Entonces se diseña de la llanta hacia arriba pero es importante hacer hincapié en un hecho específico, la parte estructural del chasis se diseña de última de resto el orden del diseño mecánico. La segunda parte en la que se quiere enfatizar es en las matrices de selección, estas son herramientas que sirven como asistencia para realizar decisiones objetivas a la hora de seleccionar las opciones que se presentan en un diseño. Estas metodologías ya están inventadas y en el documento Race Car Design de Alvaro Gracia (Anexo 1) esta muy claro como estas se deben realizar. Lo único que se quiere añadir aquí es que el usuario debe fijar muy bien los criterios de selección y los valores objetivos y no manipular los resultados ya

que de la honestidad del ingeniero con si mismo depende la calidad del producto.

EVALUACION

Una vez se fabrica el primer prototipo este se debe evaluar, pero su evaluación no es simplemente salir a dar vueltas en el, es importantísimo saber que parámetros deben ser medidos y que equipos se deben utilizar, para esto se hablara un poco de los parámetros importantes para el desarrollo del vehiculo. Se deben medir las aceleraciones, verticales, laterales y longitudinales en el carro, en condiciones de uso, abuso e impacto, estas deben ser analizadas y comparadas con las condiciones de frontera iniciales del proyecto.

De las

aceleraciones se pueden interpretar todos los datos desde números de ingeniera hasta comodidad.

CONCLUSION Cabe resaltar que el proceso de diseño automotriz es un proceso iterativo y que es difícil pretender que un vehiculo va a funcionar bien sino se le puede preguntar a BMW que tuvo en el 2004 un fallo catastrófico con su vehiculo diseñado por computador, también se debe decir que en el diseño y en especial el automotriz no existe un sustituto para la experiencia y por esto se incita a los equipos de diseño que trabajen con gente experimentada que le pueda transmitir los conocimientos a los miembros jóvenes del equipo.

BIBLIOGRAFIA •

Aird, Forbes. Automotive Math Handbook. Motorbooks International

•

Aird, Forbes. Race Car Chassis Design and Construction. Motorbooks International.

•

Adams, Herb. Chassis Engineering. HP Books.

•

Stanifoth, Allan. Competition Car Suspension. Haynes.

•

McBeath, Simon. Competition Car Aerodynamics. Haynes.

•

Smith, Carroll. Tune to Win. Aero Books.

•

Smith, Carroll. Engineer to Win. Aero Books.

•

Milliken, William F., and Douglas L. Milliken. Race Car Vehicle Dynamics. SAE.

•

Bravo, Santiago. Notas de Clase, Diseño Metodico Semestre 2007-1 Universidad EAFIT Medellín, Colombia.

ANEXO 1

Race Car Design

Study and Understand Race car design and construction demands a thorough understanding of the systems and components that make up the car, as well as an understanding of the physics involved. Before beginning design work on a car, you should understand how things work and why, so that when designing any individual component, the rest of the car's design intent can be taken into account.

Learning Sources The first source for someone without the additional funds for a university degree, should be the library. Hundreds of books and magazines exist relating to the concepts you will need for design, and the most useful of these are race car specific. Some of the most useful titles for general race car design are: Race Car Chassis Design and Construction - Forbes Aird, ISBN: 0-7603-0283-9 - A book about chassis design - excellent, with historical info. Chassis Engineering - Herb Adams, ISBN: 1-55788-055-7 - Handling, suspension design, physical forces - excellent. Engineer To Win and other 'To Win' books - Carroll Smith, ISBN: 0-87938-186-8 - Another excellent book - Metallurgy, engineering tips, nuts/bolts/fasteners, brakes, wheels, plumbing...a must have book. Racer's Encyclopedia of Metals, Fibers & Materials - Forbes Aird, ISBN: 0-87938-916-8 - Good information on materials used in race car fabrication. Racecar Engineering Magazine - Technical articles on all aspects of race car design. RaceTech Magazine - Another superb source of technical articles and technology explanations. A technique that is helpful for the designer-to-be, is to transcribe concepts that are of interest into a notebook or a computer. Later when working on the design you can easily refer to the appropriate reference, provided you categorize the information. One thing that cannot be stressed more...Reading is cheap..Redesigning is expensive.

The second source for design information comes from observation and hands-on. It is a major advantage to be able to study somebody else's work, especially if their car is winning races. Better still is the ability to work on a winning car. Good designers connect things in a logical, and sometimes ingenious way, and observing the nuances of a design with your new found knowledge is a good way to learn even more. There is also the internet. While a number of sites on the internet provide good information, it is darn hard to find. Books are the best way to learn, especially for the new student of race car design. However, there are a great number of web sites which provide valuable information in the form of guides, FAQs and tutorials. Searching usually takes a while, and general race car design principles are probably best learned from books, but sometimes you will run across good stuff.

The Best Way To Learn The best way for a would-be designer to learn is by first determining what type of car they wish to build. Learning everything there is to know about every type of car is admirable (and useful), but will lengthen the time required to ultimately complete your particular car. Learn all you can about the physics and features of the class or style you are building for, and you will have built a fairly solid basis for building a competitive car in that class. Then, be aware of other class technologies.

What You Must Know The construction of a race car is not a light matter. If you do not understand critical areas of race car design, you will likely have a critical failure at some point, which could lead to injury or death. If a grey area exists in your knowledge, refer to your books or to a mentor who has been racing for a long time in your chosen class. Here are the some of the key things you should understand before designing: Suspension / Handling • • • • • • • • • • •

Inertial forces about a car that is cornering, accellerating and braking Weight distribution and it's effect on the above Tire/wheel properties (Tread, rubber compounds, wheel materials) The relationships between tire and road The center of gravity and roll center relationship Unsprung weight Suspension geometry and handling Anti-roll bar principles Damper/shock absorber principles Suspension components, their use and placement for optimum performance Troubleshooting methods

Chassis Construction • • • • •

Structural design principles, most spaceframe design Load and forces which affect the race car Designing for the safety of the driver Materials and their physical properties (Tensile strength, elasticity, etc.) Joining methods (Welding, brazing, etc.)

Engine and Supporting Systems • •

A good understanding of the engine and drivetrain Intake, cooling and exhaust principles

• • • • •

Engine placement and mounting principles Transmission/Transaxle mounting principles Final drive components and placement Race car electrical systems Troubleshooting methods

Aero/Bodywork • • • • •

Principles of aerodynamics (Flow, pressure, etc.) Tools of aerodynamics (wings, venturis, flow redirection, etc.) Fiber/metal materials for bodywork and their fabrication Structural requirements of bodywork and aero devices Testing methods

Driver Support Systems • •

Driver safety considerations/driver support (Rollover, impacts) Ergonomics of driver and controls

Other • • •

Fuel cells and fuel delivery Fire extinguisher systems Probably a bunch more....

That about sums it up. The process of designing a race car is started with a solid knowledge and understanding. The more information you have, even without the benefit of past real-world experience, the more successful your car will be

Design Approaches Now that you have studied and understood, it is time to consider the way to approach the designing of a car. Firstly, the design process for a race car is linear, that is, each step is followed in succession. However, being as there are a million different ways to build things, the designer is quite often forced to consider other components which relate to the area being designed. For example, when designing the suspension of a car, you inadvertently affect the structural shape of the car's chassis in and around where the suspension will mount. Therefore, it is wise to construct the suspension first, keeping in mind the requirements of the things the suspension affects. Secondly, the design process demands a fair bit of estimation and compromise. Juggling performance, safety, efficiency and cost are what it's all about. This is where you want specifications ready to assist you in putting the pieces together.

Before You Get Started Computers, mostly PCs, are incredibly useful tools for race car design. A good, but optional investment is solid modeling software. Numerous companies make it, with very few selling below $5000. The lowest cost, value packed package is probably TurboCAD Solid Modeler. This software enables the designer to create virtual models of each part, in whatever required detail, and to create assemblies and finally a whole car. Sometimes training is required, but the

picture it gives is as complete as it gets. For those with a smaller budget, there are still 2D and 3D CAD programs to fit any budget. Other useful software also includes annotation and information recording/categorizing. Free tools are available for creating a simple database of information concerning your design.

The Race Car Project Steps The chart below illustrates the major steps in designing and eventually constructing a car. (See explanations below) The design steps are discussed in more detail below.

Research Of And Viability of Intended Class of Car is there to put a reality check into place before any work is done. There are many levels of motorsport and it is important that you understand the technical difficulty of the class you have chosen. Formula 1 is not a good place to start. Also consider the cost. Anyway, ask people racing in your intended class for help and you will probably get it. At this stage, it is wise to do a scale sketch of the car, in simple side, front, rear and top views. Assuming you are designing for an existing class, there will be plenty of examples of other's work, and your car's shape won't change much from the others. Information and Specifications Gathering is the step where you must go out and source the parts for your car. Whatever you cannot buy, you must fabricate, and 9 times out of 10 it costs more to fabricate, so off-the-shelf will result in lower cost. Fabrication does have it's advantages in that it allows for absolute control and optimization, an important feature in looser rule classes.

Whatever the course of action for a particular part, it is important to know the dimensions, weight, and features that apply. Catalog or record these figures for later reference. Collect brochures, and any other product information suitable--the internet has a wealth of companies offering specs and catalogs. Use a checklist of parts that the car will need, and collect several examples to chose from when dealing with race-critical parts. This checklist will be used later when design requirements meet available parts Rough Part Selection can be accomplished by first verifying that each of the available part models can do the job. If one can't, it's eliminated. Then, on the second go round, considerations such as space (volume) required, weight distribution characteristics, and aerodynamics can be evaluated and parts which don't fit can be eliminated. Repeating this process will usually get you down to 1 to 3 possible models. Always save your data. If requirements change somewhere else on the car, it may make a part you eliminated, feasible once again. The part selection process is somewhat simplified if you use your earlier sketches of the car as templates on which to draw the "spaces" occupied by each part on the checklist. Alot space, according to your research, keeping in mind weight distribution (Front/rear/left/right), safety, aerodynamics, and all the other effects the part has depending on placement. Work out several different layouts if you like, and consider later servicability. The Preliminary Design is where you translate the pictures you created above into a physical layout. You must focus on connecting all the parts, with small particulars like nuts and bolts left out of the picture, except where suspension and driveline are concerned. The idea at this stage, is to get a starting point. Then, you can use that baseline later when the REAL design work begins. It is a good idea to use CAD or Solid Modeling software for these tasks, as they are easily revised. Part Re-Selection or Re-Design is the next step. When you study your preliminary design, you should evaluate it for it's acceptability in terms of performance, safety (think impacts from all four sides, and rollover) and efficiency (how well it works for it's weight and size). If there are conflicts in the design, or areas that can be improved, make the change, but keep a baseline copy to go back to if the idea didn't work. The Final Design is not really the final design. Actually, it is the complete design. This is where you pull out the drafting paper (hard work), or start your solid modeling package (easiest). The goal of this design is to assemble the entirety of the parts you have in the design into a cohesive car. If suspension geometry wasn't considered prior, it is your last chance to consider it without redesigning. The saying "Built from the ground up" is true. No race can be designed without starting at the rubber contact patches, and working toward the chassis. To simplify life, Final Design Testing can be done if you have the right software. These tools consist of Finite Element Analysis (FEA) to test tortional and structural rigity for chassis, Fluid Dynamics to test aerodynamics, and even tools that allow for ergonomics testing. They are generally costly, but can be very helpful. Assuming everything has gone well, the Final Design Refinement and Completion will consist of small changes and verification that all parts work together, do not bind, etc. At this stage, it should be clear where every bolt goes, and how many bolts there are in the car. If it's not clear, then you need to complete your final design. More often then not, this will mean going back to research some more to find solutions to problems or shortcomings. Construction, Preliminary Testing, Car Analysis and Refinement and Future Development notes are planned for the future.

The Race Car Design Process Outlined in the table below is order in which major components of the race car can be designed, and some of the related aspects you will need to consider (There are many more than what are shown!) Order of Design

1

2

3

4

5

6

Component

Considerations

* Wheel appropriate for application * Tire appropriate for application Tires/Wheels * Wheel matches the hub/rotor * Available rotors and calipers are appropriate. * Unsprung weight is acceptable * Hub/Rotor appropriate for application Hub/Rotor * Same for bearings/spindles assembly, Wheel * Upright/knuckle design bearings, spindle, * Suspension geometry design Uprights ("At the wheel" suspension) * Loads affecting these components * Strong enough for application Suspension * Aerodynamics for exposed wishbones wishbones/axle * Mounting positions on chassis shafts, housings * Shocks/spring/anti-roll bar appropriate for application Mounting considerations Shocks/springs/anti- * roll bar * Leveraging (pivot) considerations and mounting * Spring/damping rate appropriate for travel, adjustability, vehicle weight, etc.? * Steering ratio * Left/right wheel movement (Toe in/out) through Steering suspension travel * Mounting location on chassis * Strong, intrusion-preventing safety cell for the driver * Good ergonomics for controls and seating. Good visual field * Pedals/Steering wheel positioning correct for driver Driver cockpit * Position for weight distribution * Appropriate steel tubing, bend radius for roll bar. * No protrusions that could cause injury to driver (This step could arguably be with suspension and steering, as it guides motor placement, if that course is preferred)

7

Driveline

8

Engine placement and mounting

* Determined torque handling for chassis * Mounting of differential * Driveshafts/Chain * Path of driving force not a wild angles * Proper materials used in high stress drive shafts/half shafts * Determined torque handling for chassis and mounting positions * Exhaust clearance and temperatures * Fuel and air delivery * Cooling system proximity

9

10

11

12

13

14

* Weight distribution * Transmission placement/weight * Positioned as far away from driver as possible * As close to center of gravity longtitudinally and laterally, but as close to the ground as possible vertically * Relative position to engine Fuel Cell * Fuel pump or delivery * Safety level (Degree of protection) appropriate * Mounting in chassis * Refuelling opening is located away from driver * In accessible location, for maintenance * Relative close position to engine Electrical/Engine Management/Battery * Battery located anywhere, but use for weight distribution * Chassis structure focused on handling forces generated by suspension mounts and steering Front chassis * Addresses safety, preferably through extended crumble zone(s). well ahead of the driver's legs. * Chassis structure focused on handling forces from side, frontal, and rear impacts as well as rollovers. Driver Safety cell * Anti-intrusion panelling to protect driver Chassis * Position for weight distribution * Chassis structure focused on handling forces generated by suspension mounts, as well as Rear chassis driveline torque * Addresses safety, via impact zones, or at least prevents engine intrusion into cockpit. * Light as possible * Aerodynamically attains goals of design Bodywork * Optimizes air flow allowed by class rules

There are quite a few more. The point however, is that the more you understand about the car you are designing, the more you will consider when designing. You will notice that the suspension is first in the design areas, then the engine, cockpit, electrical, and safety concerns are addressed. Finally, the chassis is designed around the requirements created before it. Each aspect listed above can be thought of as requiring you to consider every other aspect further down in the list. So, to select the tires and wheels, you must consider the entire car's dynamic requirements right through to aerodynamic shape. Two final words of advice. First, know the properties and parameters of what you are designing by consulting racers in your intended class. Second, understand the fundamental workings and physics affecting your race car. Combine the two, and you will understand what needs to be where in your car, and how strong everything needs to be to hold out for that chequered flag!

Starting From The Rules A good way to start your first race car is to build from a set of rules. While the rules won't tell you the best supplier of a part, they will simplify the variables of your design by specifying certain part models be used. Where you find the parts is your business. In sanctioned classes where a great many properties of the car design are specified in the rules, it is easier and cheaper to obtain parts because manufacturers are usually specified in the rules.

Looser rule books usually mean the freedom to explore more exotic materials and systems, a more costly proposition.

Incorporating Rules Into The Design Process Assuming you pick the class which suits your ability, you will want to make sure your design complies with the rules. For the rules-based designer, every component or part must be checked for compliance. This is not as difficult as it sounds (except in engine and drivetrain). The first place to start is by sketching out the approximate shape, in scale, of the car you intend to build (and try to follow the lead of winners in your intended class). Having diagrams of the engine and other components you intend to use is also helpful. Once the sketch is satisfactory, make copies of it so that you can annotate the (usually) many rules on them. Attach to the annotated sketch, a list of any rules which may be important but not visually drawn. All together, this will give you a picture of what you can and can't do in particular areas. As you design, refer to the sketches and lists to guide decision making. When you have found the right part, mark it, so that it is known that the rule has been adheared to. And when reselecting parts, always review the rules relating to them. Annotate any ideas which are grey areas. In the future it may be wise to get them cleared with the sanctioning body. One last comment, is that you should verify the rules aren't going to change drastically, before designing anything. There is no reason for not contacting the sanctioning body before beginning work.

Starting From Scratch Starting from scratch is not easy. The only real reason for designing without the rule book is to start a new class and usually, to gain sanctioning, you need to follow basic rules (ie. FIA's Formula Libre - Free formula). When designing without intrusive rules, one is free to explore cost, performance, and styling. However, the job is more difficult because of the vast array of available parts. What defines most cars are the cost, performance and styling requirements. If for instance, low cost is to be achieved, then all the parts and labour would have to be relatively low cost and therefore mass produced -- Hence, you might look at stock auto parts to cut costs. Every existing class has something to offer in the way of standards, parameters, etc. learned over the years. A new or inexperienced designer will pillage, pillage, pillage from everyone else's best efforts, optimizing his/her car with the information gained--no harm in that. A good designer will pillage, then improve, in order to best his/her opponents. And finally, the gurus of design, will pillage some, study a lot, and experiment with new ideas to outpace the competition. When designing without a great number of rules, still anotate them into a categorized list. In addition, you should outline aspects of the car that must comply with your vision/concept. Always refer back to these lists when making decisions.

Engineering Considerations As you design, it is important that you can gauge the requirements of your engineering work. The nature of the race car's normal operation and fatigue life depend on the structure and material composition of the car. Therefore, topics such as metallurgy and structural design are important for the designer to grasp. The whole concept of engineering considerations is that you keep in mind four aspects, where they are appropriate:

Safety

Performance vs. Strength vs. Weight

Durability (Life)

Cost

If you can optimize all four of these aspects, to select a most appropriate component, or structure for your car, then you are already winning (or at least saving your neck) Safety is a first consideration. If your car has proven safety, it will be a great confidence boost to the driver. Where appropriate, save your neck by using a quality solution. Performance vs. Strength vs. Weight is another factor that applies to every component on a car Durability comes into the picture mostly as a factor of weight penalty or cost. And finally Cost represents the ultimate limiting factor on most everything. If you can't afford it, it doesn't matter how well it performs. Each of the following sample questions ask the designer to address each of the four factors is some way, and to strike a balance between them. Sample Questions About Engineering Considerations What is the tortional rigidity of the chassis? Is it sufficient for the class you are running in? Can it be improved? What is a front/side/rear impact going to do to the chassis, at specific speeds of impact. Are sufficient anti-intrusion measures in place Is the structural design of mounts for suspension, engine and drivetrain adequate for the loads they are to carry? Is the safety roll bar adequate for protection? Is the aerodynamic body design condusive to lift or to downforce? (Good to know, especially for high speed racing) Is the body optimized for aerodynamics? Assuming the suspension and wheels are unchangeable, can any part of the body be changed, to improve aerodynamics? Don't break the rules, should you be using them. Is the suspension free of bind? Is the driveline clear of any obstructions or sensitive areas? Does spring and damper selection reflect the conditions to be expected at various tracks? What is the unsprung mass of the tire/wheel/suspension, and can it be improved within rule limits? What are the electrical wiring requirements for the entire car? A final design should include wiring, in order to evaluate potential problems. Are the most sensitive components of the car shielded adequately from elements and temperatures? Are the driver ergonomics such that control operations are all adequate, for drivers of varying heights/weights? (or perhaps just your height/weight)? Is the fuel cell compartment adequately designed to prevent fire from igniting fuel after

a mechanical failure or accident-related impact? Are the metallic and non-metallic materials used (especially in the engine bay), capable of withstanding the expected engine tempatures? Are all the appropriate critical components safety wired? Are all holes and cuts in metal properly designed so as to minimize crack propagation? Is the driver safe from head banging protrusions? This is just small example of the questions you will be able to answer, given a good study of engineering principles. You will be able to answer many more, assuming you spend a considerable amount of time getting aquainted with the knowledge.

Part Requirements For the scratch builder, probably one of the most time-consuming aspects of race car design is determining the correct part and finding a good, reliable source for it. For the builder of an existing class car, the job is somewhat simpler as other racers in that class can recommend parts and sources. Either way, the job of determining part requirements is pretty much the same. The first step is to list the parts required for your car which demand space, carry a weight penalty, or are absolutely required (either by rules or personal design). This covers pretty much everything in the car! Keeping a list of parts along with the potential sources and models that fit the application, you can build up a series of choices, from which you can optimize for the best package. Click here to see the parts checklist . It is limited to the larger, common items, and in your research you will probably come across things which are not on this list. Depending on your goals or vision for the car, you will pick potential models of parts which conform to that vision. ie. If you are building for low cost, potential models will be geared toward low cost. If a particular part is not available, then fabrication may be the only alternative. It certainly costs more than a mass produced part, but the results are very in tune with your needs. Also, you will probably encounter a situation in where in order to gain the advantages of a particular part, you must use the rest of the parts from the same manufacturer or donor car (up to a point). For instance, to use a particular bolt pattern wheel, you must have a hub that matches, suitable disc rotors, proper spindle, and suitable bearings. All these parts work in an assembly, and in the end require you to take the perfect part with the less than perfect, unless each part is optimized already. The end result of all the research and communicating, will be a short list of parts that, when used in the right combination with others, will produce superior results. Prior to design, you will need to determine which part combos work in this superior way.

Balancing Requirements In the end, all this design work culminates into a final design which balances all priorities in a neat fashion. In fact, looking back at your choices, you can gain a sense of pride in knowing that your brain has given each one it's due attention.

So how do you balance requirements? A matrix is a good tool. By rating several parts or their interactions, one can decide if a part is useful over a broader range of criteria. Consider wheels. Here is a matrix defining the criteria:

Wheel brand

Does it suit the Is the required tread of MNO hub wheel nut tire available? pattern?

Brake disc cooling by design?

Xyz, Inc. 15 x 7

No

Yes

Yes

Abc, Inc. 15 x 7

Yes

Yes

No

Simple as it is, it shows that right off the bat, the Xyz wheel is not going to work with the preferred MNO hub. However, maybe another hub would work too, a further criteria. However, disregarding this fact, it is apparent that the disc rotor would be better cooled with the Xyz wheel. This is a compromise and an integral part of balancing requirements. The designer would at this point have the choice of either scrapping both wheel models, and finding more sources and examples, or settling for the Abc wheel. It is important to keep safety at heart as well. Performance is best had in a car that can handle the lumps should something go wrong. Drivers will want to get back in if they don't have to be extracted. If cost is a limitation, then performance will have to suffer to offer the lower cost.

Balancing Your Time As a final word for this section, you should make building your race car an enjoyable experience. Sacrificing relationships and relaxation time, over the long haul required to build a car, is not healthy. Pace yourself, and enjoy life, and if deadlines really beckon, then push, push, push! Good luck!

Tips: Aerodynamics The following tips and information focus on how to optimize aerodynamics. Depending on class rules, these suggestions may or may not be valid. Always check your regulations. General Aerodynamics Principles • • • •

Drag Lift/Downforce Drag Coefficient Frontal Area

Aerodynamic Devices • • • •

Scoops/Positive pressure intakes NACA Ducts Spoilers Wings

Aerodynamics Design Tips

General Aerodynamic Principals

Drag A simple definition of aerodynamics is the study of the flow of air around and through a vehicle, primarily if it is in motion. To understand this flow, you can visualize a car moving through the air. As we all know, it takes some energy to move the car through the air, and this energy is used to overcome a force called Drag. Drag, in vehicle aerodynamics, is comprised primarily of two forces. Frontal pressure is caused by the air attempting to flow around the front of the car. As millions of air molecules approach the front grill of the car, they begin to compress, and in doing so raise the air pressure in front of the car. At the same time, the air molecules travelling along the sides of the car are at atmospheric pressure, a lower pressure compared to the molecules at the front of the car. Just like an air tank, if the valve to the lower pressure atmosphere outside the tank is opened, the air molecules will naturally flow to the lower pressure area, eventually equalizing the pressure inside and outside the tank. The same rules apply to cars. The compressed molecules of air naturally seek a way out of the high pressure zone in front of the car, and they find it around the sides, top and bottom of the car. See the diagram below.

Rear vacuum (a non-technical term, but very descriptive) is caused by the "hole" left in the air as the car passes through it. To visualize this, imagine a bus driving down a road. The blocky shape of the bus punches a big hole in the air, with the air rushing around the body, as mentioned above. At speeds above a crawl, the space directly behind the bus is "empty" or like a vacuum. This empty area is a result of the air molecules not being able to fill the hole as quickly as the bus can make it. The air molecules attempt to fill in to this area, but the bus is always one step ahead, and as a result, a continuous vacuum sucks in the opposite direction of the bus. This inability to fill the hole left by the bus is technically called Flow detachment. See the diagram below.

Flow detachment applies only to the "rear vacuum" portion of the drag equation, and it is really about giving the air molecules time to follow the contours of a car's bodywork, and to fill the hole left by the vehicle, it's tires, it's suspension and protrusions (ie. mirrors, roll bars). If you have witnessed the Le Mans race cars, you will have seen how the tails of these cars tend to extend well back of the rear wheels, and narrow when viewed from the side or top. This extra bodywork allows the air molecules to converge back into the vaccum smoothly along the body into the

hole left by the car's cockpit, and front area, instead of having to suddenly fill a large empty space. The reason keeping flow attachment is so important is that the force created by the vacuum far exceeds that created by frontal pressure, and this can be attributed to the Turbulence created by the detachment. Turbulence generally affects the "rear vacuum" portion of the drag equation, but if we look at a protrusion from the race car such as a mirror, we see a compounding effect. For instance, the air flow detaches from the flat side of the mirror, which of course faces toward the back of the car. The turbulence created by this detachment can then affect the air flow to parts of the car which lie behind the mirror. Intake ducts, for instance, function best when the air entering them flows smoothly. Therefore, the entire length of the car really needs to be optimized (within reason) to provide the least amount of turbulence at high speed. See diagram below (Light green indicates a vacuum-type area behind mirror):

Lift (or Downforce) One term very often heard in race car circles is Downforce. Downforce is the same as the lift experienced by airplane wings, only it acts to press down, instead of lifting up. Every object travelling through air creates either a lifting or downforce situation. Race cars, of course use things like inverted wings to force the car down onto the track, increasing traction. The average street car however tends to create lift. This is because the car body shape itself generates a low pressure area above itself. How does a car generate this low pressure area? According to Bernoulli, the man who defined the basic rules of fluid dynamics, for a given volume of air, the higher the speed the air molecules are travelling, the lower the pressure becomes. Likewise, for a given volume of air, the lower the speed of the air molecules, the higher the pressure becomes. This of course only applies to air in motion across a still body, or to a vehicle in motion, moving through still air. When we discussed Frontal Pressure, above, we said that the air pressure was high as the air rammed into the front grill of the car. What is really happening is that the air slows down as it approaches the front of the car, and as a result more molecules are packed into a smaller space. Once the air Stagnates at the point in front of the car, it seeks a lower pressure area, such as the sides, top and bottom of the car. Now, as the air flows over the hood of the car, it's loses pressure, but when it reaches the windscreen, it again comes up against a barrier, and briefly reaches a higher pressure. The lower pressure area above the hood of the car creates a small lifting force that acts upon the area of the hood (Sort of like trying to suck the hood off the car). The higher pressure area in front of the windscreen creates a small (or not so small) downforce. This is akin to pressing down on the windshield. Where most road cars get into trouble is the fact that there is a large surface area on top of the car's roof. As the higher pressure air in front of the wind screen travels over the windscreen, it accellerates, causing the pressure to drop. This lower pressure literally lifts on the car's roof as the air passes over it. Worse still, once the air makes it's way to the rear window, the notch created by the window dropping down to the trunk leaves a vacuum, or low pressure space that

the air is not able to fill properly. The flow is said to detach and the resulting lower pressure creates lift that then acts upon the surface area of the trunk. This can be seen in old 1950's racing sedans, where the driver would feel the car becoming "light" in the rear when travelling at high speeds. See the diagram below.

Not to be forgotten, the underside of the car is also responsible for creating lift or downforce. If a car's front end is lower than the rear end, then the widening gap between the underside and the road creates a vacuum, or low pressure area, and therefore "suction" that equates to downforce. The lower front of the car effectively restricts the air flow under the car. See the diagram below.

So, as you can see, the airflow over a car is filled with high and low pressure areas, the sum of which indicate that the car body either naturally creates lift or downforce.

Drag Coefficient The shape of a car, as the aerodynamic theory above suggests, is largely responsible for how much drag the car has. Ideally, the car body should: • • • • • •

Have a small grill, to minimize frontal pressure. Have minimal ground clearance below the grill, to minimize air flow under the car. Have a steeply raked windshield to avoid pressure build up in front. Have a "Fastback" style rear window and deck, to permit the air flow to stay attached. Have a converging "Tail" to keep the air flow attached. Have a slightly raked underside, to create low pressure under the car, in concert with the fact that the minimal ground clearance mentioned above allows even less air flow under the car. If it sounds like we've just described a sports car, you're right. In truth though, to be ideal, a car body would be shaped like a tear drop, as even the best sports cars experience some flow detachment. However, tear drop shapes are not condusive to the

area where a car operates, and that is close to the ground. Airplanes don't have this limitation, and therefore teardrop shapes work. What all these "ideal" attributes stack up to is called the Drag coefficient (Cd). The best road cars today manage a Cd of about 0.28. Formula 1 cars, with their wings and open wheels (a massive drag component) manage a minimum of about 0.75. If we consider that a flat plate has a Cd of about 1.0, an F1 car really seems inefficient, but what an F1 car lacks in aerodynamic drag efficiency, it makes up for in downforce and horsepower.

Frontal Area Drag coefficient, by itself is only useful in determining how "Slippery" a vehicle is. To understand the full picture, we need to take into account the frontal area of the vehicle. One of those new aerodynamic semi-trailer trucks may have a relatively low Cd, but when looked at directly from the front of the truck, you realize just how big the Frontal Area really is. It is by combining the Cd with the Frontal area that we arrive at the actual drag induced by the vehicle. Aerodynamic Devices

Scoops Scoops, or positive pressure intakes, are useful when high volume air flow is desireable and almost every type of race car makes use of these devices. They work on the principle that the air flow compresses inside an "air box", when subjected to a constant flow of air. The air box has an opening that permits an adequate volume of air to enter, and the expanding air box itself slows the air flow to increase the pressure inside the box. See the diagram below:

NACA Ducts NACA ducts are useful when air needs to be drawn into an area which isn't exposed to the direct air flow the scoop has access to. Quite often you will see NACA ducts along the sides of a car. The NACA duct takes advantage of the Boundary layer, a layer of slow moving air that "clings" to the bodywork of the car, especially where the bodywork flattens, or does not accellerate or decellerate the air flow. Areas like the roof and side body panels are good examples. The longer the roof or body panels, the thicker the layer becomes (a source of drag that grows as the layer thickens too). Anyway, the NACA duct scavenges this slower moving area by means of a specially shaped intake. The intake shape, shown below, drops in toward the inside of the bodywork, and this draws the slow moving air into the opening at the end of the NACA duct. Vorticies are also generated by the "walls" of the duct shape, aiding in the scavenging. The shape and depth change of the duct are critical for proper operation.

Typical uses for NACA ducts include engine air intakes and cooling.

Spoilers Spoilers are used primarily on sedan-type race cars. They act like barriers to air flow, in order to build up higher air pressure in front of the spoiler. This is useful, because as mentioned previously, a sedan car tends to become "Light" in the rear end as the low pressure area above the trunk lifts the rear end of the car. See the diagram below:

Front air dams are also a form of spoiler, only their purpose is to restrict the air flow from going under the car.

Wings Probably the most popular form of aerodynamic aid is the wing. Wings perform very efficiently, generating lots of downforce for a small penalty in drag. Spoiler are not nearly as efficient, but because of their practicality and simplicity, spoilers are used a lot on sedans. The wing works by differentiating pressure on the top and bottom surface of the wing. As mentioned previously, the higher the speed of a given volume of air, the lower the pressure of that air, and vice-versa. What a wing does is make the air passing under it travel a larger distance than the air passing over it (in race car applications). Because air molecules approaching the leading edge of the wing are forced to separate, some going over the top of the wing, and some going under the bottom, they are forced to travel differing distances in order to "Meet up" again at the trailing edge of the wing. This is part of Bernoulli's theory. What happens is that the lower pressure area under the wing allows the higher pressure area above the wing to "push" down on the wing, and hence the car it's mounted to. See the diagram below:

Wings, by their design require that there be no obstruction between the bottom of the wing and the road surface, for them to be most effective. So mounting a wing above a trunk lid limits the effectiveness. Aerodynamic Design Tips •

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Cover Open wheels. Open wheels create a great deal of drag and air flow turbulence, similar to the diagram of the mirror above. Full covering bodywork is probably the best solution, if legal by regulations, but if partial bodywork is permitted, placing a converging fairing behind the wheel provides maximum benefit. Minimize Frontal Area. It's no coincidence that Formula 1 cars are very narrow. It is usually much easier to reduce FA (frontal area) than the Cd (Drag coefficient), and top speed and accelleration will be that much better. Converge Bodywork Slowly. Bodywork which quickly converges or is simply truncated, forces the air flow into turbulence, and generates a great deal of drag. As mentioned above, it also can affect aerodynamic devices and bodywork further behind on the car body. Use Spoilers. Spoilers are widely used on sedan type cars such as NASCAR stock cars. These aerodynamic aids produce downforce by creating a "dam" at the rear lip of the trunk. This dam works in a similar fashion to the windshield, only it creates higher pressure in the area above the trunk. Use Wings. Wings are the inverted version of what you find on aircraft. They work very efficiently, and in less aggressive forms generate more downforce than drag, so they are loved in many racing circles. Wings are not generally seen in concert with spoilers, as they both occupy similar locations, and defeat each other's purpose. Use Front Air Dams. Air dams at the front of the car restrict the flow of air reaching the underside of the car. This creates a lower pressure area under the car, effectively providing downforce. Use Aerodynamics to Assist Car Operation. Using car bodywork to direct airflow into sidepods, for instance, permits more efficient (ie. smaller FA) sidepods. Quite often, with some for-thought, you can gain an advantage over a competitor by these small dual purpose techniques.

Another useful technique is to use the natural high and low pressure areas created by the bodywork to perform functions. For instance, Mercedes, back in the 1950s placed radiator outlets in the low pressure zone behind the driver. The air inlet pressure which fed the radiator became less critical, as the low pressure outlet area literally sucked air through the radiator. A useful high pressure area is in front of the car, and to make full use of this area, the nose of the car is often slanted downward. This allows the higher air pressure to push down on the nose of the car, increasing grip. It also has the advantage of permitting greater driver visibility.

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Keep Protrusions Away From The Bodywork. The smooth airflow achieved by proper bodywork design can be messed up quite easily if a protrusion such as a mirror is too close to it. Many people will design very aerodynamic mounts for the mirror, but will fail to place the mirror itself far enough from the bodywork. Rake the chassis. The chassis, as mentioned in the aerodynamics theory section above, is capable of being slightly lower to the ground in the front than in the rear. The lower "Nose" of the car reduces the volume of air able to pass under the car, and the higher "Tail" of the car creates a vacuum effect which lowers the air pressure. Cover Exposed Wishbones. Exposed wishbones (on open wheel cars) are usually made from circular steel tube, to save cost. However, these circular tubes generate turbulence. It would be much better to use oval tubing, or a tube fairing that creates an oval shape over top of the round tubing. See diagram below:

Tips: Chassis The following tips and information focus on how to optimize a race car chassis, specifically the spaceframe-type chassis. Depending on class rules, these suggestions may or may not be valid. Always check your regulations. General Spaceframe Chassis Principles Chassis Design Tips General Spaceframe Chassis Principals

Spaceframes The spaceframe chassis is about as old as the motorsport scene. It's construction consists of steel or aluminum tubes placed in a triangulated format, to support the loads from suspension, engine, driver and aerodynamics. Spaceframes are popular today in amateur motorsport because of their simplicity. Most everyone who has access to a level workshop, a saw, measuring tools, and a welder of some kind can build one.

There are also some inherent advantages to using spaceframes at the amateur level of motorsport as well. Spaceframes, unlike the monocoque chassis used in modern Formula 1 or CART, are easily repaired and inspected for damage. So how does triangulation work? The diagram below shows a box, with a top, bottom and two sides, but the box is missing the front and back. The box when pushed, collapses easily because there is no support in the front or back.

Of course, race cars need to be supported in order to operate properly, and so we triangulate the box by bracing it diagonally. This effectively adds the front and back which were missing, only instead of using panels, we use tubes to form the brace. See below:

The triangulated box above imparts strength by stressing the green diagonal in Tension. Tension is the force trying to pull at both ends of the diagonal. Another force is called Compression. Compression tries to push at both ends of the diagonal (Shown above in the horizontal yellow tube). In a given size and diameter tube or diagonal, compression will always cause the tube to buckle long before the same force would cause the tube to pull apart in tension. As an experiment, try pulling on the ends of a pop can, one end in each hand. Then, try crushing the can by pushing on both ends. The crushing is much easier, or at least humanly possible, compared to pulling the can apart. Spaceframes are really all about tubes held together in compression and tension using 3D pyramid-style structures, and diagonally braced tube boxes. A true spaceframe is capable of holding it's shape, even if the joints between the tubes were hinges. In practice, a true spaceframe is not practical, and so many designers "cheat" by using stronger materials to support the open portions of the structure, such as the cockpit opening. In contrast to spaceframes, the monocoque chassis uses panels, just like the sides of the box pictured above. Instead of small tubes forming the shape of a box, an entire panel provides the strength for a given side.

A common shape for 1960s cars of monocoque construction was the "cigar". The cylindrical shape helped impart something called Tortional rigidity. Tortional rigidity is the amount of twist in the chassis accompanying suspension movement. See the diagram below.

Tortional rigidity applies to spaceframes too, but because a spaceframe isn't made from continuous sheet metal or composite panels, the structure is used to approximate the same result as the difficult to twist "cigar car". Another reason tortional rigidity is mentioned here is that it greatly affects the suspension performance. The suspension itself is designed to allow the wheels/tires to follow the road's bumps and dips. If the chassis twists when a tire hits a bump, it acts like part of the suspension, meaning that tuning the suspension is difficult or impossible. Ideally, the chassis should be ultrarigid, and the suspension compliant. It is important to ensure that the entire chassis supports the loads expected, and does so with very little flex. Chassis Design Tips •

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Design the chassis after the suspension One of the biggest mistake novices make is to design the chassis before the suspension. It is much easier to design a tentative suspension according to the rules and good geometry, and then build the chassis to conform to suspension mounting points and springs/damper mounts. See our Design Approaches section for more information. Consider the load paths. A chassis is not about "absorbing" energy, but rather about support. When considering placement of tubes, visualize the "load paths". Load paths are defined as the forces resulting from accellerating and decellerating, in the longtitudinal and lateral directions which follow the tubing from member to member. The first forces which come to mind are suspension mounts, but things like the battery and driver place stresses on the spaceframe structure. Maximize CG placement and vehicle balance. Center of gravity affects the race car like a pendulum. The ideal place for the CG is absolutely between the front and rear wheels and the left and right wheels. Placing the CG fore or aft or left or right of this point means that weight transfers

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unevenly depending on which way the car is turning, and whether it is accellerating or decellerating. The further from this ideal point, the more one end of the car acts like a pendulum, and the more difficult it is to optimize handling. The CG is also height dependant. Placing an engine higher off the ground raises the CG, and forces larger amounts of weight to transfer when cornering, accellerating, or decellerating. The goal of vehicle design is to keep all four wheels planted if possible, to maximize grip, so placing all parts in the car at their lowest possible location will help lower the CG. Of course, in terms of spaceframe design, you have to leave space for each of the parts. Layout the tube members for easy access and maintenance. Maintaining a race car comes after construction. Placing tubes across openings is a natural way of ensuring a rigid chassis. However, in practical terms, you may be making it difficult or impossible to reach the mainenance demanding components. A good chassis design will allow quick and easy access to all components, and will not hamper removal or replacement of any part. Check out cars which are competitive in your class. Cars which are competitive are usually built well, and with appropriate materials and methods. Observe these cars at the track and in the pits, and you can infer a great deal about what makes them winners. Optimize the tubing shape for the job. Square tubing, which is known for it's ease of cutting and joining is better in situations where bending forces occur. However, round tubing is generally stronger in all other cases, albeit at a penalty in the complexity of construction. Optimize the tubing size and gauge for the job. Tubing which is used in tension, can be of a lighter gauge than that used in compression. Keeping this in mind can save considerable weight, although it requires additional joining work and variety of tubing.

Tips: Suspension The following tips and information focus on how to optimize a race car suspension. Because of the numerous types of suspension, we suggest you read some excellent books that cover this topic in much more detail. Depending on class rules, these suggestions may or may not be valid. Always check your regulations. General Suspension Design Principles Suspension Design Tips General Suspension Design Principals

Unsprung Weight Unsprung weight is a measurement of the weight of everything outboard of the wishbones or suspension links, plus 1/2 of the weight of the wishbones or links and spring/shock. It has a great effect on handling. The diagram below demonstrates why unsprung weight is so important:

The more weight outboard of the car, the more force bumps exert on the suspension (and ultimately the chassis). This force must be dealt with using springs, dampers and anti-roll bars (described below), and the more force, the more difficult it is to keep the tire planted on the road. This is especially true of lighter weight cars. In the example above, if the car weighs 1000 lbs, a 2G bump would result in a vertical force of 10% of the car's weight. This will at the very least reduce the grip of the car, because the weight of the car is what keeps the tire planted, and pushing a car up into the air with that much force will inevitably reduce the weight on the tire, and hence grip.

Tires As the first point of contact with the road, the tires work in conjunction with the suspension geometry and weight transfer dynamics to provide grip. Many different types of tires exist, but provided you are building for a specific class, you can easily select a particularly good or popular tire. The grip provided by a tire is linked to the coefficient of friction (Cf) of the rubber compound and to the tire's construction (Radial/bias). This coefficient indicates the lateral grip the tire is capable of providing for a given weight being placed on it. Racing slicks are very high Cf tires, in the range of 1.0 or more. Street radials, on the other hand, rarely even approach 1.0. So what is in a number? If you were to place 500 lbs weight onto each of four tires with a Cf of 1.0, you could expect 2000 lbs (actually a little less) of lateral grip. Without aerodynamic aids to add to vehicle weight, the car would almost achieve a 1G turn.

Wheels Of course, the wheel is what the tire mounts on. Wheels also come in a myriad of widths, sizes and materials. The primary types of wheels used in racing are alloy and steel. Alloy wheels can be constructed to very minimal weights, as alloying materials such as aluminum and magnesium can be used. They are also generally much more expensive than their steel counterparts, but they also lack the dent resistance of steel wheels. An alloy wheel, when struck by a curb will sometimes shatter, and possibly worse, crack (only later to fly apart!). Nonetheless, for most motorsports series, alloys are the choice. Steel wheels can also be constructed to amazingly low weights. Their cost is quite a bit less than the alloys, due mostly to lower cost construction. Steel wheels are deformable when struck, and will usually allow air to leak out of the tire, as opposed to shattering. NASCAR, and the general stock car scene use steel wheels due to the extreme forces encountered by 2 ton cars.

Uprights (Wishbone suspension)/Knuckles The upright or knuckle attaches the wheel, brake rotor, hub, brake caliper and steering arm to the car (of course, the wishbones and control arm(s) do the final attachment to the chassis)

The upright or knuckle determines the king-pin inclination, and the final camber, caster, and toe settings of the wheel and tire. These various factors are demonstrated in the diagram below.

Kingpin Inclination determines steering feel to a great extent. In the front view above, the red line on the right represents the center line of the tire/wheel. The kingpin inclination is several degrees, the angle between the center line and the line running through the upright or knuckle. The kingpin inclination determines steering effort, and feedback. Scrub radius is the distance from the centerline of the tire/wheel to where the kingpin line intersects with the road surface. The larger the distance, the more effort is required to turn the wheel, as the wheel has to "scrub" slightly to turn around the kingpin axis. Camber is the angle between vertical (perpendicular to a flat road surface) and the "lean" of the tire/wheel. In the diagram above, negative camber of about 2 or 3 degrees is shown. Negative camber is often used to offset the normally positive change in camber as the wheel moves up. The concept of camber is simply to keep the tire contact patch as large as possible through the complete range of suspension motion. Toe-In/Out is a slight steering angle that is preset into the suspension. Toe-in has the tires pointing slightly toward the center of the car's front. Toe-out has the cars pointing slightly away from the car. In the diagram above, there is zero toe-in/out. Toe-in/out is used to offset the natural change in toe position caused by braking and accelleration. Caster is the angle from vertical of the upright/knuckle, when viewing the wheel/tire from the side. This angle is used to create a gyroscopic effect on steering. This is easily demonstrated by turning the steering wheel in the car and then letting go of the wheel (Do this in an empty parking lot!). The caster causes the steering to correct itself back to straight ahead, instead of turning, without the need for driver input. As you can well imagine, all these factors work together to produce a varying contact patch and steering feel in the car. Software exists for designing suspensions, and a computer makes it easy to see changes and how they affect the contact patch of the tire. Like the wheel and tire, weight here plays an important part as well.

Wishbones/Control Arms Wishbones and control arms connect the previously mentioned upright or knuckle to the car chassis. The wishbones or control arms (depending on suspension type) affect the previously mentioned factors as well. Camber, castor, and toe are all affected to some degree.

Essentially the wishbones connect to the chassis with rod-ends or spherical bearings, allowing the wishbones to pivot up and down with the wheel's movement and triangulating the suspension to prevent the wheel from moving fore or aft of it's designated position. Outboard, at the upright or knuckle, there are two ball joints, one for each wishbone. See the diagram below for a better visual representation:

The toe link, shown in blue above, is attached to the steering rack at the front of the car, and to the chassis at the rear. Toe adjustments are made by varying the length of this link. Suspension Design Tips •

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Use aerodynamic wishbones on open wheel cars. Open wheel cars, round wishbone tubes will create turbulence and drag. The prefered tubing is oval, which in profile allows the airflow to diverge and converge nicely without turbulence. Use at least a little scrub radius. Some books suggest eliminating the scrub radius. In race cars this can be a problem, as much of the feel about the car's handling can be lost. The scrub radius allows the driver to feel when the tires lose traction, without being "too far gone" to recover. Use strong, high quality rod ends and other fasteners. Fasteners and rod ends can be expensive, but one of the last places you want to save money is on the suspension. These parts are often lamented by amateurs because the parts they use break too often. When selecting rod ends, bear in mind the angle, mounting and location of rod ends has an impact on their longevity. Protect the driver from the suspension. Broken chassis mounts can be deadly or at least crippling. Using aluminum plates along the sides of the chassis will prevent broken wishbones from perforating the driver's legs. Radial vs. Bias ply tires and Camber. Radial tires are more tolerant of static negative camber, or camber that is built into the suspension. If the suspension's range of motion is substantial (more than 2 or 3 inches of bump travel, and 2 or 3 inches of dip), then using more negative camber to compensate for the positive change introduced by the suspension helps. Radial tires will work better with this situation. Minimize Unsprung weight. Unsprung weight, or the weight comprised by tire, wheel and suspension affects how well the tire follows the bumps and dips in the road surface. Using lighter wheels, tires, uprights, wishbones or control arms, and other parts will reduce the weight. The weight of these suspension parts by itself is not so critical as the ratio between the car's sprung weight (chassis, driver, engine, etc) and the unsprung weight. The lower the unsprung weight in relation to the sprung

weight, the easier it will be to control the tire/wheel via the springs, dampers (shocks) and anti-roll bars.

Tips: Safety & Ergonomics The following tips and information focus on how to optimize race car safety and ergonomics. Depending on class rules, these suggestions may or may not be valid. Always check your regulations. General Safety/Ergonomics Design Principles Safety/Ergonomics Design Tips General Safety/Ergonomics Design Principals

Safety Safety in a race car is the art of protecting the human occupant, at whatever cost to the car. Designing the car to be damaged minimally while hindering driver safety is definitely the wrong approach. So how do we protect the driver? Well first we need to consider the basic physiological weak points of the human body. The diagram above shows that pretty much any part of the body exposed to the chassis of the race car is at risk. Injuries occur because the body sustains impacts beyond the G (gravities) level that it can sustain. The brain is particularly succeptible to injury, because it is really just a soft tissue mass stored inside a very solid bone container, the skull. The key to avoiding injury in the brain is to avoid instantaneous decelleration of the skull. That is, when the skull strikes something hard, it decellerates instantaneously. The brain inside unfortunately keeps on moving, causing head trauma. Neck and spinal injuries also present a serious threat to life and career. These "Connector" type elements in our body are flexible and stretchable, to a point, and can sustain tremendous G loads before breaking. However, depending on angle of impact, they can break rather easily. Other bone injuries (breakages) are not as life-threatening or career ending, but still are to be prevented. The bones in our arms, legs and spine are designed to be stressed in tension and compression along their length. In the case of impacts they are often stressed in shear or bending, and therefore snap relatively easily.

Safety In Engineering Safety in race cars consists of optimizing the chassis and bodywork to provide maximum support for normal driving situations, and maximum protection and energy absorption in crash situations. First, the driver needs to be supported, so movement under normal driving is very limited. This means a seat with lateral head support, a head rest, and good lower and upper body lateral support. Most racing seats provide these three elements.

Secondly, the car's chassis needs to hold the seat and driver in place, in all situations, driving and crashing. This is of course accomplished with a chassis mount for the seat, and a 5 or 6 point harness. Thirdly, measures must be taken to prevent intrusion into or the crushing of the driver's limbs and extremities. On formula cars, the problem of suspension wishbones breaking and piercing the driver's legs is solved by anti-intrusion panels that prevent pieces of the car from intruding into the driver's cockpit. As well, the cockpit "Safety cell" needs to be very strong. The "Safety cell" is the last piece of material between danger and the driver, and so should be well constructed, and not prone to collapsing onto the driver. Finally, the car needs to absorb the energy via structures that are crushable. As stated previously, the human body does not like to be decellerated from 80 or 100 km/h to 0 instantly. Therefore, we need to find a way that "quickly" decellerates the body. The only possibilities on a race car are the structures which surround the driver's safety cell. Designing these structures to collapse in an impact ensures that G levels are reduced because the car is literally decellerating over a small distance, instead of ZERO distance. Below is a diagram:

Ergonomics Ergonomics, or the study of human-machine interfacing, is important to race cars because the ultimate control of the car belongs to the driver. Poorly placed controls mean the driver must lose concentration on the race, and instead focus on the cockpit. The ergonomics of a race car cockpit consist of several elements: •

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The driver's line of sight - Visibility is of prime importance. The goal in design is to ensure enough of the race track in front is visible, and enough of the action to left and right is visible, through peripheral vision. Of course, the driver also needs to see behind to watch for his/her competition. The mirrors should act as an extension of the visible field. The steering wheel - The steering wheel is a tool of leverage. If a steering wheel is too far from the driver, the driver's arms will straighten, and ultimately limit the range of motion easily provided. If it doesn't stop the driver from driving properly, this situation will cause fatigue. If a steering wheel is too close, it will also limit the range of motion and perhaps cause interference with other cockpit controls or supports. The proper distance is largely a matter of comfort and clearance, and usually

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means the arms are bent at the elbows when driving straight, yet still comfortable when turning the wheel. The gauges - The gauges act as vital signs for the car, and as such should be as close to the driver's normal line of sight when looking forward. Forcing the driver to look down at gauges removes concentration from the race. In Formula 1 (a particularly good example), the RPM is displayed with a series of LEDs (Light emitting diodes) that light as the redline is approached. This light sits almost at the very top of the cockpit, in line with the line of sight, allowing the driver to change gears without ever needing to look down. A technique frequently used in racing is to rotate the gauges so that all needles or indicators are pointed to the directly vertical position when operating normally. The driver does not need to conciously scan the gauges, but can instead use his/her peripheral vision to determine the state of the car.

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The Pedals - The pedals, like the steering wheel are a leverage item. The driver's legs will tire if not given a position of leverage. Likewise, the driver's legs may tire anyway, due to an inappropriate leverage fulcrum in the actual pedal system. Assuming the pedals and levels are well designed, we can focus on the driver's legs. To be most effective the driver's legs should be bent slightly when the pedals are fully engaged, and should be bent somewhat more when the pedals are not engaged. The calf portion of the leg should probably not be at less than 120 degrees angle in relation to the thigh when the pedals are disengaged. See below:

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Other Controls - Positioning of controls such as the gear shift, kill switch, and adjustment knobs should be carefully considered. It does no good if shifting is hampered by the steering wheel, or if the kill switch is buried away from rescue crew access.

Safety/Ergonomics Design Tips •

Use energy absorbing materials in the collapsable crash structure In lower cost racing cars, most of the car is usually built from mild steel. Using that same mild steel in areas such as wishbones means that

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impacts will bend the material long before it breaks the material, meaning energy absorption takes place over a longer period. For light weight, use a stressed skin over a lightweight core material - crushable zones such as the nose cone on a formula car can be made from balsa, honeycomb or high density styrofoam covered with a stressed skin of composites. Triangulate the driver "safety cell" to prevent collapse - The safety cell can be designed in such a way that a catastrophic impact which collapses the safety cell, will make the safety cell expand away from the driver, instead of collapsing it onto the driver. In the case of a frontal impact, this would mean the sides of the cockpit would expand outward, upward and downward, instead of inward. Use a clear windscreen or bodywork to increase vision - using lexan or other non-shattering clear material can help increase visibility without compromising the function of the bodywork. In some cases, the driver can be lowered for better CG (center of gravity), and the normally opaque bodywork replaced with clear lexan, to aid in re-establishing the vision field. Keep the fuel cell and battery away from the driver and danger. Keeping dangerous items away from the driver is sometimes very difficult. In order to reduce the weight balance change over a race, designers will frequently put the fuel cell at the CG, so that no matter how empty or full it is, it does not cause a front/rear or side-to-side weight bias. However, most drivers don't like to sit next to fuel. Use secured, sealed firewalls between the fuel cell and driver compartment, and further, use the safety cell to protect the fuel cell from outside intrusions. Don't scrimp on safety. Use only top quality certified suppliers of safety equipment. The cost is perhaps high, but consider how much you value your life. Fuel cells (Sanctioning body certified), seat belts (5 or 6 point sanctioning body certified only!), and driver safety wear (Nomex, 2 or more layers minimum! -- anything less is like wearing nothing).

Alvaro Garcia