Suspension Paper

AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS

AN INTRODUCTION TO AUTOMOTIVE SUSPENSION SYSTEMS Piyush Gaur, MSc Automotive Engineering Faculty of Engineering and Computing Coventry University, UK Abstract: Suspension system is a term which is given to a system of springs, Shock absorbers and linkages that connects a vehicle to its wheels. A suspension system serves the two dual purposes. It helps in contributing of the car’s handling and braking for good active safety and driving pleasure. Any suspension system of an Automotive is classifies into rigid, Independent and combination of the above two. In this paper, a brief introduction to the suspension and is function is explained. A brief introduction to its designing procedure is also explained along with the factors affecting suspension designing. Double wishbone, McPherson Strut, Torsion bar, Quardalink, Twist beam &Leaf Springs has been discussed in detail along with the cars on which they are used. The relationship between the suspension system, the tyre and the full vehicle dynamics performance has also been discussed in this paper. Keywords: Suspensions system, Springs, Double wishbone, Hotchkiss, Adams, Vehicle Dymamics, Multibody system Analysis. 1. INTRODUCTION Suspension systems date back perhaps two thousand years or more. Early wagons were known to have used elastic wooden poles to reduce the affects of wheel shock. Leaf springs in one form or another have been COVENTRY UNIVERSITY, UK

used since the Romans suspended a twowheeled vehicle called a Pilentum on elastic wooden poles. Later, some innovative carriage designs included rudimentary leaf suspension systems. Throughout history, leaf springs would dominate as the primary suspension design until fairly recently. Leaf springs offered the benefit of simplicity of design and relatively inexpensive cost. By simply adding leaves or changing the shape of the spring, it could be made to support varying weights. As a result, major changes primarily tended to revolved around the use of superior materials and making incremental design modifications. Suspension is a term which is given to system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve the two dual purposes. It helps in contributing of the car’s handling and braking for good active safety and driving pleasure. Secondly, it helps in keeping vehicle occupants comfortable and reasonably well associated from road noise, bumps and vibrations. The suspension system also protects the vehicle and any cargo or luggage from damage and wear. The design of front and rear suspensions of an automotive may be different. If a road were perfectly flat, with no irregularities, suspensions wouldn't be necessary. But roads are far from flat. Even freshly paved highways have subtle imperfections that can interact with the wheels of a car. It's these imperfections that apply forces to the wheels. According to Newton's laws of motion, all forces have both magnitude and direction. A bump in the road causes the wheel to move up and down perpendicular to the road surface. The magnitude, of course, depends on whether the wheel is striking a giant bump or a tiny speck. Either way, the car wheel experiences a vertical acceleration as it passes over an imperfection.

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AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS

2. CLASSIFICATION OF SUSPENSION SYSTEM Vehicle suspensions can be divided into rigid axles (with a rigid connection of the wheels to an axle), independent wheel suspensions in which the wheels are suspended Independently of each other, and semi-rigid axles, a form of axle that combines the characteristics of rigid axles and independent wheel suspensions. On all rigid axles , the axle beam casing also moves over the entire spring travel. Consequently, the space that has to be provided above this reduces the boot at the rear and makes it more difficult to house the spare wheel. At the front, the axle casing would be located under the engine, and to achieve sufficient jounce travel the engine would have to be raised or moved further back. For this reason, rigid front axles are found only on commercial vehicles and four wheel drive, general-purpose passenger cars .With regard to independent wheel suspensions, it should be noted that the design possibilities with regard to the satisfaction of the above requirements and the need to find a design which is suitable for the load paths, increase with the number of wheel control elements (links) with a corresponding increase in their planes of articulation. In particular, independent wheel suspensions include: • Longitudinal link and semi-trailing arm axles, which require hardly any overhead room and consequently permit a wide luggage space with a level floor, but which can have considerable diagonal springing. • Wheel controlling suspension and shockabsorber struts , which certainly occupy much space in terms of height, but which require little space at the side and in the middle of the vehicle (can be used for the engine or axle drive) and determine the COVENTRY UNIVERSITY, UK

steering angle (then also called McPherson suspension struts). • Double wishbone suspensions or SLA (Short Length Arm) • Multi-link suspensions, which can have up to five guide per links and which offer the greatest design scope with regard to the geometric definition of guide links per wheel and which offer the greatest design scope with regard to kingpin offset, pneumatic offset, kinematic behavior with regard to toe-in, camber and track changes, brake/starting torque behavior and elastokinematic property. Broadly speaking these are the main type of automotive suspensions systems which are commonly used in different automotives today. These are  Double wish bone suspension system  Mc person Strut suspension system.  Torsion Bar  Quadra Link  Twist Beam  Leaf Springs A. Double wishbone suspension system It is the independent suspension design which uses a two wish bones arms to locate the wheel. Each wishbone or arm has two mounting points on the chassis and one point at the knucle .The shock absorber and coil spring mount to the wishbone is used to control the vertical movement. This allows the suspension designer engineer to control the following parameters:  Camber angle  Caster angle  Toe pattern  Roll center height  Scrub radius, scuff and more. The double wishbone suspension can also be referred to as double 'A' arms and short long arm (SLA) suspension if the upper and lower arms are of unequal length. SLAs are

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AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS

very common on front suspensions for medium to large cars such as the Honda Accord, Volkswagen Passat, Chrysler 300, or Mazda 6/Atenza, pickups, SUVs, and are very common on sports cars and racing cars. A single wishbone or A-arm can also be used in various other suspension types, such as Macpherson strut and Chapman strut. The suspension consists of a pair of upper and lowers lateral arms. The upper arm is usually shorter to induce negative camber as the suspension jounces (rises). When the vehicle is in a turn, body roll results in positive camber gain on the outside wheel. The outside wheel also jounces and gains negative camber due to the shorter upper arm. The suspension designer attempts to balance these two effects to cancel out and keep the tire perpendicular to the ground. This is especially important for the outer tire because of the weight transfer to this tire during a turn. The advantage of a double wishbone suspension is that it is fairly easy to work out the effect of moving each joint, so you can tune the kinematics of the suspension easily and optimize wheel motion. It is also easy to work out the loads that different parts will be subjected to which allows more optimized lightweight parts to be designed. They also provide increasing negative camber gain all the way to full jounce travel unlike the MacPherson strut which provides negative camber gain only at the beginning of jounce travel and then reverses into positive camber gain at high jounce amounts. The disadvantage is that it is slightly more complex than other systems like a MacPherson strut. Prior to the dominance of front wheel drive in the 1980s, many everyday cars used double wishbone front suspension systems or a variation on it. Since that time, the Macpherson strut has become almost ubiquitous, as it is simpler COVENTRY UNIVERSITY, UK

and cheaper to manufacture. In most cases, a Macpherson strut requires less space to engineer into a chassis design, and in front wheel drive layouts, can allow for more room in the engine bay. A good example of this is observed in the Honda Civic, which changed its front suspension design from a double wishbone design, to a Macpherson strut design after the year 2000 model. The changes was made to lower costs, as well as allow more engine bay room for the newly introduced Honda K-series engine.

Fig 1: Double Wishbone suspension system B. MAcPherson Strut McPherson struts are popular struts that are used mainly in the front suspensions on vehicles especially cars. This strut contains different types of components into one package making them ideal for front-wheeldrive cars. The McPherson struts are used in different types and models of cars. The strut is used for both rear and front suspension but mainly used in the front suspension because it provides a steering pivot. The subframe of the strut is capable of providing the lateral and longitudinal location of the wheel. The strut was designed by Earl S. McPherson. This strut was used in Ford Vedette in 1949. The strut consists of a wishbone or a compression link which is Page 3

AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS

stabilized by a secondary link. The secondary link is important for providing a bottom mounting point for the hub or axle of the wheel. The lower arm of the strut is helpful in providing both lateral and longitudinal location of the wheel.

Fig 2: Mc Pherson Strut The body is suspended on the coil spring whereas the shock absorber, which is usually in the form of a cartridge mounted within the strut. The assembly is simple and can be preassembled into a unit. Moreover, it allows for more width in the engine bay by eliminating the upper control arm. This is useful for smaller cars particularly with engine having transverse orientation just like most front wheel drive vehicles have. C. Torsion Bar System- A torsion bar suspension, also known as a torsion spring suspension or incorrectly torsion beam, is a general term for any vehicle suspension that uses a torsion bar as its main weight bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end terminates in a lever, mounted perpendicular to the bar, that is attached to a suspension arm, spindle or the axle. Vertical motion of the wheel causes the bar to twist around its axis and is resisted by the bar's torsion resistance. The effective spring rate of the bar is determined by its length, diameter and material.

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Torsion bar suspensions are currently used on trucks and SUV from Ford, Dodge, GM, Mitsubishi and Toyota. Manufacturers change the torsion bar or key to adjust the ride height, usually to compensate for heavier or lighter engine packages. While the ride height may be adjusted by turning the adjuster bolts on the stock torsion key, rotating the stock keys too far can bend the adjusting bolt and (more importantly) place the shock piston outside the standard travel. Over-rotating the torsion bars can also cause the suspension to hit the bump stop prematurely, causing a harsh ride. Aftermarket forged torsion key kits use relocked adjuster keys to prevent overrotation, as well as shock brackets that keep the piston travel in the stock position.

Fig 3: Torsion bar action D. Twist Beam Suspension- The Twistbeam rear suspension is a type of automobile suspension based on a large H shaped member. The front of the H attaches to the body via rubber bushings, and the rear of the H carries each wheel, on each side of the car. The cross beam of the H holds the two trailing arms together, and provides the roll stiffness of the suspension, by twisting as the two trailing arms move vertically, relative to each other. The coil

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springs usually bear on a pad alongside, or behind, the wheels. Often the shock is colinear with the spring, to form a coilover. This location gives them a very high motion ratio compared with most suspensions, which improves their performance, and reduces their weight. The longitudinal location of the cross beam controls important parameters of the suspension's behavior, such as the roll steer curve and toe and camber compliance. The closer the cross beam to the axle stubs the more the camber and toe changes under deflection. A key difference between the camber and toe changes of a twist beam vs independent suspension is the change in camber and toe is dependent on the position of the other wheel, not the car's chassis. In a traditional independent suspension the camber and toe are based on the position of the wheel relative to the body. If both wheels compress together their camber and toe will not change. Thus if both wheels started perpendicular to the road and car compressed together they will stay perpendicular to the road. The camber and toe changes are the result of one wheel being compressed relative to the other. This suspension is used on a wide variety of front wheel drive cars, and was almost ubiquitous on European superminis. It was probably introduced on the Audi 50, which was rebadged as the Volkswagen Polo. This suspension is usually described as semiindependent, meaning that the two wheels can move relative to each other, but their motion is still somewhat inter-linked, to a greater extent than in a true IRS. This limits the handling of the vehicle, and VW have dropped it in favor of a true IRS for the Golf

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Mk V in response to the Ford Focus' Control Blade rear suspension.

Fig 4- Twist Beam Rear Axle E.Leaf spring Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles. It is also one of the oldest forms of springing, dating back to medieval times. Sometimes referred to as asemi-elliptical spring or cart spring, it takes the form of a slender arc-shaped length of spring steel of rectangular crosssection. The center of the arc provides location for the axle, while tie holes are provided at either end for attaching to the vehicle body. For very heavy vehicles, a leaf spring can be made from several leaves stacked on top of each other in several layers, often with progressively shorter leaves. Leaf springs can serve locating and to some extent damping as well as springing functions. While the interleaf friction provides a damping action, it is not well controlled and results in stiction in the motion of the suspension. For this reason manufactures have experimented with mono-leaf springs. A leaf spring can either be attached directly to the frame at both ends or attached directly at one end, usually the

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front, with the other end attached through a shackle, a short swinging arm. The shackle takes up the tendency of the leaf spring to elongate when compressed and thus makes for softer springiness. Some springs terminated in a concave end, called a spoon end(seldom used now), to carry a swiveling member.

faults. Following are the parameters which are kept in mind for designing of suspension systems 3.1 Wheel Camber Angle- Wheel camber is the lateral tilt or sideway inclination of the wheel relative to the vertical. When the top of the wheel lean inwards towards the body the camber is said to be negative, conversely an outward leaning wheel has positive camber.

Road

wheels

were

originally

positively cambered to maintain the wheels perpendicular to the early cambered roads. Fig 5- Leaf Spring

Practically for most of the suspensions

3.1 Suspensions Geometry& Tyres role for Effective vehicle Handling

system wheel cambered has been reduces to

The stability and effective handling of the

slightly more cambered then the other, due

vehicle

may be to body roll with independent

depends

upon

the

designer’s

0.5 degrees to 1.5 degrees. if one wheel is

and

suspension or because of misalignment ,the

suspension geometry which particularly

steering wheel will tend to wander or pull to

includes the wheel Camber, Castor and King

one side as the vehicle is steered in the

Pin inclination. It is essential for the

straight ahead position. To provide a small

suspension members to maintain these

amount of understeer, the front wheels are

factors throughout the whole life of a car.

normally made to generate a greater slip

Unfortunately,

the

angle then the rear wheels by introducing

swiveling joints are both subjected to the

positive camber on the front wheel and

wear and damage and must be periodically

maintain

checked. With the understanding of the

perpendicular to the ground.

selection

of

optimum

the

steering

pivoting

and

the

rear

wheels

virtually

principles of the suspensions geometry and their measurements it is possible to diagnose and rectify the steering and suspension

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hits the road on the outside of the centre line of the tyre contact point. The Kingpin Angle, along with the Castor, dictates the self-centering action of the steering and the affect the steering will have under braking. Fitting larger wheels can alter the Scrub Radius if the correct offset is not chosen which in turn can affect the handling.

3.2 King pin inclination – king pin inclination is the lateral or inward tilt from the top between the upper and lower swivel ball joint or king pin to the vertical. If the kingpin is perpendicular to the ground ,it’s contact centre on the ground would be offset to the centre of the tyre contact patch, the offset between the pivot centre and contact patch centre is known as the scrub radius. When turning the steering the offset scrub produce a torque T created by the product of the reactionary force f and offset radius .A large pivot to wheel contact centre offset requires a large input torque to overcome the opposing ground reaction, therefore the steering tends to be very heavy. A positive Scrub Radius or Kingpin Offset is when the Kingpin Angle hits the road surface on the inside of the centre line of the tyre contact point (see the diagram below), a negative Scrub Radius is when the Kingpin Angle COVENTRY UNIVERSITY, UK

3.3 Castor Angle- Castor Angle is the angle to the vertical plane on which the steering axis sits as viewed from the side. In other words if we imagine looking at the side of the front wheel, the Castor Angle is the angle an imaginary line makes that is drawn through the centre of top ball joint (or top mount of a suspension) and down through the lower suspension arm ball joint. Looking on the diagram, if we follow the Castor Angle line down we can see it hits the ground in front of where the tyres contact with the ground, this is Positive Castor. This means the tyres will always follow the steering input or in other words act just like a normal furniture castor wheel. Castor Angle determines the amount of self-

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centring the steering will have, influence the straight-line running and with the Kingpin Angle it will influence the camber change when cornering as a function of the steering input. Castor Angle traditionally used to be very small as large amounts of Castor Angle created heavy steering,. Large Castor Angles mean greater, dynamic camber changes can be created and that means better negative camber when cornering and smaller camber on the straight, ideal for both performance and wear of the tyres unfortunately too large

wheel rim and the rear of the wheel rim. Total Toe is the overall distance for a pair of wheels whereas Individual Toe is half the Total Toe and relates to individual wheels. Toe-in increases lateral stability but can lead to wear on the inside shoulder of the tyre. Front end toe-in dampens turn in response but improves the self centring action of the steering while rear toe-in helps to reduce oversteer due to the improvement in lateral stability. Toe-out reduces lateral stability and can lead to wear on the outside shoulder of the tyre. Front end toe-out can improve turnin response while rear end toe-out encourages oversteer due to the reduction in lateral stability. Toe can be altered on the front by adjusting the track-rod ends and on the rear by adjusting the toe control arms.

a castor angle can lead to poor turn-in.

Fig 9- Toe geometry

pattern

on

suspension

Fig 8- Negative & positive Camber 3.4 Toe Pattern - Toe describes the angle at which a wheel sits on a horizontal plane relative to the longitudinal axis of the car. In other words if we imagine looking vertically down on top of a wheel mounted on a car, if the front of the wheel is angled inwards more than the rear of the wheel then it is said to have ‘toe-in’, if it’s the other way around then the wheel is said to have ‘toeout’. If the wheel is parallel with the longitudinal axis of the car then it has zero toe. Toe can be measured in degrees but more commonly, it’s measured as the distance difference between the front of the COVENTRY UNIVERSITY, UK

3.5 Roll center Analysis – One important property of the suspension relates to the location at which lateral forces developed by the wheels are transmitted to the sprung mass. This point, which has been reffered to as the roll center, affects the behavior of both the sprung and unsprung mass, and thus directly influences the cornering. Each suspension has a Roll center, defined as a point in the transverse vertical plane through the wheel centers at which the lateral forces may be applied to the sprung masses without producing the suspension roll. It derives from the fact that all suspensions have a roll axis, which is the instantaneous axis about

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which the unsprung masses rotate with respect to the sprung mass when a pure couple is applied to the unsprung mass. The roll center is the intersection of the suspension roll axis with the vertical plane through the center of two wheels .The roll center height is the distance from the ground to the roll center .The suspension roll axis and roll center can be determined from the layouts of the suspensions geometry in the plan and elevation views. From the analysis, we draw the concept of Virtual Reaction point. It is another word of Instanteous centre.

Fig 10a- Roll Center of McPherson Strut Fig 10 b- Roll centres of other automotive suspensions 3.6- Tire behavior in Vehicle handling - A tire is a simple visco-elastic toroid which serves the three basic functions -1. It supports the vertical load, while cushioning road shocks 2. It develops longitudinal forces for acceleration and braking and also develops lateral forces for cornering. To facilitate precise description of the operating conditions, forces and moments experienced by the tire, a SAE has defined the axis system shown in fig below-

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AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS  Over







Fig 11- SAE axis systems



turning Moment (Mx)Moment acting on the tire by the road in the plane of the road and parallel to the intersection of the wheel plane with the road plane. Rolling Resistance Moment (My)Moment acting on the tire by the tire by the road in the plane of the road and normal to the intersection of the wheel plane with the road plane. Aligning Moment (Mz)- Moment acting on the tire by the road which is normal to the plane of the road. Slip angle (α) - Angle between the direction of the wheel heading and the direction of the travel. Camber angle (γ) - angle between the wheel plane and the vertical.

 Wheel plane – Central plane of the   







tire normal to the axis of rotation. Wheel center- Intersection of the spin axis and wheel plane. Center of tire Contact- intersection of the wheel plane and projection Loaded Radius- Distance from center of the tire contact to the wheel center in the wheel plane. Longitudinal force(Fx)- Component of the force acting on the tire by the road in the plane of the load and parallel to the intersection of the wheel plane with the road plane .The force component in the direction of the wheel travel is called Tractive force. Lateral Force (Fy) - Component of the force acting on the tire by the road in the plane of the road and normal to the intersection of the wheel plane with the road plane. Normal Force (Fz) - Component of the force acting on the tire by the road which is normal to the plane of the road .the Normal force is negative in magnitude.

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3.7 Mechanics of forces Generation in tires- The forces on a tire are not applied at a point, but are the resultant from normal and shear stresses distributed on a contact patch. The pressure distribution under a tire is not uniform but vary in X and Y direction. When rolling, it is generally not symmetrical about the Y-Axis but tends to be higher in the forward region of the contact patch. Because of the tire’s visco elasticity, deformation in the leading portion of the contact patch causes the vertical pressure to be shifted forward. the centroid of the vertical force does not pass through the spin axis and therefore generates rolling resistance. With a tire rolling on the road both tractive and lateral forces are developed by shear mechanism. Each element of the tire tread passing through the tire contact patch exerts a shear stress which, if integrated over the whole area is equal to the lateral /tractive forces developed by the tires.

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producing a differential between the tire rolling speed and it’s speed of travel. The consequence is production of slip in the contact patch. Slip is given by S= (1- rώ) × 100 where, V R= Tire effective rolling radius ώ= Wheel angular velocity V= Forward velocity

Fig 12- Tire Deformation in the contact patch 3.8 – Forces Developed on the tires and their effect on vehicle handling 3.8 a- Tractive properties- Under acceleration and braking ,additional slip is observed as a result of the deformation of rubber elements in the tire tread caused as they deflect to develop and sustain he frictional force. As the tread elements first enter the contact patch they cannot develop the frictional force because of their compliance-they must have bend to sustain a force. This can happen only if the tire isd moving faster than the circumference of the tread. As the tread element proceeds back through the contact patch its deflection builds up currently with vertical load and it develops much more friction force. However, ,approaching the rear of the contact patch the load diminishes and there comes a point where the tread element began to slip noticeably on the surface such that the friction force drops off, reaching zero as it leaves the road. Thus acceleration and braking forces are generated by

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3.8b.Effect of tractive properties on Vehicle Handling- Longitudinal traction properties are the properties of the tires system that determine braking performance and stopping distance.Beacuse of the weight transfer during deceleration, all wheel cannot be brought to the peak traction condition except by careful design of the braking system so as to proportion of the front and rear braking forces in accordance with the prevailing loads under these dynamic conditions. Since it is practically impossible to design a conventional braking system that can achieve exact proportioning under all conditions of load, center of gravity location, and load condition, it is inevitable that the driver will experience lock up problem. Therefore, the sliding coefficient of friction is an important tire performance property. With the use of antilock braking system thr brake system maintains the wheel near the peak of the traction curve and does not allow lock up . 3.8c. Cornering properties and It’s effect on the Vehicle Performance- One of the very important fuctions of the tire is to develop the lateral forces necessary to control the direction of the vehicle, generate lateral acceleration in corners or for lane change, and resists external forces such as wind gusts and road cross slope. These forces are generated either by lateral slip of the tire, by lateral inclination, or a combination of the two. The integration of

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all the forces acting on a contact patch yields the net lateral force with a point of action on the centroid.The asymmetry of the forces build up in the contact patch causes the force resultant to be positioned toward the rear of the contact patch by a distance known as Pneumatic Trail. By SAE convention the lateral force is taken to act at the center of the tire contact. At this position net resultant is a lateral force,Fy and aligning moment ,Mz.The magnitude of aligning moment is equal to the lateral force times the pneumatic trail. Vehicle stiffness is one of the primary variables affecting steady state and transient cornering properties of vehicles in the normal driving range.Understeer gradient, the characteristic commonly used to qualify turning behavior, is directly influenced by the balance of the cornering stiffness on front and rear tires, as normalized by their loads.A higher relative cornering stiffness on the rear wheels is necessary to achieve under steer.

rolling at a non-vertical orientation, the inclination angle being known as camber angle.With Camber, a lateral force known as “Camber Thrust” is produced. The inclination angle is defines with respect to the perpendicular from the ground plane, positive corresponding to an orientation with the top of the wheel tipped to the right when looking forward along it’s direction of travel. It is the primary cornering force by which motorcycles and the other two wheeled vehicles are controlled. On passengers and trucks, camber thrust contributes to understeer behavior, but normally as a secondary source. On vehicles with independent suspensions where significant camber angles may be achieved, this mechanism may contribute up to about 25 percent of the under steer gradient. On vehicles with Solid axles, little camber can occur such that its contribution to turning performance is very less.

Fig 14- Lateral Angle vs Camber Angle

Fig 13- Lateral force vs slip angle graph 3.8d- Camber thrust and it’s effect on vehicle handling- A second means of lateral force generation in a tire derives from COVENTRY UNIVERSITY, UK

3.8e- Aligning Torque And its effect on Vehicle performance- Aligning torque as a torque acting on the vehicle contributes a small component to the understeer of a vehicle. The fact that positive aligning moments attempt to steer the vehicle out of the turn means that they are understeer in direction. Overall, the direct action of the moments contributes only a few percent to the understeer gradient of a vehicle. The aligning moment has a more direct influence

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on understeer by its action on the steered wheels. The moment is normally in the direction to turn the steered wheels out of the turn. the steered wheels out of the turn. Even though the steer deflection angles in response to aligning moments may be small, this is normally an important contribution to under steer gradient. 4. Simulation and Analysis of Suspensions Systems- Computer aided simulation of vehicle handling characteristics is nowadays universally acknowledged as an efficient method in the process of developing new vehicles. Simulation software tools are used both by automobile manufacturers and suppliers to an increasing extent. The outstanding quality of simulation results for chassis development is acknowledged without exception. ADAMS® as a multybody-simulation-tool is in service in automotive engineering all over the world. The dynamics of rigid bodies can hereby be analyzed mathematically very exactly.

Fig 15- ADAMS model of a car The main use of ADAMS within the automotive industry is to simulate the performance of suspension systems and full vehicle models. The analyst will often wish to validate the performance of a suspension model over a range of displacements between full bump to rebound before the assembly of a full vehicle model. The final model may be used for ride and handling, durability or crash studies. A detailed model

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may include representations of the body, sub frames, suspension arms, struts, roll bars, steering system, engine, drivetrain and tyres. The main analysis code consists of a number of integrated programs that perform threedimensional kinematic, static, quasi-static or dynamic analysis of mechanical systems. In addition there are a number of auxiliary programs, which can be supplied to link with ADAMS. These programs can be used to perform modal analysis, model vehicle tyre characteristics, pre-process using a library of macros, automatically generate vehicle suspensions and full vehicle models, or model the human body. Once a model has been defined ADAMS will assemble the equations of motion and solve them automatically. It is also possible to include differential equations directly in the solution, which allows the modeling of active suspensions or steering, braking and speed controllers. Programs such as ADAMS have developed to such an advanced stage that they form an integral part of a modern computer aided engineering installation. The program will, for example, link or interface with CAD systems, finite element programs, software used for advanced visualization or additional software modules such as those used for tyre modelling. The combined use of these systems can lead to the development of what may be referred to as virtual prototypes, which is computer models that can simulate the tests and conditions that a real prototype would be subject to during the development of a new engineering product.

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AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS CAD/Solid Modelling Mass properties

IGES Translation Human Factors Modelling

Body properties, geometry, postures monitors

Geometry

Bond-graph Models

ADAMS System Model Definition

Hydraulic, Pneumatic Subsystem Modelling

Suspension models, tyre models, drivetrains Differential Equations Mass, stif ness, damping models

Control laws

Control System Modelling

Vehicle Modelling

ADAMS Data Language

Interactive Real-Time Kinematics

(i)

The use of kinematic or quasi-static analysis to simulate the motion of the road wheel relative to the vehicle body passing through the full range of vertical movement between the rebound and the bump positions. The output from these analyses is mainly geometric and allows results such as camber angle or roll centre position to be plotted graphically against vertical wheel movement.

(ii)

The use of static, quasi-static or dynamic analyses to simulate the diffusion of loads from the contact patch through the suspension system and into the body mounts. These types of analyses are used to represent typical in service loads that need to be considered to provide the required durability. Typical load cases will include those due to driving, braking and cornering leading on to the simulation of the more severe cases to which a prototype vehicle would be subjected such as driving through a pothole. The output from these analyses will be the peak loads produced at locations such as the suspension arm to body mounts and the spring seats. These results can then be used as inputs to finite element models in order to determine the structural stresses and strains required for the design of the components and to perform further fatigue assessments.

(iii)

The third type of analysis is the use of dynamic analyses to determine the natural frequencies in the suspension system required for the consideration of the ride performance of the

Actuator Modelling

F.E. Flexible Body Modelling

Kinematic Path Optimisation

Equation Generation Assembly/ Initial Condition Analysis Kinematic Analysis Static/Quasi-Static Analysis Dynamic Analysis Plant Model

Loads Boundary Conditions

Linearisation/Model Analysis ADAMS System Simulation Modules ADAMS Results Files

High-speed Shaded Image Animation

Signal Processing

Data Tabulation

Configuration Display

Results Plot ing

Superimposed Display/Animation

Photo-realistic Rendering Film-recorded Animation

ADAMS Simulation Results Processing Modules

Fig 16- Integration of Adams with CAE

The types of analyses that can be performed and the use in design will be addressed in three main areas:

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vehicle. An example of this would be to recreate test procedures carried out in the laboratory such as the input of an oscillatory load at the tyre contact patch where the frequency is varied with time. This is often referred to as a frequency sweep and will identify which frequencies will excite the suspension leading in severe cases to problems such as ‘wheel hop’ where the resulting excitation of the road wheel can lead to violent bouncing.

and aligned through the front and rear roll centers. The four suspension arrangements are shown schematically in Figure18.

Modelling of suspension system consist of the following four types of model which are used with ADAMS. They are— (i) The Linkage Model where the suspension linkages and compliant bush connections are modelled in detail in order to recreate as closely as possible the actual assemblies on the vehicle.

Fig 17.1 Linkage model

(ii) The Lumped Mass Model where the suspensions are simplified to act as single lumped masses which can only slide in the vertical direction with respect to the vehicle body.

(iii) The Swing Arm Model where the suspensions are treated as single swing arms that rotate about a pivot point located at the instant centres for each suspension.

(iv) The Roll Stiffness Model where the body rotates about a single roll axis that is fixed

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Fig 17.2- Swing Arm Model Page 15

AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS

5. Physical Testing of Simulation SystemBasically two tests are commonly used to test the exact geometry of suspension system. These two test are- 1. K and C test 2. Shaker’s Rig test. 5.1- K & C Testing - For the understanding of vehicle handling characteristics, investigations on suspension kinematics and compliance steer are of major interest. Kinematics means the movements of the wheel relative to the body that result from spring travel. Compliance steer results from additional forces in the contact area of the tires. These forces caused by lateral or longitudinal accelerations of the vehicle deform the suspension parts and its bushings and lead to additional camber and toe angles. Compliance steer of axles has a great influence on the handling performance of vehicles. By a specific interpretation of the suspension elements, the engineer is being forced to get a compliance steer that supports a controlled road performance of the complete vehicle. Some types of axles have however conceptionally caused disadvantages regarding compliance steer such as twist- beem rear axles .These shall be minimized in the most effective way by constructive features. Because of the diminution of vehicle development time it is necessary to get object measuring results from new axles very fast and easily. These results are also necessary to validate the compliance steer of vehicle models for the simulations of vehicle dynamics. The quality of the model compliance steer influences very clearly the results of the multy-bodysimulations. Pure static mechanical models do not deliver adequate simulation results for modern vehicles.

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The IKA kinematics and compliance Test Rig can be used for the measurement of the influences of vertical deflections and both lateral and longitudinal forces on the axle geometry of complete vehicles or of axlesystems. By the help of four hydraulic cylinders that are fixed to the four wheels arbitrary wheel suspension positions can be realized. The test bench mainly consists of 12 hydraulic actuators (one for longitudinal, lateral and vertical force generation on each wheel) that can be operated individually.. In order to simulate a contact zone between wheel and the ground the test bench can be equipped with aerostatic bearings. Highly sophisticated sensors, amplifiers and measurement data acquisition systems record any value in the course of time that might be of interest. Fig. 17c shows the optical Autocollimator sensors that are used to measure the camber and toe angles. A large number of fastening devices, which serve to fix the vehicle body to the test rig, eliminate the influence of body stiffness on the measurement. Moreover, the fastening systems allow the easy fixing of any car to be tested without the need to produce costly adapters. Apart from that it is also possible to fix and to examine single axles and wheel suspensions without examining the complete vehicle. The complete system is controlled by a reliable computer system to reduce the operator's influence on the results and to achieve an optimum reliability and repeatability of the measurements. Extensive routines shall exclude a malfunction of the test bench to avoid a damage of the vehicle. This system is presented in Figure 16 . Typical characteristics which are supposed to be examined are: roll axis position, roll stiffness or steering compliance. But also complete driving maneuvers such as 'steady state cornering' or 'breaking maneuvers' can be simulated at the test bench. Knowledge

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can be gained about the self steering properties of the vehicle by using this method. The technical data of the test rig are: · variable wheelbase: 2000 to 3250 mm · variable track width: 1180 to 1650 mm · max. vertical displacement at the wheel: 400 mm · max. wheel load: 14 kN · max. lateral force (per wheel): 10 kN · max. brake force (per wheel): 10 kN · max. Traction force (per wheel): 5.5 Kn

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AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS

Fig 18- Measurement of Toe and camber angle 5.2 Shaker Rig Test- Dynamics and vibrations are much harder to understand than static forces. Ever since man has been building and driving cars, the complex systems of springs and dampers have created a complicated symphony of noises and vibrations. The passenger car industry has been mounting cars on four-post shaker rigs for years, since it allows for more precise evaluations of body and suspension dynamics than running on a road. The inputs can be simple repetitive vibrations (sine waves), or they can be representations of real roads. While undergoing input from the road, sophisticated dynamic measurement devices provide insight to how the system is working. Generally, unwanted noises and vibrations entering the passenger compartment are the focus of these investigations. In racing, the only objective is to go fast. One of the main limiting factors is how well the tires stay in contact with the track surface. The basic use of the shaker rig is to optimize the springs and shocks to minimize tire load variations while maintaining reasonable body motion control.

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We have all seen a car running down the highway with a bad or missing shock absorber. The body is bouncing up and down like a boat on big waves, and the tire may also be hopping up and down showing daylight on each up cycle. This is a representation of what happens when the system of springs and masses is very underdamped. The shock absorbers are the key element here. They have to do the job of controlling the body motions as well as the wheel motions. One device (normally a shock absorber) mounted between the chassis and the suspension is asked to control a spring connecting two different masses, each with its own natural frequency. To further complicate the issue, the four corners of the car work independent of each other but are tied together by the body structure. The wheels basically move straight up and down relative to the chassis while the body has several motions relative to the ground. Engineers call these body motions “heave (movement up and down), pitch (forward and back) and roll (side to side).” Each of these motions is resisted by the springs at the four corners of the car. Resistance to these motions causes force variations between the tire and the road. The trick is to find the balance point. Tie the car down too tight and the force variation goes up, but freeing it up too much can do the same thing in the opposite direction. There has to be a compromise for the correct amount of damping that gives the best load control. Finally, there is one last, but very important, variable to throw into the mix. Driver preferences come into play here in a big way; some drivers like very little body motion, while others don't mind a car that moves around a little more.The seven-post shaker rig is used in racing work because aerodynamic downforce and track banking add to the wheel loads. The amount of load added can be more than the static initial weight, so it must be included in the test

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procedure. The seven posts are hydraulic cylinders. Four of them have flat pans the tires sit on and support the car. The other three are called the aeroloaders and attach to the sprung mass. Normally, two are mounted to the front of the chassis some distance apart while the third one is mounted at the rear on centerline. Loading on these cylinders is done to pull the car down, opposing the four wheel pans. The aeroloaders simulate other forces on the car such as the squashing from inertia loads as the car rolls through a banked turn or deflections due to aerodynamic loading. By adjusting the load on the three downforce rams we can simulate any combination of roll, heave or pitch displacement to recreate specific conditions seen on the track and repeat that condition. Normally, wheel travels from actual test-session recordings are re-created in the lab. By using the correct deflections indicated by wheel travel with the same springs and bars as those used in the track test, the loads will be correct. Deflections are used because race teams seldom have vertical loads as a measurement.

types of cars on which the suspension systems are used has also been discussed. Particular Emphasis has also been given on the mechanics of tyres, Suspension Geometry and how it affects the vehicle performance. Various physical test like K & C Rig test and Shaker’s Rig test has also been given and fully explained. More emphasis is given on the Modelling and analysis of the Automotive Suspensions.

References1. Advance Vehicle Technology by Heisler 2. “ Vehicle Dynamics” by Thomas D.Gillespie 3. www.howstuffwork.com 4. Advance Race Dynamics Milliken & Milliken

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5. www.sidebrake.net/forums/index.ph p?topic=841.0 6. http://www.circletrack.com/techarticl es/seven_post_shaker_rig_suspensio n_dynamics/index.html

Fig 19- Seven post shaker rig

Conclusion- Suspension systems are one of the most important system of an automotive .In this paper, Various types of suspensions and their functions has been introduced. The COVENTRY UNIVERSITY, UK

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