Lesson 8_formula One Car Design

Lesson 8 – Using Dynamic Simulation In this lesson, you use the tools of the Dynamic Simulation environment to determine front spring properties for a race car suspension.

Objectives After completing this exercise, you will be able to  Access the Dynamic Simulation environment.  Create a spring/damper joint and external force.  Run a simulation.  Use the Output Grapher.

Exercise: Dynamic Simulation In this exercise, you do the following:  Prepare an assembly for Dynamic Simulation.  Access Dynamic Simulation.  Create a spring/damper.  Determine the spring constant.  Determine spring free length.  Apply an external force.  Run a simulation.  Edit the applied force.  Run the Output Grapher.  Change the damping.

Displacement vs. Time Graph

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Prepare the Assembly Before taking an assembly into Dynamic Simulation, it should be simplified so that it includes only the parts and subassemblies you want to study. The starting assembly in this exercise has been simplified but still requires a few components to be deleted. 1. Open DynamicSimulation.iam.

You create a simulation of the front suspension under straight line braking. Since the suspension is symmetrical, the simulation can be performed with just one side. In the following steps, you remove the components on the right side of the suspension: 2. Delete the suspension parts on the right side, including the control arms, pushrod, upright, bellcrank, and damper.

3. Open the frame subassembly and remove the suspension attachments for the right side. Autodesk® Formula Car Design

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Simplified Frame

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Enter the Dynamic Simulation Environment Dynamic Simulation does not use assembly constraints like the assembly environment; instead, it uses joints. There are different joint types corresponding to the many possible connections found in mechanisms and machinery. By default, when the Dynamic Simulation environment is activated, assembly constraints are converted to joints. Joints can also be created manually. 1. Click Applications > Dynamic Simulation.

2. Expand the Standard Joints node in the Dynamic Simulation browser.

3. Click and drag the outboard end of the upper control arm to view the motion. The mouse cursor behaves as though it is connected to the assembly with a spring damper.

Create a Spring/Damper Joint 1. Click Insert Joint.

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2. Select Spring/Damper/Jack from the joint list.

3. Select the inner circle on the damper head.

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4. Select the circle on the damper body.

5. Click Ok. A representation of a spring is placed in the damper. The spring extends to the planes inferred by the circles you selected.

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6. Right-click the Spring node in the Dynamic Simulation browser (located underneath the Force Joints section). Click Properties on the context menu. 7. Expand the dialog box and enter the following values:    

Radius: 24 mm Facets: 10 Turns: 6 Wire Radius: 4.0 mm

8. Click OK.

Note: These values change only the screen display of the spring, not its working properties.

Determine Spring Constant Under hard braking, the front suspension of the car undergoes compression. By design, the amount of compression must be limited to maintain the tires contact patch with the road and to prevent the leading edge bodywork from contacting the road surface. For this exercise, assume the compression to be no more than 15 mm. Calculations show that the static load on each of the front wheels is 600 N, and that under threshold braking, it is 900 N. Therefore, the 300 N difference must result in no more than 15 mm vertical wheel motion. Initially, you might conclude the spring constant would be: kF

x

 300 / 15  20 N / mm

However, 15 mm of wheel motion may not correspond with 15 mm of spring compression. It depends on the geometry of the pushrod, bellcrank, and spring. The Measure Distance tool can be used to determine the relative displacements, but in this exercise, you use tools within the Spring Joint dialog box. 1. Switch to the front view.

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2. Drag the lower control arm until the upper control arm is horizontal.

3. Right-click the Spring/Damper node in the browser and open the Properties dialog box. 4.

Click Update Free Length. This computes the current distance between the seating surfaces of the spring. The result is approximately 148 mm.

5. Click OK to close the dialog box.

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6. In the front view, drag the lower control arm upward until it is approximately horizontal.

7. Open the Spring/Damper Properties dialog box again. Click Update Free Length. This time it is close to 135 mm. The spring displacement is approximately 148-135=13mm. This value is close to the wheel movement. Therefore, you can use the 20 N/mm spring constant calculated earlier. Furthermore, race car suspensions typically have considerable adjustment designed in, so this approximation is acceptable. 8. Open the Spring Properties dialog box again. Set the spring constant (Stiffness) to 20 N/mm. Note: The terms spring constant and spring stiffness both refer to the same spring characteristic. Stiffness is the term used in the spring dialog box.

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Determine Spring Free Length The spring constant you just set ensures the wheel deflection under braking will be acceptable. Now the spring free length needs to be set so that the static load of 600 N yields the desired ride height. An early estimate can be made based on the spring constant. x  F

k

 600 / 20  30mm

Add this value to the spring length at the desired ride height. FreeLength  148  30  178mm

1. Open the Spring Properties dialog box. 2. Enter 178 for Spring Free Length.

3. Click OK to close the dialog box.

Apply an External Force In this sequence, you apply a 600 N external force to the suspension to verify the spring parameters will result in the correct ride height. 1. Click Force on the Dynamic Simulation panel. 2. Click the outer hole of the lower pushrod mount.

3. Set the Use Vector Components. For Fz, enter 600. Autodesk® Formula Car Design

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4. Click OK to close the Force dialog box. 5. Activate the front view. 6. Drag the lower control arm so that it is angled slightly below horizontal.

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Run the Simulation In this sequence, you set up and run a simulation. 1. In the Simulation Panel, set the simulation time to 2.0 seconds and the number of images to 400.

2. Activate home view.

3. Click Play. The suspension assembly compresses and extends repeatedly since it has no damping. All the energy put into compressing the spring is fully returned as it extends. Next, you add damping. 4. Click Construction mode. This exits the simulation and enables editing joints and forces.

5. Open the Spring Properties dialog box. Enter a Damping value of 2 N s/mm. 6. Activate the front view. 7. Click Play in the Simulation Panel. Autodesk® Formula Car Design

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The suspension compresses slightly and stops with the upper control arm very close to horizontal, which is the desired static ride height. This verifies the settings made earlier for the spring.

Edit the Applied Force In the following steps, you change the applied force from 600 to 900 N, corresponding with full braking. 1. Right-click the Force within the External Loads node. 2. Select Edit Force in the context menu. 3. Change Fz to 900 N.

4. Click Play to run the simulation. The spring compresses until the lower control arm is close to horizontal, which is the desired orientation under full braking. 5. Click Construction mode.

Output Grapher The Output Grapher provides a graph of simulated values. 1. Click Output Grapher in the Dynamic Simulation Panel.

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2. Expand the Force Joints node. Select the Extent_Length parameter.

The graph shows the variation in the spring length with time. Note that it starts at approximately 153 mm. This corresponds with the starting position of the simulation. The static ride height is about 148 mm, as seen earlier. Therefore, the total displacement during braking is about 14 mm.

Change the Damping Changes to damping do not influence the final displacements, but they do influence how long it takes to reach them. 1. 2. 3. 4.

Leave the Output Grapher window open. Click Construction mode. Set the spring damping to 6 N s/mm. Run the simulation.

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The starting and ending displacements remain the same; however, the higher damping value increases the time it takes to reach the final displacement.

Spring Displacement vs. Time

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