Suspension Geometry

Owned by Devon Copeland
Last updated: Oct 31, 2018 by Saif Hafeez

Modeling Suspension Geometry

Key Parameters

MSXII's suspension will be driven by 11 key parameters: 

Parameters:

  1. UCA mount (x position)
  2. UCA mount (y position)
  3. LCA mount (x position)
  4. LCA mount (y position)
  5. UCA length
  6. LCA length
  7. Upright length
  8. Spindle height
  9. Wheel offset
  10. Wheel diameter
  11. Spindle angle

Note: this geometry is fully defined. 

MATLAB

Source

The source code for the simulator is available in the mech git repo. 

Types of simulations

Using MATLAB, a simulator has been created to simulate the behaviour of the above geometry. The inputs are the above parameters and the output is the behaviour of the suspension under different conditions. There are two main conditions that must be simulated:

  1. Vertical movement of chassis as a result of braking, accelerating or hitting a bump 
  2. Roll/rotation of the chassis during cornering 

Sample outputs

Below are examples of the outputs that the simulator can generate: 

 

Other Resources

Optimization

There are a number of features, we should consider when optimizing the suspension for MSXII

  • Minimal lateral wheel scrub
  • Minimal change in camber
  • Minimal scrub radius 
  • Minimal bump steer

Process for selecting parameters 

A Solidworks sketch was created and the following process was used to determine the suspension geometry parameters for MSXII: 

  1. The tire+wheel diameter was set as 533mm (21in)
  2. The spindle was defined to be a perpendicular bisector of the wheel
  3. The bottom tire was set to be coincident with the road 
  4. The wheel track was set to 1720mm (per DWG-00001)
  5. The distance between the upper and lower control arms was set to 230mm (per the recommendations of The Winning Solar Car)
  6. The length of the lower control arm was set to 250mm (per the recommendations of The Winning Solar Car)
  7. A slight positive camber of 1 degree was set 
  8. The scrub radius was set to 20mm to reduce sensitivity to braking while maintaining low rolling resistance while cornering (read more about this trade off on page 263 of The Winning Solar Car) 
  9. The horizontal distance from the center of the car to the mounting point of the lower control arm was set to 555mm. 
  10. The vertical distance from the road to the mounting point on the lower control arm was set to 190mm. This provides adequate ground clearance while leaving enough for for the lower control arm rod end. This also defines the spindle length as approximately 60mm which is comparable to what we achieved with MSXI
  11. Construction lines were created to constrain the geometry for zero lateral scrub at 20mm of upward and downward travel of the wheel
  12. The above constrains were removed as well as the constraint on the scrub radius. The UCA mounting point and the UCA length were rounded to the nearest mm. 

By following the above steps, the suspension geometry is fully defined. See DWG-00002 for detailed suspension geometry.