Suspension Geometry and Kinematics

Track width

This has 4 main considerations:

• If the track is too narrow, the car may fail the 60deg tilt test
• Narrower track increases load transfer, reducing lateral grip
• Narrower track reduces rear wing, front wing, and undertray width, reducing aero potential
• Narrower track tightens the racing line - can be very beneficial on a manuverability emphasized course, such as the autoX and endurance.

These need to be analyzed for an adequate tradeoff.

Wheelbase

Considerations:

• 1525mm minimum set by rules
• Shorter generally makes a more compact car, that will at least in theory be lighter and have less yaw inertia
• Shorter reduces steering angle requirement per a given corner radius, geometrically speaking
• Shorter makes the car less stable at high speeds, but the minimum set is well above any sort of safety concern
• Longer can allow more downforce, at least in theory, from undertray and side aero. But, is it worth the penalty?
• Longer can allow more flexibilty with CG longtitudonal location

An adequate compromise must be made.

Caster

Caster Offset

Caster is the side-view angle of the steering axis to the vertical. It affects:

• Mechanical trail
  • Steering effort
  • "sense of direction the wheels want to go to" - i.e, where would the wheels steer if you let go of the steering wheel
  • High speed stability
    
• Jacking
  • Low speed oversteer inducing, as a tuning mechanism
  • Another variable in steering effort
  • Consider it as adding roll also!
• Steer camber
  • Negative camber on outside wheel, positive on inner - both good!
    • But, do you want more camber for a tight hairpin than for a wide sweeper? I dont think so! in practice, this can be neglegible if caster is low enough, but it is a factor.

Remember, you can have "Caster offset", to independently affect mechanical trail (the most important one) from the other 2.

Also, you dont want to drown out the pneumatic trail with too much mechanical trail. Youll just end up with super heavy steering all the time, instead of giving the driver a signal of understeer like the pneumatic trail should.

CAD Tips

Its helpful to set the CAD up for easy adjustment.

That can be done by:

• Integrating the suspension geometry sketches into the frame file
• Having a few levels of sketches, set up for ease of parameter adjustment:
  • A 2D "geometry sketch", that isnt movable, and has all the critical angles and dimensions set, as well as static roll center visible with driven dimensions
  • A 3D "geometry sketch", adding in caster
  • A 3D "linkage sketch" - movable, with equal length relations to the "geometry sketch" - so that it can be cycled through travel, while getting geometry updates from the geometry sketch.

It can look like this:

Movable Sketch

These sketches would then have in-context relations to the bellcrank, a-arms, uprights etc to have their geometry update accordingly to geometry sketch changes.