Back to Geometry

With the information garnered from the upright model, I've been able to return to SusProg and try and finalise the suspension geometry, so that I can then engineer the chassis hardpoints and then get on with the design of the chassis itself. The picture above shows the (theoretical) final positions for the suspension arms, together with the best location for the steering rack to give minimal bump steer. For those who want a feel for the scale of the drawing, the thicker grid lines are 200mm apart and you're looking rearwards from the front centreline of the car.

I've rolled and bumped the suspension and got some feel for the geometry. As expected, there's very little change in roll centre height during roll, something of the order of 0.08 degrees inward for both wheels at 2 degrees of body roll. As I've said earlier, the net result of putting the instant centre on the centreline of the car is that you get a relatively shorter equivalent swing arm length (695mm in this case) compared to having less inclined suspension arms. According to SusProg, at 40 mm of bump travel, there's 3.5 degrees of negative camber. The roll centre doesn't move at all during roll, and is fixed at 132 mm above the ground.

Using SusProg to do all the hard maths for me has resulted in a total bump steer of 0.15mm toe-in at 100 mm bump, and around 10 microns over the range of +/-40mm wheel travel from static. One thing that I havn't considered up to now is the amount of Ackerman that the car will have. Ackerman is the method by which the inside wheel turns more than the outside when cornering. It's especially useful on circuits with tight corners, such as the hairpin at Mallory Park. At the moment the tie rod end is effectively outboard of the virtual kingpin, so the car has anti-Ackerman (277% for those who like meaningless numbers) and the outside wheel turns more. If I had sensible tyre data, I could actually work out if this was a bad thing or not. I don't and I doubt anyone will give me £200,000 to do some testing at CALSPAN, so I'll put it in the 'to be determined' pile for now. It's not an enormous issue, as I can do the same job on the steering arm as I did with the top rose joint mounting to offset it inward. In fact doing it that way would enable a range of Ackerman to be easily engineered and you could have a different geometry for Mallory than for Thruxton or Silverstone.