Rally Car Suspension Optimization

Tim,

This is a rather broad question but I'll try to add some context for direction. I run in open class in the American Rally Association in a GD Subaru STI. While it is open, I'd consider the car to be more of a Group N + in that it has added power, is lighter, and has a sequential. Beyond that, the suspension points are unchanged. I can however adjust these to a large degree, make different control arms, links, etc.

The general question is, what would you look to change from the production geometry to optimize performance in a rally application?

I currently am planning on machining new front top hats to get approx 8* of caster and increasing overall track width by approximately 120mm. Beyond that, I am short on ideas of what to improve. Like in Chris's discussion, the R5 is just on rails everywhere it goes. Granted it has longer travel, but what tricks are folks at M-Sport, etc. employing to get such impressive performance? Thanks!

-Sam

I'm unsure how much difference increased track width will make on gravel rallies, certainly on tarmac is should be an advantage but IMO needs to be approached more intelligently than simply bolting on low offset wheels (as many do) or even just extending the lateral links (and front arms) as to do so affects the whole geometry of the suspension including things many don't usually consider like camber gain.

I have had the opportunity to look over and take a few measurements from an S12b WRC and without giving too much away they run less camber gain than the road car - that may be because they have better control of body roll through ARB selection/low speed compression damping and/or changes to roll centre... that much I haven't fully established and in any case the post 2016 and R5 cars are a step forward in development even from an S12b.

The increased travel helps for sure on gravel, longer travel allows softer springs which in turn gives better traction but the thing I've never worked out is that longer travel with softer springs at the same ride height is going to mean more frequent visits to the bump stops or worse still bottoming the car on the under-body guards... there must be a trade off between the softer set-up and increased ride height (the underside of car to road distance doesn't look massively increased) unless the dampers are being relied up with progressive damping/hydraulic bump stops to prevent the worst of the bottoming although would still unsettle the chassis if it ramped up too much too quick after a big compression.

It's an area which I think is still far more of a dark art/misunderstood than any engine calibration strategies!