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How to run a test session

General Car Setup Discussion

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So I'm building my new rally car and the thing I would like to spend some time this year is testing. How would I go about setting up the testing. I've a rallycross track nearby I can run laps at. The things that I can change on the car are

- Tire pressure

- Camber

- Toe

- Possibly caster

- Springs

- Ride height

- Brake bias

With these parameters that are adjustable, do I want to start with adjusting the wheel alignment to be as close to even temperatures over the tire as possible and then change ride height and springs, or should I start the other way around?

What I'm asking is in what order should I go about changing settings on my cars? And in the future possibly adjustable dampers as well, when do I start playing with those?

If you're planning on re-engineering your entire suspension to some degree, the easiest route is to start by calculating all of your weight transfer, and your cars response to lateral and longitudinal force. This is probably gross overkill for a modified stock car, however everything suspension is interconnected, if you change one, the effects cascade through the whole setup making formerly correct settings incorrect.

A little bit simpler, if you change the spring stiffness, or ride height, you change the amount of body roll. That then changes the amount your suspension compresses when going around a turn, and the amount of camber change you'd expect to see. So you should start by changing the ride height and springs before tuning the alignment to match.

So the order of significance that you've posted, from the biggest impact, to the smallest overall is: Springs/Dampers - Ride Height - Caster/Camber/toe - Tire Pressure - Brake Bias. Any of these in extremes can cause a bigger issue than others.

Onto Rallycross. This poses a somewhat irritating hurdle in determining what alignment, or settings to run. A car on pavement's answer to any question suspension related, is simply give the tire whatever it needs. If the ground is the limit, not your tire, the goal is now whatever keeps the ground from giving out. If you run a large amount of camber, does the loading of the inside edge carve a groove for the tire to push against and increase grip? Or does it force it to slide out much quicker(most likely the latter).

With all of that covered, I would start with springs, and some very soft, adjustable sway bars(to avoid buying a half dozen springs to setup correctly). Set my ride height. From there, find the average corner of your expected rallycross track, and find out how fast you can go through it. Then, I would mirror that radius, and speed on asphalt (skid pad), and take temperatures, adjust camber, and pressure based on those.

Toe is very sensitive, and for anything racing, is sensitive enough that bushing compliance, front or rear mount steering rack, and scrub radius come into play. There's also a relatively rally specific use in mitigating camber thrust. If you hit a bump with only 1 wheel, that wheel rises and gains an amount of camber. That camber(negative camber) creates a thrust similar to toe in. Rally cars could/do benefit from small amounts of bumpsteer/rollsteer/toe curve designed into them, causing just enough toe out to keep the car straight. Ultimately, nobody can answer what to run for you without almost complete knowledge of your car. Lots of testing, and keeping notes is going to be important to set this correctly.

Brake bias should be fairly simple to set, car manufactures have an interest in keeping the back from locking first, so you can almost always increase the rear brake bias. Take small steps, get the rear to lock first, then back off some.

Caster and Kingpin Inclination(Steering axis inclination) are largely self centering forces on the steering. How it's accomplished is by changing the camber of the wheel when any steering angle is introduced. When you see someone drifting, the wheel doesn't center, it snaps to counter steer where there is the least amount of resistance. I'm unsure of how to determine this for rally cross, as you do end up countersteering but can't solely focus on it. You'll have to weight the pros and cons of self centering force against better camber across a large range of steering angles yourself.

Hopefully this is more helpful than confusing to you