Suspension setup for Autocross

[Jesse D]

Asking here as I don't see much action in the Autocross section (heck I only scrolled down that far for the first time  last summer). Move to the autocross heading if appropriate.  I have a 1993 Caterham HPC with a Vauxhall that I have been autocrossing the last several years , using Avon slicks on 13x8 wheels with a square setup. Last summer my grandson joined me and we are working on the setup of the car. I drive maybe 1,000 miles on the road for fun . Our locations are bigger than most- a racetrack in NH, Loring AFB with a 2 mile runway and the apron of the old Navy airport in Brunswick. I just bought a new de Dion rear end and a wide track front end with new Nitron adjustable shocks all around. We have hub stands, plates and scales and I'm looking for toe in and camber settings mostly. and any other advice. I have no idea how it was setup before and we have replaced everything.

 

As an aside, SCCA has begun running events in Brunswick, so they consider it a good location if anyone wants to come to Maine in the summer. Loring is a long drive but an amazing place (an abandoned SAC base) and Northern Maine is unique.

 

 

[Davemk1]

Are the tires radial or bias ply?

 

dave

[Jesse D]

They are Avon AV3146s, which I believe is bias ply. On their second year, I bought them from Roger Kraus just as Avon went out of business. Replacement is one of our issues at some point this summer.

 

[Davemk1]

I don't know that tire and I had no luck searching for it online.

 

I asked if it's a radial or bias ply because these two different tire constructions work best with different alignments. Generally speaking a radial tire will work best with more camber (2° to 4°) and less toe than a bias tire.

 

A bias tire needs little camber (about 1°) and works better with more toe due to its inherently higher slip angle.

 

 

So....VERY generally speaking...

 

 - bias tire front... 1° camber and 1/8" toe out

                  rear.... 1° camber and 1/16 - 1/8" toe in

 

 - radial front... 2° - 3° camber and 1/16" toe out

              rear... 1.5° - 2° camber and 1/16" toe in.

 

 

 

That is a very rough suggestion and only meant to be a starting place. There are many variables that influence the specs and one of the biggest ones is your style and experience as a driver. You'll want to set an alignment and try it on the autocross course and see how it feels and possible change it over time as you get used to the car and the set up. The confusing part is that there's more than one thing that can make a car behave a certain way. For instance....Sevens can tend toward understeer from mid corner to corner exit and this could be due to not enough toe out in front, too much toe in in the rear, too soft rear shocks, too stiff a front sway bar....etc.

 

It really comes down to experimenting and seeing how it works for you as a driver and you may find that your set up evolves as your skill as a driver does.       

 

I hope that helps more than it hurts. Let me know if you have any questions.

 

dave              

[Jesse D]

Thanks Dave, that helps a lot. I'm 76 and have never developed much experience in translating how the car feels and how it is set up. I've never autocrossed another car except for fun. My grandson, who comes from karting, was faster than me at his first event, and has much better  ideas, so we'll begin and see what's up. Avon is out of business, soon to be Nova (see the thread in tires and wheels). I put Hoosiers on as my first slicks, but much preferred the Avons. 12 lbs of pressure all around wears them out evenly switching front and back between events. We run mostly on concrete runway aprons.

[sf4018]

What's the process to adjust the camber on the rear? I just assumed with the solid axle it wasn't possible.

[pethier]

1 hour ago, sf4018 said:

What's the process to adjust the camber on the rear? I just assumed with the solid axle it wasn't possible.

The car in question is not a solid axle, but deDion.  The rear hubs are bolted to flat tabs that extend forward from the deDion tube. There are four bolts, I believe, and by shimming one could adjust both camber and toe.   I can't seem to lay hands on the latest report from thealignmentguy.com but I do remember that my Caterham has some negative rear camber in it now. It has a lot more negative front camber, which is not adjustable.

 

My Birkin has a live rear axle.  The front is adjustable for camber and looks about zero.  I'm planning to adjust it to match the Caterham and go from there.

 

BTW, you can get some negative camber in the rear of solid-axle cars.  It tends to  cause a lot of wear on the parts.  Back in the stone age, a guy I knew raced a Mustang notchback.  He was going to enter a TransAm race and took the Mustang to a tech to get some negative camber.  He told the guy he wanted to watch.  The tech advised he not watch because it would be disturbing and might give him nighmares.