Suspension set up basics...

[WJM]

...for those who are curious....

 

ADJUSTMENTS | DECREASE UNDERSTEER | DECREASE OVERSTEER

 

Front Tire Pressure | Lower | Higher

Rear Tire Pressure | Higher | Lower

 

Front Tire Section | Larger | Smaller

Rear Tire Section | Smaller | Larger

 

Front Wheel Camber | More Negative | More Positive

Rear Wheel Camber | More Positive | More Negative

 

 

Front Wheel Toe | Toward Toe-Out | Toward Toe-In

Rear Wheel Toe | Toward Toe-In | Toward Toe-Out

 

Front Wheel Caster | More Positive | More Negative

 

Front Springs | Soften | Stiffen

Rear Springs | Stiffen | Soften

 

Front Anti-sway Bar | Soften (Thinner) | Stiffen (Thicken)

Rear Anti-sway Bar | Stiffen (Thicker) | Soften (Thinner)

 

Weight Distribution | More Rearward | More Forward

[subyrally]

wjm, you have too much free time.

[WJM]

wjm, you have too much free time.

 

Yeah, and im sitting at work right now.

Actually...doing THIS, is actually part of my job....aint it cool having a job that browsing and posting on USMB is something you are supposed to be doing?

[lagwagon]

I think you got it backwards according to this page http://www.kormanfastbmw.com/thandling.htm

at least I know from experience the tire pressure info is backwards. everything else I'm not sure about.

 

Also with tire pressure you never want the difference to be more than 10 psi.

 

For auto-cross I run 46 psi up front and 38 psi rear this set up with induce oversteer for sure I'm not guessing.

[bushbasher]

wouldnt higher pressure in the front reduce contact area and reduce traction? if you've got less traction in the front it seems to me that that should = understeer.

[Warp3]

Ok...the correct answer about tire pressure is:

 

There is (in theory) an ideal pressure for each tire on your car given the load, conditions, road/track surface, etc. Going either over or under this pressure will cause a loss in traction, thus causing "slip" on that end of the car. So if you happen to have all 4 tires at "ideal" pressure (good luck with that, btw...lol) and you change the rear tire pressures, going EITHER WAY would increase oversteer/reduce understeer as you are reducing grip at that end of the car.

 

So you are both right and both wrong...LOL

 

Also, remember that raising tire pressures also increases the "wheel" rate and has an effect similar to stiffer springs since it stiffens the sidewall.

 

It's still quite the black art (even among some of the fastest autocrossers in the country) and some people just prefer the feel of "slipping high" vs "slipping low" (most common on Stock class cars where springs cannot be modified so reducing rear grip is done intentionally to assist in car rotation).

[bushbasher]

well slipping high might mean less drag as well, and would give more sensitive feedback I imagine. Hmmm.

[TomRhere]

This is where I get all cornfuzed. Just never got a "grip" on set-up of a vehicle. Really get confused watching the NASCAR races. "I'm tight in, and loose off". But the anouncer says just the opposite of what I'm thinking the driver is saying. Always think of being tight is; the car wants to turn to quickly, and loose as the back end wants to lead the car around the corner. Then they throw in the pushing bit, ??!?. Is that the understeer part? And tight being the oversteer part? Then they throw in the 1/2 pound of air in this tire, and raise/lower the track bar, do something with the "wedge"??!?, what ever that is, ... I'm lost.....

[bushbasher]

The best way to learn car setup is to ignore what nascar drivers say Whats best for doing a left turn fast, in a vehicle with a solid rear axle is not going to be the same as for the typical car that has independant all around and has to turn both ways

 

My guess is "wedge" would be about the angle of the track bar on the rear axle, which would control the amount of lateral movement in the rear axle when the suspension moves up and down.

 

My guess for "tight"/"loose" would have to do with toe in and steering geometry, affecting how abruptly the car wants to turn/return to center.

 

Push would be understeer

[TomRhere]

NASCAR is doing Watkins Glen this weekend, Road course action. I loves it, wish they did more. So now they do have to do the right turn stuff. And still, it's loose here, tight there, pushing here.

 

On another note.... I used to do some road coursing myself many moons ago when I was a much younger puppy. Always the family ride that Dad had at the time, but I could drive it, good. Never gave any thought to different tire pressures or spring rates, front-end geometry, what not. Just a stock car, doing the Esses..............

 

Have I kinda Jacked this post? Ifso, I appologize......................................

[lagwagon]

wouldnt higher pressure in the front reduce contact area and reduce traction? if you've got less traction in the front it seems to me that that should = understeer.

I think if you were going in a straight line lower psi would increase traction to a point, but a tire that is under inflated going throught a hard turn will produce understeer because of tire role which will turn into a slide if pushed. On the other hand an over inflated tire will be so hard it won't get good traction and will understeer. For example when auto-crossing. I have locked up the brakes and left a big long skid.This heats up the tire which actually increases tire pressure. This situation will cause the car to have understeer for the rest of the run.

 

Like someone said already you want to find the ideal psi for your tire and then adjust psi to induce understeer or oversteer. Tires with a bigger side walls need a higher psi to hold there line and shape. but there are too many factors to have just one rule.

 

But usally higher psi in front and lower in back will cause your car to oversteer. I'm not sure why you would want your can to understeer but, usally lower in front and higher in back would give you this result if the tire were near there ideal psi. I have read a few books on racing and that was always thier advice as well as some good auto-crossers gave me the same advice.

 

 

P.S. I believe this site advice is based on you useing the ideal psi for maximum traction and then adjusting psi to make a car over react or under react.For auto cross it is useful to drift some obsticles so if you set the car up to oversteer your back end won't clip the cone and you can hold a tighter line.

http://www.kormanfastbmw.com/thandling.htm

[subynut]

But usally higher psi in front and lower in back will cause your car to oversteer. I'm not sure why you would want your can to understeer but, usally lower in front and higher in back would give you this result if the tire were near there ideal psi. I have read a few books on racing and that was always thier advice as well as some good auto-crossers gave me the same advice.

I agree. I had a rear tire go low on me (about a 15 pound differance between L and R) and that made the rear VERY over reactive. Turn the wheel a little, and the back wants to go first!

Just my 2 cents.