Perry 951

I've said it before, my intelligence level on suspension setups sucks. For this weekends AX, I want to dial in a touch more understeer to compliment my somewhat aggressive oversteer.

I'm dropping my front pressures a pound or two so I can get a little more bite on the really slow corners, but need a touch of understeer to help on the low 2nd gear corners.

Would adding a touch of rebound stiffness on my Koni's add a small amount of understeer?

Current Setup:
Sways: 30mm Front/19mm Rear, set at middle
Koni's: Yellows at the corners, Front at 1.3 turns (5/8 stiff), rear at 1/2 turn (1/4 stiff)
Pressures: 37psi front, 33psi rear
Alignment: -.7 front camber, -.25 front toe
-.2 rear camber, .0 rear toe
Tires: RE-730's, 225 40 17's front, 255 35 17's rear
Bars: 25.5 stock
Springs: 180lb stock double yellows

The car handles very well as it sits, however, I want to take the opportunity to tweak at this event. Unlike my previous AX on a guard rail infested drag strip, this one is on very large open lot. I might spin it on one of the runs, just to know where the edge is. Sounds like fun huh???

Let me know your thoughts.

iloveporsches

Yes, generally a stiffer front will increase understeer (or decrease oversteer as it seems in your case). Now, actually setting up the car, I have no idea.

Dave

Sounds like you should already have a decent bit of understeer already, but if you want more you can always soften the rear bar or crank the front Konis a little stiffer.

Perry 951

The tricky part for me is the Koni's. They are rebound adjustible only... so the compression side is set. If I could add in stiffer compression, I could get all kinds of understeer, but I don't know the effect of adjusting rebound.

I have tweeked it to where it is just by feel and by the info found here. Pretty good I must say!

Perry 951

Dave -
That's the thing.. you'd think it would understeer bad with the factory 180lb springs. The front end is pretty firmly planted, with just a slight amount of understeer on slow flat corners. I adjusted the rear bar to full soft once before and it was not good. It was so soft that the additional body roll would upset it and provoke a spin. At the middle setting, it's very well planted and stable, until the tires are on fire.

I think the pressure adjustment will help.

Ken

This chart might help. I look at it alot to get ideas.

Ken D

Here's another suspension site that contains lots of good information. I referenced this when setting up my suspension to turbo s specs.

http://www.davidsims.ukgateway.net/944t/suspension.htm

MXM

2Ken:
Chart seems logical, except for rear tire pressure. IMHO higher pressure should cause overstear (less contact surface).

Oddjob

Do you have M030 suspension on your 87? The T-bar diameter and front spring rates indicate turbo S specs?

The cars wont understeer that bad with the stock spring rates. They are pretty balanced with some slight oversteer, but nothing terrible.

Are you experiencing power-on oversteer? Or neutral throttle tail drifting?

Yes rebound adjustment will increase stiffness. If you want to dial in more understeer, adjust the front struts to full stiff (back off a 1/4 turn or so after turning to lock).

If that is still not enough you can adjust the rear sway to the full soft position.

Good luck.

Ken

Shoot, you're right. The chart is wrong. I'll fix it. Thanks!

Waterguy

You have more than stock front negative camber (although probably not alot for AX), but less than stock rear negative camber. Adding some rear camber may help combat oversteer and improve handling. Just a thought.

Geo

All things being equal (which seldom are), stiffening the front suspension will create more understeer.

You shouldn't need to jack with your Konis. You shouldn't be trying to make the dampers do the work of the springs anyway. You generally want to not mess much with compression damping. Rebound is far more important. Except perhaps in autocross.

My recommendation for autocross is a stiffer front swaybar. It will speed up the load transfer at the front and will make the car feel better making quick direction changes as you find in autocross. If you have a stock front swaybar, stiffening it will be a bit difficult. If you have an adjustable front swaybar, go stiffer.

keith

Perry - try INCREASING your front tire prssure a few pounds, and LOOSEN, or better yet, DISCONNECT your rear swaybar (Just pull the bolt in one of the droplinks).

This should help correct your oversteer and allow the understeer you've ALREADY built in to your suspension equation show itself.

Post back and let us know about it.

keith

Originally posted by Perry 951

Current Setup:
Koni's: Yellows at the corners, Front at 1.3 turns (5/8 stiff), rear at 1/2 turn (1/4 stiff)

Too much rebound on both front and rear for your stock springs. I run almost the same with #350/28mm.


Pressures: 37psi front, 33psi rear

Might be a little high here. - try 35psi front, 31psi rear, cold to start - that works very well for Adam and I.


Alignment: -.7 front camber, -.25 front toe
-.2 rear camber, .0 rear toe

not enough camber for autocross - ideally, you would add a whole degree neagative to your front and rear settings. - your tires are rolling over on the sidewall, I guarantee it.


Tires: RE-730's, 225 40 17's front, 255 35 17's rear
Bars: 25.5 stock
Springs: 180lb stock double yellows

With stock M030 (soft) stuff - you need to pay attention to your ride heaights - a low 13" front center of front wheel to fender arch will commonly bottom out the shock while autocrossing, causing the front end to skip around instead of letting the suspension work properly. I added a 1/2" win height to the front and levelled the catr in the rear when I was running the M030 setup and really improved grip and handling.



The car handles very well as it sits,

Try the changes I recommended and see if you still think that it handled pretty good the way it was! You may not even be getting into the really good grip zone with those settings! Try it, you'll like it!


Adam Richman, care to chime in here??

Manning

I think it can go both ways. You're not only concerned with contact patch but sidewall stiffness. I've experienced some oversteer issues in the past with rear tire pressure too low. I think what happens with lower rear tire pressure is the sidewalls are able to deflect/deform causing the slip angle to change to the point where you get oversteer.

On the other hand if you over inflate you reduce the size of the contact patch and make the tire stiffer and more susceptible to being upset by road surface irregularities.