jabbadeznuts

Perry, after attending this morning's AX school, I hear that the course is pretty tight. I don't know if that has any impact on your setup or not, but I thought that I'd throw it out there.

See ya tomarrow!

Matt

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

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

Agreed - try taking some of the rebound out and let the suspension work. I've tried the whole range, and I run about those settings (actually a little softer in front) with 375# front/29mm rear, though with a somewhat lighter car (2780# wet).

Matt

Adam Richman

Perry, I tend to think like keith on the rebound. I think you are far better off making the dampers work properly and change the understeer/oversteer characteristics through other means. I would suggest that you use an auto-x/track day to figure out where you want the dampers first. My suggestion, take them all the way to full soft on both ends and slowly stiffen the rebound to get the oscillations out of the car (like a car would feel on blown shocks). For me, its harder to differentiate an improper setup from having the car jacking down so I would avoid starting from full firm and working down - that's my butt-ometer though.

In essence, I believe you would see increased understeer by firming up the front rebound. I think however it would be for the wrong reasons - the front end would tend to stay jacked down. If they are too soft, you should experience an oscillation in that end. I'd shoot for something where the dampers are working properly for your driving style/needs.

Ken, I think that chart is overly simplistic - tire pressures well in excess in either direction will cause a loss of grip, tire widths change the rate at which you can build heat in the tires so wider doesn't always mean more stick over a given time period (its also dependent on tire type - perhaps wider is more stick on a street tire, I do not know), spring rates vs. grip are entirely dependent on damper valving, camber needs vary by spring rates, front airdams/splitters can make the entire car settle down due to changing the airflow under the car - they do not always create oversteer on production cars.

JMO and good luck with it.

Alan C.

Perry,
If it would help I can send my suspension down

Eyal 951

Sorry for the OT, but perry, I was reading the ill fated rebuild story, and though i know of everything from the fire on, i didn't know of the first part. That is ****ty! Your right, its not for the faint of heart. You need to update it though, your car is amazing now.
~Eyal

Perry 951

I'll get a tracking number. Oh.. send your brakes.. I'd like them too.

Thanks for all the tips guys. I sat down early this morning and read up. I agree that the proper way to set suspension on an AX car would be new springs/coilovers and adjustable Welts. Since this car is 90% streets, 5% car shows, and 5% track days, I don't want to get aggressive with it. It's a perfect street car.. the intention when I built it.

So.. working with what I have on the car, I took your tips and decided to have some fun at the track.

1st Run: I added a 90 degree turn (90% firm) on the front Koni's and dropped the rear tire pressures to 30psi... front still at 37. I pushed pretty bad, but the *** was well planted.

2nd Run: I went the opposite on the front, going 1/2 turn (20% firm) and dropping the fronts to 35psi. Much much more stable, however, I could lock up a front going into corners.. something I have not had a problem with before.

3rd Run: Went about 60% firm and dropped the tires to 33psi on the front, and got after it. I was out of shape in 1 section from power, but short of that, it ran really well. It has a very slight amount of push on slow tight corners, and that is what I wanted. Also, no brake lockup... and I was not being gentile on this run.

Next event.. I'm gonna toy with the rear so I can use the power I have.

Thanks again everybody!

Perry 951

I do need to get an updated side to that site. I'm pretty busy with the new job.. but I'll do what I can to get a good photo gallery up. I appreciate the kind words!

Yup... this car has kicked my *** 3 times. How about the total noobie mistake of leaving the ring gear off my new pressure plate!!! I was pissed!
All us "Guru's" were noobs once. LOL.. I still make some pretty stupid mistakes!

I've said it before.. this is the best automotive site I know of. There is no way in hell I could have rebuilt the car to where it is without Rennlist. Group hug!

Now go to bed you pansies.

Danno

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

" You're not only concerned with contact patch but sidewall stiffness. "

I'm with Michael on this one. Friction is a tricky thing to understand, it has more to do with loading than surface-area. Notice in F=mu that contact surface area has nothing to do with it. Also the same thing with brake-pads. Larger pads don't give more friction, it's the clamping force from the pistons. If it was all about surface-area, we'd all be using 5psi or even 2psi in our tires for maximum grip.

It really comes down to sidewall stiffness and how stable the tire can keep the contact patch flat; this means higher pressures up to the point where you start lose traction from curving the contact patch into a convex shape. Every tire has an optimum pressure-range that it can operate in, say...32-40 or 35-42 hot. You'll find maximum grip occurs at the high-end of this range. So it's actually possible that if you're already at the high-end of this range and you increase front tire-pressure, you'll lose front-traction and get more understeer. And it's also possible at the low end of the range to decrease pressure and lose grip and get more understeer as well. A within the operating range of the tire, increasing pressure will get more grip at that end.

So that chart is correct; don't change it. And as with Perry's testing and getting imperical data shows, decreasing front tire-pressure will reduce grip and cause more understeer making the car slightly more controllable.

Z-man

Perry:
I am a little surprised that you are so afraid of too much oversteer! I'm not sure why you need to dial in so much understeer into your car. (My attitude is the opposite: I can't get enough OVERSTEER! )

My setup is mostly stock: I've got the sport shock option (Option #474) and I've upgraded my REAR sway bar to the 968 19mm 3-way adjustable. It is set to full firm. When possible, I try to run with wider tires upfront in autox, again in order to help reduce understeer. While my setup may not be perfect, I feel it is a very good autox/DE track setup: it's probably as close to a neutral handling car that I can achieve without really getting into the suspension. (But I digress....)

If you go too firm up front, you may be limiting the suspension's ability: on a track driven car, a firmer suspension will still compress due to the higher speeds and forces that are being acted upon it. However, in autocross, with slower speeds, you may not be getting the suspension in it's 'sweet spot,' and it won't help you much at all!

Back the the oversteering issues you have: if your rear suspension is too firm, as you know, more oversteer will result. But also: if you are throwing your car into the corners instead of a smoother approach and less abrupt transitions, you can also acheive massive oversteer! Focus on how you car driving instead of how the car is behaving.

One more small suggestion: I think you are adjusting too many things too often! Start with a baseline: pick some numbers that you feel comfortable with, then run a whole event with that setup! (I am a huge advocate of the 'set them and leave them' mantra) By doing ONE run with a given setup and then drastically altering it for the next, you're not giving yourself the opportunity to really feel how the car is behaving! Also: you're probably too concerned about how your car is reacting, and that's slowing you down, and giving you inaccurate results. There have been several times where I forgot to even check my tire pressures in the morning, but I went out and the car felt great, so I left well-enough alone.

Just my $0.42,
-Z-man.

Perry 951

Z-

I plan to run the next AX without changes. This last setup feels good. You are 100% correct in that I need to be behind the wheel more with where it sits.

I'm not against controlled oversteer at all.. it's fun, and if done correctly, it's fast. A little understeer seems to help prevent the *** end coming around.

MXM

2 Michael & Danno
A little for my defence. I wasn't saying that lower pressure gives more grip in curves. I just noticed that higher pressure (higher than "normal") on rear wheels was marked as causing understear (i.e. more grip than at front). And obviosly overinflating tires does not give you more grip.
Underinflating CAN give you more grip in drag-race i think... When sidewalls stiffness is not an issue.
So chart, I think, is just giving directions. I wouldn't take it too seriously.