trackjunky

OK all you engineering junkies, start your engines. Here's the question:

After much droving and fiddling with suspension bits, I finally seemed to dial in my car's suspension. As I'm running a 996 tester (of sorts) I am running 800# springs in front and 1000# springs in the back. Wheel/tire combination is front 8x18 w/225/40/18 and Rear 10x18 w/275/35/18. GT3 sways Front full stiff and Rear full soft. Camber is -3.1 front and -2.6 rear.

Based on this, it would seem that my spring rates front to rear are wrong, as I am really balancing suspension via sway bar. After researching cup car rates, they run a 92% split front to rear with the front rate being 92% of the rear rate. This means that I would either run a 920# front spring or change to an 875# rear spring.

Thoughts? Which would you change?

Bill Verburg

softer front spring rates will induce less understeer/more oversteer which I think is what you want
or
harder rear will do the same

using a stronger spring also reduces chassis roll, so that seems to be the way to go as long as traction can be maintained.

Larry Herman

Like I should tell you...going to protest my old car...grumble, grumble, grumble.







Actually Bill, I think that your combo should be ok. I think that you are under- tired. I think that you should be running either a 235 or a 245 in the front and a 305 or 315 in the back. At worst case, leave the fronts and go to a 295 in the back. Then report back on how it runs.

IMHO if you go too soft in the back, the car will "sit down" on the rear suspension coming out of the corner and will understeer like a pig in slower corners.

Darren

Ewww 225 front. Let me guess it pushes a little

Larry Herman

See, now I took it that the car was loose but is now balanced because he has his swaybars set to the extreme. Maybe we should have started here; what exactly does it do Bill?

trackjunky

Well, the problem is that it hasn't pushed at all! Been very nervous at the rear and not very confidence inspiring.

Larry, the tire combination has been forced by overall tire diameter. The 225/275 combo means that both front and rear have the same diameter. When I ran the 245/285 combo the fronts were 1" taller than the rears - took all of the rake away.

I think a 235 would be great for the front but again that would leave me with too tall a front unless I jumped up to a 315 in the rear. I may go back to a 285 at the rear with is only .3" difference in diameter.

I think going to a 900# spring up front might give me more roll bar adjustment down the road. My gut tells me that being full stiff on the font bar (while full soft at the rear) is just masking an unbalanced condition.

My general premise is that most people over tire their cars. Gt3's come with a 235/295 tire set up and that's a much more powerful car. Cup cars are something like 240/285 - or very near that.

Bull

I used to run R6s in 245/40x18 and 295/30x18, which were less than .5" diff overall. Worked well with my suspension without extreme bar settings.

Larry Herman

Rake, huh? How low is your car set? Having ANY rake when the car is very low will cause all sorts of strange handling due to vastly mismatched roll centers. Also, how much rear toe-in do you have? Are you running solid bushings? That can have a marked effect on rear-end stability.

BTW the 245/35 Hoosier fronts work nicely with the 285/30 rears. That is what I ran on my RSA. Most guys up here run the 245/40s with the 295/30s on their 996s.

forklift

For my RSA I started at 750/900 and too much OS and couldn't dial out w/ sways or my double JRZ, so went to 900/750 and that was better but still too much US.....so went to 800/800 and is just right w/ the front sway at full soft and mid rear sway bar. Might try 800/850 or 800/875 at some point.

This is w/ a 245/275 17 combo.

forklift

Also, thanks Lary for the rebound tips for Summit the other weekend, it worked great. Car is finally handling well at Summit.

trackjunky

Rear toe 3/32ds per side
stock bushings
ride hieight is about 9mm lower in front (well within spec per manual)
245 and 295 still gives a 50mm difference front to rear, so a 225/275 should still be balanced. Is this too much logic?

Here's the funny part, with two separate cars set up with the same basic alignment and exactly the same spring rates, the cars require different sway bar settings. Mine is very extreme on both ends while the other is mid way on both bars. Strange? (I'm on Ohlin's and he's on Moton Club Sports)

Darren

If it doesn't push then my guess is that he could be compensating by making the rear grip less Partially this is happening by the also too-small rears, partially it's because of the sway bar settings. This all leads to that nervousness feeling, or a car that goes easily from understeer to oversteer almost unpredictably. If it feels nervous then neither end is working well

These are big heavy fat pig cars, they need tires! The 225 IMHO is just a stupid thing they do from the factory to make the car push so that idiots don't spin on the street. Similar to why on a stock 996 you can't even get -1 degree front camber!! They dumbed it down.

The GT3 is also a street car, probably also under-tired at the front. The Cup Car? I don't know what comes on it but I know what the Grand Am Spec is!
2-18.9 Tire sizes are 285/645-18 front and 305/685-18 rear.

Have you ever seen a FAST 996 with 225 front tires? Just for braking alone I think you need a bigger tire. I'd fix the tires first then try to dial the car in.

2BWise

If you're full stiff on the front bar and full soft on the rear you definitely can change spring rates without issue. Either bring the front rate up or rear down. It will give you more ability to adjust the bars later on.

Veloce Raptor

Bill, IMO, if you went with a wider tire up front, you might be able to go to a middle setting with front & rear bars, giving you more range of adjustment when you needed it for different tracks or weather conditions.







Professional Racing and Driving Coach

Darren

So with that, 900/1000 or 800/900? Both sound like too small of a difference, so the problem is probably something else, not springs.