mitch236

I am a student at heart and when I began my tracking hobby, I quickly began reading and analyzing my driving style. For the first two years, I would spend my down time debreifing with my pit buddies. These three drivers would become my mentors and I couldn't have chosen better people. My first DE instructor, who would become my best friend over the years instilled safety into me when I needed it most. Bill Buff, who was a factory Porsche driver in his day, gave me alot of insight into the technical aspects of going fast. I read books. I feel that at this point I am an above average driver.

Now I have taken it upon myself to learn about car (suspension) setup. As luck would have it, at two events last year, Michelin representatives attended and gave me alot of insight into alignment. Now I am trying to learn about springs and dampers.

I started out with stock PSS9's on my '04 C-2. It wasn't bad but there was still significant understeer. Then I switched to 700lbs front 900lbs rear spring rates. This worked fairly welll at Homestead but had too much oversteer. Then I lowered the rear rates to 800 and went to Sebring. Woa!!! That track is so bumpy, I couldn't keep the car on the track! Turns 17 and one were a disaster with my car skating all over the place!!

Does anyone have any advice to offer about choosing spring rates for a bumpy track? I don't want to keep trial and error for months.

Thanks

SundayDriver

You should probably be focusing a lot more on shocks and less on spring rates. That is going to have a much greater impact on how your car handles bumps. If you stayed with the same shocks - single adjustable with a fairly narrow adjustment range, while making pretty big changes to spring rates, then they are going to be way out of balance with each other.

As a reference, we have DSR's running up to 1400lb springs with 1:1 motion ration for a car weighing 1000lbs on bumpy tracks. It is the shocks that control the contact patch.

penguinking

if the car weighs 1000lbs and the springs are 1400lbs...then how much movement is there in the spring? 1/4"? yikes...

SundayDriver

Yup - that is pretty much it. A big reason is to keep the ground effects working.

Geo

No pun intended?

I agree with Sunday. Everything I've read/heard is that it's the dampers that do the lion's share of the work keeping your tires in contact with the racing surface (my friend keep trying to tell me the grass is not a racing surface....). Even if you change springs, if your dampers are not up to the task, you're still going to have issues keeping the tires in contact.

Adam@Autometrics

You can take the shocks off the car and sebring will still be bumpy if your springs are too stiff. But 700 and 900 on a street car is nowhere near too stiff. Softer shocks would likely be in order.

Many factors affect the ideal spring rates, not least of which is bumpiness of the track.
Rule of thumb for a bumpy track is to drop the spring rate (10-20%) and stiffen your sway bars. Taking out some comnpression is probably a good idea as well.

mitch236

Ok. I changed the shocks to full hard in the front and full soft in the rear after I could barely drive the car due to excessive over-steer. The car was still skipping all over the place.

BTW, before I swapped out the springs, I had no problem with skipping around. Just too much under-steer

kurt M

I though ajusting the sways rather than the shocks to tune over/understeer was the way to go on the simple susp systems.

Larry Herman

Adjustable sway bars are a good way to make a gross adjustment to the balance of the car. If the car has too much oversteer or understeer, you start with the sway bars to get it close. Then you will probably have to make a more limited change to affect the handling in one aspect of the corner.

For example, my car had too much understeer for my taste; I really don't like any. So first I softened the front sway bar, and that helped. Then I stiffened the rear sway bar and now the car is fairly neutral coming off the corner, but still has some understeer at turn-in and mid-corner. Now I am going to fiddle with the rear shock rebound to try and get rid of just that portion of understeer, hopefully without screwing anything else up.

SundayDriver

There are probably a few things going on. First, with street type shocks, you will have very little adjustment, no matter how many clicks they put on the shock. The difference between full soft and full stiff can be pretty small. When you change spring rate, it may well be impossible to adjust that shock to the correct range - it is just not valved for those springs. Going full soft on one end and full stiff on the other indicates that you are way out of the correct range for the shocks.

The other issue is that by going to much stiffer springs, you have shortened the shock travel. Most street type shocks need a good bit of travel to do their job. I suspect you have basically needle and seat shocks. Shim stacks are much better for short travel installations.

If your shock is not doing the job, then the tire is not going to stay in contact with the road surface and there will be loss of grip. With single adjustable shocks, it can be really hard to tune on a bumpy track. You want stiff compression to keep the tire in contact with the road, but need soft rebound to regain contact patch after a bump. It may simple be impossible to tune when the shock valving is too far off - this is what I think your situation is with the stiffer springs and the bumpy track.

RedlineMan

Hey Mitch;

I'm with Mark here. No way those shocks can dampen those stiff springs. Who ever suggested those rates should get hosed. You probably can't do much more than 1-200lbs without exceeding most shocks adjustment range. You'd need to revalve, but then you'd be too stiff for softer springs. A call to the manufacturer is in order to find out what range those shock will dampen properly. Only when you get into the right range will tuning be truly effective and linear.

Ed Newman

700/900 for all tracks 1100/1300 if all I ran was the Glen (smooth track)

mitch236

That seems to be the right thought since the original springs that came with the PSS9's are around 200lbs. Maybe that's all they can handle. Now tell me, if I swap the dampers for a proper 2 way adjustable, would the car stay on the track in the bumps. It really is disconcerting coming around both turns one and seventeen at Sebring and having the car skipping all over.

SundayDriver

The answer is maybe (sorry). If the shocls are properly valved, then they will be able to do their job. But springs and dampers are not all the equation. There are also the spring rates of the tires and chassis flex as well as bushing compliance.

Ed's comments indicate that your spring rates should be able to work, but you need to check with someone who knows your chassis, in stock form, and has used those spring rates. A race car, with cage and heim joint suspension, will be much stiffer than your car. If you go too stiff on springs, then it is being controlled more by chassis flex than springs. In that case, it is pretty much impossible to get the shocks to do their job. Sorry to be a bit difficult here, but you are asking good questions abou complex systems whose parts interact with each other in often strange ways.

AudiOn19s

Mitch,
If it makes any difference I spoke with TRG on the phone about suspension for my 996 and one route they suggested for a street car was re-springing PSS9's. They said that you can go up about 200 lbs on the springs and still have the shocks working well with the current valving. The stock spring rates for the PSS9's are ~300 f / ~550 R or approx. the GT3 rates. Maybe go down to a 500/700 combo and you should be back in the proper adjustment range for the PSS9's to work...or send them off to be re-valved.

My impression is that the PSS9 shock itself isn't a HORRIBLE piece...it's no race shock but isn't bad for street and DE duties but can only handle soo much spring rate.

Now if I could just make up my mind on what I want on my car....great info in this thread though.

Andy