Brian (MADMAN) has helping get the initial tuneup on the car. He's out in Memphis right now so I'm posting here to see if I'm on the right track.
The bold is what I found interesting. Over the phone Brian told me to shoot for a 53% bias, but didn't say front to rear or rear to front. Tom at Coach chassis though I meant rear to front. Brian is referring Front to rear. I'm making this assumption based off of what has been written down on paper from what we measured. The corner weights were as follows.I have had alot of calls and emails about choosing the correct spring rate for their car so I will give a rough overview of spring rates in hopes this will help.
First this is how spring rate is measured. The springs are measured in lbs per inch. An example of this is the QA1 spring for the front os our cars. It is rated at 315 per inch. This means that each inch you compress the spring changes by the rating. 315lb is 315 for the first inch, the second inch would be 630lbs and so on.
We will start with the front.
Most of the 4th gen f-bodies weigh in at 3500 at the high side, the weight bias(front to rear) is usually 54% front 46% rear. This equals 1890lbs front weight. If you run a QA1 spring at 315lbs usually the spring is compressed 5 inches per side. This comes to 3150lbs of spring on the front. NOW comes the trick. The spring shock setup is actually inboard of the spindle on our cars unlike strut front ends. The spring has to actually hold up more weight because of the leverage of the control arms. This comes to 40% more spring to hold up the car. Here lies the problem. The more you compress a spring the more "stored energy" the spring holds. When you launch the spring tries to throw the car up and relieve the stored energy. This ends up being a wheelstand situation if the cars hook.
The way to remedy this situation is to run a softer spring. We normally try a 275 lbs spring compressed to 5.5 inches. This lowers the total weight to 3024lbs BUT the car runs out of stored energy quicker so it will slow a wheelstand down. You could actually put a 225lb. spring on and compess it to 7 inches and hold the car up but kill the weight transfer.
Tightening the shock up on the front will not control the wheelstands as the shocks work to slow to overcome the spring. Only a softer spring or a limiter will control the front end.
Hopefully this will explain some of my spring theories.
LF......RF
919....876
LR......RR
754....650
43.89 Rear bias (56.11 FRONT)
Corner weights as I drove the car, no adjustments
Added 83lbs of lead to simulate a full tank of gas
LF......RF
900....882
LR......RR
827....682
45.85 Rear bias (54.15 FRONT)
3291 Raceweight
Adjusted LF shock to move weight towards the RR tire (adjustment maxed out)
LF......RF
928....852
LR......RR
794....717
45.91 Rear bias (54.09 FRONT)
3291 Raceweight
The RR tire should have 10-15lbs more weight than the LR. So somehow I need to get 94 more lbs of weight on there.
I never saw the paper as he was writting the numbers down, so I didn't pick up on the confusion. He had a calculator that just spit out the numbers as the corner weights were entered.
So my thoughts based off of what I saw.
-Loosen up the RF shock to balance out the LR side
-Loosen up both front shocks to shift more weight to the back, at the cost of ride height.
Anybody else agree?
I'm not to crazy about running at a full tank of gas. I may just live with the 56% as long as I can get the rears to balance out.
Billy? Ed?