Larry Herman

Geoffrey, I was having trouble with braking going into 1 and 8 at WG. Both braking zones are downhill. I was actually triggering the ABS as I was building braking pressure on initial application. Chris and I discussed it and I felt that I was not getting the weight to transfer fast enough onto the front tires, so increasing the low speed compression in the front would transfer the weight faster as the body pitchs forward under initial braking. With softer LS compression, I have to wait for the body to pitch down and compress the springs. With stiffer LS compression, as soon as there is any movement, the shock loads the tires while the pressure on the springs build. Make sense?

I need to know what you learned about that.

Geoffrey

Larry, I think you have it reversed, the spring rate is what transfers the weight around the car. If the car had no springs at all, it would effectively have springs of infinite spring rate and the weight transfer would occur immediately. The function of the dampers is to resist the weight transfer, so, increasing the compression in the front will slow down the weight transfer onto the front tires. You went the reverse of your desired outcome.

I'll let you think about that one for awhile...

Veloce Raptor

So, a question: how did you intrepret this ^^^ information? Why did your car behave this way with only 1/4 of the spring rates?

Geoffrey

Answer to the first question:
Chris's car is a 993, mine is a 964 with 993 front suspension, so the fronts are identical and the rears are not since the rear motion ratio of a 993 is <1 and a 964 is > 1. We both have MoTeC ADLs with suspension position sensors which measure damper position and a math channel for wheel travel which is calculated from the motion ratio and damper position. At the time I was running 600/650 and chris at 2000+ F/R (I cannot remember exactly). My car is about 200lbs lighter than his car at the time.

Veloce Raptor

Thanks. I did not realize you had a hybrid suspension with different motion ratios.

So how do you think your car would have behaved, had you had the same spring rates?

Geoffrey

Differently

Larry Herman

Geoffrey, I would argue that the shock does not resist weight transfer, it resists movement over time. The stiffer the shock is set, the faster and harder it resists this movement. In my understanding, the body will compress the spring and the shock. The force from both is transferred to the tire, however, the force from the spring will increase in a somewhat linear amount with movement, while the shock will exert force immediately respective of the amount of dampening it is set for.

Larry Herman

Probably comes down to roll centers. Even though you have the same front suspension, your rears are different, your ride heights are probably different and so the roll centers in your car are higher, therefore less body roll, allowing for lighter springs.

Geoffrey

Exactly, my car was approximately 25mm higher than Chris's, so although my center of gravity was higher, the suspension geometry had the roll centers closer to the center of gravity and therefore had less roll, not to be confused with weight transfer.

wanna911

I'm a nub at this but I was going to say the same thing, suspension doesn't prevent chassis weight transfer, it simply controls those motions and translates them to tires. You'd still have weight transfer if you were riding with no springs and hard rods.

Larry Herman

I have raised up and re-aligned several cars that were IMHO too low, all with good results.

TheOtherEric

I'm surprised nobody commented on that. Isn't there disagreement here? My shock rebuilder (reputable) told me that measurements are done wheels-on-ground.

Larry Herman

I was letting that go for someone else.

I agree about all wheels on the ground. That is the operating condition.

Geoffrey

It will depend on the way the damper works. The Moton / JRZ / Protrac shocks are built where the canister pressure adds effective spring rate to the suspension. They need to be filled and tested with the damper unloaded (wheel off the ground). If you have airjacks it really isn't an issue. I check and set mine on a regular basis.

Accelerator

[QUOTE=Geoffrey;6302255]It will depend on the way the damper works. The Moton / JRZ / Protrac shocks are built where the canister pressure adds effective spring rate to the suspension. They need to be filled and tested with the damper unloaded (wheel off the ground).

That is precicely why I jack up each side to measure canister pressure. I was wondering when someone would pick up on and question that procedure. There is also the issue of ambient. My measurement for this weekend at 32 ambient will be a different reading from the one taken at Road Atlanta in August, assuming no system leakage, which in my case is a poor assumption.