| I have seen alot of posts discouraging people from buying coilovers... while I don't disagree with the good intentions behinds those posts, there is alot of misunderstanding about the performance of lowering springs/shocks. I think this came about because there are guys here who track regularly running just such lowering spring/shock combos, but IMHO their suspension tuning is inappropriate. The main problem is with the front suspension on the Z. Take a look on the front and you will see that the shock is very short. That means its not possible to have a very long stroke (the length the piston can push into the shock before it bottoms out) on the shock. Here's a little calculation: I approximate there is not more than 3" of compression available from static ride height on a stock suspension before something bottoms out (suspension arm hitting, tire hitting, etc). And this is being pretty generous. With the bumpstop in place, that means about 2" of compression before you start riding on the bumpstop. You don't want to be riding on the bumpstop, that makes the effective spring rate non-linear and radically higher, also virtually eliminates the shock absorber's ability to control the suspension movement. This translates into poor behavior in corners and hard braking, the car will be sensitive to imperfections on the road surface. Stock spring rate is 2.8kg/mm. For a 0.8" drop, that means your spring rate needs to increase by 67% to avoid riding on the bumpstop. So minimum spring rate is 4.7kg/mm to have *proper* control over the suspension movement. For reference, I found the Tein H-tech front springs to have a spring rate of 3.3kg/mm for a 0.8" drop. Not enough. Tanabe GF210 springs rated at 20-30% increase in spring rate, again not enough. I couldn't find the spring rates for Eibach, but I know for a fact its not enough. Can you crank up on the dampening on the shock to compensate for lack of spring rate? No, because of 2 problems -- a) most single adjustable shocks primarily adjust rebound on the dampening, so it does little to change the compression stroke. b) this is a poor way to adjust roll rates since shocks control only transient behavior, this does not change how the car behaves on long sweepers. The bigger problem is that the shocks (AGX, etc) are all designed with stock length bodies. What you really need when you lower the car with a lowering spring is a "short stroke" shock, which AFAIK, only Koni offers and perhaps not even in the 300ZX application. That means that with a shorter stiffer spring (ie. all lowering springs), the amount of shock compression stroke is reduced. So the shock could bottom out before the rest of the suspension, causing the shock to blow out much sooner. What about the Ground Control "coilovers"? The problem with that is you cannot adjust the ride height without changing the spring preload. So you have the same problem of the reduced shock compression stroke as a lowering spring. Most coilovers now have adjustable lower brackets, so you can set your ride height *and* spring preload accordingly so you have the appropriate amount of shock compression stroke to match your suspension travel. That said, I am sure 99% of the people with coilovers did not set their preload properly. But properly setup, this will be the correct way to lower the car. So IMHO, lowering springs with shocks are in most cases NOT the proper way to lower the car. They would ride fine on the street, but it will not perform correctly on a track.
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15:13:50 10/19/07
