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Rob 930 Rob 930 is offline
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Join Date: Dec 2002
Location: Seattle
Posts: 661
Miguel,

I can answer your second question about why one would leave the torsion bars in place. In my case, it's becasue I don't want to load the shock towers with the full brunt of 650 lb/in from a coil spring. If I felt good about using only coil springs, my T-bars would be gone in a flash. And some people claim to have good luck with full coilover conversions on street cars this way. But there have been reports of structural cracking of the chassis (and I've seen some myself on other 930s). Those shock towers were never intended to carry so much load through that path. Some people reinforce the shock towers (a la early RSR) to alleviate the problem, but I've never been sure that's enough. Because my car is intended for the harshest load case (a hard pounding on the track), I'd rather not subject it to heavy spring loads through the shock tower at all. The "right" way to do it (IMHO) is to install a cage that picks up the rear suspension points and attach the shocks (carrying coil spings) right to the cage. But that's one notch more hard core than I want to go right now. So, the T-bar plus helper spring gets me the stiffness I want without loading the chassis in an unfavorable way. Not an elegant solution, but one that has been shown to work surprisingly well.

The first questions you asked are essentially the same questions I have. More specifically, I'd like to know just how much to "preload" the coil springs when the car is static. If I install the coils so they just contact the perches, then the ride height won't be affected, and theoretically I'll have their full benefit under pure acceleration (squat control). But under cornering or braking, the suspension will lift off the coil springs on one or both sides, which doesn't seem optimal. To improve that, I can first lower the car on the torsion bars (re-index them) a bit first, then bring the car back up to height on the coils, which shares the load between the T-bars and coils under static conditions. Under acceleration, these two setups should yield the same spring rate. But for cornering or braking, the latter solution seems better, to avoid having a sudden increase in spring rate at some point when the coils "engage."

But how much preload is optimal? I'm not sure. And I'm not sure how much it matters -- two springs, acting in parallel, are additive in spring rate. Hence, a 400 lb/in (equivalent) T-bar plus a 250 lb/in coil, will give 650 lb/in, as long as they're both being deflected simultaneously. I would think you'd want keep the coil spring between two limits -- so that it always carries enough load that it won't lift off the perches, and so that it never carries too much load which could cause it to "coil bind" or reach solid height by bottoming out from too much deflection. Other than that, does it matter? I dunno. I'd like to hear what others think

Apologies to the original poster for the slight thread hijack -- I hope this is relevant enough to the general topic of squat control on 930s...

Rob
Old 02-16-2009, 01:49 PM
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