Rear torsion bar sizing?
 

What size torsion torsion bars would be appropriate for a ~2100lb car used primarily for the track, running 11" wide rears and 9" wide fronts? I have 29mm now, and was looking to upgrade to 30mm, maybe 31mm. I've been offered a set of 34mm, and was wondering if that might be too much spring rate. What would be an appropriate front to go with that? I have 22mm now.

You know I'd give this answer a try !

Common convention for street/track upgrade of the later, heavier cars is 22/28.

You have 22/29 which is movement in the right direction.

I *think* all your change will/should be in the rear. 911's have characterisitic high roll resistance in the front ( note the inside front wheels lifting)...so either a 31 or 34 rear would be good.

I understand how 31 works but am unsure of the 34 since it would be plenty fat and maybe even thicker than the root spline diameter..so you wouldn't get the full benefit of a the 34's spring rate.

Anyway, to gauge this somewhat...the 31 would be (31/29) *4th power stiffer than the 29...about 30% stiffer. A "pure" 34 would be (34/29)*4 th power stiffer...or about 89% stiffer ( but not quite that much because of what we said....maybe 70-80% stiffer).

Wil

I have a NEW pair of 31's on the parts area

your 22's will be fine, i am running 22/31's.

How does one calculate what spring rate is correct for a given setup? Or is it just swap->try->swap->try->swap->try?

I have offers of 30mm, 31mm and 34mm bars in a fairly narrow price band. Maybe I should just buy them all and experiment... Or not!

Cheaper to experiment with springs.

In your shoes, I'd go with the 31's, and tune from there with shocks and swaybars.

Thom, one determinant will be the bumpiness of the tracks you run on. Bumps, esp. if combined with corners, will make you want a smaller bar real fast. You don't want to upset the attitude of the car at the limit -- or maybe you do (I don't know how much fun spins in the dirt are for you....).

I think I'll stick with my original plan and go with 31's - thanks!

I have 22/29 now, and my rear isn't firm enough, pink velour jumpsuits notwithstanding. Yes, on the street it can be 'interesting' but on the track it's not as much of an issue.

What are the shocks your running? I have sports front and back. I need to upgrade my torsions because I'm still running the originals. I was wanting enough spring to push the shocks back into place but not a whole lot more.
I guess I'm sort of hijacking...
Thanks
Or sorry, whichever applies

FYI,
It's late here in Norcal, but I just saw these after a long night in the garage:

http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/200595-fs-torsion-bars.html

Jim

You may want to talk to the folks as Smart Racing Products. The do allot of Toyo Cup cars (spec 911 series) and can help you with torsion bar questions. (I'm not affiliated with them.)

Day late, dollar short :( - that's 1/2 the price of the other set of 31's I was offered!