36mm Rear Torsion Bars - 24mm Fronts?

[jamesjedi]

Hi,

Feels like a silly question that I think I know the answer to, but I want to be sure.

I have bought a pair of 36mm rear torsion bars. There is a set for sale that are 24mm fronts. Simple question is - would the 24mm fronts be acceptable? I have big sways to help compensate/balance. I would think the answer is yes, but I thought I would ask from people with more practical experience to be sure. Thanks.

[Helix8]

"borrowed" from a previous PP post on this topic - maybe helpful - maybe not?

[Driven97]

My calcs, ymmv:

[jamesjedi]

Ok. Thanks for the charts!

Looks like we have in theory - 394 fronts and 619 rear. Which I can hopefully balance out with sways.

[3rd_gear_Ted]

I use this tool to figure out what the lever arms on the sway bars should be set at to achieve added rate desired for each end of car.
Start for me was 60/40

https://swayaway.com/tech-room/torsion-bar-wheel-rate-calculator/

[Evan Fullerton]

36mm rears work fine, the problem you have is 24mm fronts aren't really what they calculate out to be when you factor in the reaction twist of the front control arm. It has been several years since I ran the calculations but I remember it being around 220 lb/in front as the maximum wheel rate you could get in the front without sleeving the Arm T-bar reaction tube to up its resistance to twist. The other downside of big torsion bars is you loose droop travel. If you want to go stiff, coil springs just work a lot better. Not only can you get some droop travel with tender springs, get higher wheel rates, but it also reduces suspension bushing friction by taking the spring loads out of the pivot point. There are just a lot of good reasons to go coil over if not bound by class rules.

[jamesjedi]

Thanks Evan! I truly appreciate this response.

The rules and cost are a consideration. Thanks again.

[jamesjedi]

Corrected the name - woops. The phone has small font. Thanks again for the responses!

[Evan Fullerton]

24/36 works. You will need a 28mm or larger front bar and none or close to no rear sway bar to make it work. Some people have shortened the front torsion bars and had them resplined and then modified the heck out of the control arms to make it all work but...... ideally you need more front wheel rate. The kinda lame but it still works rout is to run really long bump stops so the front of the car is pretty much always on the bump stops and you use that for added wheel rate. If you search around you can find a lot of different styles of bump stops and mix and match a few to get a little bit of a progressive rate out of them.

Went from 700 lb/in springs to 36mm rear T-bars and the car oversteered. If you are stuck with torsion bars in the front, 24/34 I think is probably a better balance unless you like a loose car (which I do) but 24/36 is on the edge of what swaybars and driving style can handle I feel. If you run a bigger tire stagger it might work better. This car ran 225/245 RA1s for Spec911 or 240/280 Cup slicks off a 997 when it didn't. The balance was better on the Cup slicks.

[jamesjedi]

At a certain point it becomes more practical to get the coil conversion for the the struts and buy the rear coil overs. I have kept with the torsion bars in hopes of doing a PCA race one day. Truthfully I just need to get it running.

Thanks again for all the responses.

[stownsen914]

I didn't see it mentioned, but be sure to account for whether the bars are hollow or solid. I believe the hollow ones are often sized by "solid equivalent" size. I mention this because the stiffness calculation is different for hollow than for solid bars if you are measuring the actual OD of a hollow bar.

[jamesjedi]

Yes hollow. I believe they call it "effective rate".