P Cars With Torsion Bars

Ron Earp

Ron Earp

Administrator
You folks that are racing P cars with Torsion bar rear suspensions can you give me a run down of how you make these work for racing? I'm curious about them. I assume that you get a larger/stiffer torsion bar that is essentially like stiffening a coil spring. I have seen some kits for rear spring kits, coilovers that is, but that would not be legal in IT racing.

Seems like the torsion bars would be something of a pain in the butt to replace, and, without the proper (not sure what the right word is) preload or indexing it looks like the ride height would be, well, for lack of better word all wonkered up.

I practice do they work okay? Why did Porsche choose to use these instead of a seemingly much more simple coil spring design? VW origins?

Ron

PS - Found this:

http://en.wikipedia.org/wiki/Torsion_bar

Some useful stuff there but not Porsche specific.
 
You're pretty much correct on all counts, there.

Torsion bars can result in a lower CG - they're basically horizontal on the floorpan. But yes, I think a large part of it is VW origins... though they do help contribute to some impressive trunk space vs. some multilink suspensions... that coupled with a hatchback, it's amazing - I once took a queen-size futon home in my old 924... ;)

As for racing with them, it's no different than using coil springs - they're just coil springs that have been unwound. All the spring in a coil spring comes from torsion anyway.

You just try to find the size you want first time, cause you only want to have to change them once! I've been through a couple sets, and it does get easier with time, but still a PITA.

Ride height adjustment is actually not bad at all; in addition to reindexing the t-bars, you can also adjust with an eccentric bolt. That comes stock, BTW! No aftermarket hardware needed, at least not on the '44's (early 911's I guess didn't come with this feature, but it can be added).

The PITA comes in at aligning the rear - you've got camber and toe moving at the same time, with the same set of 3 bolts... again, it's all adjustable from the factory, but takes quite some time to learn how to do it quickly/efficiently/right the first time. Another downside is that few alignment shops can do it, or at least do it well enough to hit the track. Not that this will stop them from taking your money...
 
Not P-car specific but we had torsion bars on the back of the Renault Cup cars. They were actually pretty easy to work with, once you figured out how to diddle them up and down. We used to be able to cornerweight those little cars pretty effectively.

We actually drove our Alliance from Seattle to Riverside (CA) Raceway, loaded down with tools, camping gear, and other stuff - and jacked the rear end up into hog heaven for the trip, so we wouldn't be on the bumpstops. Lowered it back down, reset the static ride height at the track, and went racing.

The biggest issue is that they are expensive to make, so you can't change rates by just calling your Eibach dealer.

K
 
Like Vaughan said T-bars are just like coil springs.

The 944 rear suspension works farily well although there 3 issues.

1) tendancy to tow out under braking (bushing material and toe settings are critical)
2) T-bars are not easily changed (figure 3-4 once you learn all the tricks, 8 hours for the first time)
3) interdependance of ride height, camber & toe in the rear


The great news about 944 rear suspension however

1) Low CG
2) Wide ranger of camber & toe -you can get over 3 deg negative camber in the rear with no special parts
3) Spring plate adjustment allows for "quick" +/- 1/2" ride height changes if you inde the the bars properly
4) indexing allows you to run any ride height you chose with factory parts (no special kits needed to lower the car).

Spring rates increase by the 4th power of the bar diameter. Here is a link to proceedure I developed to swap 944 t-bars and get them indexed properly.


http://944spec.org/944SPEC/content/view/42/25/
 
I just came back and got to read this, very nice information. That setup guide is a nice piece you put together Joe.

The rear suspension is relatively simple, it appears, but I'm still not sure I completely get it. The torsion bar, it is about what, four feet long and lives in the tube that the trailing arms are connected to? (Check Joe's setup photo of the rear on the floor)

Is it really one bar that has splines on each end for the "lever" to attach to? If that is the case then does it affect the other wheel? Seems like it'd have some effect for sure, one end gets twisted as the cornering wheel twists it, seems the other wheel, the inside wheel, would be picked up some?

Wait, it appears it is two independent bars, I'll leave the above paragraph for train of thought.

I would imagine that some fairly good torsion rates have already been worked out by the multitudes of 944 racers in the world?
 
Correct, the bars are splined into a hard point in the center (inside the torsion bar carrier). They're each closer to 2.5' long (half the width of the 924, really).

Most '44 racers use 30mm, 'cause they're the biggest bars readily available off the shelf. They're also specced out by the spec classes.

Mine are the biggest available, super stiff, another 50% more than the 30's at 34mm hollow. :D
 
One of the joys of racing a Porsche is that the cars are very well set up from the factory, and while they sometimes solve problems in unusual manners, they solve them! So the rear suspension is very adjustable.

And another joy of Porsche car racing is the relative ocean of knowledge on how to set them up. The 944 has a ton of info out there, so it's pretty much plug and play. once the rear bars are in, you can do all the adjustments easily.

Pelicanparts.com is a great parts source and their forum has very knowledgeable posters. http://www.pelicanparts.com/944/944world.htm

Elephant Racing is a great source of handling and race bits for the 944. Call Chuck Moreland. he's super friendly, and engineer, and very knowledgable. heres a link to his suspension page:
http://www.elephantracing.com/suspension/d...siondiagram.htm

And theres always Rennlist as well.

Also, PM Geo, and chris Cammadella here, as they each have ITS 944s.

The only downside is the typical Porsche owner expects really high quality stuff, and the better companies provide sweeeet stuff..but of course, nicer parts cost more money. Do NOT be tempted by Weltmeister...cheap stuff, and it shows.
 
According to Jon Milledge there is a way to modify the suspension so that you can adjust toe and camber independently. He says soemthing in his notes about welding something I think. Apparently IT legal. Does anybody know how this works? A diagram woudl be great!!!

BTW - Jon is more or less retired. He is closing his business and moving to CA

Maybe you can mark this up...

[attachmentid=1040]
 
The mod I'm aware of to separate toe and camber adjustment, based on what the factory guys did way back when, is not to my knowledge IT-legal. They didn't do it at the spring plate, but at the inner end of the trailing arm, modifying the attachment point IIRC so the pivot can be moved around.

Sounds like moving a suspension pickup point to me...

Using the stock adjustments isn't that bad, once you've got some practice, not so much that I'd consider making a car illegal! As was pointed out, you can get enough adjustment from the stock adjusters to more than align your car to race specs...
 
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