Bigger torsion bars on a cabriolet

I'm thinking of putting bigger t bars on my '86 Cabriolet. They car is used for a combination of street, DE, and autocross.

I am considering 27/21 mm. Will this be too stiff for the cabriolet flexy-flyer chassis?

Currently the car has 25/19 mm.

Chuck


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'86 Cab, '77 Targa, Family Truckster

I think you'll be fine. They are fairly soft to begin with. Better that you go with stiffer torsion bars than sway bars. That's not too dramatic of a difference so it's not like you'll be twisting the chassis or anything. I think you'll really like the ride to handling trade-off. It definitely favors handling as the ride doesn't suffer as much as you might expect.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

Tyson,

What do you have in your cab, are you talking from experience? I find the existing setup to be solid already, and haven't had the benefit of riding in a car with bigger bars for comparison.

The stiffness of the torsion bar increases with the square of the radius (I think), so I am expecting the 27/21 to be significantly stiffer than my current setup. I can detect some flex now, especially over bumps, and fear that the larger bars may be too much.



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'86 Cab, '77 Targa, Family Truckster

I think you'd be surprised at how little a difference you'll experience in ride quality, even with the Bruce-Anderson-reccommended 22/28 combination (for later cars). So long as your shocks are matched to your bars, it should be very tolerable, and a lot better for DE and Auto-X. I went from 19/23 to 19/26, and then to 21/30. I'd try even stiffer, now, if it didn't cost a lot to find out.

The only downside I've seen is in low-level vibration -- the sort of thing that the occupants of the car never even notice (since, for them, the seat padding absorbs it all). But the new bars make my in-car video camera shake more, and I've noticed more of the hot, dangerous, splashy dance in the coffee in my cupholder. Small price to pay for precise handling and rock-solid stability over 130 -- the next time I race it, I'll just ask for a little more room for milk.

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Jack Olsen
1973 911 T (3.6) sunroof coupe
jackolsen@mediaone.net

[This message has been edited by JackOlsen (edited 05-21-2001).]

I'm interested in this one.

Jack said something very important,
"So long as your shocks are matched to your bars".

If you go too large your dampers will need a revalve to match the new spring rate.
Because of this I went for a very conservative 24.1 up from 23 on the rear only.

This is 20% stiffer & the size chosen by Porsche GB for their track & rally RS of the seventies.

I noticed in the Boge application guide they have different rear dampers listed after 85 when the rear bars went to 25 or 26mm.

From this I guessed that 26mm bars were too stiff for my standard shocks.
I have not tried the 24.1mm bars yet, I can swap the bars for 26 with the shop if neccessary.
Am I wasting my time with the 24.1 bars?
I find the standard car OK but just want to cut down the viscious transition to oversteer when exiting corners at full steam.

I can't be bothered with changing the front bars, for my ability I don't think it is needed.

Can the factory have got it so wrong that a 60% increase in spring rate is needed?

JG

Thanks for the tips guys. I beleive the ride quality will be acceptable to myself with the 27/21.

My real concern is if the flexible cabriolet chassis will be up to the task. Most cars getting upgraded T bars are track-bound Coupes. None of the discussions I've seen on Tbar selection have addressed question of Chassis type. After all the Coupes are more rigid.

Should I be sticking with lighter bars for a Cab?


Jon,

I think you will find going to heavier Koni or Bilstein shocks will make a world of difference. I couldn't believe the improved feel and control putting Koni's on my otherwise stock suspension made. With stiffer T bars they would be all that much more valuable.

Chuck

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'86 Cab, '77 Targa, Family Truckster

83 SC cabriolet.....

I have the 22/28 torsion bars and upgraded to the larger Carrera sway bars. Great upgrade. I also have a four point roll bar. I give up a little weight penaly to the other guys, but rag tops kick ass....

Chuck,

The stiffness actually goes up by the fourth power. This might help a little bit...

http://www.pelicanparts.com/techarticles/911_torsion_bars/911_torsion_bars.htm

FWIW, here's my observation, which I think is shared by others. I drove a car with 28mm rear sways, considered by many to be full-race and inappropriate for street. I found them to be quite tame, still offering a fairly smooth, quiet ride.

I have been in full race vehicles before. Cars can be made intolerably stiff for street purposes, for sure, but torsion bars alone are not likely to produce those undesirable characteristics, IMHO. IMHO, in order to get a truly offensive ride, you need stiff shocks and sway bars.

I intend to put stiffer T bars on my car when funding permits. I'll choose at least 28mm bars for the rear, with utter confidence that my car will be fine for daily driving. If the stiffer bars are too lively for the old stock shocks, then I'll follow up with a shock upgrade, but I'll be careful not to get shocks that are too stiff, as I feel this is where race cars get most of their suspension "bang."

I also take a slightly more dim view of sway bars than many folks. I believe they mostly fix a problem brought by torsion bars that are too weak. I believe that with proper torsion bars, sway bars are not needed, or are only really needed for purposes of adjustability. And I'm not alone in that opinion.

Finally, I can see that your question really deals with the cabriolet shell. I don't know the answer to this one, but I suspect your frame is plenty strong enough to avoid getting damaged by the stiffer bars.

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'83 SC

Well, it looks unanimous. Everyone and every source I find seems to think 27/21 is fine for both my backside and the flexy-flyer chassis.

If I can get all the peices together, this will be my project this weekend.

Mikez, I am with you on this one. Unless it is a dedicated race car, Cabs kick ass. The top on mine goes up maybe twice a year.

Chuck

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'86 Cab, '77 Targa, Family Truckster