Still confused about torsion bars

dd74 · 06-26-2002, 12:28 AM

Hi all:

I'm still in a quandry about torsion bars. So far, I have to my knowledge, stock 19mm up front and stock 23mm in back. It's on a '74 911, but with a 3.0 engine. The shocks have been replaced; Bilstein HD in front, Bilstein Sports in back. But the rear is spongy while the front is very solid. What would be the ideal torsion bar size, in anyone's opinion, for agressive street driving and next to no track work? I'm looking at rear torsions that are 28mm, but have also know of a 21f 33r set up, which I think is pretty stiff, though I can deal with stiff as long as everything handles. I also plan to replace the rear bushings. And since the front on my car is already stiff, should I leave it alone while I work on the rear? Or should I do everything at once, front and rear?

Sorry about the long-winded question. Nonetheless, thanks in advance for any and all responses.

Superman · 06-26-2002, 08:16 AM

If the car will not see any track time, then I would think 33mm torsion bars would be a bit stiff. I'd guess you'd be happy with the 28mm bars. Yes, you can deal with the rear and worry about the front later. I would stay away from the hard plastic bushings. There are some good bushings out there that don't damage your fillings (Neatrix?).

911nut · 911nut's Garage

Quote:

Originally posted by Superman
If the car will not see any track time, then I would think 33mm torsion bars would be a bit stiff. I'd guess you'd be happy with the 28mm bars. Yes, you can deal with the rear and worry about the front later. I would stay away from the hard plastic bushings. There are some good bushings out there that don't damage your fillings (Neatrix?).

If the rear of the car is stiffened without stiffening the front it's going to unbalance the car in a big way; severe oversteer. Wait until you have the cash and do both ends.

Superman, lots of guys here on the bb have installed the Weltmeister "street" durometer plastic bushings and don't report any unacceptable harshness.

Superman · 06-26-2002, 09:05 AM

Nut, with 19mm fronts and 23mm rears, his car is already substantially out of balance. I'd expect severe understeer. And with the added weight of the 3-liter, this problem would be exaggerated. I think 21/28 would probably be a better balance, but if there's an end that needs attention, it is the rear. I know some folks with plastic bushings too, and most of them tell me that this is not something they would recommend for a street car. In decreasing order of harshness, they say plastic bushings, then stiffer sway bars, then stiffer shocks, then stiffer torsion bars. On the other hand, some folks are okay with harshness on the street. I don't mind a little harshness myself, but will probably do a torsion bar and shock upgrade, plus Neatrix bushings, and see how that goes. I've been in full race cars, and while I want my car to have a race car ambiance, I don't want it to actually be like a race car. Real full-race cars feel like the axles are welded to the frame. But hey, everybody is different. Handling and ride harshness is pretty subjective.

dd74 · 06-26-2002, 10:44 AM

The torsion bar sizing is a good debate, as are the bushings, and I appreciate the feedback. What really threw me for a loop was the fact that I installed new shocks in the rear, and it was nearly as soggy as before. Why would that be?

Thanks for your replies.

Jdub · 06-26-2002, 10:58 AM

Jim:

Standard Tbars on the SC are 19mm front and 24.1mm on the back. Are you saying the stock setup is out of whack, even though we are talking 19/23 on a '74?

JOhn

jphillips-998 · jphillips-998's Garage

Don't know the effect of the heavier 3.0 L engine, but I have 23/29's installed on my '74 and it is very rough on the road. I use it primarily for track, but I know that it would not be comfortable for everyday driving.

If the car won't see track time are you sure you need/want larger torsion bars? I would guess it is pretty difficult to get to the limits of the stock suspension on the street. Maybe larger anti-sway bars would be a more useful upgrade?

dd74 · 06-26-2002, 11:35 AM

Jim: I really wouldn't say it's out of whack. It does wander a bit, which leads me to think it might need thicker torsions in the rear, and some bushings. The front's fairly solid, but I haven't touched that at all except for new shocks, which didn't seem to do much since the front was always quite stiff. It's just the rear that needs some firming up, I think. My mechanic suggested 26mm in the rear, and did not bring up the front suspension. He didn't mention a thicker rear sway bar, either.

Thanks.

Randy W · Randy W's Garage

Your mechanic is right on the money - 26mm rears with your 19mm fronts would be my recommendation for your car and stated puposes (this is what the factory used on the first RSRs). By the way stock 19/24 on early 911s is balanced toward understeer, not neutrality, probably for liability reasons.

Superman · 06-26-2002, 02:04 PM

Well, that's my sense too. That these cars are often set up for understeer. My car understeers in spirited handling, which I reduce using tire pressures. I believe that the effect of stiffer rear torsion bars would be to reduce the understeer. I think 19 and 26 would probably be a very good balance. Or 21/28. Personally, I'd probably prefer to use torsion bar sizing, rather than sway bars, to maximize handling.

Jack Olsen · 02:26 PM

dd74, I've got a pair of rear 26's I'd be willing to sell you. I started with 19/26, and then moved to 21/30.

I wouldn't do the 28's without stiffening the front to 21 or 22. This isn't primarily because of the ratio between the two -- Bruce Anderson acknowledges that there's a whole school of thought in track circles that holds that you can go much stiffer in back and only improve (high speed) handling. My reasoning is because the 18.8 bars you've got in there now are so soft, relative to performance handling expectations, that a big upgrade in the back is going to leave you with a car that's half street-cushy, and half track-stiff.

Seat-of-the-pants judgments about tight and loose suspension feel is not very reliable. There are so many factors that contribute to the suspension's 'feel' (from tire pressures, tire type, wheel size, shocks, springs, sway bars and (often crumbling) bushings -- all the way to suspension settings that have spent many years drifting off from stock specs (which themselves aren't optimal for what a lot of 911 guys consider serious driving). It gets very complicated to try and sort it out without taking into account the whole picture.

dd74 · 06-26-2002, 02:45 PM

Hi Jack: I'd be interested in the bars. Can you send me an email, or can I have yours, and we'll talk.

Thanks.

rattlsnak · rattlsnak's Garage

Do the RIGHT thing,....... Get Coil-Overs and be done with it.!!

dickster · 06-27-2002, 01:01 AM

Quote:

What really threw me for a loop was the fact that I installed new shocks in the rear, and it was nearly as soggy as before. Why would that be?

you need to read up on shocks, i had an article - can't find it. i'll post if i do.

basically shocks just control the movement of the suspension. ultimately if you have more weight you need stiffer springs not stiffer shocks.

on the road (more bumpy) shocks would be on a softer setting (more suspension movement) to smooth out the bumps. stiff shocks would give a choppy, uncontrolled ride.

on a track they can be set alot harder as the surface is much smoother and there is less movement in the suspension.

theres more to it than that, but it gives you the idea.

jluetjen · jluetjen's Garage

With shocks; what matters on the track is often not so important on the street. Why?

On the street the most common job of the shock is to keep the car from oscelating after going over bumps. This is the function of the "hi-speed" valving in the shock.

On the track, hi-speed valving is still important to keep the tires firmly on the road over bumps, but the primary function drivers tend to be most concerned with is how the shocks control the transistion of the car through corners. This is a function of the "low-speed" valving. This is the part of the shock set-up that has the biggest impact on understeer/oversteer while entering and exiting a corner, not to mention TTO.

Going back to one of the "baseline" set-up is always a good idea to start. By "baseline" I mean either a standard factory set-up or one of the common alternative configurations that were discussed earlier. If you are only going to do part of the job (let's say the back only), I'd recommend that before you start switching stuff that you have a clear idea what it is that you are trying to fix. It's obvious that compared to (the often right) conventional wisdom that your car is set up to understeer.

Wondering can be the result of many factors. In some respects it contradicts intuitive understanding of your set-up since cars that push often do not like to change directions - which is what wondering is. (BTW - this conclusions assumes constant push, if you car darts in and pushes in mid-corner, you could have an entirely different set of problems!) To really fix the problem without wasting a lot of money throwing parts at the problem, you might want to confirm the basics to start:

1) Is the car aligned correctly?
2) What does the car do when the tire pressures are set to spec? When does it do it - corner entry? mid-corner? corner exit? On the gas? On the brakes? You want to make sure not to mask the problem with the tire pressures. It's fine as a quick fix at the track, but you don't want the resulting handling to lead you down an entirely different (and possibly dead) rat-hole.
3) Is the tire wear reasonable?
4) Are the corner weights reasonable?
5) Once you've done that and the car still doesn't handle to your liking, then I'd start to disect the what? where? and when? to improve the handling.

I'd strongly recommend this book from Carroll Smith. The rest of his library I'd also recommend for anyone considering making any changes to their 911's for track use.

71T Targa · 06-27-2002, 07:45 AM

Is there a good place to see what T-bars were original on different models and what would fit different years? There is a set of 930 bars for sale here but they’re 26mm/30mm. From this thread, this would seem to be too stiff in the front for the back.

Would 930 bars even fit my early ‘T’?

Jack Olsen · 06-27-2002, 09:58 AM

Stock sizes are listed here.

71T Targa · 06-27-2002, 11:08 AM

Quote:

Originally posted by JackOlsen
Stock sizes are listed here.

Thanks Jack, what about compatibility? Do they all have the same length, number of splines, etc?

dd74 · 06-27-2002, 11:28 AM

As always, a lot of the advice here has some very good points and is very helpful. I probably should clarify that my car, in my opinion, is not handling strangely. It does, however, have that soggy rear end that I plan to compensate with larger torsion bars. One interesting fact is I have 15x7 Fuchs on the rear with 205-60-15s. This is on a narrow-bodied 911 BTW. Thing is the rear-right tire is rubbing slightly somewhere on the fender, something that has been suggested can be remedied with alignment, camber change or even a 55 series tire, to which I'd be replacing the other three as well. Question: it seems as if this right rear tire/wheel protrudes a bit farther from the fender than the left rear. Another Pelican member told me this is common. Is it? The last thing I want to do is "roll" the fender to stop the rubbing. What are your thoughts? Might alignment/camber settings fix all this?

Thanks in advance.