Torsion Tube

[BobTucker]

Hello all,
I need some help sorting out a torsion tube issue. Any and all advice welcome!
(long read)

Car is 1973 911 RSR look.

I've seen many posts on replacing the torsion tube, and I've got some great ideas and helpful info so thanks everyone. I think I am about to replace mine and I have a few questions about the process and hopefully learn from those who have been there. Earlier this season I was hit in the right rear by a rogue BMW bending the right control arm and ultimately the torsion tube. I removed the engine/trans and front and rear suspension and took it up to Damon for "unbending". It fit on the Celette bench once he straightened it and all the mount points aligned. The curious part is when I put it back together I couldn't get enough toe-in in the rear. I decided to go with coilovers and 935 spring plates to try and get more adjustment. After the install I was able to get just enough toe-in however the 935 adjustments were all the way forward and the eccentric on the control arms were too. What was worse is that the coil springs were(are) hard up against the shock towers and once the car was back on the floor the springs are rubbing.

Those of you with coilovers on early cars what do you have for clearance to the inner shock tower? And were are your 935 adjustments? How many threads are showing?

Those of you who replaced the torsion tube:
Did any of you use a new tube? Where did you get it from?
How about used tube? Source?
How far did you cut the old one out?
Have any of you gone with a later tube in an early car to use a G50 trans?

Thanks for clues and info!
Cheers
Bob

[Walt Fricke]

Bob - running coilovers on these cars in the rear requires some modification to the sheet metal in the area where your springs are interfering.

With coilovers the torsion tube has less to do, as it isn't being twisted in the middle by the torsion bars any longer. But that part of the torsion tube still takes up significant braking forces, and acceleration forces, and those tend to try to twist the tube first one way, and then the other. In general, I'd say it is simpler to weld in reinforcements by the inner suspension pickup points to counter these twisting forces.

Do you suppose your steel banana arms, or one of them, were bent in the crash?

You say that you have to run both sides full tight, so to speak, to get any toe in? That is really odd. However, there isn't any such thing as a standard number of threads exposed and so on. My setup is sort of home made and the machinist who did the work just copied the lengths of the parts I gave him (one piece spring plates, like yours were, as the base) and I have plenty of toe adjustment.

An easy fix here could be to elongate the two slots in the rear of the spring plate so the banana can be moved just that little bit farther forward. Since you don't need the eccentric for adjustment, you can replace it with a bolt and some washers, and keep the fat adjuster out of the equation. I think you will still be able to deal with the camber this way. And is the jam nut on the Heim joint quite thin? That might be a way to gain some more forward movement. I'm not sure that you even need a jam (or lock) nut if your setup is just a heim screwed into a threaded hole in the spring plate equivalent end. That adjustment can't change all by itself as the plate and other clearances prevent rotation by more than a hair.

Chuck Moreland replaced a torsion tube and did a great write up on it - maybe you have found that? It is a very substantial job, and you need to cut openings into the firewall so you can get full circle welds, and then weld them back shut again so you are mucking around and weling in the passenger compartment, pulling upholstery and seats out, and so on.

Me, I'd spend the money on 935 style adjustable inner mounts and reinforcement to the tub, and switch - if you haven't already - to aluminum bananas.

[chris_seven]

There is a detailed description of how to change the Torsion Bar Tube in Volume ll of the original workshop manual that is not bad.

There may be a few other issues.

The towers maybe small and in the past on early cars we have used modified spring perches and used the dampers ‘upside down’ so the coil springs could clear.

The RSR Trailing Arms which used coilovers on the 1973 shell had the damper mounting bracket extended




If you need to strengthen the centre of the torsion bar tube the RSR Plate is not a bad idea.





The mounting for the torsion bat covers were also reinforced on the RSR.





We make all of these strengthening plates in Domex 700MC Steel for the torsion bar centre and Tenform XF450 for the side pieces.

[BobTucker]

Walt, good to hear from you again, thanks

What specifically should I do to modify the shock tower?


You can see here that the spring is up against the inner tower.

Yes I was planning to add strength supports to the tube.

I have changed over to aluminum arms about 10 years ago. I changed the right side just in case it was bent. I am unable to get enough toe on both sides or more so on the left side which is a bit odd.

Yes I am aware that there is no standard number of threads showing I thought it may be a easy way to "measure" it.



If I were to elongate the spring plates the spring position won't change, I think although the car fits on the Celette bench the tube needs to move to the rear at least 1/2 inch to allow clearance and toe in I just don't know if I weld in a new tube I can get there.

Yes I saw Chuck's write up on changing the tube and many more too. Chuck suggested I machine the lower shock mount points to give more room.

Looking to get back to the track in the spring-

[BobTucker]

Chris

Thanks for the info, I will try flipping the shock and see if that helps. I will also check the volume II manual. I have already welding in gussets on the shock tower and sides plates.





Thanks
-bt

[Dan J]

I'm a little confused. Did Damon say that the tube was bent or not?
I know he is very particular on his measurements so if the trailing arm mounts are in the
correct spot then it must be bent control arms

[Tom '74 911]

Originally Posted by Dan J

I'm a little confused. Did Damon say that the tube was bent or not?
I know he is very particular on his measurements so if the trailing arm mounts are in the
correct spot then it must be bent control arms

This is what I came here to say. If the chassis sits on the Celette as it should and is deemed 'straight', and you still have non-symmetrical alignment settings or can't achieve the toe or camber settings that you should be able to, I'd start looking at bolt-on parts (control arms, spring plates, a-arms, struts etc...) very carefully to see if something else is bent.

[Shaun @ Tru6]

What year is your car?

[BobTucker]

Dan

Tube was bent, Damon straightened it, fits on bench now. Replaced right control arm. I am confused too which is why I am asking for help. I was thinking that 935/coilover setup would have done the trick.

You have seen a lot of different cars in your shop what do you see for clearance when you setup a car with coilovers in the rear?

Thanks, haven't seen you for a while, I don't get down to Lime Rock much..too loud.
Bob

[BobTucker]

Shawn

1973

[BobTucker]

Tom

Thanks for your response. I replaced the control arm on the right, the side that was hit. I got the 935/coilover setup after the accident so all those parts are new/straight never seen a lap. I have been doing just as you say checking everything and yes if the car fits.. that's why I came here.

[Shaun @ Tru6]

Originally Posted by BobTucker

Shawn

1973

Thanks, I thought it might a 901 box car that had smaller shock mount tubes making inverted shocks a necessity. That would be your solution here too. There's also the different shock mount control arm geometry for 69 to 71 arms versus 72+ used with 915s to consider but unless you swapped to earlier arms, that's not a consideration here.

[Walt Fricke]

Bob
My PCA GT4 car (base tub a '68 Targa) came to me as a roller with the "hole" area already modified. The inside, up to about the level of the cross piece, had been cut out, sheet metal welded in the across the car direction on both sides, with a piece of thicker steel on the inside reclosing it - something like that.

It had the steel bananas, and they had the shock mount moved. When I switched to aluminum pieces nothing special needed to be done for the shock mounts.

This car has been in coilover configuration for a long time, and any issues with the tub and mount at the outsides of the torsion tube have been minor despite there being no reinforcing there, knock on wood. Had I known the RSR had a reinforcement I might have done that, but I didn't, and since there have been no issues beyond maybe rewelding one of the nut inserts/standoffs, I don't think I'll add that to my list. And I use only three of the four bolts - the lower rear isn't used for clearance.

If you had anticipated this problem (but how could you), you could have had the bolt which holds the Heim to the front of the spring plate moved forward. I had two holes drilled and tapped into the slug welded into the ends of my tube - one in the center, and one above it. I wish I had had the upper one installed at the absolute upper limit, as there are advantages to moving that pivot up. Some cars are modified to where the pivot is actually a bit above the top of the tube.

If you cut the spring plate rear slots out some more in the forward direction, that would give you more toe in. I elongated mine, though I think I did so rearward, because we may not have hit the optimum length just right with this one off system instead of the manufactured one you have. Looks like you have the same clever adjustment (forward threaded heim in a reverse threaded larger bolt, or vice versa) my machinist came up with.

If you disassemble your toe adjusting Heim/bolts/nuts, you could mill off some of the face of the head of the large bolt. That should allow the Heim part to get farther in. You don't need much width on the bolt head - just enough to hold it while you tighten the lock nuts (which also could be thinned.

I know that these sort of kludes are not as satisfying as kn0wing how this problem arose, but they all sound like they are a better use of time and money than replacing the whole torsion tube.

But if your inner mounts are a bit forward, that would move the toe out. Are those stock? Or are they sphericals? If the mounts on the torsion tube are bent inward some, that too would move the toe outward.

If you convert to adjustable camber boxes in there, along with reinforcement, you can move the pivots aft a bit - not as far as the Turbos (and RSRs?) - but enough to get you your range of toe adjustment back.

If you do replace the torsion tube, that would be the time to do something fancy with the center. There a large slug of metal (maybe two) inside that tube toward the center - to hold the ends of the torsion bars. Just dead weight in the rear with coilovers. Some cut out the center of the tube and notch it upward - or join the two halves with a short I beam or something, all to allow the transmission to be mounted a bit higher without the shift linkage bumping into the tube (helps axle/CV angularity with lowering, especially with taller rims). I've seen and saved pictures of how several clever shops have done this. That would go with adding the RSR style inner adjustable mounts. Turns out you still want to adjust camber the old way with the eccentrics, as the inner mount adjustment is used to deal with a different suspension parameter (roll center), but they are nifty to have.

[BobTucker]

Walt
Thanks for your time and great info. Yes I have seen many of the other posts detailing the pivot point modifications. Since I need only a few mm adjustment rearward I'll take your advice (and others) and keep the original tube and make the mods.
Cheers
Bob