Repairing torsion bar tube (rust)

[scottbooth]

My 78 911SC track car has developed rust holes in the bottom of the torsion bar carrier tube, where the tube passes through the longitudinal panels that form the "frame rails". The holes are inside the box section of the longitudinal, so they are only visible from inside the tube. I found the rust spots when I removed the spring plates to upgrade the bushings.

After grinding out the thin metal, I have a slot, about 8mm wide, extending across the bottom 1/2 or so of the circumference of the tube. You can see the inside of the inner longitudinal by looking through the slot. Not good.

Here is a picture of the left side, which is the worst. The rusted out area is highlighted in red.



Everything else in the area looks fine, and I have looked pretty thoroughly. The tube has no rust inside except for what I've already removed. I have stripped the undercoat and PVC from the outside of the tube and there is no rust there either. The rest of the tub is OK except for some comparatively easy fixes to the fenders and the left side outer rocker. Front pan / fuel tank support area is rust free.

I have read all of the threads I can find here that relate to torsion tube failures, fixes, and chassis reinforcement. But I have not seen any cases where the tube was bad at the ends.

Since the alignment is so critical here, and most of the tube is still in good shape, I would like to try to patch up what I have instead of replacing the whole tube, or even leaving the center section attached and replacing the ends only - which is probably almost as much work (a guess).

So, my plan (so far) is to weld up the holes as best as I can, then try to add some curved plates inside the ends of the tube to cover and strengthen the affected areas. Then add more reinforcements to the outside of the tube where it meets the inner longitudinal, and maybe some of the other RSR style torsion tube reinforcements (bracing to the rear seat area). This tube already has the "eyebrow" braces from the center section to the rear edge of the tunnel.

Any suggestions, comments, better ideas? (I have of course considered the obvious answers of buying a different tub or an entire car but am expecting those suggestions anyway...)

Thanks
Scott

[Porsche_monkey]

I had a similar problem on my '74. I welded the area you show in red, then made a tight fitting sleeve from tubing and inserted it into the area where the rubber bushing goes. I then welded the inside (again) and the outside and made a 'custom' bushing from urethane. I never had an issue after that.

[scottbooth]

Thanks PBH!

Good to know that it is indeed possible, and that it can work.

Where did you get the tubing for the sleeve? I was thinking about doing that, but could not locate tubing with the correct outside diameter. Standard DOM seems to come in half-inch increments and the ID of the existing tube is 2.9" / 74mm if my measurements are correct.

I could use thick wall 3" DOM and have it turned down by a machine shop. But if there is an easier way...

Thanks
Scott

[Porsche_monkey]

I work in a machine shop, that is the easy way. Try a muffler shop? Or Pep Boys, they have tons of muffler tubing adapters, maybe something will fit.

[scottbooth]

Good tips. I don't know the thickness of the torsion bar tube so was not sure that muffler stuff would be thick enough to be of much use. But I haven't looked to see how thick the adapters are, just figured the tubing was thin.

The machine shop is the way to go. I have an ex machinist friend who lurks on this list, who helped me out when I was rebuilding my engine many years back. Sadly he has since retired from machine work. Too bad for all of us.

I invited him down to analyze my problem a couple weeks ago and we ended up going out to play pool, drinking too much beer, and then he broke my welder trying to blow holes in 1/8" bar stock to see if he could. I was not paying attention (trying not to get flashed) and had not explained to him that it is a bad idea to set the wire speed very low and the voltage very high. Now we both know why that is true. Tip, cable liner, and wire all become one. Which is the MIG sign for "last call" I guess. (Note to all: I do not advocate drunken welding. This $30 parts cost should be a warning to everyone. Nobody was hurt this time but it could have been much much worse.)

That was a pointless story but I had fun telling it. I plan to do all the work on my car very, very sober.

Scott

[dyhunter]

Bumped.

[dyhunter]

Bumped.

[DW SD]

support the torsion tube by welding gussets to tie it into the body and go to coil overs. It is my impression The torsion tube integrity becomes less critical with coil overs. In addition, I would replace the rusted metal with a focus on restoring integrity, as opposed to functionality of the T Bars.

Add the gussets for coil over mounts, etc.

Doug

[scottbooth]

Thanks Doug. I was wondering if it might make sense to do that. Somewhere on here I read an opinion that the tube is still under significant loading even when running coil overs, but it seems to me that the largest loads would indeed move to the rear shock towers.

I am actually planning to add the rear shock tower gussets anyway, since the time is right (engine is out and everything is stripped to bare metal). I figured it would add some stiffness to the tub and make things easier if I decided to go to coil overs later on.

But maybe it makes sense just to skip a step and do it now. Maybe that deal I got a couple weeks ago on a set of used (larger) torsion bars wasn't all that it seemed. Oh well. (How can we build up garages full of Porsche parts if we don't buy stuff that we don't need?)

At the risk of hijacking my own thread, any suggestions on spring rate for a coil over setup? This is a track-only car but I still drive it to the track, for now anyway. I was going to run 22/31mm torsion bars if I put it back together that way.

Does it make more sense to change the fronts to coil over as well? Or does that not matter?

Thanks
Scott

[304065]

It's impossible to get a high enough spring rate in the rear end of a torsion bar 911. That's why the factory went to coil springs in the first place.

That being said, there's a point of diminishing return at which you throw up your hands and backdate a 964. . .

[randywebb]

I thought that "das Lawyah" found a loophole in the rules letting them add on accessory springs - hence the coil overs.

That doesn't mean that the above issue was not part of it too.

Further comments??

[scottbooth]

Good point John. If the funds were currently available I would be doing my own version of a 964 RS right now. The RSA is too pricey (and too heavy) but I think with a proper donor 964 and a Sawzall and a welder something could be done.

However, my own reality is that all of this year's money is going into a back yard landscaping / deck project. And now, it seems, to pay the lawyer because the neighbors are intent on stopping the project. Whee.

So it makes sense to me, right now, to get this SC put back together as best I can. And it turns out that there is more to deal with than I expected (surprise surprise). But I was going to replace the entire suspension anyway, so coil overs are not too far out of line cost wise.

So I will (without trying to sound pissy) re-ask my initial question, which is, what would be a good spring rate choice for a reinforced, otherwise gutted SC track car? I suppose I want the stiffest springs that will work well with that chassis, knowing full well that a 964 etc. would be stiffer, and thus would accept larger springs.

Thanks
Scott

[DW SD]

I would expect your coil over spring rates to be between 400 and 600 lbs / inch.

Check with Socal70RSR (Fernando) and Clint Smith (911CTS) about coil over solutions. Fernando found one at a reasonable cost for his project via a supplier out of San Diego who sells coil over packages which can be adapted to Porsche and Clint sells coil over solutions at a very reasonable cost.

Doug

[randywebb]

Scott - the tracks you use are completely flat?

or do they have "fun bumps" like in Portland...

[scottbooth]

Thanks Doug. I will check it out.

Randy - not sure what to take from your post. I drive once or twice a year at Mid-Ohio, which has been recently repaved, so it's smooth (now), but more often at smaller, closer tracks (Gingerman and Grattan) that, because they don't get a lot of big-name events, and probably are operating on smaller budgets, do have their share of bumps. Nothing out of hand but not quite smooth either.

So if you can suggest anything based on this, I would be interested.

(BTW I was in Portland a few weeks ago with my family, visiting my sister in law and driving a rented Camry. We did not get to the track...)

Maybe there is an element of philosophical debate in this. I've been following suspension threads for a while here, and I can tell that there is a "stiffer is best" school and a "let the suspension move" school. I am not sure where I belong since so far I have been driving on stock T-bar sizes and, though I've been doing the DE thing for a few years, I am nowhere near "experienced" as a track driver. Slow learner, slow driver, whatever.

I know that it's frowned upon to improve the car until the driver is all sorted out, but the reality is that my car is wearing out and needs attention, so instead of doing the same job twice, I'd rather upgrade now, instead of replacing now and upgrading later. I suspect that whatever I do, I will need to learn the car all over again, once I get it put back together.

Thanks for the input guys. This is good stuff.

Scott

[randywebb]

just that the car can be too stiff for a bumpy track

[Kroggers]

Bringing this thread back - I now have the same problem (Rear torsion tube, not good - now what.), and it looks like coil overs is the best option here. But I have just completed the front end of my car and do not really want to start all over again at that end. Is it possible to run torsion bars on the front and coil overs in the back?

[Tom '74 911]

Originally Posted by Kroggers

Is it possible to run torsion bars on the front and coil overs in the back?

I think BB2 has t-bars in the front and coil overs in the rear. Check:

Black Beauty II

[bcketner]

Scott
have 84 targa tub sitting bare if you decide to try to put original parts back in and just up the road in Comstock Park
Bret

[TRE Cup]

Scott
it is a large undertaking to replace those sections and the tube as well, but not impossible. Careful measurements and some home built jigs will allow you to cut and replace it. There are several chassis laying around so cal that are rust free and excellent donors for your project. You will need a sawzall, torch, welding equipment to get it all done.

We have replaced many inboard sections of tubes in cars over the years (still have an excellent G50 section available), but its been close to 25 years since we did one as extensively as yours (labor was a whole lot cheaper then) . But my point is that its doable and encourage you to dig further on the rear "horns" that support the tube to find un rusted metal. That will determine how deep the job will get

good luck