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Torsion tube failure - and repair - the project

Received an interesting phone call about a historic racecar that had suddenly dropped down on the tires mid-turn at Laguna Seca. The failure was sudden and without warning.

The first thought of course is a broken torsion bar, but this car dropped on both sides - seemingly at the same time. While it is possible that one torsion bar failed and the double load now carried by the remaining bar caused a rapid failure (kinda like a zipper), I figured this was unlikely.

I suspected something else was at play, perhaps a failed torsion tube. The car is a '68 and rust was a serious possibilty.

The car was trailered to our shop.



Beautiful car. But it gives new meaning to the words 'Tail Dragger'
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Old 11-24-2011, 07:11 PM
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hmmm....
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Old 11-24-2011, 07:29 PM
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Sub'd to see the repair
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Old 11-24-2011, 07:38 PM
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the trepidation builds...
Old 11-24-2011, 07:54 PM
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Popped the car on a lift to have a look.

This torsion tube seems to have a little extra contour to it:



If you are not familiar, it shouldn't have that bulge in there as above. It should be straight.


Woah! WHAT IS THIS >>>



That's a pretty big gash in the tube. That's not supposed to happen. That's the driver's side.


Not quite as obvious, but here is a similar rip on the passenger side:





This is what you call a problem.

That torsion tube has gone kaput. In a big way. I assume there is rust in there.



Repair work, and specifically rust repair, is not what we do. But I liked this car, the owner seems like a good guy, and the project sounded like fun. So we agreed to take it on.

more to come.........
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Old 11-24-2011, 08:01 PM
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Sounds interesting, Chuck. Take pics so we can follow along.
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Old 11-24-2011, 08:31 PM
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cool. stuff... thanks
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Old 11-24-2011, 08:36 PM
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Is a chassis jig needed to replace the torsion tube or can you get by without one
Old 11-24-2011, 08:42 PM
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Yow, that looks an awful lot like work!
Old 11-25-2011, 05:55 AM
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Well, on the bright side the owner can have a few 'While yer in there' fixes....it's 43 years, prolly gonna need a few things.
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Old 11-25-2011, 10:19 AM
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Old 11-25-2011, 10:34 AM
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Old 11-25-2011, 02:32 PM
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Chuck,

Thank you for bringing this issue to the forefront.
This is a problem every early car faces.

Porsche learned a lot over the years but the most significant improvement came with the ‘73RS and RSR.
With these cars Porsche (first?) reinforced the center of the torsion bar tube to the chassis at the rear end of the tunnel.
Not only did this transmit the cornering loads from the trailing arms (‘bananas’) but it also provided an additional path for the torsion bar (spring) loads.
This was continued with every production 911 from 1974.

There used to be available a L&R pair of these reinforcements as a spare part.
These should be made available again.
These are relatively easy to retrofit without significantly disturbing the chassis.

The 3rd part of the ‘73RS/RSR feature was a horizontal flat plate from the torsion bar tube through the seam at the bulkhead and spot welded to the top of the tunnel, around the shifter adjust opening.
This would be extremely difficult to retrofit.

I can’t tell you how strongly I recommend everyone this deep into their chassis apply this retrofit.
This is not an easy job on a complete 911 as the welding is adjacent to the wiring harness and other temperature-sensitive components.
However, it is easy as part of a torsion bar tube failure repair.
IMHO, every torsion bar tube failure repair should include this update.

This is increased in importance because many of these cars also have rust compromising the strength of the connection of the torsion bar tube to the side chassis.
Most of the obvious failures are twisting (and usually due to additional bending from cornering) of the torsion bar tube.
In Chuck’s example, note the tube has been forced forward, probably from cornering loads from the ‘banana’.
The combination of the ‘twist’ and ‘bend’ initiates the failure.



When I installed larger torsion bars in my (then new and today still rust-free) ’68 911 in early 1969, the inside of my (sealed) torsion bar tube was full of surface rust from the Factory.
This was 11 months after manufacture and most here in dry & high altitude Colorado)!
I resealed with a combination of gear oil and Tectyl. Never any additional rust (the torsion bars having been in and out and adjusted many times).
Since ’73, I have always considered retrofitting the center reinforcements. Perhaps soon.

Chuck,
What is the degree of rust vs. high track loading?
Sliding into a 'sand trap' can provide huge loads.

I have posted extensively on this subject. There is a lot in Pelican archives.

Best,
Grady
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Last edited by Grady Clay; 11-26-2011 at 07:49 AM.. Reason: accidently included some Cub Scout info.
Old 11-25-2011, 04:00 PM
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Grady makes good points, as usual. The RS reinforcements will be a part of the plan.

The first question at hand is 'how extensive is the rust'.

If the problem is confined to the center section that failed, life will be a lot easier. We can then section the tube leaving the portion welded in the unibody box sections alone.

If the problem extends into the box sections, the torsion tube replacement becomes an order of magnitude harder. The box section has three layers of sheet metal, each welded to the tube. Replacement through the box sections requires cutting back substantial sections of the box section sheet metal to gain access to the inner layer.

To answer this question we pulled the spring plates to get a better look.



This was an interesting perspective. note the torsion bar is significantly off-center in the torsion tube. This tells us the torsion tube is tweaked, of course that was already obvious.

However this is a good tip and test for torsion tube straightness. While this car had obvious and catastrophic torsion tube damage, far more common is a relatively minor bending of the torsion tube. This often happens when the car slides sideways into a stationary object, like a curb. The sideways impact is transferred down the trailing arm to the torsion tube, forcing it forward and bending it.

This is a fairly common problem affecting a sizable percentage of cars, some fairly mild, others severe. The problem manifests as an inability to get the toe setting correct in the rear. Specifically it will toe out too much and the problem will exist on both sides.

When this happens, the torsion bar gets off-center in the torsion tube just as above - though typically not as severely.

This is especially common on the pre-78 cars. In '78 all the cars got the reinforcements Grady describes. And somewhere around '84 the cars got a further reinforcement securing the torsion tube to the seat bottoms. The early cars had none of this.

Next step was to pull the torsion bars and have a look at the bushing cups and the ID of the tube.



Wow! The cups look solid. Looks like we got lucky on this one.




Surprisingly the tube ID looks good too! There is some superficial surface rust giving that color, but it doesn't appear to be structural. But that tear sure does stand out.

Hmmmm. Looks basically rust free.

The plan is to proceed by cutting out the center of the torsion tube, leaving the part in the box section alone. Once the offending center section is removed, a more complete accessment of the rust and damage can be made. But at this stage it sure looks like we might not have to go into those box sections.
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Old 11-25-2011, 06:06 PM
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Next we need to make some room to get in and cut that tube out.

The rear suspension has to come out to go to get access.



The owner decided to use this opportunity to do a suspension upgrade, so the front suspension comes out too.



And the full underside:



Engine and trans gotta go too:




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Old 11-27-2011, 08:31 PM
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This car has really nice interior. Almost too nice for racecar.




We need to access to the front side of the torsion tube. That means we have to cut access holes in the rear seat area.

the seats come out, and the rear seat upholstery need to be removed.



Here is the stripped car, the suspension is sitting on that rack alongside the other removed parts:



Now that we have a little working room, we can get down to business.
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Old 11-27-2011, 08:39 PM
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When I look at that pile of parts, it occurs to me that the torsion tube is the most deeply buried part of the car. It's the very core.

It's as if the torsion tube is the very first part they started with when building the car. They just put it there, suspended in space. Then they built the rest of the car around it.

Everything else on the car is a sub-assembly of one sort or another. They all just bolt on. Engines, Control arms, Struts - you name it. Everything else is just hanging on the car, and for the most part can just be unbolted.

But not the torsion tube. It is truly buried inside the chassis, welded to multiple layers of metal in the box sections. Then layered thick beneath one sub-assembly after another.

To work on the torsion tube, you basically have to gut the car. Everything off, and out of the way.

Okay, enough thinking about the how the world revolves around torsion tubes. Let's get cutting.
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Old 11-28-2011, 09:06 PM
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We are going to section the torsion tube. We'll cut inboard of the chassis box section, but far enough out to be clear of the damaged areas. This will leave stubs protruding from the box sections.

A doaner part will be sourced from a local salvage yard and butt welded to the stubs. Then we'll add the RS/SC type reinforcements described by Grady.

One of the challenges is to make a clean cut square to the tube on the stubs. This is important because we want a tight fit to the doaner section. The problem of getting a clean cut is further compounded by the close proximity of the seat bottoms.

To get a square cut, I used a piece of paper with a straight edge. The paper was about 3 inches wide, and long enough to wrap around the tube about 1.5 times. The paper was then wrapped around the tube, with the overlap used to ensure it was square.

Then a sharpy pen was used to scribe a line all the way around following the edge of the paper. I should have taken a picture of this, but the description should be pretty clear.

Now we had a line all the way around the tube that was squared up and straight.

Just a simple matter of cutting......


Sawzalls are impressive tools. I've got a Milwaukee and it cuts everything, without flinching. When handled with care it can make surprisingly clean cuts too. But not clean enough in these tight confines, when the sharpy line can't be seen easily on the backside of the tube while cutting.

So the Sawzall was used, but about 2mm+ shy of the sharpy line. An angle grinder was then used to trim right down to the line. It was tight.






The grease on the seat back was due to a failed CV boot and had nothing to do with the torsion tube problem.
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Old 11-28-2011, 10:12 PM
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Old 11-28-2011, 10:19 PM
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And I've gotten a bit ahead of myself....

Before cutting we shimmed the four lifting points to make sure the chassis was evenly supported. The goal was to eliminate any loads that might twist the chassis when the tube was cut.

This car has a rollbar, which helps to brace it after the tube is cut (though this is probably more a feel-good then actually needed support).

We then took careful measures to ensure the back of the car was square. It was. And when the cuts where completed we looked at the gaps of the cut to ensure the chassis did not shift about post cutting.
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Old 11-28-2011, 10:24 PM
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