Testing bent torsion tube theory
 

On the presumption that a bent torsion bar tube might be the source of my toe-out problem, here are the results of a preliminary test. Not yet enough to prove that the bent tube is the problem, but it seems to support the theory.

It had been suggested that another way to approach bending a bent tube back into place was to spread the rears of the banana arms apart toward the outside of the car. If nothing in the geometry of the triangle formed by the banana arm, the spring plate and the distance between the banana arm support and the mounting point of the spring plate and the torsion bar tube occurs, then the torsion bar tube must bend in the desired direction to correct the toe-out, that is, toward the rear of the car.

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I figured that a brute force approach could not harm anything at this point, so I proceeded to spread the ends of the banana arms using a hydraulic PortaPower. To measure progress, I made a reference measurement of the exposed part of the hydraulic ram, and dropped a plumb line to the floor under the lift, attached to the transmission mount. If the transmission mount moves, it is because the torsion tube moved.

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There are some intermediate measurements, but a beginning and end reveals that when the rear ends of the banana arms are forced apart by 1 1/8", the transmission (and torsion tube supports) move to the rear of the car by 7/32", almost a 1/4"!

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Now how does that affect the toe, you may ask. Since the car was on the lift during this process, I had to resort to eyeball "measurements" to see the result on the toe. So I sighted down the outside edge of the rear tire towards the front of the car. Before the "Spread", I could not see the front tire. After, I could see a portion of the front tire. I checked this for both rear tires. The Portapower was relaxed, and the test was repeated - same results. The transmission moved back to the starting point when I relaxed the PortaPower, then back to the same 7/32" when it was extended. "Toe sightings" were repeatable.

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The toe problem remains at this point, however, as everything moves back to it's original position as soon as the spreading force is removed. What has to happen now is to drop the engine, weld up a fixture to hold the PortaPower, and try to take the 7/32" bend out. I'll post photos of the progress.

By the way, the fixture will consist of an "A" frame, with the legs of the A at the ends of the t-bar tube and the point of the A at about the distance of the rear engine mounts. The pull on the t-bar tube will be at the center of the tube towards the rear of the car.

You can file the slot in the spring plate larger to accomidate more movement of the banana arm forward. Many racers do this to the camber slot to get more camber.

-Andy

Hi Andy,

I've given some thought to modifying the spring plate. To change the toe adjustment, one would actually have to move the hole in the spring plate forward. The toe eccentric bolt uses a hole in the spring plate, and the slot is in the banana arm. Filing the slot larger in the banana arm would make the toe-out only worse, so the solution here would be welding up the hole in the spring plate and re-drilling it closer to the torsion bar end. Or, one could weld up and move the toe slot in the banana arm, moving it more toward the wheel.

The camber adjustment on the other hand, has it's hole in the banana arm, and the slot in the spring plate. I haven't thought through the camber mod you speak of, but if the racers use this I'm sure it must work for camber increase.

My eventual plan for this car is to replace the steel banana arms with aluminum banana arms I already have on hand, along with the adjustable spring plates, also on hand. So if I have to resort to modifying my current spring plate set-up, I'll have to modify my adjustable spring plates as well.

So I'm hoping to address this problem by going to the source if at all possible. I still haven't entirely ruled out the delrin bushings as the source, as some have suggested, as the root of the problem. But testing out this theory would require that I replace them with either the rubber bushings or the monoball ones, and I won't know until the replacement is complete whether this would solve the problem.

I would highly recommend that when you put the alum banana arms in you use the mono balls. Not suggesting that will fix your problem. For that one bushing, delrin is not good. I had the poly graphite and they would resist any suspension changes I tried to make.

Jim, I'm learning a great deal from your systematic research into the problem. Thanks for sharing this with us.

John,

I would agree that from all I can find, monoballs would be the best choice for this particular bushing. My car came to me with delrins in ALL possible places to put suspension bushings, and they ALL will eventually be replaced. However, I am not convinced at this point that the delrins in the banana arms are the problem. If I force the torsion bar tube to move back to where I think it belongs using the PortaPower, the resulting movement toes the wheels in.

If the delrin is really as hard as it seems to be, then the bushing is probably not deforming under the load the PortaPower is subjecting it to, and thus the tube is what is being deflected, as the movement of the transmission shows.

I double-checked the available adjustment room provided by the slots on the spring plate/banana arms on both wheels, and there is no more adjustment available at the spring plate.

So... the basic issue at hand is fixing the toe problem so I can drive the car, and finish the break-in of my engine.

I am having the same issue with not getting enough toe at the back of the car. The mechanic told me to grind out more clearance at the adjustment slot, as they are at the end of their travel. Any one ever done this?

Jim,

I agree, you need to verify and fix the problem you have with the steel arms now. It sounded to me like you were going to upgrade to the alum stuff later. I was only saying when you do that, you should use the mono balls. If you are going to the trouble to upgrade the suspension pieces, don't use the wrong bushings again. This has been a great read, and can't wait for the final installment when it's all fixed.

Is it possible to weld some strengthening brackets to the torsion tube to maintain its positions when the banana arms are spread? I know the factory did some gussets / brackets for strengthening the torsion bar tube.

Doug

After looking at this for a few weeks, I don't think that you can simply grind out a slot and fix the toe-out problem if the adjustments are at the end of their travel. The toe adjustment slot, in the banana arm itself and not in the spring plate, needs to be moved forward. This means not only removing material from one side of the slot, but filling it back in (by welding) on the opposite side. It is the filled-in side that causes more toe-in when the eccentric is turned.

Also the two slots at the end of the spring plate will need to be ground out more (lengthened), that have the locking bolts passing through, as they will be at the end of their travel . Then, the camber slot in the spring plate will need to lengthened so that there will still be a full range of adjustment for camber.

Another approach would be to cut a slice out of the spring plate, and weld it back together. You would d@mn sure want to trust your welder, though. A broken weld here would be a disaster at much of any speed at all on the road.

I would advise researching all that would need to done before going down this path.

It would be interesting to see what the factory did. Do you have any info on this? One issue might be getting access (with a welding torch) to a solid place on both the body and the tube to do a decent weld. The bracket would need to be near the center of the tube, right above the center of the transmission. I have learned from the factory manual that replacement of the torsion bar tube requires cutting open part of the flooring in the car above the ends of the tube, in order to complete the required welding. The photos show cutting open a flap and folding it back much like opening the top of a can of beans, then welding the flap back into place. Welding in a bracket might require the same. I don't have the engine out yet, so I can't really visualize/remember what access there might be above the transmission.

Very interesting thread. Subscribing,

ianc

You don't actually have to fill in the unused end of the toe adjusting slot. Just file in more room on the end you need and then bolt the thing down. The bolts on the end of the spring plate and the two adjusting bolts will hold the arms inplace once they are tightened. In other words the slots don't have to maintain the position they only have to allow it.

-Andy

Jim, sorry I don't have photos. I have seen Pelican threads relating to "RSR" chassis strengthening techniques. I posted within one of the threads. Tyson Schmidt shared some information within, as well. I'll do some searching for a link.

I cannot remember to what areas you have proper welding access with the engine in (it has been a while).

I was thinking you could weld two brackets on either side of the transmission mount between the torsion tube and the body.

Does the 7/32" movement of the torsion bar offer enough to set your alignment toe properly?

Jim,
Check out this thread
http://forums.pelicanparts.com/porsche-911-technical-forum/259557-structural-reinforcement-post3195258.html#post3195258
And this one:
http://forums.pelicanparts.com/showthread.php?t=342844&highlight=RSR+weld+plate&p age=2

this might sound like a dumb idea after i skimmed over the thread, have you considered taking the car to a panel beater who has a 911 jig to check that everything body wise is straight.

i remember seeing a car that didn't have any castor adjustment left (it was a mercedes however) and that was mainly because the body was bent.

Andy,
One of us is not thinking the same as the other. The toe slot is in the banana arm, opposite the hole for the toe adjuster in the spring plate. For toe-in, the eccentric, rotating in the hole in the spring plate, has to push the banana arm forward, that is toward the front of the car. On my car, the eccentric has pushed against the side of the slot which is toward the front of the car, as far as it will go. In order for the eccentric to have something additional to push against, this side of the slot must be filled in. The side you *need* doesn't need filing, it needs filling in. If you have some way to adjust the relative length of the spring plate *without* the adjuster (eccentric), then you wouldn't need it at all. The other thing you are overlooking is that the bolts at the end of the spring plate will not allow the banana arm to move any further forward. They are already at the end of their travel.

There are no 911 jigs within, I'm guessing, 200 miles of my car, likely even further. I do have the factory plan and elevation drawings which spell out the locations of the points I'm measuring from, namely the torsion bar tube centerline and the transmission mounting bolt holes. From that I have determined that the tube is bent. The next step is to try and remedy that. Since I have no idea how to locate anyone with a Celette jig, I figure if this is going to happen (straightening the tube) I'll have to do it myself.

signing-up. Thanks for the reports on this Jim.

Jim,

Very interesting thread. Really enjoy the creative ideas.
I was wondering how the torsion tube was bent forward. Could the car been hit from the rear before?

Have you consider using Elephant Racing's adjustable spring plate or just use coil-over with 935 adjustable spring plate? Expensive, but less work.