torsion bars ok to use

[red83911]

Had some squeaks from the front, pulled the torsion bars and they are bright where they have rubbed, are they ok to reuse. Going to fit new poly bronze bushing to the a-arms.

[Rot 911]

I wouldn't use them. Too much pitting.

[tonythetarga]

It looks like they have been worn down quite a bit near the splines. If there is a definite notch there, then I wouldn't run them either. If you do any spirited driving they could shear at that weakened spot.
If you just want to replace with stock bars, I am sure there are plenty of good used ones out there for cheap.

[red83911]

Thanks for your input, new torsion bars ordered.

[burgermeister]

I'd have said as long as the notch is sanded & polished out, they're probably OK, especially if the car has been lowered - bars are likely designed to live for US ride height cars. Lowering would limit the maximum twist imparted to the bar, thus giving quite a bit of margin. Then again, that's pretty good steel, so the sanding & polishing could be time intensive...

[tonythetarga]

Originally Posted by red83911

Thanks for your input, new torsion bars ordered.

presumably you are going with same size bars? Going larger in front without going larger in rear will alter the handling characteristics as you probably know.

[Kemo]

are these torsion bars solid? What is the worst that could happen with a rubbed off torsion bar? As long as the grooves are not damaged at the end, wouldnt it still function ok?

Im no engineer by any means, I just thought that the ends were more important than the bar itself.

[dad911]

Originally Posted by burgermeister

I'd have said as long as the notch is sanded & polished out, they're probably OK, especially if the car has been lowered - bars are likely designed to live for US ride height cars. Lowering would limit the maximum twist imparted to the bar, thus giving quite a bit of margin. Then again, that's pretty good steel, so the sanding & polishing could be time intensive...

Why would lowering reduces suspension travel? If you are hitting the bump stops, the car is too low.

Stiffer shocks, sways, or torsion bars would reduce travel.

[Steve@Rennsport]

Those are throwaways,....

[island911]

Originally Posted by burgermeister

I'd have said as long as the notch is sanded & polished out, they're probably OK, especially if the car has been lowered - bars are likely designed to live for US ride height cars. Lowering would limit the maximum twist imparted to the bar, thus giving quite a bit of margin. Then again, that's pretty good steel, so the sanding & polishing could be time intensive...

How does lowering a 911 take weight off the T-bars? (hint: it doesn't)

Send those to the scrap heap.

[island911]

Originally Posted by Kemo

are these torsion bars solid? What is the worst that could happen with a rubbed off torsion bar? As long as the grooves are not damaged at the end, wouldnt it still function ok?

Im no engineer by any means, I just thought that the ends were more important than the bar itself.

Torsion bars are like income tax revenue. The material farthest down contributes nothing, The material at the top does the most heavy lifting.

[Chuck Moreland]

Trash.

Good used stock size bars are a dime-a-dozen since many people upgrade the bar size, then have no use for the take-offs.

[arbita1]

I just replaced mine...so let me know if you want them.

[red83911]

Originally Posted by Steve@Rennsport

Those are throwaways,....

Thanks, new std. bars ordered

[red83911]

Originally Posted by Chuck Moreland

Trash.

Good used stock size bars are a dime-a-dozen since many people upgrade the bar size, then have no use for the take-offs.

Thanks Chuck, gone for new from Porsche dealer in uk (good price surprisingly), just waiting for UPS to show with your poly bronze bushes.

[burgermeister]

Not arguing with the decision to trash them.

Torsion bars are not damaged by the applied moment (weight supported) per se. They are damaged by stresses created by rotating them. Lowered car = less possible rotation = less damage.

All cars hit bump stops during large impacts. It's why the bump stops are there.

The grooves in the bar are stress risers (crack initiators). This is where metal fatigue will occur. Sanding and polishing the grooves out will remove most (but not all) of the stress riser. As the bar is locally smaller now, there will be locally higher stresses. Also, the residual stresses induced by the factory pre-stressing will be altered in a non-beneficial way. So, lower durability in the bar. OTOH, the bars in mine are WAY worse (will be replaced) and they showed no evidence of fatigue cracking, so there is some margin in these things.

[Quicksilver]

Originally Posted by burgermeister

Not arguing with the decision to trash them.

Torsion bars are not damaged by the applied moment (weight supported) per se. They are damaged by stresses created by rotating them. Lowered car = less possible rotation = less damage.

All cars hit bump stops during large impacts. It's why the bump stops are there.

The grooves in the bar are stress risers (crack initiators). This is where metal fatigue will occur. Sanding and polishing the grooves out will remove most (but not all) of the stress riser. As the bar is locally smaller now, there will be locally higher stresses. Also, the residual stresses induced by the factory pre-stressing will be altered in a non-beneficial way. So, lower durability in the bar. OTOH, the bars in mine are WAY worse (will be replaced) and they showed no evidence of fatigue cracking, so there is some margin in these things.

In a spring application like this the "evidence of fatigue cracking" is called breaking. I have found that most things work right up to the point where they break.

What does sanding do to the surface treatment of the bar? Wow this is just bad advice.

The shape of the bar is important which is why there is such a huge tapered fillet leading up to the splines but surface treatment is extremely important on a part that is as highly stressed as this.

A crack has to start somewhere. If it doesn't start then it doesn't start. Cracks always start at the surface. The "surface" can be an internal bubble or a flaw but on a piece like this it will always be on the outside surface. To make it so a crack can't start to form on the surface there are a number treatments used that basically compress the surface. If the surface is compressed then when the part is stretched under load, the surface will only be relaxed so it will resist cracks.

Shotpeening obviously compresses the surface mechanically.
Chemical processes such as nitriding compress the surface by increasing the size of the molecules on the surface.

Sanding only leaves a reduced stress riser and an accident waiting to happen.

A bar that has any visible wear on it is trash and should be tossed.

[Chuck Moreland]

This one snapped and it was visually perfect.

Sanding out that groove is not advisable. You will reduce the spring rate, and a disproportionate flexing will happen at the thinned area. It's likely to fail.

And as a practical matter, consider that is hardened allow steel. If you think polishing out a set of fuchs is hard work, try sanding hardened alloy steel. Fooorgeettaboutiit!

Again, good used OEM bars are available for next to nothing. There is just no sense in using a questionable torsion bar.

[burgermeister]

Like I said, not arguing with the decision (or suggestion) to trash them...

Fatigue test specimens are usually machined to size and then polished to remove surface defects...like I said, it would be detrimental to factory pre-stressing. I think it consists of an overload in the case of these bars (the reason they are LH/RH specific), not shot peening. Don't know if they use nitriding.

If I wanted to be on the road tomorrow, I'd clean 'em up a bit and put 'em back in, and then order a set for replacement another weekend. But that's just what I would do, and many people think I'm an idiot

[island911]

Originally Posted by burgermeister

...
If I wanted to be on the road tomorrow, I'd clean 'em up a bit and put 'em back in, and then order a set for replacement another weekend. But that's just what I would do, ..)

you make some good points. My apologies for suggesting you were way off base about lowering -- indeed, less winding 'til hard stop means less T-bar stress potential. (somehow I thought you were saying "lower meant less wind, just sitting there" - my bad)

I'll also note, that you are right is saying that they could be cleaned-up enough for a "get by" mode. ...Kind of like how some get by with cast fake Fuchs. Just don't do any of those "Why rocket scientist prefer Porsches" poses.