Torsion bar wear - cha-ching?

[steely]

I found scratched paint and a wear mark to bare metal on one TB.

I think I have to replace them, but at more than $350 a pop (for 19mm stock) , I thought I'd double check.

What do you guys think?

Thanks.

[michael pagano]

I say keep'em. but what do i know. You will need front a-arm bushings soon by the looks of that bar.

[James Brown]

Stock ones are cheap, used.

[JWLindsey]

Those don't look too awful but agreed you need bushings. My car with 175k on it and shot bushings had grooves in the bars so I junked them. Partsheaven is a great source and pretty reasonable on the bucks for good used bars.

-JWL

[steely]

Thanks guys - I am already in the middle of the FE bushing R&R now - that's why I had these out to clean and grease when I discovered what I originally thought was crud, was this scar.

That's great news about the good used bars.

No disrespect intended, but what kind of longevity could I expect with used bars?
The price is compelling after all.

I appreciate it.

[llangston1]

So what kind of front bushings are you using? rubber or more exotic like polybronze, etc? If rubber which manufacturer are you using?

[michael pagano]

It all depends really, they dont really "wear" so you cant judge the life thats in them. I would paint up the bare spot for rust protection and go. The bars i pulled from my bar were 100x worse with rust and deep scars and had been driving great for 15 years until i happened to notice them in a bushing replacement.

[steely]

thanks michael - I was concerned about load sag - but I may still go the used route just because I am in there already. There are 2 places near Allentown Pa.

llangston1 - I went with the uro rubber ones from CSP (classic and speed parts)

[RWebb]

I would NOT do that - you can search on stress riser for more info.

Your t-bar has something that could cause it to let go catastrophically while driving some day

[steely]

UPDATE

What are my chances of getting a pair of used 19mm front TBs that are clean?

I searched the many posts and was convinced that a used pair could be safely substituted.
The assumptions here are: good and available.
Today I drove to a salvager to pick up some replacements.
It was disappointing since they were worse looking than my original.

I am inclined to run with my originals until I find some good ones, but can they be had?
Thanks

[island911]

While it's true that you have a stress riser there, It doesn't look that bad to me - barely thru the paint.

There is a very good thread on the topic here http://forums.pelicanparts.com/porsche-911-technical-forum/366945-torsion-bars-ok-use.html

btw, even new ones can break. A lot depends on how much abuse they need to take.

[gregwils]

I paid $60 for a near perfect set this week from the forum. You could get a set from a porsche salvage yard for about $100.

[LJ851]

If you want to run those torsion bars pictured i would sand the sharp edge off and reuse them. Without the stress riser i dont think they have been compromised much.

[steely]

island911 - thanks - yes I saw that thread this past week - we were even discussing it in work (the FEA) - This is 'just' for street use - no DE - I appreciate the input / re-assurance.

Greg - so I am told - I just ran into some difficulty (as I posted) getting the good ones you mentioned - the hunt resumes. Good to know they're out there...somewhere.

[Steve@Rennsport]

Dan,

JMHO, but I'd replace them as I've seen them fail with such damage. One can source good used ones without any damage for pretty reasonable $$.

Just make sure any used ones were marked before removeal so you know which side is which.

[steely]

Steve - saw your post late - thanks - finding them was a trick, but gregwils had just what I needed - THANKS Greg.

It arrived yesterday and I installed it tonight - she is finally coming together a long winter project. Too bad work gets in the way of a good time. :-)

[Hendog]

Dumb question, but what's a "stress riser"?

[Hendog]

I found the answer by reading through an imbeded link

Originally Posted by burgermeister

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 Hendog

Dumb question, but what's a "stress riser"?

Not a dumb question. That is a very complicated and important question.

A crack has to start somewhere. If you can keep a crack from having a place to start it can multiply the toughness many times. The most important component in stress resistance is to try and make the stress act equally as possible across a part.

For a basic example take a coat hanger, straighten out the wire and grab each end of it and flex it. You can do that for hours and it won't break. Now take a 2" diameter bar of the exact same type of steel that is the same length and turn it on a lathe so just one spot is the same diameter as a clothes hanger. You could say that you have beefed up a cloths hanger every where except just one spot. If you flex that a one or two times it will break. All the stress is concentrated in one spot. That is your basic stress riser.

The part where this starts to get more counter intuitive is really important...
A crack has to have a place to start.
This is REALLY important. Cracks always start on the surface. If you can make the surface extra resistant to cracking you can make a part WAY tougher. A very common way to do this is to compress the metal at the surface. That way when the metal is starting to flex the surface isn't be stressed, It is being relaxed! This is why shot peening (physical compression) and surface treatments (chemical compression) like nitriding improve toughness so much.

Corrosion can be a major contributor to initiating cracks which is why Porsche went to such lengths to make sure the torsion bars couldn't corrode.
This isn't the reason I would automatically reject a slightly marred bar like was shown here. The real question is what did the marring do to the surface treatment? The answer has to be "Nothing good" so just toss and replace.