Do torsion bars wear out?

Rouxzy · 12-17-2005, 04:58 AM

I'm going to rebuild my front suspension this winter because everything is pretty much worn out. Now is it necessary to replace the torsion bars? Do they lose their ability to do their job over the years? I plan on upgrading the rears because I've replaced the 2.7 with a 3.6. But should I replace the fronts or leave the originals?
Tom

Bill Verburg · Bill Verburg's Garage

They do get old, tired and rusty, and have been known to snap. Depends very much on the enviroment and past usage.

It's not necessary to replace the rears on later models because of the 3.6 , the weight difference is minimal.

But, if doing the front, I would do both ends w/ matched bars. Don't forget that matching shocks and sways are also wanted.

Joe Bob · Joe Bob's Garage

Snap yes....wear out? Dunno about that but if converting to a 3.6 it's a good idea to consider bigger/stiffer bars. I went with 29s in the rear and later Carrera sway bars on my 83.

randywebb · 12-17-2005, 08:03 AM

No they won't wear "out" -- not within your lifetime anyway.
The metal may get worked enough that its properties alter at some point. Where that point would be, I don't know. Many tens of thousands of flexes at least.

More important is to inspect them carefully for any pits, cracks or other surface defects. All rubber and plastic items will need to be replaced - maybe every 5 or 10 years. That includes brake hoses.

In NH ore anywhere in the northeast or midwest, I would worry a lot about rust and pitting. Those lead to breakage.

If you replace the bars, I'd get a quality hollow bar (Sanders). They cost little more and save a bit of wt.

Nine9six · Nine9six's Garage

What do you plan on doing with the car? Any AX/DE/track time planned?
If so, upgrade accordingly.

island911 · 12-17-2005, 08:21 AM

good post by Randy Webb. THough, on the hollow bars, one thing to consider is that the stress on t-bars is a function of diameter. Hollow bars bump-out the diameter slightly. Usually it's not a problem, unless you stat going to larger t-bars. (see widebody911s posts on cracking)

randywebb · 12-17-2005, 09:00 AM

So, there'd be greater stress at the surface I guess - is that right (?)

One other guess: the bars made today are probably a different alloy and better than the older ones.

tubwreck · 12-17-2005, 09:14 AM

Even on stock cars with low miles, there seems to be enough deflection in the bushing at the back end of the a-arm that the top of the torsion bar gets all of its paint scraped off, like this:

Under harsh conditions, corrosion can cause pits to form, and as they deepen, they can form stress risers, and ultimately the bar can break.

Also, like all springs, t-bars will sag over time. This isn't much of an issue, at least in theory, since you can keep adjusting the ride height to keep up with developing sag.

Brand new genuine Porsche front t-bars are available from our host for $110 each.

island911 · 12-17-2005, 09:20 AM

RW- the further away from twist center, then the higher the stress. (and the stress rises exponetially with dia.) That's why hollowing is done . .. the metal in the center is contributing little. But, as the hole gets bigger, the amount of "spring" taken away also grow (exponetially) . Of course, what has been taken away from the center can be added back to the outter most diameter in smaller amounts, and have the same net stiffness.

as far as metals go, unless someone is making the t-bar out of so-called 'liquid metal' I doubt that the choice of metal is something "new and improved."

island911 · 12-17-2005, 09:28 AM

Another thought:

As Ed's pic shows, t-bars will start rubbing/scraping when the bushings wear/distort.

. . .just one more reason to NOT go with hollow bars. (that is; the smaller the bar OD, the better.)

randywebb · 12-17-2005, 10:10 AM

Hmmm food for thought re hollow bars.

But you don't think metalurgical techniques for these sorts of items have improved in the last 30 years?

island911 · 12-17-2005, 11:58 AM

I would be glad to hear HOW any metalurgical techniques, for these items, have improved in the last 30 years. One hellava lot was known about metalurgy 30+ years ago.

I suppose that the question maybe, has the price of high-end alloys (Inconel, ect) come down enough in price 30 years later?

I don't know what material(s) Porsche used for the OE t-bars, so hard to say if something 'better' is being offered.

At the end of the day, it's just a spring. Between the different common alloys, the physical parameters change little. The cost differences, though . . . yikes.

fwiw, I designed/sized a torsion spring, yesterday, for a medical-device arm. Tho' only ~14 ft-lbs @ 90°.

Nine9six · Nine9six's Garage

As far as metalurgy goes, what was known may have little bearing on what was incorporated for purposes of mass production and cost control.

island911 · 12-17-2005, 01:29 PM

yep. that was pretty much my point.

the physical property differences are small, but the price...

Rouxzy · 12-18-2005, 04:16 AM

Thanks guys, there is alot of good advice here. Without actually pulling them out and inspecting them I won't know the condition. The buschings are sagging so I would venture to guess that there is some scraping going on especilly when I can here squeeking going on. So my plan will be to pull them out and inspect.
Thanks
Tom