Do torsion bars wear out?

[silver912e]

I'm going to refresh all of my suspension bushings, and was wondering about the torsion bars. I'm not interested in upgrading them, but they are almost 30 years old. If they look straight is there any reason to replace them when I do the bushings?

[IROC]

In theory, they shouldn't wear out, but a torsion bar is nothing more than a coil spring that has been straightened out (again, "in theory") and I have seen coil springs sag over time, so maybe I'm actually wasting your valuable time even typing this response? :>)

One of my favorite adages is "don't fix it if it ain't broke". I think that would apply here.

Mike

[chrisp]

there are torsion bar testers on the market. Some circle track shop in your area may have one. It might be worth checking the rates of your stock bars and more important than their ultimate rate is the equality from left to right.

[Jgordon]

I agree on the right-left parity, but if they're weak, can't you simply torque them more? I thought that it was possible to simply increase the tension on them even in place on the car. I could be completely off base here though.

[Thrasher]

assuming (nearly) equal (50-50) left-right weighting, aren't you effectively preloading them to be equal by setting the ride height the same left to right?

If they need to be indexed differently in the spring plates to achieve equal ride heights, that would indicate different spring rates. Unless the spring rates are linear, that is a different story once the suspension gets loaded.

[v9ff]

I've snapped one once.

[MotoSook]

Steve, what was the casue of failure? Corrosion, pitting in the failed area?

[v9ff]

I don't think we ever figured out why it happened. Then again, the bars were 17 years old.

[MotoSook]

Thanks Steve. I have a discussion going on with a couple of materials engineers about t-bars...looking for data.

[project935]

Torsion bars (or any spring) deal with material yield and fatigue limits. Yield and fatigue limits can change due to thermal cycling (seasonal and operational environment), working (torsional loads), aging ("creep" under load), and surface condition (corrosion, scoring, etc.).

Combine all that up on 30yo torsion bars that have seen yearly temperature changes, operational cycling, and just plain age at a loaded condition and the do begin to "wear out" - meaning the yield and fatigue limits are effectively dropping.

Over-torquing them is a temporary "fix" - they were initially designed to operate within a specified angular rotation range based upon material fatigue life estimates.

Rust pitting/scoring (surface imperfections) will also reduce life or cause failures. I would think anything deeper than 0.5-1mm would be a danger sign.

Try to buy "matched performance" products too - costs more, but it's consistant (within spec). Material properties (ie. strength limits) vary between different batches of base material - even down to the time of day is was made (different thermal/processing conditions). You would be surprised to see how much the yield limit changes on the same batch of steel based upon what time of day it was poured, at what rate, etc.

If they are straight and torque test within spec (I'm sure there is one out there) they are probably OK to reuse.

[Grady Clay]

See, when we have materials engineers chime in, we all learn (or remember) something.

From a practical standpoint, let me translate some things.

We all know that when a 911 is stored for a period of time, the ride height tends to raise. This is probably “counter-working” or ”anti-aging” as the torsion bars are not stressed above the static weight of the 911.

My personal opinion is it is impossible to over stress to yield a 911 torsion bar. They will break first.

I won’t claim to be an expert about the manufacture or thermal cycling.

I think the biggest issue is surface damage and corrosion. A single nick will leave a location for corrosion or crack failure. My torsion bars are additionally coated with a heavy layer of grease. Careful installation will prevent nicks and damage to the surfaces.

These are not particularly expensive parts. If in doubt, replace them.

Best,
Grady

[MotoSook]

The discussion, which is open between myself and two other engineers, is that normal use and reversal of parity (?) is not going to change the material or it's performance (age and other defects have to be quantified). Cycling is generally not at a high enough frequency and amplitude to generate heating to the point that the material property will change.

We are not talking about a whole lot of strain on our t-bars. The angle of rotation is fairly small, and the deformation is elastic...even after years of use!

From my metallurgist, "There is a phenomenon called the Bauschinger effect that essentially is a lowering of the yield point when you load a specimen in one direction, for example under tension, then load it in the other direction, compression." But this may not apply to our torsion bars for the reasons I stated above.

[Lukesportsman]

"Bauschinger effect" that was on the tip of my tongue

[dickster]

in answer to your question, a few guys here had theirs break - probably due to age/corrosion.

personally i would change them after 30yrs but the decision is yours