Hollow torsion bar questions

[ALEX P]

I’ve got some hollow torsion bars to fit to my '85 coupe but do have a few questions that I was looking for an opinion on.

The bars have no manufacturers marking on them and also nothing to tell me if they are left or right handed. I’ve been through lots of threads on here and there seems to be a real difference of opinion on whether this matters or not. Because there are no markings on these can I assume it won’t matter?

The bars also are a bit strange in that they have both 25 and 27 stamped on top of each other but they measure up at 27mm diameter – do you think that they’re 27mm dia but are effectively the same as 25mm dia solid bars?

How do you do a comparison between hollow and solid torsion bars? Is a 27mm hollow is the same as a 25mm solid – is that how it works?

Thanks in advance.

[bkreigsr]

Chuck At Elephant Racing sells his new ones with no markings. When I questioned him on this, he said they don't take a 'handed' 'set' until they are installed.
There are plenty of threads here on the subject, the consensus is that they should be installed on the same sides they came from.
If you search the net there is a comparison table on solid vs hollow diameters and their ratings. (Elephant racing has something similar)
Bill K

[Craig_D]

So what you're saying Bill, is that they're universal until they're installed. Once they've been used, they are now definitely a Left and Right side only?

[zippy_gg]

Originally Posted by Craig_D

So what you're saying Bill, is that they're universal until they're installed. Once they've been used, they are now definitely a Left and Right side only?

Yes, that's pretty much it.
It would be a good idea to paint a sign to identify the proper side in re-installation later.

[Walt Fricke]

Alex, you ought to be able to find the formula for torsion bars. Since you have them out of the car, you can measure the distance between the splines, and the OD and ID easily. The formula involves the radius of the bars, and you can solve for a solid bar of the larger radius, and then for a solid bar of the inner radius, and essentially subtract the phantom inner bar. Because bar rate is something like the 3d power or whatnot of the radius, it turns out that removing the inner part of the bar has less effect than you might think - it is more like a neutral axis.

If you don't find where someone has already done that fairly simple math for you.

These seem like pretty soft bars for a 3.2, though, if their purpose is to stiffen up the ride for track or autox use.

[ALEX P]

Thanks for the replies chaps. The only problem is I have no idea which one was on which side and as there are no obvious markings it just turns into a 50/50 gamble I guess!

Walt, that makes perfect sense, I'll try to measure them tonight.

They are a fairly 'soft' mod but I was just going to lower the rear a bit following the cars slight weight loss program and will need to re-index the TBs when these came up as a mild upgrade from the stock 24.1mm dia TBs.
The car is use is mainly for fast road driving with the occasional track day so it doesn't need to be too extreme especially with a bit less weight on the back.

[bkreigsr]

as far as the 50/50 odds - here's how you can tell if you guessed wrong.
Bill K

[ALEX P]

I know - It's not pretty is it!

I've read lots of horror story threads about snapped torsion bars which is why I was looking for a bit of advice!

[chris_seven]

There is a great deal of mythology about metals having a memory for the direction in which they experience elastic deformation and I am sad to say there is just no truth in this 'fact'

The manner in which metals elastically deform is controlled by the basic crystal structure of the metal and in general terms the slip planes are evenly distributed throughout the material.

These slip planes have no sense of direction.

If we now consider a typical rotating shaft then force can be applied in two senses of rotation and two senses of torque - known as 4 Quadrant operation.

Half Shafts typically operate in one sense of rotation (we don't spend too much time reversing) and see both acceleration and braking torque and they seem to cope quite well.

A torsion bar is much more straightforward as it doesn't rotate only twists. In an unstressed condition a torsion bar will deflect in either torque helix (Left or Right hand) identically up to the material's yield point and it will behave perfectly as a bi-directional spring within these limits until it suffers from a fatigue failure.

In practice it would be normal to limit working stresses to about 50% of yield as this would ensure that the material was operating below its endurance limit and never suffered from fatigue unless some defects were present.

Porsche torsion bars are normally 'scragged' as part of their production process and scragging involves loading a bar in a specific sense of torque until just beyond its yield point.

All materials when they are first manufactured will contain small areas of weakness or zones where crystal structures are not entierly stable.

In a spring this means that the first few time the spring is fully excercised some microplasticity occurs and the spring will change position. In a coil spring it will lose height and in a torsion bar it may develop an offset which again will translate to a loss of ride height.

Scragging ensures that all of this microplasticity has been used up before the vehicle is used so ride height is stable - or at least for many years until other structural damage, such as fatigue softening, takes place within the material.

The process of scragging produces an unusual phenomenon - known after the guy who discovered it - called the Bauschinger Effect.

Most steels work harden when they are plastically deformed and the increase in yield stress caused by work hardening in one direction leads to a loss of yield strength in the opposite direction.

In practice this means once a spring has been scragged it will then be more likley to yield when loaded in another direction and in very hard use cars that are driven over poor surfaces and using large amounts of wheel travel could be running into an area where the bar could suffer from fatigue failure.

I think in practical terms the chances of this happening are reasonably slim and I have seen early cars with front torsion bars incorrectly installed that have clearly been in place for years.

The failures we see in old bars are most likely related to fatigue initiation from corrosion pitting or the damage that can result from a worn out spring plate bush.

With aftermarket bars it is more likely to be either manufacturing defects or material quality issues.

If it a good quality bar that has not been scragged by the manufacturer I wouldn't be too concerned about which side it was installed.

I spent many years manufacturing torquemeters that worked in a Bi-diretional manner and never had one break.

With regard to torsional stiffness the following may help:

The torsional stiffness of a solid bar is proportional to the 4th Power of of the diameter (D^4).

The torsonal stiffness of a tube is proportional to (D^4 - d^4)

If you make both calculations then the perectange difference in the products of both equations gives you the percentage difference in spring rate.

If the products are the same then so are the spring rates.

I have just missed out the parts of the equation that produce the dimensions so you can't calculate real spring rates.

[bkreigsr]

see also: old threads on broken (almost) new bars:

Record for shortest torsion bar life?

[KTL]

Those are Sander Engineering torsion bars. I recognize their number type and the 911 X marking. I think there is an S/E on there too?

Very good torsion bars, that are among the best you can buy for the 911

[IROC]

chris_seven: excellent write-up. You didn't work for Norbar did you?

[70SATMan]

Originally Posted by KTL

Those are Sander Engineering torsion bars. I recognize their number type and the 911 X marking. I think there is an S/E on there too?

Very good torsion bars, that are among the best you can buy for the 911

Yep and the number stamped is the effective rate not the measured diameter.

[chris_seven]

Originally Posted by IROC

chris_seven: excellent write-up. You didn't work for Norbar did you?

No we used to make quite different types of system and torquemeters. Torquemeters

[ALEX P]

Some really good answers there, thanks all & that's some excellent detail chris_seven, thanks for taking the time to post.

I did a quick & dirty amateur calc on the volume and that didn't really tell me much - If the standard TBs are approx.

575mm between splines and 24mm dia then that gives a volume of 262296.1 cubic mm

and the hollow bars are approx.

575mm between splines and 27mm o/dia, 19mm i/dia then that gives a volume of 83673.2 cubic mm

My maths may be way off there so feel free to laugh at me if it is!

Well, there is certainly an 'X' on each bar and an 'S/E' so it's great to know what make they are (Sanders) & that they're good quality. As I don't know which sides they came from I can only put them in and see how it pans out.

Just to finish off - If the 27 stamped over the 25 is referring to the effective rate not the size then how does this compare to the stock 24.1mm dia solid bars?
Is it a fairly 'tame' upgrade or is it going to make the rear of my car pretty stiff?

[KTL]

27 is fairly tame. Might not notice a difference much at all.

[Shaun @ Tru6]

not sure if this will help:
Need help ID'ing rear torsion bars

[Quicksilver]

Originally Posted by bkreigsr

as far as the 50/50 odds - here's how you can tell if you guessed wrong.
Bill K

The area in red is where the crack started:


The failure is most likely a machining/surface treatment issue or a corrosion issue.

[Walt Fricke]

Chris pointed to a great shortcut in comparing relative stiffnesses.

Take 24.7 to the 4th power. Write that down.

Now measure OD (of the shaft, not the splines) of what you have. And the ID (the hole diameter)

Let's say the OD is 27 (that's what you told us), and the ID is 20. Take both these numbers to the 4th power, and subtract the second (d) from the first (D). Compare that number to your first number from the 24.7s. You can do that as a percentage if you want. Higher number = stiffer. That will tell you the relative differences. Without knowing the ID, you can't do this calculation or I'd be tempted to see what I have which will easily do 4th powers. Maybe using Excel?

If you are curious, and have a calculator which can do 4th powers and, more to the point, take 4th roots of numbers, you could easily calculate what the ID of a 27mm hollow bar would have to be to equal a 24.7mm solid bar.

I'm glad I joined this discussion, else I'd not have learned that scragged was a legitimate technical term. All posts by Chris are worth reading.

[chris_seven]

Walt,

Just to try to help all calculators can do 4th powers for example:

(24.7 x 24.7 x 24.7 x 24.7) = 372098

(27 x 27 x 27 x 27) = 531441

(20 x 20 x 20 x 20) = 160000

531441 - 160000 = 371441

Tedious but manageable.