questions on indexing front torsion bars
 

1- Last time re-indexing, I just turned the torsion 1 click to the left or right to raise or low it a bit. This time, I am taking the whole thing off the car include a-arms, torsion bar, etc. The torsion bars are completely off the a-arms. My question is if there is any trick on installing it or I just have to try it from a random point and turn on order to get to the height I want?

2- On the torsion bars, is there a front or rear end, or either end can go in either way?

Thank you

I don't think that front bars index as they have the same spline count on each end but if you want to roughly set them then let the suspesnion droop with the caps off and refit the caps with the arms pointing to 4 O'Clock and 8 O'Clock respectively.

You will see that both front and rear bars are marked L & R (Left and Right) and they should be correctly installed as they have been 'scragged' in a particular direction and this needs to be maintained.

Front torsion bar spline numbers.......
 

rnln,

The front torsion bars have different spline count or size. The smaller diameter spline goes inside the A-arm and the larger diameter spline stays outside. Pay attention on the markings at the big end. It has R & L markings. It is critical that you install them at the correct side of the car.

Tony

I am fairly confident that front bars are the same both ends - 24.5mm approx diameter over splines and 30 splines on each end.

You adjust ride height using the screw in the end cap. If you need to adjust outside this range you just need to re-position the cap. There isn't a 'vernier' type adjustment as with the rear bars.


The 914 has a similar torsion bar but with a different number of splines (29) so they are not interchangeable with the 911.

They are the same splines but I think the factory bars have been preset as the last pair I saw were marked L and R, so in that regard they do have a correct orientation.

You are indeed correct.......
 

Chris,

After reading your above post, I went back to the garage and measured the torsion bars. I picked the rear torsion bars instead of the front when I checked them earlier. You are indeed correct. Plus the rear TB was 24 mm from the SC car. My bad.

Tony

Max,

When manufacturing springs it is normal practice to carry out a procedure known as 'set removal' or 'scragging' and this has clearly been translated as pre-stressed as I mentioned in my first post.

They should be installed so that the L & R are visible with the adjuster cap removed.

Springs are loaded to just past their yield point to ensure that any metallurgical changes take place before springs are installed on the vehicle.

This process is normally integrated into the spring test routine.

With leaf and coil springs subsequent loading is always in one direction so there is not issue with the spring.

As a Torsion bar that can be loaded in either a Left Hand or Right Hand Torque Helix there is a potential issue due to a process described by the 'Bauschinger Effect'.

Simply put this means that in you yield a material in one direction it will strengthen in that direction but will weaken slightly when loaded in the opposite direction.

I think it is unlikely that this would cause any problems but theoretically the fatigue strength of the bar could be lower if the operating helix was reversed from the test direction.

It is good practice to maintain the correct loading direction hence marking bars left and right and installing them correctly.

There will be no effect on the spring rate and in reality the shear stresses in the bar are relatively low so that they are unlikely to fail in fatigue unless they suffer from surface damage or corrosion.

If Torsion Bars have not been scragged during manufacture then there is no need to 'hand' them as they will only be loaded elastically and should be quite capable of operating in either helix.

Thanks guys,

This thread if on the front torsion bars only and these are existing bars on my car, not new.

1- If both ends of the torsion bar are the same size and have same numbers of spline, then there is no adjustment on the front end of the bar, adjustment is only at the cap? Meaning, the torsion bar can go in once, then adjust the cap to the height you like?

2- I just checked my left bar and see that the it has the R mark. Meaning someone already put the right bar on the left (driver side). Should I leave them where they are, or should I swap them left to right to have them on correct side?

The only adjustment is the cap position relative to the bar and it is best to try to keep the lever horizontal with the screw centralised in the normal ride position as this allows the best scope for fine tuning.

I think I would swap the round to the correct side but this is probably part of my OCD. :)

Thanks Chris,
I wonder what is the diff between the L and R bars. Is the L bar stronger, more stiff, to handle the driver weight? Don't swap them, I afraid that my driver side will be weaker or anything else that I don't know. Swap them, then I afraid they might break because both bars have settled for the wrong side after sitting in there a long time.

+1 do it correctly Brother R = passenger & L = Driver

draco,
I remember I've read somewhere in here of the similar situation. Someone said do not swap old torsion bars because the steel is already settle and will easily be broken when twisting into opposite direction... can't find that thread again. arg

That is true. maybe that could be your source of the noise, metal fatigue???

I have my original T-bars. what size are yours?

thanks draco,
my bars are 3/4" measuring in the middle of the bars.

question: the bars marked R and L. The other end marked M and K. What M and K mean?

I need MM, please :-D

Mighty King???

Maybe German???

we are american, you need to learn inches Draco :D jk
19mm. Actually, my measured to be 18.9xmm

I had hard time learning to get used to inch system when I first came here. Lately, the hospital start using metric. Now, I have hard time switching back. WTH ...

Okay 3/4" (19.0MM) OD front T-bars :D. So, you are set. Put in yours in properly "L" "R", if it snaps then you have back ups :D

ok, thanks Draco

There are many 'old wives tales' about the mechanical behaviour of steel bars some have merit and others are simply wrong.

If a bar is loaded elastically it will deform equally in either sense of torque and provided that the stresses do not exceed the material's Endurance Limit it will never fail.

The TBs used on the 911 have clearly been scragged, which is why they are marked L & R and this procedure involves a small amount of plastic deformation.

It also means that the bar will tend to be stable in terms of ride height over a long period of time.

A bar that has not been scragged may 'settle' in terms of ride height and this is due to certain metastable metallurgical effects being influenced by initial loading.

Retained Austenite would be a good example and the mechanisms involved are well understood.

The 'settling' has no effect on the material's fatigue life nor the stiffness of the bar nor its long term behaviour.

When the bar 'ages' it is likely that the effects of corrosion and any pitting will lead to fatigue crack initiation which then cause failure.

I believe the 'old wives tale' regarding driveshafts has some foundation.

Many cars with semi and 3/4 floating axles show signs that the driveshafts have been plastically deformed.

This causes two potential problems.

The first is that it allows the Bauschinger effect to reduce the materials yield strength in the opposite direction of loading.

The second is that severe plastic deformation uses up the material's ductility so when it is reverse loaded it fails quickly.

If the shaft never deforms plastically this will never occur and shafts can run in either torque sense.

The sense of rotation makes no difference whatsoever.


The stresses in a 911 TB are well within the steel being used Endurance Limit even with a raised ride height and all deformation is well within the elastic regime.

If it were my car I would swap them to the correct side without worrying. :)

The Left and Right Hand bars also should have identical spring rates - at least within the specified tolerance range.