Are Front Torsions bars L/R

[rwest]

Hi,

On our host site, they list OEM Porshe bars as left and right. I bought Elephant Racing hollow bars and they appear to be identical. So my question is: Does it matter what side a torsion bar goes in?

Thanks in Advance!

-Rutager

[Bob Kontak]

Factory parts catalog lists left and right part numbers as well for a 77. Elephant has to know. Call them.

[equality72521]

Yes, they are left and right. They are marked on the end (OEM). It does matter which way they go. As Bob said, call ER and have them verify what you have.

[rwest]

Thanks guys,

I sent an email to Chuck at ER, but haven't heard back yet, but being the weekend, he's probably not at work. I was hoping to put them in this weekend (of course!) so if anyone is familiar with ER Hollow quick change bars, I would appreciate any input, or what I need to look for on the bars. The markings appear to be exactly the same, so either l/r doesn't matter, or I have two of the same side, but the markings don't seem to have an indication about which side.

-Rutager

[Flieger]

The issue is in reversing the strain. If the bars are pre-stressed at the factory to remove the initial non-elastic strain, then the L/R marks matter. If they are not pre-stressed, then you can install new bars on either side. Just do not switch them after that. That can make them more prone to snapping by reversing the strain and increasing the stress.

[Steve@Rennsport]

These bars are not L or R specific until after they have been installed and driven, since they don't become "directional" after they are "set".

One you drive on them, they should be marked "L" and "R" upon removal to ensure they are reinstalled on the correct sides.

[rwest]

Thanks Steve!

I kinda figured that to be the case since the markings were the same and there was no mention on their web site about L/R.

-Rutager

[rwest]

Hi Guys,

Just a follow up. I received an email from Chuck at ER and he confirmed that the bars are the same until installed.

Thanks to all, I'll be able to keep working this weekend!

-Rutager

[chris_seven]

I have always been interested in the Left/Right marking of OEM 911 Torsion bars and how well the various aftermarket bars compare in terms of materials and long term behaviour.

During the last few weeks I have been looking for some relatively 'soft' bars to use and a long travel suspension we need to fit to a car that will be used on the London to Cape Town Rally and I must say I am not too excited by what I have learnt.

IMHO the reason that the OEM torsion bars are marked L and R is that they have, indeed, been preloaded as part of the final test procedure.

I believe that the bars have been twisted through a sufficiently large angle to cause a small amount of yielding (plastic deformation) to take place.

The effect of this yielding will be to slightly increase the yield strength of the material in the direction of twist and this will help to improve the torsion bars long term resistance to sag and also improve fatigue life.

Unfortunately the yield strength of the material will be reduced if it is loaded in the opposite sense of torque and it will be more likely to sag and will have reduced fatigue life.

(We do need to clealry differentiate between the sense of torque and the sense of rotation).

This reduction of strength in the reverse loading direction is known as a 'Bauschinger Effect' and was first observed in the late 1860's. The effect is due to the complex behaviour of the crystal structure of steel that occurs during strain hardening.

Modern spring steels used for leaf spring and torsion bars use materials that demonstrate significant Bauschinger Effect as they are very reistant to the long term cyclic softening that causes sag. Material used in this area tend to be relatively high in Silicon 54SiCrV6 being a good example.

If we now consider the aftermarket bars there seem to be two types being offered.

'Road Bars' which have not been preloaded and 'Race Bars' which have been preloaded.

Some of the road bars I have looked at seem to manufactured from 4340AQ steels.

I am sure that this is an excellent quality material and has a high level of 'cleanliness' but I am not convinced that it can be classified as a 'Spring Steel'.

The fact that these bars have not been preloaded is probably not a signiifcant problem.

In normal duty a torsion bar will operate in a entirely elastic manner. This means that apart from a very small degree of hysteresis, the loading/unloading behaviour will be very repeatable.

The bar may be subject to some medium to long term cyclic softening which will cause sag but as the bars we are considering have a higher spring rate than the OEM bar and are probably used on smooth tracks and for relatively low mileages it is unlikley to be a problem.

As bars of this type will never have been subjected to any plastic deformation marking them left and right will not do harm but will not provide any benefit as materials have no 'memory' of sense of rotation or sense of torque when loaded elastically.

The 'Race Bars' I have seen tend to be manufactured from 300M which is a very high quality Aerospace material and is commonly used for the driveshafts used in FI cars.

It is very similar in to 4340AQ but has slightly more Carbon, Silicon, Vanadium and Molybdenum. It has much beeter fatigue life than 4340.

Its main application is for aircraft landing gear and fasteners but I do think it is a very good driveshaft material.

I wouldn't tend to select it a a spring steel but it may not be too bad for race car torsion bars.

One of the companies that uses 300M clearly state that they preload their torsion bars to 90% of yield stress.

This means the bars are only loaded elastically and I don't believe that this procedure provides any benefit . The SAE, for example require that is a spring be twisted 3 times through an angle that will produce a surface strain of 0.022 and at this level yielding would certainly have occurred, hence the need to mark the bar.

My conclusion is that providing the aftermarket bars have a good reputation and don't sag or fail due to fatigue it really makes no difference which way they are installed on the front of the car and changing them from side to side doesn't matter much... but being a bit old-fashioned I would still prefer to buy bars made from a spring steel and which had also been correctly 'scragged' and tested.

[Flieger]

Thanks for another of your very informative replies. It is nice to know I was on the right track in my thinking.