Can torsion bars go soft / weak ?

[MarkT]

Been setting up ride height on my 76 911 after a complete nut and bolt resto and had to twice re-index the right hand torsion bar as after about 20 miles driving the ride height drops about 8-10mm, I have also noticed that that I have had to set the spring plate angle when free to ~34 degrees so now needs a lot of pre load to get it passed the spring plate cover hole in order to get the spacer and bolt in. During the initial set up the LH and RH were set at about 25 degrees, the LH ride height has remained constant/correct and can get the spring plate cover bolt and spacer in with very small amount of pre load.

The torsion bars are 45 years original however, look OK/clean as far as I can tell.
Spring plates and bushings all new.
All splines look good and feel firm when installed.

My only thought is that the RH torsion bar is weakening, is this possible ? or could it be something else ?

Question I have: why would it drop by the 8-10mm and then hold and not drop anymore ? maybe anti role bar helping and/or other 3 corners taking load.

Cheers

[Leicestershire]

It is a possibility but more likely that the bushes have settled during the initial 20 miles.

What shocks are you running?

Oil / gas like Bilstein give around 30 Kg of lift. If the shocks have deteriorated this would impact on ride height.

 

[MarkT]

LH and RH bushes are all the same and new, don't think bushes as they would not be so different L vs R and spline indexing angle significantly different between L & R.

New BOGE as original fitment.

 

I have probably done about 150 miles now, the ref to 20 miles is it takes about that many miles to drop the 8-10mm before holding at the reduced height.

[SilverWT]

56 minutes ago, MarkT said:

My only thought is that the RH torsion bar is weakening, is this possible ? or could it be something else ?

If old Tbars change in any way they usually get brittle/stressed from overwork, corrosion, or if used on the wrong side (they are marked L & R) and then they snap. As they say, anything is possible, but old Tbars sagging would be unusual.

Defective new bars that have been manufactured poorly, not been tempered properly can gradually sage, a few on here have had this problem with OEM replacements.

Snagging bushes during indexing?

 

Mark

[MarkT]

Many thanks for the responses however, I keep coming back to the difference in spline indexing angle is getting bigger on the RH spring plate only i.e. since the initial set up at about 29 degrees L & R to get correct and even ride (+/- 2mm) height, after which it take about 20 miles for the RH side to drop by 8-10mm causing me to have to re index the RH torsion bar (Increase the angle after I think 3 times now the angle has needed to be increased, it is now set about 34 degrees to get back to the same ride height as the LH side !

[Chris_911]

It will only change if the bar is yielding.

The bar will work within its elastic limit if it's good and its fatigue life hasn't been used up.

Something is going on that needs a strip down and examination.

[MarkT]

I have ordered a replacement used rear RH Torsion bar I thought worth a try, its already posted so may get it before the weekend, if it fixes the problem I'll probably buy/fit a new set, if  it does not fix it I'll have to think a little harder !

Chris, are they known to yield ? 

I'm looking for comfort any recommendations on size / brand (NLA from Porsche) mine are standard 76 911 23mm.

Cheers

[Chris_911]

I don't believe I've ever heard them described as a weak point.

Any spring can yield if it's over-stressed, damaged or at the end of its fatigue life.

Be interested to see what you find.

[Leicestershire]

TB's usually fracture without warning.

Everything's been checked and the torsion bars, spring plates, rear arms etc all match, TB inserted in the correct side???

New spring plates with the bushes vulcanised to the steel?

If this was my car I would reset everything again and ensure the spring plates have been inserted without any the need to 'wind up' the rubber bushes. Initial setup with ARB's disconnected. 

TBH using an odd pre owned torsion bar would be close to the bottom of the list.

Always had 100% reliability with sway-a-way / elephant solid TB's.

[MarkT]

9 minutes ago, Leicestershire said:

TB's usually fracture without warning.

Understood

Everything's been checked and the torsion bars, spring plates, rear arms etc all match, TB inserted in the correct side??? YES

New spring plates with the bushes vulcanised to the steel? YES

If this was my car I would reset everything again and ensure the spring plates have been inserted without any the need to 'wind up' the rubber bushes. Initial setup with ARB's disconnected. 

Spring plate inserted without need to wind up rubber bushes, Initial set up was done without ARBs connected

TBH using an odd pre owned torsion bar would be close to the bottom of the list. Considering responses above using pre owned TB is next on the priority list.

Always had 100% reliability with sway-a-way / elephant solid TB's. Thanks, do I presume that smaller diameter the softer the ride ?

Pre owned TB arrives tomorrow, as you suggest I will treat this as a reset/initial set up and check everything again. 

[Chris_911]

If the setting is changing over time, something is yielding. There's relative movement somewhere.

Agreed that TB's do tend to have a brittle failure mode.

[johndglynn]

New Porsche bars are notorious for a tendency to drop early in life. Should settle after you reset/reindex going by previous experience.

I had a pair of new rear bars that a so-called mate helped himself to. The likelihood that they would sag after a while and earn him some grief was the silver lining.

[MarkT]

Fitted another RH Rear TB, Problem still there !

A thought ! could it be the RH shock absorber losing pre load, they are new BOGE ? I guess I could swap them over.

[Jonny Hart]

I believe there were bad batch of Porsche 26mm bars.  
 

I had a pair on my car.  It dropped about 70mm in ride height (!) over the course of a year or so.   Baffled the experts.  Bushes changed, torsion tube inspected.  Adjusters fiddled with. Re indexed the car twice.  
 

Eventually got rid and fitted ER bars.   Not had a problem since. 
 

 

[GrahamTompkins]

I think you’ve eliminated the TB as being the problem by swapping them, so shock absorber has got to be the next suspect. Change them over and 🤞🏻 That will nail the problem down to probably a bad valve in the Boge .

[sopor]

Boge shocks shouldn't affect the ride height; it's Bilsteins that do that. Could easily be confirmed by removing the lower fixing bolt - ride height should not change. Wouldn't recommend driving it like that though!

[MarkT]

8 hours ago, sopor said:

Boge shocks shouldn't affect the ride height; it's Bilsteins that do that. Could easily be confirmed by removing the lower fixing bolt - ride height should not change. Wouldn't recommend driving it like that though!

I'll try that however, I'm sure the new rear BOGE's I fitted do have pre-load, so must be giving the TB some assistance, the original BOGE's did not have pre load !

Another thought could it be the interface rotating between the outer spline pocket and the spring plate ? are they welded or press fit..... Its difficult to see that interface with the rubber bushes in place. Bushes, Adjustable Spring Plates are all new.

[MarkT]

Just been to check the old/original rear shock absorbers (Porsche Logo sticker but can't read the Part number) and on reflection these are Oil filled shocks without pre-load (They stay in place if you move the piston) and not Gas shocks that have lost their pre-load (which I thought was the case before today).  The new fitted rear shocks (BOGE Part Number 91133305117/1) are defo Gas Pre-loaded !!!

So I guess the exam question is could the fact I have pre-loaded Gas shocks and not Oil shocks (As original) cause L & R ride height differences after Spring Plate angles are set identical.

 

[sopor]

They may have preload but I suspect it’s negligible.

[GrahamTompkins]

When I changed my Boges, as you say old ones oil filled, new ones gas filled.My guess is one of them is defective and has a leaking valve . I guess if you swap the sides, this will confirm it as being the case. 🤞🏻

[MarkT]

So this morning I disconnected the shock absorbers both sides and then bounced the car remeasured ride height absolutely no difference ! For information the pre-load in the shocks is not significant and the piston can easily be pushed in with the palm of your hand.

So, either later today or tomorrow I plan on re-setting the Right/lower (13mm) this is the current difference L/R when measured from Floor to center of torsion tube to obtain a 13mm increase I expect this will take 2 or 3 spline increments, if this does not fix it, in order to eliminate the spring plate/outer spline pocket moving I will fit the original RH spring plate and spline pocket .

I will have then Run out of ideas.

Q. Is it acceptable to have a different L/R free spring plate angles in order to achieve same LH & RH ride height ? if yes how big can the difference be before you consider you have a problem, it just may be I don't have a problem if the difference can be +/- 2-3 spline increments (1 increment being = inboard 1 anticlockwise and 1 outboard clockwise............).

[MarkT]

Re baseline close of play yesterday:

After realising my rear TB to floor measurement differences were incorrect i.e. yesterday I actually started with a 7mm difference not 13mm (Previous post needs correcting!?). After yesterdays efforts playing with RH spline adjustments and also recognising that there is a couple of mm relative difference in wheel arch heights i.e. the RH side is lower than the LH side, I am now only using the bottom edge of TB to Floor as relative ride height, the L/R difference now is about 2mm which today I hope can be further reduced by a) Recognising my floor is not flat flat, b) Maybe Wiggle room on front Left ride height and c) My spring plate adjusters are in the mid position so some scope here.

[Jonny Hart]

On 13/08/2020 at 13:02, MarkT said:

Pre owned TB arrives tomorrow, as you suggest I will treat this as a reset/initial set up and check everything again. 

Hmm.  You replaced one side only?  Anything brakes or suspension should be done as a pair.

[MarkT]

Agree, however I could only find 1 pre owned available, as new are NLA. 

Will probably put original back in. 

Not sure why you mentioned brakes they are all new. 

[MarkT]

So today a slight lowering (1/2 turn) of the front left adjuster all 4 corners torsion bar pockets to floor perfect just where I want everything, go for 40 mile drive and all 4 corners 6mm lower !  Normal ?