DDK - Die Deutschen Klassiker

I have an '82 SC with Sport Bilsteins and new 26mm torsion bars at the rear. All the suspension is new or refurbed. I have Neatrix rubber bushes.

The car has drooped a total of 80mm in around 2 months at the rear. It has been put back up once already using the spring plate eccentric adjusters.

Today I am re-indexing the bars so the adjusters come back to the centre. I noticed that the glue had failed on the Neatrix bushes but even so I cannot see how this would allow the suspension to droop like this. Can the grip of unstuck rubber really prevent the car from rebounding? I don't believe this since the car doesn't 'recover' from its squat position when jacked.

Is it normal for new torsion bars to sag? They are genuine Porsche and fitted to the correct sides.

Doubt the bars are at fault.
The eccentric bolts must be working round after setting.
The one bolt is eccentric to set ride height and the other is a plain bolt to clamp the long spring plate to the adjustable arm which has the rubber bushes and torsion bar socket to it.

It has been a long time ago since I tweeked mine but they have to be quite tight.
Re-adjust and make a very clear mark between the adjusting short arm and the longer spring plate.
The marks should stay matched.
My marks are still true after 60K miles and 18 years....

Nope. that's not it. The tide-mark of spray wax is clearly visible and has not moved. All bolts are torque and paint marks intact.

splines, t tube condition ?

The TBs are handed, check they on the correct sides.

Hmm ............ odd

No - new torsion bars don't "sag" - they are (in effect) springs ... so to work correctly they must behave as such ... and as such will go back to where they were (assuming elastic limit of material has not been passed, by putting small elephants in the back of the car)

Handing ? - don't think this is relevant. My thought would be the bars are manufactured the same , and are handed (stamped) to make sure they always go back in the correct side ... NOT a good idea to swap them !! ... I stand to be corrected here.

All suspension will "settle" after use. Was the 80mm difference between starting out and where things are now ? (in fact where is the 80mm measured? : vertical height at correct datum point , or wheel arch centre , or where ?) .... or was it after things had settled ?

Something is moving somewhere (depending on answer to the settling question) .... you just need to find out where. I hope for your sake, it's not in the middle of the torsion bar tube.

keep us posted.

TBs correct sides as per original post.

Car is solid and rust free, no weakness to any mounts or tubes.

80mm measured from floor to wheel arch lip on level ground. Basically I wanted a 'finger' of gap above the rear tire. A month later,the arch was well below the top of the tyre, so it was raised on the eccentric adjusters. Another month passes (now) and it is down below the tyre again.

An IB'er suggested that the brand new Bilstein Sports have been 'assisting' the suspension but have now settled in. I think this is the most likely explanation but am surprised that this has not been mentioned before.

I have re-indexed today (and centred adjusters) but yet to put it on the deck and see how it looks.

I am assuming the drop by 80mm is a gradual affair and not sudden.
The spring plates are tight so any radial change giving the 80 mm drop is the 2 torsion bars yielding due to poor manufacture, the external splined cup has a weld broken to the spring plate ( doubt that) or the centre bar splined sleeve slipping round in the torsion tube but that is crimped and plug welded iirc. Bit strange that both sides are doing the same drop.

Sorry, just re-read op and it has dropped over 2 months.

The dampers would add little to the suspension ride position, they damp the springs oscillations not suspend the car.

What else can gradually slip by giving the 80 mm drop?
That is a huge amount, about 5 to 10 degrees yield, almost the full suspension deflection from static to compression.

Real pain, but you could remove the torsion bars and draw with an indelible felt tip pen a straight line down the whole length of each bar. If the bars are yielding, then the set will show as the line being twisted over the bar length.

Would be good to change the rear dampers over to another set and drive it to see if this changes anything to eliminate the thought?

Ok, '80mm' is me approximating the height and it has gradually gone down, not sudden. perhaps it's not quite as much as that but certainly more than 60mm. Both sides the same. The TBs are brand new genuine Porsche.

My money is on the new green Bilstein Sport dampers. They had a lot of spring fresh out of the box.

Maybe, when the Neatrix bush glue failed, it provides no 'twist' so the assistance to rebound is lost. Combined with settling dampers reducing assistance further, perhaps that explains it?

Two things to explore then!
Change the rear dampers and see if the same drop happens.
The rear bushes are originally vulcanised to the torsion bar splined sleeve on the smaller diameter, the out dia of the buses are free to locate and twist in the tube or the end caps.
It is a case of the ensuing friction being greater than the torsional stiffness of the bush itself, but as most times these bushes do wear out I think says they move (slip?)in the mounting locations.

Good greasing on assembly will assure the bush slips in their housing diameters.
I have hard plastic bushes in my car (73T) and so the parts all slip to allow movement.

I have custom valved Bilstein sports fitted a very long time ago and I've never had this issue, and to be honest, have not heard of such a failure before in my 27 years of 911 ownership.

Interesting, and intrigued to see what has caused this.

You clearly suspect the dampers. Do you still have the old ones you can use if serviceable for a few thousand miles? I wonder if the new ones removed stay 'short' or extend fully as per new condition?

Jonny, when I changed my rear shocks from Konis to Billies it jacked the rear of the car up a lot. Enough to warrant an adjustment - can't remember if it was just on the adjusters or re-index though. However I don't remember them subsequently dropping, but maybe I just didn't notice....

P.s. I'm in the office tomorrow, I'll fire over the dosh for the fuse board.

Thinking of the layout, geometry and physics I think there can only be 4 possibilities:

1. Something was preventing the rear end of the trailing arm going up to its 'correct' position and that something (shock absorber or bush) is slowly finding its correct position - unlikely in my view given the weights involved, but possible)
2. The spring plate is bending - even more unlikely, and it'd be pretty obvious
3. The torsion bars are slowly giving way - equally unlikely. Their metallurgy means catastrophic failure is more likely that this.
4. The 'block' in the centre of the torsion tube (with holes for the torsion bar splines to mesh into at the inner end) is slowly rotating in the tube. Corrosion or weld failure or poor manufacture are all possible causes. Never heard of this ever happening. The fact it has gone down the same at both sides also supports this possibility as it is one piece of metal I believe.

So in my view if you eliminate 1 by your current checks then it has to be 4. Hope it's 1!
No idea how you'd check 4 either.....

Mick

I have the same thought as mick.
It is the only part that can keep on moving and affect both sides equally, it is the only link between both sides!

On my 73T the boss in the tube is crimped heavily into place and iirc there are substantial plug welds too so any failures should be readily spotted?

Did you talc up the rubber bushes when you fitted them? It need to 'relaxed' (not twisted) at normal ride height. When you initially fit the covers the rubber is relaxed at max droop & the talc allows it to relax when lowered down & settled with a drive around. Without the talc the rubber will bind and stop the suspension settling fully until a few heavy bumps break the grip.

Cortina wrote:No - new torsion bars don't "sag" - they are (in effect) springs ... so to work correctly they must behave as such ... and as such will go back to where they were (assuming elastic limit of material has not been passed, by putting small elephants in the back of the car)

Handing ? - don't think this is relevant. My thought would be the bars are manufactured the same , and are handed (stamped) to make sure they always go back in the correct side ... NOT a good idea to swap them !! ... I stand to be corrected here.

The reason they are handed is due to the fact that all good quality springs are 'scragged' as part of the manufacturing process.

Scragging is a process which slightly yields the material and prevents any further deformation from occurring in service.

Slight amounts of deformation would otherwise occur due to some unstable metallurgical processes that scragging helps to overcome.

Coils springs are taken to an almost coil bound condition and leaf springs are also permanently deformed. This process is also referred to as 'setting'.

Large Railway and Truck Springs are 'set' in very sophisticated machines which measure deflections and plate angles in a very precise manner.

Torsion bars are simply scragged by causing them to yield by a few degrees. This yielding process also causes a small increase in the strength of the material.

Unhappily there is a phenomenon known as the Bauschinger Effect which means that if you cold deform and hence strengthen a material in one direction it become weaker in the other direction.

This mans that if you scrag a torsion bar with a Left Hand Helix its yield strength will reduce if you then load in in a Right Hand Helix.

This will not affect its stiffness but may affect its fatigue life.

If the shaft has never been scragged it can operate in either sense of torque and in either sense of rotation without issue.

Unhappily the problem does sound like a torsion bar tube failure.