Can torsion bars go soft ?
 

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

I think all of the pieces are settling. Spring steel can fatigue I suppose, but even with age, I think the bars stay intact. I have never encountered what I would call a fatigue issue, but a few broken ones.

It's more likely that the bushings and related components are compressing and settling in.

leaf springs get sagged out over time i dont see why torsion bars couldnt do the same

Did you install them back to the same side and orientation they were before they restoration?

Many thanks for the responses.

The thing that I have trouble with is the the left side is rock solid set it once 3 weeks ago and has not moved at all, the right side on the other hand continues to slump after a few miles driving and I have needed to increase the height twice now and already next day its about 8mm lower than it was yesterday after increasing the angle on the spring plate 1 increment. When I did the initial set up both left and right spring plate angles were at 29 degrees when free and initially provided the perfect left/right ride height, now after a few slumps on the right side spring plate angle is 33 degrees ! now making it difficult to get back on to the trail arm.

I'm sure I did, but will check again tomorrow.

If it's just one side and keeps getting worse after several adjustments I'd pull the bar and inspect..

Thanks, will do tomorrow.

I wonder if your torsion housing is the part that is failing. Was the car rusty ?

They can break ... happened to a friend with an 84 , one of the rear bars just cracked when he was backing out of his driveway.
If you have any doubts , replace both of them !

Not in the inner torsion housing it and the splines were well greased when the TB came out.

If you search the forum you'll see many statements about torsion bars not wearing out...until they just suddenly break.

I'm not talking about the splines, the entire torsion tube can fracture and rotate. The car sits lower and lower.

No sign off that as for as I can tell and would that not impact both left and right side ?

Left side is spot on at correct ride height.

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

happened to my 85. had noticed the car was not sitting normal, sagging slightly on that side just before it broke.

I had the same situation but it was created by a set of weak shocks.

A standard gas shock adds about 45lbs of static spring rate to the suspension , as the shock wears
the rate decreases
Ian

Another thought I had was movement in the interface of the outer spline pocket and the spring plate, its difficult to see this with the rubber bushes on, I'm not sure if its welded or press fit or.... if its possible it could move it would give the same symptom.

However, "45lbs of static spring rate to the suspension" is my favorite, will check this Left & Right Monday, Albeit whats the chances of a brand new Shock failing after a
~100 miles.? I guess if I disconnect both and the rear settles even Left & Right it must be the RH side Shock absorber lost its spring rate.