Soggy bottom. Help with rear suspension please.

[911hillclimber]

Good call on the dampers.
I've heard that the adjustment can 'jack' a car down on racing dampers (2 or 3 way adjustable) but have never experienced it.
Rears are a cinch to change, so borrow a pair, and ordinary type like bolt-on Boge, non adjust gas type, re-set the ride height and see what happens.

[Jonny Hart]

The dampers I have fitted were brand new green Bilstein Sports when the car was put back on the road 2 years ago. Not adjustable, just the standard ones.

My gut feeling is they are defective so will have to come out (again!)

[fetuhoe]

Guys,

Dampers don't affect ride height unless they are siezed and not working.

The gas charge in a Bilstein will raise a car that has been set with a standard damper as the spring force generated by the gas will add to the spring rate at the wheel.If you fit a Bilstein as a direct swap for a Boge the car will raise slightly.

The 'jacking' effect of high rebound rates ONLY happens whilst the car is being driven along a bumpy road.

When you hit a bump the wheel moves up and then the high rebound keeps it down for a specific time - which depends on the damper rate - but when you stop the stored energy in the spring will return the car to its statuc ride height. The 'jack down' only occurs when you coontinue to drive on a bumpy road.

This would always be considered as an 'overdamped' condition.

If you look the graph I posted showing force versus velocity of a damper you will clearly see that when velcoity is zero force is zero.

This is not really a damper issue with a 2 year old Bilstein.

[Gary71]

I was implying they were seized (however unlikely that is with new dampers). The jacking I experienced was exactly as you describe, bumpy road, aggressive driving, and eventually find the car on the bump stops and undrivable.
However it would also remain there for some considerable time afterwards.

[fetuhoe]

However it would also remain there for some considerable time afterwards.

Gary, Why do you think this happens?

If you look at the basic numbers then shortly after the car stops moving the force generated by the damper drops to zero.

It is possible that when tested on a 'bell crank' driven damper test machine you can see an force offset in the rebound direction of a damper but this is due to the nature of the test system.

When you test a damper with a sinusoidal input the time spent at zero velocity is too short for the stack and bleed system to equalise pressure across the piston hence this anomaly.

If the damper is stationary for more than this short fraction of a second then the bleed system generally has time to operate.

Reducing the bleed on the rebound side of a damper tends to make the presence of a 'nose' more pronounced but this is generally occurs at a velocity on around 0.5mms^-1 and below this the force needed drops steeply.

If the car has lost the 3" of ride height described in the early post then the force needed to hold down the rear of the car would be around 500lbs per wheel - if you look at it the other way the force available to restore the ride height would be 500lbs which I would think should counteract any stiction unless the damper were completely FUBAR.

I am sure that there must be a failure happening somewhere in the car.

Either the torsion bar is cracking and losing stiffness, something is happening within the tube or the spring plate adjuster is slipping.

I just can't think of any other cause.

[911hillclimber]

A seized/seizing damper was in my mind too, but have never experienced this on any car, always a first time.

The bars have to come out to check for cracking.
If nothing is apparent then mark the length of the bar with a fat indelible marker straight along the length.
I f re-fitted and used and sagging continues, remove the bar and see if that marked line is twisted. It should be straight still.

What can progressively move?

The spring plate/external splined tube are welded together, failure would be sudden I think if the welds were to fail.

The internal splined boss is welded inside the torsion tube (and crimped?).
If the welds fail, the failure will be sudden but might if crimped move gradually?

If the bars themselves are failing by a crack getting longer, spring rate getting weaker, then the 'failure' could well be gradual and eventually suddenly collapsing to the damper bump stop, very nasty.

What else is left?
Banana arm bolts on the move, not by much even if finger tight and movement could be sudden, but not 3" at the apex of the wheel arch.

Intriguing!

[Jonny Hart]

Either the torsion bar is cracking and losing stiffness, something is happening within the tube or the spring plate adjuster is slipping.

It is happening evenly on both sides of the car. This has only happened since 'upgrading' the bars, it was fine on the originals. Gotta be duff bars.

[fetuhoe]

If you consider the metallurgy it is very unlikely to be the bars.

If the bar is yielding then it would need to have been manufactured to around 30% of its specified tensile strength. This could be caused by steel being made to either a very incorrect chemistry or to have been heat treated very badly.

In modern steel processing this type of issue almost never occurs to the degree required to cause this type of problem as process control on excellent.

I can understand that steel can either fail to meet the required strength by a few percentage points or that it can have been heat treated to a 'brittle' condition which would causes the bar to snap with less twist than expected but not to continually yield.

If the bars are developing helical cracks then there must be planes of weakness within the bar such as a rolling lap.

Nowadays most bar mills inspect for this type of defect using a Magnetic Sorting Bridge so bar is rejected before being delivered.

Rolling laps also tend to develop in a specific helix so they would behave differently when loaded in either a Right hand or Left hand helix. This means that this type of defect would lead to different behavior on either side of the car.

Having manufactured and supplied Spring Testing machines to most of the OEM Spring Makers in Europe for a period of around 20 years I would expect that the presetting operation - 'scragging' - would identify bars that yield at low stress levels and any modern semi-automated test system would reject a bar of this type. It is also likely that this operation would reveal the presence of any significant defects.

For a 'duff' bar to have made it though the system would mean that several quality system failures have occurred and I have to say I find this unlikely.

OEM Springs are extremely well controlled.

The excuse of 'bad material' is a bit of an old wives tale and in my experience of working with fracture and fatigue for more than 40 years most failures result from poor design or lack of maintenance.

It will be very interesting to learn the cause of this problem.

[GARY RICH]

I am having my bars reset again next Monday and hope to be there to watch how it,s done. The car and two large passengers drove to the Spa Classic after a reset, and dropped but not by as much as the first time so hopefully it will just need tweeking. Fingers crossed.

[fetuhoe]

Gary,

While the bars are out can you add a fine paint line along the length of the bar.

If they permanently deform you should then be able to see if there is a twist.

[GARY RICH]

Simon is very busy and can only ask

[Jonny Hart]

I was at Tuthill's on Monday. I was speaking to one of the guys there and he said that they have had endless problems with 26mm bars from Porsche. They just keep on sagging so they do not fit them anymore. Looks like it is the bars then.

[Jesse]

Any updates on your issues?

[Jonny Hart]

I still have a 'soggy bottom' and am now at the end of the adjuster. I'm gonna take the t bars out and fit the ER hollows as recommended by Tuts.

[GARY RICH]

Did you not keep original torsion bars