Rear Torsion bar issues, GURU help required !!!

[brentrussell]

So this situation has been plaguing me for a week now!

1973 911 RSR Replica with 81 SC engine
Aluminium Trailing arms with adustable SC Spring plates which are set in the middle.
All New Rubber/neatrix bushings and Bilstein Yellow Shocks
31mm Hollow rear bars
Below link to full build duration if you need background.
NZ- 73 RSR Clone build

Reckon I have indexed the rear Torsion Bars about 20 times throughout different stages of the project for various reasons, mainly when sorting rear fender placement, measuring wheel offsets sometimes with motor sometimes without... so I would call myself capable with this... I'm down to 45mins per side.

My issue over last week has been that the Left rear sits too low, pratically sitting on the Tyre. I had set my rear spring plates at 20degrees as I want it low, this works good on Right Side.
I re-indexed the left side to 21deg, no difference, then 22 degrees no different, then 24, no different, finally 26 degrees.. NO FRIGGIN Different.,... by this stage it should be jacked off its head on the left side!
This is all been done on 4 stands with the sills at a ZERO Plane.

So today I removed both torsion bars and inspected, no visual issues with either. 100% not snapped in two
Triple checked that I hadn't passed the bar through the inner spline and it was free running on the inner spline.. this is really impossible when both bars are in as you can hear them tap each other when inserting. I removed end caps to make sure they are also on outer spline which they are and both sit about 3mm in from outer edge.
I reset them at 21.8 degrees both sides and sits the same as it did with 26 on left.

Front sits even at rest, however when jacked in middle under engine it does drop more on the Left hand side by 20mm maybe... I'm not discounting some kind of tripod trajectory issue here but I can't help but think my massive 6 degree difference in spring plate angle would counter this in a major way.

Apoligies for the long description, trying to get it all down to cover off any queries.... I'm totally out of ideas of why this could happen.

Possible scenarios I can thing of...
1) My inner spline in the torsion bar has sperated? This would be bad on soooo many levels! plus I can't imagine it, the car hasn't even been drived yet since a 7 year build! I have tried with all my strength to pull up Torsion bar without Trailing arm connected and it barely moves. The re-indexing has been working previously in past years.
2) Could the Shock be pulling it down? I doubt this as I have to push up/shorten the shock to bolt it to the lower mount, if anything it should be giving it some extra height? I could disconnect both shock lower mounts and see if this does anything?
3) Front Left pulling it down or Front right pushing up crossing it to Left rear...? When jacking the front in the middle and Wheels off ground it doesn't change how the rear sits.
4) Considering removing rear torsion bars and swapping back to the factory 18mm to see if that does anything?

Please help!, I'm booked for a Wheel Alignment Wednesday night and at the moment the Left rear is sitting on the Tyre.

Brent

[Steve@Rennsport]

Hi Brent,

Offhand, I think you may either have an inner spline issue or a bad torsion bar. You can prove the latter by re-installing the factory rear ones to see if you can restore ride height adjustability to the chassis.

[jpnovak]

Are you sure the front ride height is somewhat consistent? A large difference in front height will load the rear corner more than you think.

Have you tried to jack up the rear via the trailing arm or spring plate? Does it hold spring tension the same on both sides? Are the torsion bars the same size on both sides?

Can you temporarily swap torsion bars side to side and isolate chassis vs torsion bar issue?

[chris_seven]

The splined section in the centre of the torsion bar tube is a one-piece 'dog bone'

If there is a difference side to side then the splines would have to shear on one side rather than the other.

This would suggest a huge amount of corrosion as the amount of torque that the standard torsion bar will transmit is very significant.

You just need to calculate the basic shear area for a first order guess.

The force to fully compress a rear Bilstein is around 50kg so the wheel rate created by the gas in the damper is around 16lbs/in - which I guess would add about 10% to a 26mm bar.

[brentrussell]

Thanks Steve,

Yeah I'm thinking I'm reading this direction... I'm doubtful it's the splines on the Chassis but not totally sure on the current T-Bars...

Brent

Originally Posted by Steve@Rennsport

Hi Brent,

Offhand, I think you may either have an inner spline issue or a bad torsion bar. You can prove the latter by re-installing the factory rear ones to see if you can restore ride height adjustability to the chassis.

[brentrussell]

Thanks JP,

Height is right, small rake forward at the moment. But I'm going to remove rear wheels and drop down to approximate height and re-index fronts to be spot on level while pivoted at rear centre engine.

Originally Posted by jpnovak

Are you sure the front ride height is somewhat consistent? A large difference in front height will load the rear corner more than you think.

Have you tried to jack up the rear via the trailing arm or spring plate? Does it hold spring tension the same on both sides? Are the torsion bars the same size on both sides?
Good idea also, will try that.
Confident they are the same, have the same part # and are installed Left and Right.

Can you temporarily swap torsion bars side to side and isolate chassis vs torsion bar issue?

Will probably install my old ones this morning to clarify this at this stage but I can do that once I get past tonights booking

Thanks again!

[brentrussell]

Hi Chris,

I agree it can only be a one side issue.
The car didn't have much rust at all and none in Torsion bar area, it was also checmically dipped and inspected.
The original bars were easy to remove and also installing the current ones are pretty simple too.

Not to familair with calculating shear force.. but I'm thinking I will disconnect rear shocks and see what happens when applyinf force to trailing arm.

Thanks

Originally Posted by chris_seven

The splined section in the centre of the torsion bar tube is a one-piece 'dog bone'

If there is a difference side to side then the splines would have to shear on one side rather than the other.

This would suggest a huge amount of corrosion as the amount of torque that the standard torsion bar will transmit is very significant.

You just need to calculate the basic shear area for a first order guess.

The force to fully compress a rear Bilstein is around 50kg so the wheel rate created by the gas in the damper is around 16lbs/in - which I guess would add about 10% to a 26mm bar.

[brentrussell]

So today I spent some time making sure the Front was 100% correct without the back influencing it's heights, basically tripod type setup from centre of engine crack case with no rear wheels and lowered to approx ride height, it was out maybe 10mm at Fender so I fiddled around to get it 100% right, which it is now.

Repeated the process for the rear with the front balanced on a socket, rear shocks removed and sitting on Tyres, interestingly when measuring bottom of Spring plate to floor the Left hand side was 18mm lower which I woud have expected the opposite!

Jacked the Front and put on axle stands, jacked rear onto stands removed rear spring plates again, installed standard SC Torsion bars and set to 22deg, dropped to ground without shocks attached and Left rear came to rest on Tyre and Right sat about 25mm off Tyre.
Re-indexed Left rear to 25 Degrees and it made NO Difference.

My Theory so far!!!; Inner Spline has seperated from Torsion Tube, I have no idea how, when or why.
How have I not noticed in the past? The majority of all my previous re-indexing that appeared to work (Prior my issues of Left not working) was done without engine/ gearbox, Glass, interior etc... my thinking is there is enough Friction with the rubber bushings to keep the height where I would expect it to be????
Now I have the engine/box, fitted out this Friction is not enough to hold up the weight so regardless of my spring plate angles it drops to the point where the Shock can hold it up.
Now to prove my theory, tomorrow I'm going to remove both spring plates and Torsion bars, put left bar back in, make a bush to sit where the rubber would normally rest, reverse the Sping plates and cover so it fits to Spline then either jack the spring plate or lower the car while I have the spring plate propped on stand.
With another set of eyes looking in the Right hand torsion hole to see if the inner spline is turning or maybe put the right bar in and see if that turns, probably both just in case it's split in two?

What do people think? I can't imagine any other reason for it to be doing it... its a very clever yet simple system how it works... just never seems possible for that inner spline to seperate.

Does anyone know how the inner spline is attached to the torsion tube? I'm guessing its a cast piece slid in and welded?
If my theory is right and I'm kind of hoping I'm wrong... its quite a big job

Brent

[boyt911sc]

Brent,

The inner spline section located in the middle of the torsion bar tube is one-piece and common to left and right torsion bars. You could test the inner spline section by pushing one side (torsion bar). If you slide or push one of the torsion bars inside, the other would be displaced. Just make sure you don't drop the TB inside and get it stranded (more headache).

The torsion bars are easy to inspect for any physical defect. My bet would be a worn out inner spline housing/bushing. Let's hope the inner spline is not the problem because that's a major undertaking to replace it. Keep us posted.

Tony

[brentrussell]

Hey Tony,

You know more stuff

I also hope it's not that too as I have a fair idea of what's involved to sort!
Added with my CSI issue you're helping me with it's a bit over whelming!
I almost hit the wall and ordered PMOS and rear coilovers last night!

Will report more tonight.

Originally Posted by boyt911sc

Brent,

The inner spline section located in the middle of the torsion bar tube is one-piece and common to left and right torsion bars. You could test the inner spline section by pushing one side (torsion bar). If you slide or push one of the torsion bars inside, the other would be displaced. Just make sure you don't drop the TB inside and get it stranded (more headache).

The torsion bars are easy to inspect for any physical defect. My bet would be a worn out inner spline housing/bushing. Let's hope the inner spline is not the problem because that's a major undertaking to replace it. Keep us posted.

Tony

[chris_seven]

If the bar is slipping with the force being applied then the inner spline must be in a very poor state.

I would estimate that when you lower the car onto its wheels it will move by between 4 and 1.5 inches - assuming your hollow bars have an equivalent rate to a solid 26mm bar.

If the spline on the bar is engaged correctly into the spline in the dog bone this would result in a shear stress of around 2000 psi under the spline.

We are in the process of making some new dog bones and the hardness of the standard part would suggest that it has a tensile strength of around 150ksi so its shear strength would be around 82.5 ksi. This means the stress being developed in static condition is less than 2.5% of the material's capacity.

At full compression the shear stress may increase to 5000psi and again this is a very small percentage of the capacity of the dog bone.

These approximate calculations assume that all splines are equally loaded and they are in pure torsion.

I would also expect that the spline in the dog bone was likely to be stronger than the spline on the torsion bar.

The only explanation is that the inner spline has corroded significantly and again it is surprising that only one side is affected.


Can you get an endoscope to have a look down the tube?

[brianlay]

If your observations are correct it must be slippage on the dog bone on that side. Do the the splines on the TB show any signs of distress or slippage?
You've tried multiple TBs correct?
I'm guessing the acid dipping may have affected those splines

[brentrussell]

Crisis diverted !!!!
Some stupidity involved but also a issue to take into consideration.

I dismantled all rear again inserted T-bar with spring plate in reverse on spline and jacked it up and there was no way the inner spline was spining or sheared off! Awesome So had to be to do with the outers or my measuring process.... it was me

Havn't got explanations for all that happened.. some are still mysteries.

Basically what I think was happening with me repeating the angle increase of 2 degrees each time was that I would disconnect the trailing arm so spring plate was relaxed then measure. I would then remove from splines and readjust adding 2 degrees.
What was happening was... from the first relaxed measurement I took to then removing rubber/ spring plate from chassis and spline it would release tension and raise 2 degrees so when I adjusted and added 2 degrees I was basically putting it back where it was before, hence I was getting no change!

First I zeroed car level and set both at a new relaxed 28 degrees, finally the car sat high off the tyre by about 50mm. Then I dissasembled and took extra measurements of relaxed spring plate and removed from chassis on splines but no tension.. almost 2 degrees different, then adjusted down 3 degrees from the removed relaxed point to get me closer, pretty much bang on.
Drove it around the block and it rubbed a little Left rear so repeated process again and lifted 1 degree, has come up about right.. well close enough that it can be adjusted with adjusters.

Off to wheel alignment after work tonight so get everything within spec, I expecting everything to settle a little as it hasn't been on the road for a very long time and everything is new.

Thanks for the comments and ideas, some good things happening above if someone does have a Torsion bar issue down the track... thankfully its not me!

Brent