Failed (bent) WEVO Spring Plate - cause?

[jaydubya]

First, I want to say that I have emailed both WEVO and the shop that installed the spring plates and I am confident that they will get back to me with some answers and advice on how to proceed. I just want to get some additional opinions on what may have caused this problem.

I had the WEVO SPS spring plates and Elephant Racing rear trailing arm monoballs installed on my car about a month ago by a very reputable shop. The car is a 1982 Porsche 911SC race car, prepared for the PCA "G Stock" class. The rear of the car has Elephant Racing trailing arm monoballs, 29 mm torsion bars, a "Charlie Bar" sway bar and Bilstein Sport shocks. Rear tires are Hoosier R6 (245-45-16).

I have driven the car with the WEVO spring plates during about 7 track days now.

I was in the middle of a race yesterday, in the middle of a corner, when I heard and felt a "bam" from the rear of the car. I came into the pits and noticed that the rear of the car was sitting very low. I went back to the garage, jacked up the car, removed the wheels. Here is what I found:



As you can see in this picture, the height adjustment plate was bent outward so that it no longer rested on the height adjustment screw. Also, there is evidence that the spring plate had been rubbing on the tub. It appears that lateral motion of the spring plate bent the height adjustment plate outward. This had to be a substantial amount of motion given how much the adjustment plate has bent, since the height adjustment plate would recoil elastically after plastic deformation.

Here is how the RHS spring plate looks with the car resting on the wheels:



Here is a shot of the right-hand rear wheel:



Finally, here is the LHS spring plate. As you can see, the height adjustment plate is bent outward and is close to "failing".



I am at a loss as to what caused this to happen, although I can think of only three possible categories of problems: 1) the spring plates are not stiff enough under lateral bending loads for this application, 2) there was an error in the installation of the spring plates, or 3) there is a specific problem with the setup of my rear suspension. Given the substantial expertise of the personnel at the shop that installed the parts and the relatively straightforward procedure for installing the product, it seems to me that option 2) is unlikely.

Seems like there is either more lateral motion of the trailing arm than there should be or the forces were just to high.

I should mention that I increased the stiffness of my rear swaybar by 10% yesterday morning (reduced moment arm by 10%). However, I noticed the rub marks on the tub prior to this adjustment.

I'd appreciate any thoughts as to what to look for.

Thanks,

Jeff

[911ctS]

I have yet to install a pair of these SPS spring plates so I cannot fully conceptualize what could have gone wrong during the installation process. With that said... I think its obvious that your spring plates did have way too much lateral motion under those driving conditions. Actually you had ALOT of motion going on back there to bend them that far past their elastic limit.

In my experience Wevo products have been top notch!

Have you double cheked all of your nuts/bolts for tightness in the rear?

Food for thought- I know its a 30+ year old design with room for improvement but Porsche chose to use bananna arm bushings with very limited lateral movement and adj. spring plates that were fastened together, never allowing them to seperate in any manner.

Interested to hear what others think...

[jaydubya]

Originally posted by 911ctS
Actually you had ALOT of motion going on back there to bend them that far past their elastic limit.

Yes, I agree. It would seem that the entire trailing arm must have moved over.

So where could the slop be? Trailing arm attaches to spring plate. Trailing arm attaches to chassis pivot at front with monoballs. Trailing arm attaches to axle.

Have you double cheked all of your nuts/bolts for tightness in the rear?

I did a quick visual inspection of the attachment bolts for the spring plate to the trailing arm and the monoballs. They looked good. I pulled HARD on the spring plate and got no movement.

Food for thought- I know its a 30+ year old design with room for improvement but Porsche chose to use bananna arm bushings with very limited lateral movement and adj. spring plates that were fastened together, never allowing them to seperate in any manner.

The thing is - the WEVO design is not substantially different than the OEM design, with the exception of the roller bearings. The rubber trailing arm bushings were designed to accommodate motion of the trailing arm as it moves during suspension travel - the path of the trailing arm is not just straight up and down.

Cheers,

Jeff

[john walker's workshop]

what does the inner bushing look like? they do keep the stock plate off the bodywork.

[jaydubya]

Originally posted by john walker's workshop
what does the inner bushing look like? they do keep the stock plate off the bodywork.

I'm sorry but I don't have any photos of the actual install on my car. These pictures are from the installation instructions - hope WEVO / Smart Racing don't mind.

A schematic:



Inner bearing carrier (without bearing) installed in tub:



Picture of inner bearing carrier with bearing and lip seal:




The complete installation instructions can be downloaded from Smart Racing's web page:

http://www.smartracingproducts.com/pdfdocs/561300_wevospringplate.pdf

[analogmike]

The spring plate is not designed to resist side loads. It is the banana arm that does that. I think something is wrong with your banana arm(s) or inner monoball which attaches near the torsion bar tube.

[Grady Clay]

Jeff,

Here is the Factory adjustable spring plate.
"
"
© Dr. Ing. h.c. F. Porsche A.G.

As you can see the adjusting hardware also has a clamping function.

The WEVO adjustable spring plate and bearing is better for competition and locating the rear suspension. The adjustment range is also far greater. I have always considered adding a countersunk clamping bolt and nut to hold the two pieces together.

Looking at your adjusting screw, I would have adjusted the torsion bar splines to minimize the amount of thread through the threaded piece.

Do you have a recess for the adjusting bolt to seat in?

The purpose of the spring plate is to be able to twist along its long axis as the suspension travels. With race spring rate torsion bars that shouldn’t be an issue (smaller travel). On the other hand is there a lot of travel past static (rebound)? If so then the spring plate can twist a lot. Could this be a cause of this problem?

Have you limited the downward travel of the suspension past where the torsion bars are effective? If the suspension can travel where the adjuster gets clearance then the adjuster can become mis-located and possibly cause this problem.

Many do this with a cable or chain but I think the correct way is a rebound elastic stop in the shock (just the way there is a bump stop). Many Porsche race cars allow the suspension to move a lot in rebound and accommodate this issue with very soft rebound springs that collapse (spring bind) for normal operation.

From your pictures, can the spring plate assembly move axially causing the contact with the chassis? Some prying to push and pull axially will tell with a dial indicator on the torsion bar cap.

These should be easy issues to solve.

Best,
Grady

[jaydubya]

Thanks to analogmike and grady for your responses!

Originally posted by Grady Clay
The WEVO adjustable spring plate and bearing is better for competition and locating the rear suspension. The adjustment range is also far greater. I have always considered adding a countersunk clamping bolt and nut to hold the two pieces together.

That would make sense. I guess you would have to slot one of the two plates to allow height adjustment though.

Looking at your adjusting screw, I would have adjusted the torsion bar splines to minimize the amount of thread through the threaded piece.

I didn't set the ride height, but my guess is that it was set like that to allow the rear to be lowered later. I assume that the length of the adjustment screw was selected appropriately given the design of the system.

Do you have a recess for the adjusting bolt to seat in?

There is no recess on the top flat surface of the height adjustment plate to seat the adjusting bolt.

The purpose of the spring plate is to be able to twist along its long axis as the suspension travels. With race spring rate torsion bars that shouldn’t be an issue (smaller travel). On the other hand is there a lot of travel past static (rebound)? If so then the spring plate can twist a lot. Could this be a cause of this problem?

I assume that the spring plate is free to move downward without limitation in my current setup. I have not limited the suspension travle in droop in any way.

If the suspension can travel where the adjuster gets clearance then the adjuster can become mis-located and possibly cause this problem.

I'm starting to think that this is in fact what happened. Perhaps the height adjustment plate was slightly bent previously and the adjustment bolt slipped off the platform during hard cornering. I suppose that it could be related to droop travel. If I recall correctly, the right side "failed' in the middle of a series of esses (left-right-left), as I exited the second of three esses. So the tub wanted to move to the left relative to the tires/suspension.

From your pictures, can the spring plate assembly move axially causing the contact with the chassis? Some prying to push and pull axially will tell with a dial indicator on the torsion bar cap.

I tried to move the spring plate by hand axially but there was no slop or motion. The car is stored at the track an hour away from my house, but I will either check that myself the next time I go out there or have the shop that did the work check for axial play when they take things apart to diagnose and fix this problem.

Cheers,

Jeff

[analogmike]

Originally posted by Grady Clay

The WEVO adjustable spring plate and bearing is better for competition and locating the rear suspension. The adjustment range is also far greater. I have always considered adding a countersunk clamping bolt and nut to hold the two pieces together.

[/B]

I use the sway away spring plates and have often thought of the same thing. They normally allow a lot of "loose droop" and the adjustment bolt gets unloaded when that happens, which is not a good thing IMHO. That could be what happened to yours.

A clamping bolt would not have to be super strong. I was thinking of just drilling through the two pieces, and tapping the hole and installing a strong screw. Something like the screws we use to mount the seats on the old cars would work. If you need to reset the ride height, just remove it and do it again. The more holes you drill, the more weight you save

Good luck!!!

[911pcars]

Originally posted by analogmike
The spring plate is not designed to resist side loads. It is the banana arm that does that. I think something is wrong with your banana arm(s) or inner monoball which attaches near the torsion bar tube.

I agree with Mike. The control (banana) arm and the attached spring plate form an open triangle or A-arm; two independent ends pivot; the third end moves up and down with the wheel/tire. The spring plate controls the movement of the suspension in a somewhat vertical arc (with the exception of some camber angularity), not to counteract horizontal loads. I assume the spring plate pivot bearing has provisions to allow for slight angular movement as the suspension rises and falls.

If the control arm pivot point flexes for some reason, e.g. during high G-load cornering, the triangle could distort. Excessive play could place side loads on the spring plate, and the the temporary side load on the spring plate could bring it closer to the chassis and scrape it as well as create more space between the height adjustment lever and the adjustment bolt. From the photos, it looks like the spring plate took a permanent set, or the suspension is caught in a distorted position.

Disconnect the sway bar drop link on that side and verify this isn't contributing to the way the spring plate has bent, then disconnect the spring plate from the control arm. When first installed, the control arm/spring plate mounting surfaces should have met fairly closely. Once disconnected and the spring plate springs back to a more or less normal position, do these mounting surfaces remain fairly close? If not, something is happening with the control arm.

My WAG,
Sherwood

[jaydubya]

Originally posted by analogmike
I use the sway away spring plates and have often thought of the same thing. They normally allow a lot of "loose droop" and the adjustment bolt gets unloaded when that happens, which is not a good thing IMHO. That could be what happened to yours.

Yeah, that's what I was trying to say above. It will be interesting to hear what the folks at WEVO think. If there's nothing wrong with my car, and everything is installed ok, seems like for the price of the spring plates this ought to have been addressed in their design.

A clamping bolt would not have to be super strong. I was thinking of just drilling through the two pieces, and tapping the hole and installing a strong screw. Something like the screws we use to mount the seats on the old cars would work. If you need to reset the ride height, just remove it and do it again. The more holes you drill, the more weight you save [/B]

I'm already underweight w/o the spare tire and a half tank of gas

[jaydubya]

Originally posted by 911pcars
I agree with Mike. The control (banana) arm and the attached spring plate form an open triangle or A-arm; two independent ends pivot; the third end moves up and down with the wheel/tire. The spring plate controls the movement of the suspension in a somewhat vertical arc (with the exception of some camber angularity), not to counteract horizontal loads. I assume the spring plate pivot bearing has provisions to allow for slight angular movement as the suspension rises and falls.

Thanks for this explanation! Makes sense.

What do you mean by the "spring plate pivot bearing"? If you mean the needle bearing in the torsion tube that allows the spring plate to rotate around the axis of the torsion bar, then no, there is very little play there at all other than rotation about the axis of the torsion bar. My guess is that the spring plate would have to twist/bend. I believe that this design is similar to what one gets when the ER polybronze bushings are installed in the factory spring plates.

If the control arm pivot point flexes for some reason, e.g. during high G-load cornering, the triangle could distort. Excessive play could place side loads on the spring plate, and the the temporary side load on the spring plate could bring it closer to the chassis and scrape it as well as create more space between the height adjustment lever and the adjustment bolt. From the photos, it looks like the spring plate took a permanent set, or the suspension is caught in a distorted position.

The adjustment plate has definitely deformed plastically. There is no load on the suspension in the photo - the car is jacked on a tow point jack pad on the rear of the engine, so there is no sway bar load either.

[B]Disconnect the sway bar drop link on that side and verify this isn't contributing to the way the spring plate has bent, then disconnect the spring plate from the control arm. When first installed, the control arm/spring plate mounting surfaces should have met fairly closely. Once disconnected and the spring plate springs back to a more or less normal position, do these mounting surfaces remain fairly close? If not, something is happening with the control arm.

I will ask the shop to check the alignment of the spring plate and control arm after disassembly, it's gonna have to come apart anyway to repair or replace the height adjustment plates. I just want to make sure this doesn't happen again. So much for my alignment and corner balance...

Cheers,

Jeff

[911pcars]

Being steel, the spring plate can be bent back straight. Not sure about any damage to the bearing end. Your tech can verify.

Let us know what you discover.

Sherwood

[Grady Clay]

Guys,

I disagree.

“The control (banana) arm and the attached spring plate form an open triangle or A-arm; two independent ends pivot; the third end moves up and down with the wheel/tire. The spring plate controls the movement of the suspension in a somewhat vertical arc (with the exception of some camber angularity), not to counteract horizontal loads. I assume the spring plate pivot bearing has provisions to allow for slight angular movement as the suspension rises and falls.”

The spring plate is designed to twist along its long axis to compensate for the camber change. It is designed to be rigid in compression and tension along the long axis to be part of the triangle created by the trailing arm (banana), spring plate and torsion bar tube. Clearly it has to be rigid in the angular direction to transmit the torsion bar forces to support the weight of the car. Obviously appropriate bearings at the connection between the trailing arm and the torsion bar tube (2-axis) and the spring plate and torsion bar tube (1-axis) are critical to maintain this rigid triangle. The bending of the spring plate accommodates what you refer to as “angular movement.”

While I haven’t studied it, I think the Elephant links may address this issue and more.

Mike,
“A clamping bolt would not have to be super strong. I was thinking of just drilling through the two pieces, and tapping the hole and installing a strong screw. Something like the screws we use to mount the seats on the old cars would work. If you need to reset the ride height, just remove it and do it again.”
I agree exactly. Once you have the suspension adjusted and settled where you want, simply drill a hole through both plates. Disassemble and countersink the inboard side. Reassemble with the nut on the outside. If the same adjustment can’t be achieved, elongate the outboard hole.

Everyone,
I think we should pay more attention to an easy conversion to rear coil-over shocks. One advantage is the easy ability to move the rear suspension through its entire travel while not under load. (Yes, with WEVO and other you can do this with the torsion bars not in place.) A very good set-up practice.

Best,
Grady

[jaydubya]

Originally posted by Grady Clay
The spring plate is designed to twist along its long axis to compensate for the camber change.

Grady - when you say "twist along its long axis" - do you mean that it should be allowed to rotate about that axis somewhat, or do you mean that the spring plate should actually deform in torsion? It seems to me that with the stock spring plate bushings, they are going to deform before the metal spring plate actually undergoes any torsional deformation.

However, with needle bearings, that rotation about the long axis of the spring plate would have to be accommodated by actual torsional deformation of the spring plate.

If I'm visualizing this correctly in my head, when one of the rear corners goes into compression, we get a camber increase, and this would cause the top of the spring plate near the control arm to want to rotate toward the chassis; the opposite would occur in droop. Is that correct?

I agree exactly. Once you have the suspension adjusted and settled where you want, simply drill a hole through both plates. Disassemble and countersink the inboard side. Reassemble with the nut on the outside. If the same adjustment can’t be achieved, elongate the outboard hole.

Could you just tap the hole in the inboard part and avoid the need for a nut?

Cheers,

Jeff

[911pcars]

A slotted and arch-shaped hole through both height adjustment lever and spring plate would serve the same purpose and allow infinite (within-a-range) ride heights but with only one clamping area, otherwise you wind up with a swiss cheese effect and many drill-through holes only conforming to a set ride height setting.

Sherwood

[Grady Clay]

“- when you say "twist along its long axis" - do you mean that it should be allowed to rotate about that axis somewhat, or do you mean that the spring plate should actually deform in torsion? It seems to me that with the stock spring plate bushings, they are going to deform before the metal spring plate actually undergoes any torsional deformation.

However, with needle bearings, that rotation about the long axis of the spring plate would have to be accommodated by actual torsional deformation of the spring plate.”

Correct. That is why Porsche designed it as a flat plate. The same is true with swing-axle 356s.



“If I'm visualizing this correctly in my head, when one of the rear corners goes into compression, we get a camber increase, and this would cause the top of the spring plate near the control arm to want to rotate toward the chassis; the opposite would occur in droop. Is that correct?”

Yes.

“Could you just tap the hole in the inboard part and avoid the need for a nut?”

I prefer a nut-bolt arrangement for strength and serviceability. If you use a grade 10.9 ISO countersunk Allen bolt on the inside and an appropriate washer & nut outboard, you will have the best clamping force and easily serviceable.

Best,
Grady

[shbop]

signing-up

[jaydubya]

Originally posted by Grady Clay
“- when you say "twist along its long axis" - do you mean that it should be allowed to rotate about that axis somewhat, or do you mean that the spring plate should actually deform in torsion? It seems to me that with the stock spring plate bushings, they are going to deform before the metal spring plate actually undergoes any torsional deformation.

However, with needle bearings, that rotation about the long axis of the spring plate would have to be accommodated by actual torsional deformation of the spring plate.”

Correct. That is why Porsche designed it as a flat plate. The same is true with swing-axle 356s.

Sorry but I'm not sure which statement you are saying "correct" to or perhaps you are saying correct to both

In the OEM configuration with rubber bushings, why would the plate deform in torsion? Wouldn't the rubber bushing deform first? Why would Porsche use both a rubber bushing at the torsion bar end AND a spring plate designed to deform under torsional loading? Seems redundant.

Cheers,

Jeff

[ZOA NOM]

Great thread. I just looked at my Weltmeister adjustables, and they are indeed flexed a tiny bit, resulting in an increasing gap between the main plate and the adjuster arm. I see a clamping hole in my near future. I suggest any other adjustable plate owners check theirs.

Great posts as usual, Grady!