In order to tighten up the handling, I am replacing all the suspension rubber bushings with “poly” bushings. In this case, poly is a generic term for polyurethane but other materials can be used. For instance, Racer’s Edge uses Delrin, which is probably a little bit stiffer than polyurethane.
I usually find the parts I want at Paragon Products and this case was no exception. I went with the 944 Spec suspension bushing package, which includes a mixture of Weltmeister and Racer’s Edge bushings. The current price is just a little over $400, which gets you all the bushings you need for the front and the rear.
The rear suspension is out of the car. I put it up on saw horses to get better access. The key bushings are those that allow the trailing arms to rotate. There are 2 of these at the end of the torsion bar tube. These are called “spring plate” bushings on the Porsche PET. There are also bushings at the front end of the “banana arm”.
I have a Harbor Freight press, which is pretty useful for pressing out the bushings. A bench vise can also be used. The rubber center section of the banana arm bushings pushes out very easily. Unfortunately, it is wrapped in a metal sleeve that must come out and is usually rusted into place. I wound up slicing the sleeve with a metal cutting blade in my SawzAll, in order to peel it out. Once it was out, insertion of the new bushing was straightforward.
Next it was time for the spring plate bushings. These are supplied by Racers Edge and are made from Delrin. The directions for the installation are correct in that they warn that removal of the old rubber bushings can be arduous and that new bushings may require some trimming. My method to remove the old rubber bushings is to use a wire wheel to abraid the rubber off. Works fine but makes a mess. I test fit the smaller inner bushing in the end of the torsion tube. After cleaning up the the ID of the tube it fit fine. The directions regarding how to heat the bushings to achieve an interference fit gave me pause but that part went exactly as described. I performed the heat/shrink fit of both bushings on the rear axle strut with no issues.
I test fit the larger outer bushing in the ID of the bearing flange, after cleaning up the bearing flange. I thought I had it fitting pretty well, although in retrospect it may not have been fully seated into the flange socket.
As I assembled the pieces and started to draw down the 4 bolts on the bearing flange, I was bound up hard with about a 1/4″ to go (on pulling the 4 bolts down completely). I went ahead and used brute force to pull the 4 bolts flush and it was now totally bound up. Sensing this was not right, I pulled it apart but had to use my press to get the bushings extracted.
The outer bushing appeared to be the problem. It appeared to have too large of an OD. Micrometer measurements indicated maybe 0.025″ too large. Being a tapered profile it was hard to see exactly where the problem lay. I had not worked with Delrin before. I started to to work the OD with the wire wheel with no effect. Then I tried my grinding wheel, which only removed material if I bore down. I don’t own a lathe. Best material removal was obtained working the OD of the bushing with a 3″ wafer disc on a die grinder. This removed the material OK but it was hard to get a good profile. Once I had the bushing to where it would drop into the ID of bearing flange, I felt I was ready for re-assemble, so I re-installed the bushing using the heat/shrink fit process.
A tip when grinding on the Delrin is to wipe it down with a greasy rag first. Then as you remove material, the black turns to white and gives you a good indication of your progress.
The final step was to re-assemble the bushings and bolt them up. As I tried to pull the bolts down, the strut arm still jammed. I inspected it closely and found that the bushing was too long. So I tackled the end of the bushing with the die grinder to make it shorter.
All of this finally did the trick and I was able to get the 4 bolts pulled down and still obtain rotation of the strut arm with reasonable hand force.
After all this effort to fit the spring plate bushings, I wondered if my install was typical. Here is what Karl Potel at Racers Edge had to say. I do appreciate Karl offering his thoughts.
I have done over a dozen of these(MC-3000 & MC-3001) installations personally and I can attest that it is by far the worst job on the car. However it sounds like Harvey has experienced it to an extent I never have. I do make the bushings on the larger side but not to the degree which Harvey assessed. The Porsche factory, in their infinite wisdom, never put much time in those castings,… they are rough castings with no machining because they knew they would be installing rubber bushings so for them, close was good enough. When you try to install a solid bushing the inexactness of the castings presents a significant problem. To overcome this the only way I could was to make the OD of the bushing large enough where it would fill up the torsion tube end carrier bushing location in almost all instances. When I designed these I got about a half dozen junk yard castings and tried to figure out a best size for the bushings,… all the castings I have seen are of different ID to some degree. The width of the assembly has only been different to any large degree in only one application I installed, so roughly 10% has given me that problem as well. The bushings have an approximate 2 degree taper,… I forget the exact numbers but that means (in rough numbers) that if the bushing doesn’t seat by 0.10”, then you have to increase the ID of the housing by approx. 0.004” (or 0.002” off each face of the housing). I have never seen a bushing stick out more than an approximate ¼” and I have had a couple that fall right in,… so the variance between housings is large and problematic. The installation instructions I have prepared and send along with every set of bushings is pretty clear that these have to be fit on a custom basis and I even give recommendations as to how to do it. Over the years I have received about half a dozen phone calls of people who did not fit them and just put them together and tightened the bolts and then went to the track,.. of course the cars were un-drivable because they had no rear suspension movement and they bounced around on their sidewalls. When fit properly the bushings give good performance and long life. I hope Harvey finds the time well worth it when he hits the track.
How did your using Delrin bushings on both the spring plate and trailing arm work? It seems to me that the trailing arm is meant as the horizontal control arm and take higher loads and the spring plate is for vertical location and with the centers being different they run in different arc. There needs to be an allowance for misalignment/deflection somewhere in the system.
Hey Mike. The short answer is I don’t know, or at least I haven’t thought about it. You are right, rubber bushings offer some compliance in multiple planes and Delrin can’t behave in the same way. The cop out answer is I bought them from a reputable guy that apparently knows what he’s doing, since he has some very fast cars. But just thinking about it, you are right the trailing arm (the diagonal member) has to swing through a shorter arc than the spring plate (the part that has the large, conical bushing). I think the spring plates job is pretty straight forward. It swings through the axis of the torsion bar. No need for it to pivot about any other axis. …. I’ve just come back from looking at the car. I think the answer is right under our noses. The “spring plate” is actually very thin in one dimension. It will certainly offer limited compliance about its vertical axis, i.e. it would allow the wheel to pinch in as the suspension droops. Probably why they named it the spring plate!