Poly Bushing Cold Flow

[iismet]

I purchased my car in Nov of 03 and in early 04 installed Front MM control arms and poly bushings. The car has less than 25K on it and I started tracking it last year. Of the 25K, less than a 1000 miles is track use. After adding some aero it was apparent we did not have enough suspension to run the kind of speeds we were starting to hit so I decided to upgrade springs, dampers, and front bushings.

In Alan Staniforths "Competition Car Suspension" he talks about Poly, Cold Flow, and Bushing Loading. He makes the point that the max loading of the bushing typically occurs at the rear bushing. I believe this has a lot to do with brake torque. Needless to say I was quite interested in the condition of the bushings installed in my car.
Below are pictures of the Prothane poly I installed in 04. I am not sure the resolution is good enough, but I hope you will get the idea. The bushing on the left is out of the rear of one of the control arms. Notice the gap between the bushing and the sleeve. The bushing is wallowed out though it still has a circular form. I contribute this to cold flow from extreme pressure.

Here I have removed the sleeves. The bushing on the right is the oversized one. I was hoping you can see it is no longer a "square" bore. Clearly you can see the difference in inside diameter.

Some measurements -

Sleeves => .878 OD

Square Bushing

Minor Dia => .867
Major => .883 - This one shows the same kind of wear trend

Ruined Bushing

Minor Dia => .896
Major Dia => .974 - WOW!


Even the square bushing exhibits signs of flow. I always kept them lubricated and they never made any kind of sound. From day one, they were night and compared to OEM.

I have no idea if this is track related or not.

Side Note - If I had it to do over, the following NVH things I worried so much about, I would do without hesitation - Solid Steering Shaft, Solid Diff Mounts, Delrin Control Arm Bushings, UHMW Cradle Bushings. None of the NVH has meant anything compared to the benefit from these upgrades.

YMMV

chr

 

[Tractionless1]

So in other words your replacing with Delrin?

 

[iismet]

So in other words your replacing with Delrin?

Yes - we already completed it and a couple of my younger guys ran the car today at PIR. We did a lot of other work, but one of them had never driven the car and was hitting in excess of 130 at the end of the front straight. This is not because of Delrin, but controlling toe and camber is a big deal with respect predictability. Either end of a control arm displaced in any axis is not the way.

Now check this out - I am still running the original OEM bushings in the rear control arms. This will change soon, but the car is very fast and very predictable. The changes in the rear are a welded rear shock tower brace, welded upper IRS brace, Poly in the cradle, and a bump steer kit.

Getting the front to work in such a heavy car is time and money well spent.

Latest rev is 525lb front/ 700 rear on the stock K member with the car dropped to 26 1/4 to top of wells. I would not be afraid to try a 425/600 on the street with Delrin. If it was too mean, I would go back to the 375 setup. The car would be sick for the street, but I don't run straight lines. The car is not usable on the street with its current ride rate, but it sure does corner flat.


chr

 

[SlowSVT]

Urethane has it's place but in an articulating joint isn't one of them.

nice write-up

 

[svtshadow]

Do you have some of sort of procedure on how to change the front arms bushings?