PowerFlex Urethane Bushings, What you need to know.

[victorgallas]

So i am just updating everyone on how does urethane bushings hold on after some years of use and miles.
I wrote in the past a very long response about suspension work and what i had done to my e36 325i 1993. (Look under product reviews).
Back then the car had 125k and original suspension. I replaced most of all of the suspension components and for most of them, everything has been working great apart from the urethane bushings, specifically the subframe ones.
The problem with the urethane bushing is on how poorly they are designed and how instead of providing a stiff setup on my car, it allows for the subframe to move vertically.
Horizontally, meaning forwars and backwards direction on the car, the bushings do what they are designed for, they have no flex, and therefore a much tigh feeling, only when going forwards. The problem with the bushings is that they are not restrained vertically, meaning up and down motion of the subframe. Which is the worst feeling when ever going trough a transition such as cornering, braking or any sutiation on which there is dynamic weight transfer accross the vehiacle. When cornering, the kinematics of the rear suspension, will apply vertical forces on the subframe coming from the suspension arms attached to wheels and then to the subframe, and when a force compresses the spring between the body and the suspension arm, the arm pivot point will force the subframe downwards.
The factory rubber bushings have a steel construction on its outside shell with a lib on the bottom. This contruction makes that the bushing will not move under load on the subframe, the only movement allowed comes from the compliance of the rubber material itself, not from parts that do not fit properly and allow for movement, such as the powerflex bushings.
The powerflex bushing, come new with a two pieve urethane bushing, top and bottom part for the subframe and a stainless steel insert that going in the middle between the bolt and the urethane. When installed on the subframe, this parts does not have any mechanical constrains built into the bushing to prevent the bushing from moving vertically or the subframe itself from moving. When installed, the subframe and bushings where held in place by using the nut and washer that is in place to hold the subframe. But keep in mind that this washer is not flat, it has a small flat section where the nut goes onto the bolt and also the flat area covers only the steel insert mentioned before, and therefore the urethane part, which flexes, has no support, and anytime there are vertical loads, the subframe moves freely up and down, not only generating noise, but also giving terrible drive feel, since the whole rear suspension mounting points are shifting under load and therefore not doing its job properly. Also, urethane bushings, because of the use of a steel insert, will creek and make noises once the provided lubricant, wich is just a grease, dry outs and then there is that nice dry rubber rubbing noise.
My fix consists of machining a couple of spacers and washers that will fit between the bushings and the bolts that will eliminate the gap while providing a mechanical constrain vertically for the whole assembly.
I will be machining these myself using T2024 aluminum, this is an aircraft grade aluminum that is also very resistant to corrosion. (Yes aluminum corrodes... its a white powder)

I will provide pictures of the fix and the parts itself, so you can understand exactly what i mean.
I should be getting the material in soon, and then i will have to do installation.

Any questions, ask me,
Ill keep everyone posted.

Victor

[drz]

Thanks for your thoroughnes, Victor. It is good to know you haven't forgotten about us. It is good to see you stop by

[///M Power]

I've got the same bushings but to be honest didn't feel any sort of weakness in vertical motion. It might be the fact I dont drive my car hard anymore but simply I havent felt your weakness. I enjoyed the stiffness of these bushings and they provide quite a proper feel.

Keep us updated

[94BMW325i]

I wish I at least went ot OEM m3 bushings, oh well..

Great little writeup, havent seen you post in a while you are really helpful in the e36 section.

[victorgallas]

So i just wanted to update everyone that i have been looking into the problem. I have received my new subframe bolts, i will then model them into SolidWorks, and design the spacers. I will post pictures of the models and all. I havent have time to work on the car, since my final are approaching, so as soon as i am free, i will be able to work on it.

Also if you like formula cars, i am strong member of the Clemson Formula SAE team. Clemson FSAE - Clemson Formula SAE
We design and build a race car to race with other schools, its a lot of fun and i learn so much. My main area of work is all of the electrical system, design and construction and some of the engine tuning..
anyhow ill keep you guys posted.

see ya