This one has to do with material properties: When looking at materials to use for suspension bushings, what properties would one look at to determine how stiff a bushing would be. This is for the purpose of comparison between materials, I don't need to calculate actual deflection at a certain load or anything fancy. I'm just trying to figure out what empirical data there is on how stiff different bushing materials are.

This isn't entirely academic, I do have a reason for asking. A discussion came up recently on another board about some autocross classing rules: basically, metal bushings were specifically disallowed, but anything non-metal (delrin for example) is allowed. What wasn't clear was if, with the current crop of plastics, this rule made sense anymore. So, I wanted to do a little research about how 'hard' different materials were, and see if aluminum bushings were still a significant step up. Mind you, spherical bearings aren't an issue. They are illegal for different reasons. This is all just about replacing rubber bushings like subframe bushings, where compliance of the material is really the only thing allowing motion.

Anyway, I found some decent info out on www.matweb.com, and managed to get a Dupont spec sheet for a particular type of Delrin, but I have no idea how to compare them. Hardness seemed to be the most usefull property to compare, but plastics and metals seem to be measured using different, and un-comparable test methods. There are some things like tensile and fexural strength that could be compared, but I don't know that those really represent how hard a bushing is. That seems to be more about how much force it takes to break them.

Basically, I am not a materials engineer, and have no clue what numbers I should be looking to compare to get what I want to know. If any of you mechanical types could point me in the right direction, I'd appreciate it. Comments like, "Aluminum is still alot better", are welcome if they come from experience, but I'd like something a little more concrete if possible.

Thanks for the help,

-Bret

I think one of the biggest differences is that spherical bushings and rod ends allow a solid mount while still allowing rotation in multiple planes. Generally, a "solid" plastic bushing is only going to allow rotation in a single plane. (Edit: Just reread your post and it seems like you've got this part...my bad).

On an E36, this isn't necessarily a big deal for something like the front control arm bushing, but would be a big deal for something like the rear upper and lower control arms (if I remember the movement of those arms correctly).

When dealing with subframe bushings and other bushings that don't move at all (diff, etc), there's probably not a lot of difference between a good hard plastic and an aluminum machined bushing. I know race teams who have used both and didn't really have an opinion on which was best. If anyone has a quick link to a comparison of the physical properties of various plastics and aluminum, it would be interesting to see.

This is what I've been looking at myself:

Delrin (One form of it anyway) (http://www.matweb.com/search/SpecificMaterial.asp?bassnum=P1SM04)

Aluminum (6061 T4): (http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061T4)

-Bret

Since the deformation is limited to the elastic portion of the stress-strain behavior, i.e., there is no permanent deformation, I think the property you want to compare is the elastic modulus. There are ways to translate between the various hardness scales, but I don't think this will be of much help for this type of loading.

a common bushing stiffness is measured by a durometer. The number associated with that is the "Hardness, Shore D" on matweb.

some friends have started a small business making bushings and go through a few different durometers before finding the best one for the application.

The unfortunate part about hardness is that the tests used on palstic and rubber bushings seem to be different than those used for metals, and there is no way to directly convert between different tests. The closest thing I found was a chart that compared the general ranges of several harndess tests. Not very scientific though.

-Bret

check your PM. Delrin is a very poor choice compared to others.

You are right, THIS (http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PSPEC02) Sounds much stronger :) Now, I just have to figure out how to make bushings out of it!

-Bret