Here's a quick explanation about polyurethane as I know of it. I've made my own bushings from a rod of 85A polyurethane so I have some hands on experience here.
Polyurethane is commonly used for automotive applications to replace rubber bushings. Anywhere from 87A to about 93A durometer is an acceptable replacement for rubber when a stronger and tougher bushing is desired. 95A has been tested before and has been found too hard to be applicable, atleast for street use. The durometer scale is used to determine how hard a substance is. To help you understand how hard 90A durometer is think about this: 40A is about the same as a rubberband and by 70A it is as hard as a shoe heal. From about 25A until 91A material is considered a rubber and from 96A until 150 rockwell R is a plastic. There are other scales such as durometer B, C, and D to my knowlede. 96A is equal to 55D and 50 Rockwell R. So thats how the hardness of polyurethane is measured.
So the concept is to replace the rubber with this material that has less flex to it becuase it is harder. This can firm up your suspension and steering considerably if you do all the sway bar bushings and control arms. Another thing to replace is the motor mount bushings.
So are there any bad points to polyurethane? In cold weather or after a while of wear they can develope squeaks. Also, some factory parts might not be strong enough to handle the tougher material well. For example, on Berettas, if you put on polyurethane end links on the front swaybar you could break the swaybar ends off. Mine has those endlinks and nothing has happened yet, but my mechanic warned me about that when I put them on.
Manufacurers: Suspension Restorarion Parts Co.
Making your own bushings is not easy if you don't have the right tools. You need machines common in machine shops, such as a lathe or drill press. It is not cheap at all either. It cost me $71 for a 2 3/4" rod two feet long. The bushings for the rear sway bar were cake. I had the center hole drilled on the lathe. Then I cut that into 4 sections, since I needed 4 bushings. Next I just sawed out the shapes I wanted around the hole I had drilled. I traced a line to follow of corse. The saw was a faster automated kind, I didn't mean a hack saw as you might have though. That worked great, but to give it a nice smooth feel I just took it over to a high speed grinder which to my surprise gave it a very smoooth finish. Cake, just like I said.
My original plan was to make control arm bushings as well, since no one makes them for a Z26 suspension. After much consideration and investigation I decided against making them though. The urethane is simple to make high tolerance pieces out of, but when persicion is key, then it becomes difficult. I still have enough urethane left to make myself a set, but I don't know if I would risk doing it. It is a risk to play with your control arms like that too.
