AlexS:
Be careful what "hardness" of Poly that you get. Not sure what the correct term is but somebody here will probably be able to tell us.
Making suspension parts out of the wrong stuff may cure the squeaks and groans, but it could also make whatever you're putting them into ride like a lumber wagon. It may corner like it's on rails, but it could ride very rough. Of course if you're building a racer and not a cruiser then you probably WANT to feel every bump.
Don