1. What are Poly Bushings?
Poly is short for Polyurethane, which is simply a specific type of polymer. Polyurethane is a blend of polymers that can be made in many degrees of density, generally referred to as the 'durometer' of the bushing. "Higher durometer" simply means "harder" in layman's terms. It so follows that higher durometer bushings are less prone to “compliance”.
2. What is Bushing “Compliance” or “Deflection”?
In short, compliance is a bushing’s tendency to allow movement due to the force applied to it. Thus it is complying with the force and direction exerted upon it. The bushing can also distort from the twisting motion of normal suspension movement. Generally, compliance and deflection are bad because they do not allow your cars suspension system to work positively 100% of the time. Poly bushings are designed to hold their shape, resisting compliance under load much better than the OEM rubber parts. This reduced bushing deflection helps maintain a more precise suspension geometry resulting in improved handling.
3. What is Wrong With Stock/Rubber Bushings?
The suspension bushings on most cars are a midrange density rubber compressed between two metal sleeves with many tons of pressure. Despite this compression the rubber still (intentionally) moves under force from the suspension system. This can lead to sloppy handling, numb feedback and so fourth. In some bushings, this movement and “cushion” is designed in to the bushings for different NVH issues that the manufacturer was trying to solve.
4. Where Can I Get Poly Suspension Bushings For My 300ZX (Z31)?
Motorsport Auto is highly recommended (Link). They are not only the leader in Z car parts and accessories, but they actually had the push to get the ‘Prothane’ (Link) bushing kit made for the Z31. Yes, that’s right. They actually gave ‘Prothane’ a Z31 for about a year so they could design bushings for it in addition to all the other Z31’s (some of which have slightly different anti-roll-bars and tension rods). An alternate source that comes highly recommended as well is The Specialty Shop (Link). Currently only the ‘Prothane’ bushing kit is made for the Z31, for 87-89 cars graphite impregnated polyurethane tension rod bushings are available.
5. What Are The Kinds of Poly Suspension Bushings?
Along with the density of the polyurethane used, there are different colors (red, black, blue, green, etc are all plausible) and different mixtures. Graphite impregnated polyurethane bushings are just what they say they are. This name is intuitive; the bushing actually has little bits of graphite inside of it. The graphite mixture inside the bushing is supposed to be self-lubricating and prevent the bushing from squeaking as the suspension moves through its range of motion. This helps because the polyurethane in the bushings actually does not 'twist' like rubber bushings do; rather, they actually pivot on an inner sleeve lubricated with a special grease.
6. What Can I Expect To Gain From Installing Poly Bushings In My Car?
If your rubber bushings are worn out, it will be a massive improvement. In place of rubber bushings that are still good the difference is almost as massive. You should expect much better feedback from the suspension (better ‘feel’ as people call it) as well as the partial elimination of rear toe out during cornering (from stock bushing compliance on a semi-trailing arm design) which causes over-steer (where the rear end of the car ‘comes around’). From my personal experience these differences are most notable on sticky tires, the car becomes much different overall. Turn-ins are much more crisp and decisive, it is also more stable in high-speed cornering.
7. How Do I Install These Bushings In My Car?
They are a Do If Yourself job, but you could call around and find a shop that will install them for you if you are that unmotivated to do it yourself. Keep in mind it is quite labor intensive, so don’t expect it to be any reasonable price. I’d guess $500 easy.