by Alain Hoffmann
Up to the 50's pivot points on lorries and cars were made of greasable bronze bushings. From then on neoprene (synthetic rubber) was introduced, giving a much nicer ride but had a short life. Since then the quality of those bushings has developed to the point that thez last as long as the car- in street cars. Even medium off roading reduces their life drastically. So a few years ago PU bushings were introduced. They offered a much extended durability at the cost of ride comfort being much harder. So if ride quality was more important then extended life rubber was still the way to go. A couple of years ago manufacturers began to close that gap so you can now buy PU's that are very close to rubber parts but still give the extended lifespan.
Left: One of the best known makers is Polybush
The softness of compounds is measured using a durometer and rated according to a scale (left). The second quality is tensile strenght. An rubber bushing has between 25A and 50A durometer rating and a tensile strenght of 500-1500 psi. Current PU bushings have a tensile strenght equal or above 1500 psi but a 70A-95A durometer rating. New materials improve qroduction all the time so bushings with a 3500-4000psi rating and 73A durometer are now available. Working is done on the same tensile strenght but aiming for 60A and less.
The way bushings are made is vital for their durability. During the mixing process the materials will absorb water. Any moisture trapped inside will form bubbles or pockets and weaken the part. So the best manufacturers mix their products in total vacuum. Generally you can say that a rough looking PU is inferior in quality to a smooth one. Best rule of thumb is to buy from a well known manufacturer who gives a warranty on his parts.
Coil sprung Land Rovers are especially prone to failing bushes as their trailing arms front and rear are located in thick rubber plates. If those wear you get all sort of wandering and bump steer as well as tire wear. This is a slow process so you won't notice it unless you switch over to another Land Rover.
PU bushings for coil sprung vehicles are advertised as having a 3 times longer life if used off road and to last as long as the car if used only in street applications.