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Rubber verse Poly Bushes
MICHAEL HAYES EXPLAINS THE DIFFERENCE BETWEEN RUBBER VERSE POLY BUSHES!
Suspension bushes play a large part in a 4WD's suspension flex and ride comfort - they're an important part of tailoring your suspension for your needs, but there's a lot of misinformation out there.
How the bushes are positioned and what material they are made out of and the range of movement they allow before binding up all have significant effects on a bush's life and functionality.
A few issues back I spoke about Superior Engineering Superflex Arms and explained how repositioning or even changing the type of bush can alleviate a lot of the binding issues and have significant advantages for suspension movement and articulation. However when it comes to bush materials and where or what to use, there is not much reliable information out there.
A bush's flexibility comes from its hardness (duro). This is a measure of its compressibility and is done by a Durometer. Traditionally rubber bushes have a lower more linear hardness, where poly bushes are utilised for their greater resistance to oils and abrasion.
Which one is better
is actually dependent on what you intend to do with your 4WD. There are pros and cons to each. Both can be manufactured with ranging durometer. Both serve their purpose well. The main differences are that rubber won't ever squeak where polyurethane will if not greased well, and rubber tends to deteriorate easier than polyurethane.
Traditionally polyurethane has been associated with being a hard material, giving excessive NVH (Noise, vibration, and harshness). While the cost of buying some brands of poly bushes may be significantly more than an OEM rubber bush, if designed correctly for its application can save you money in the long run with a long service life and performance that doesn't degrade over time.
These aftermarket bushing parts
look nice enough on the shelf! They are designed to fit in the same housing as the original rubber part, and are subjected to the same forces, which they can handle, except for when it comes to huge deflections due to load and rotation, which OFTEN they cannot. Rubber is very, very stretchy, poly generally is not, and therefore the bush has to be very carefully designed. OEMs are very good at getting the process right so that the rubber sticks to the metal housing where it should. Aftermarket suppliers, well that's another story!
Most OE rubber bushings in real suspension links act more like a spherical joint at least to the extent that they don't create much resistance when made to rotate or resist forces in other planes or directions. Polyurethane is a poor choice when there is multi-axis rotation at the bushing, whereas compliant rubber bushings accommodate this by their own deformation.
Most aftermarket poly bushings
are pretty much held captive by the outer housing and the chassis brackets. There often isn't much space for the material to "squish" to. A lot of poly bushes aren't vulcanised to the spacer tubes, so when flexed up off-road, the gap between the spacer tube and the poly bush allows dirt, rocks or sand to get in and become an abrasive that will increase wear and reduce life of the bush. An OE style rubber bush is vulcanised on both the inner and outer and will not allow any material to cause wear.
However, poly bushes do have their place. In suspension joints that are subjected to movement in one direction or compression bushes on shock absorbers they can perform far better than the rubber equivalent. Combine these with greaseable pins in a shackle and they can provide an excellent alternative to rubber.
As with any suspension modification
it pays to talk to your suspension expert about the type off driving you want to do, how much wheel travel you're chasing and what will work best for your application, before handing over the credit card and your hard earned cash.