Due to the proliferation of urethane suspension bushings in the hot rod market most enthusiasts assume they perform the same function as steel sleeved rubber bushings only better. Not necessarily so.

It's true that urethane bushings will 'tighten up' a suspension due to their higher density and lower deflection, however they are not a universal solution. Urethane bushings are ideally suited to torsional loads where a suspension component rotates around a mounting point. A good example of this is in the ends of coil over shocks or leaf springs, where the bushing's movement is around the mounting bolt or shackle pin.

   • Click here to see conical vs. torsional load on urethane bushings


Contrasting this, when urethane bushings are used on front or rear radius rods, these are subject to twisting, conical loads. With minimal deflection, urethane bushings have limited ability to rotate around the axis of the radius rod to which they're fitted. As a result, the twisting forces get fed into the radius rod - and something has to give.

If you've ever wondered why the lock nuts on a four-bar front suspension with urethane bushings continually try to undo themselves, read on....

When urethane bushings were introduced on four-bar front ends, it was claimed that their function transformed four-bars into a form of anti-roll bar. While true in essence, this can only ever happen if the lock nuts on the rod ends don't loosen. And even if that's achieved, you have to ask yourself if feeding twisting forces into radius rods is a good idea, as repeated loading could induce component failure in the rod ends or where steel bushes are welded to the ends of the radius rods.

   • Click here to see 4-bar front rod ends under load


An anti-roll bar is very different from a four-bar, being a torsion spring that's designed to twist and made from steel with very different qualities. Due to their greater deflection, when steel sleeved rubber bushes are used on radius rods they don't transmit as high twisting loads, evident from the fact that lock nuts don't work themselves loose so often.

   • Click here to see urethane vs. steel sleeved rubber bushing


Nylon provides lower friction than urethane, which tends to stick to metal surfaces in rods ends and steel bushes....

Lesser-known and appreciated, nylon suspension bushings actually do a better job in applications where a urethane bushing is suitable. That's because nylon provides lower friction than urethane, which tends to stick to metal surfaces in rods ends and steel bushes.

Both nylon and urethane bushings should be lubed with grease, however this tends to be squeezed out quickly - and 'graphite' impregnated into some urethane bushings doesn't really function as it's trapped inside the polymer rather than coating the contact surface.

There certainly is a place for low deflection urethane and nylon bushings in applications where torsional loads occur. When it comes to rubber suspension bushings they still serve a worthwhile purpose given they usually allow for greater deflection.

A good compromise is to use urethane bushings on the axle end of front radius rods because they reduce the transmission of road vibration to the chassis compared to a solid clevis. And where radius rods attach to the chassis, steel sleeved rubber bushings will provide more deflection to reduce twisting forces being fed into the radius rods..........read more

 

Read more about: