Location Bushings

By Kev Rooney

Urethane bushings are great units, cheap and durable, and good in the correct application. Unfortunately, familiarity breeds contempt and they get used in inappropriate places. They are designed to be used where all bolts lie in the same plane so are ideal for independent front/rear set ups, including shock absorbers in this scenario. They work fine in 4 bar set up, but suffer somewhat as there is a slight diagonal movement when the axle moves up and down. They are not, however, suited to panhard rod application as they have to operate in a different plane to the axle motion thus placing hard loads on them. As the bushes are very stiff the load is transplaced onto the weakest point of the set up, usually the bolt or bracket weld. The same applies to shock absorbers particularly coil overs on the rear. I know of at least 6 cars where the rear shocker brackets have failed or the shockers have sheared. This occurs at the top where they have the top eye welded or bolted on. The problem is that the coil overs are set up with the bolts running lengthways compared to the 4 bar which runs across the chassis. Due to the inherent strength of the urethane the loads become imposed on the weakest part of the shocker as above. Now you’re going to say that if you look under any beam axle production car shocks they are mounted in the same way. Not quite!! Check out the production units and you’ll see large rubber bushes which allow the stresses to be absorbed by flexing. If you turn the shocker round so the bolts run in the same place the 4 bar, you have a much more usable set up. As there is still some movement the preferred bushing medium would be rubber or rose joint in at least one eye.

Coming back to the panhard rod application, upon inspection of O.E.M. units, the same rubber bushings can be seen. Once again rubber or rose joints are preferable in our applications, due to the fore/aft movement during suspension travel. On a triangulated 4 bar rear it is a far better idea to use rubber / rose joints in the top arms due to the binding experienced because the bolts do not run in the same direction.

These misalignment problems will manifest themselves as ovalling of the urethane bushing which will become apparent with use. These should be checked during the winter off season to allow forewarning of an impending problem. Mileage/duty cycle will have a great effect on the speed of failure i.e. you may not spot it on a 1500 mile per year rod for 2 or 3 years, but 6000 in a year would certainly cause worries.

Coming on to non industry standard urethanes, i.e. those supplied for Cortina, Capri etc. front ends, the same problems apply. You may have noticed that many adverts in mags now specify “for race use only”. There is no doubt that they firm up the handling, mainly due to a degree of binding. If using these items in rod/custom applications, one rubber bush per side should be left to allow compliance. Taking a Macpherson strut type front end, my personal preference would be to urethane bush the front (chassis) anti roll bar mounts, and also the inner bushes where they are bolted into the cross member. I would leave the anti roll bar/track control joint as rubber. The Mac strut set up doesn’t just move up and down but also fore and aft as the anti roll bar link moves through an arc, when viewed from the side, pivoting around the front chassis mount. It also pivots in an arc under suspension travel, at the inner tca mount, when viewed from the front. You can then see the need for compliance at the outer tca bush.

Always remember that suspension misalignment will place the loads that should be absorbed by bushings onto the weakest link. The best way to ensure that they are reduced to the minimum, is by correct choice of bushing. Your life could be placed in jeopardy by the wrong choice, take your time and get it right. Kev Rooney

Tech Section