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Suspension Problem Diagnosis

Suspension Problem Diagnosis

Jan Wikstrom

Noah wrote: Recently someone wrote in that they had bad rear bearings, and this was evident by a tail wiggle when they took their foot off the gas at high speed. I'm having the same problem with my car, and I'm wondering which parts to order. The front suspension also seems very loose. The car is slightly hard to control, etc. I need to discover which areas may be at fault, any ideas?

Back end

There are lots of parts back there, and the same symptoms can be caused by several of them (eg., worn diff output bearings). The best way to diagnose the rear suspension in my opinion is to remove the shock absorber/spring units, then push/pull/tilt/twist in every direction at the hubs and note where the play appears.

A point to remember here is that many of the rear end repair jobs require you to take the rear sub-frame out (eg., tightening the upper diff mounts). Once it's out (grunt job for one, easy for two) it's a golden opportunity to replace the diff output seals, which tend to deteriorate due to rear brake heat.

Front end

To diagnose the front suspension, put the front end up on stands, then remove front wheels and apply jack at bottom ball joint to take spring pressure. With the joints unloaded in this way, push/pull/tilt/twist in every direction at the hubs and note where the play appears (don't forget the wheel bearings).

Poor directional control can also result from bad steering rack mount bushes. To diagnose this, put the front end on ramps and crawl under to observe rack movement while a helper twists the steering wheel strongly back and forth.


Randy Wilson responded to a similar problem at a later date ...

Often this is worn upper c/arm bushings.

To test them, and ball joints, you should really support the car by the lower c/arm such that the wheel close to normal ride height position. The reason for this is the spring operates on the lower arm, and the droop stops are on the upper one. If the wheel is off the ground, the spring is contained via the lower ball joint, the upper joint, then the droop stops with the inner bushings acting as a fulcrum point. You will not be able to overcome the preload induced by the spring.

It is possible and acceptable to test with the car sitting on it's wheels by grabbing the top of the tire and shoving/pulling in/out. This will "test" the upper c/arm components only; not the lower ball joint, as it is still seeing spring load.

For the upper bushings, often a visual test will suffice. If the bushing ends (mushroom heads) are visably offcenter to the arm, or there is rusty dust around the area, then they are bad. These are not rubber bushings as normal. They are rubber encased nylon bearings. The presence of rust dust (fretting residue) means you've gone through the nylon and are chewing into the steel distance tube.


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