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Clunk versus somewhat worn
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Sun, 16 Mar 2008 17:01:37 Share Post by Email
In Reply to: Re: Need help diagnosing "throbbing" in front left of 9000, dbreid [Profile/Gallery] , Sat, 15 Mar 2008 18:56:26
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Yes, if the bushing is totally busted, it'll clunk over bumps, etc.

But think about this - the rubber is shot, so you get more compliance (movement) that stock. But not so much it's bouncing around over every bump.

The A-arm carries suspension forces. Something pushes on the front of the tire (slight braking force, for example, or a slight change in road surface). Instead of this force being transmitted to the steering rack where it gets absorbed, some of the motion gets transmitted to the rear A-arm bushing which moves. It shouldn't, but it's worn. This allows the tire to change direction slightly. this change isn't what the suspension was designed for, so the change in suspension geometry causes the wheel to 'steer' back the way it came. This unloads the rear of the A-arm, which then compresses the other way. Again the suspension geometry changes, and the tire changes steering angle.

Put another way - the steering is set up assuming that the bushings are as rigid as specified, so things that aren't supposed to move don't. When the bushings get too loose, the suspension geometry can dynamically change, and that can show up as a vibration in the front end, the rate of the throb depending on the geometry, bushing deflection, and speed. Very complex. And it doesn't require the car to steer left when the wheel is straight - this is the bushings moving more than they should, but not at the 'totally shot clunk' level.

posted by 75.58.11...
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