The MGA With An Attitude

At 09:40 AM 4/26/04 -0400, Barry Dowell wrote:
>"My front end wobbles starting at 60 and stopping at 70 and above."

You can start by reading Diagnosing front end shimmy, but I suspect your problem will be in the wheels or tires, not in the suspension or steering parts.

This sounds like a mild case of wheels out of balance, or the tires are not running perfectly true. The front suspension mass and springs have a resonance frequency that matches the tire rotation period at about 60-70 MPH. So if there is something causing a slight wobble at that speed, the resonance effect amplifies the problem so it is felt through the suspension and/or steering at that speed.

>"All bushings have been replaced, front end professionally aligned and wheels balanced (unsure if static or dynamic), shocks not broken or loose. ....
>P.S.   48 spoke rims, probably original."

A very common problem is that a lot of tire shops do not have the proper fixtures to mount a wire wheel in a spin balancing machine. The contact point for the balancing machine has to be the outer (male) conical surface at the outside end of the splined hub. If they use a male cone to fixture on the inside of the hub at that point, the wheel may not run true in the balancing machine. When the wheel then wobbles in the balancing machine, the machine will give a false reading for location of the wheel weights, and the wheel will then be out of balance when mounted properly on the car.

Another thing that can cause a similar sort of jiggle at road speed is a tire running slightly out of plane, even when it may be perfectly balanced, and maybe even perfectly round and concentric. This can be caused by warped wheel that wobbles, or the tire may be badly constructed or defective so the tread may run laterally out of plane as it turns.

Mount the wheel in question on a front hub and support the car so the wheel is free to turn. Place a reference pointer physically near the tire, and give the wheel a spin. Check lateral run out of the edge of the wheel rim first. If the wheel runs true within 1/8 inch all the way around on both sides, this is probably not a problem. If the wheel rim runs more that 1/8 inch out of plane, then you should repair or replace the wheel before you worry too much about the tire(s). Lateral run out as much as 1/4 inch at the edge of the wheel rim will definitely cause this sort of shaking problem at road speed.

Assuming the wheel itself runs true within 1/8 inch, then place the pointer near the tire tread, spin the wheel again, and check for lateral run out of the tire. Tires are not all perfect, even when new. Some tires occasionally have an internal construction problem that may make the tread appear to wobble laterally as the wheel turns. This tracking problem with the tire tread may cause a steering system wobble with each rotation of the wheel that can induce a chassis shake very similar to a wheel out of balance. The shake caused by this tread tracking error may be felt more at the steering wheel, and maybe less in your seat. The only reasonable cure for a crooked tire problem is to replace the tire.

If you can determine which tire or tires are affected in this manner, you might try running the straighter tires on the front of the car and the worse ones on the rear. The solid rear axle seems to be better at damping out that sort of tire wobble, while straighter tires on the front are less likely to send a wobble through to the steering wheel.

Another indication of this type of tire irregularity can be a small shake in the steering wheel that comes and goes at regular intervals of half a minute to one minute as you drive along a normally smooth paved road at normal road speed. This is caused by a slight difference in rotational diameter of the front tires. Over the distance of half to one mile one tire is likely to rotate one more turn that the other tire. As two slightly wobbly tires get together in sync to add their little wobbles together you may feel the effect in the steering wheel. As one tire then gradually rotates one half turn out of position relative to the other tire, the wobble effects of the two tires may cancel each out through the suspension and steering connectors, and you may not notice any wobble at all. Here again, putting the tires with the worst wobble on the rear axle may help. Otherwise the only full cure would be to replace the tires (or wheels if necessary).

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