Why do you say the alignment is fine? There are three things to look at. What most people notice is if the steering wheel is off-center when driving straight. Fewer people pay attention to whether the car pulls or drifts to one side when you let go of the steering wheel. And the the last thing is improper tire wear. That can take a long time to show up.
In addition to those things, worn steering and suspension parts can make it impossible to keep a wheel in proper alignment. It will shift position from bumps in the road, the stresses of cornering, and the forces from braking. The best thing to do is have the front end inspected at a tire and alignment shop. They will look at ball joints, tie rod ends, struts and shock absorbers. GM products also seem to have more than their share of worn rubber control arm bushings. Another thing that only pertains to GM cars is the front cross member. That's a part of the frame that goes under the engine. It must be unbolted and removed when removing the engine or transmission for repair. Some mechanics are not aware of how critical it is to reinstall that cross member in the exact same location where it was. Shifting it sideways will move the bottoms of both wheels to the side which will cause a pull. The mechanic might try to correct that pull by readjusting the alignment but that just covers up the apparent problem and doesn't address the underlying incorrect suspension geometry. Even though the alignment numbers on the computer say everything is good, a less-understood angle can still cause instability on bumpy roads. That angle is called steering axis inclination, (SAI) and can be measured with all alignment computers. There is no right or wrong value for that angle. All that is important is they are the same on both sides. That cross member is shifted sideways to correct unequal SAI. Think of a person with one leg shorter than the other one so they wear a tall shoe on one foot. The unequal length of their legs represents unequal SAI, and the corrective shoe represents readjusting the alignment to compensate for that difference. They might walk just fine but they will look kind of funny if they try to run. That instability is similar to what unequal SAI causes. By the way, if you have not had any engine or transmission work done, chances are the car just has a few worn out parts. Don't fret about unequal SAI unless major repairs were done to the car recently.
Binding upper strut mounts can also cause the problem you described by not allowing the front wheels to return to center easily. Those mounts hold up the weight of the car but they have bearings inside to allow the wheels to turn left and right. Sometimes when they bind you can hear a banging sound when you turn the steering wheel while standing still, but very often the tightness can not be detected until they are removed during strut replacement. Mechanics and service advisers hate them because that means they have to sell you more parts after they gave you the estimate to replace the front struts. Some shops include the cost of new mounts up front in their estimates so they are covered, then hope they can surprise you with a bill lower than expected if they are not needed.
There is also a strut mounting hole in that upper mounting plate. That hole can rust out allowing the shaft of the strut to move sideways. That shaft is what holds the wheel straight up and down. Here again, it is almost impossible to know that mounting hole is damaged until the struts are replaced. The mounting plate has to be removed to be inspected.
Thursday, November 25th, 2010 AT 5:44 AM