You're overlooking one important point. A constant wobble can only be caused by something that is rotating. If the tires have been eliminated, that IS most likely to be related to brake parts. The tires can be checked on a road-force balancer. That identifies the result of uneven curing of the sidewalls that is a characteristic of all tires. One section of the sidewall is harder than the rest. That makes the spindle and wheel move up and down a little once per wheel revolution. Those road-force balancers will allow the tire to be reoriented on the wheel. Most wheels also have high and low spots. Turning the tire on the wheel can match the high and low spots to reduce the effects of the uneven sidewalls.
I too used those tire shavers for a couple of years in the late '80s. They automatically found where to grind off the rubber, but they only removed it from the very edge of the tread. That was to weaken the sidewall until it was even all the way around. We had never heard of that before, but in the training class, we were told that was commonly done way back in the '30, or '40s, or '50s. Those have gone out of favor now, thanks to the road-force balancers.
The place to start is by measuring lateral run out on the lips of the front wheels, with a dial indicator. I'd want to see less than about 0.025" to 0.030". If it is excessive, remove that wheel, reinstall the lug nuts to the correct torque, then measure the run out on the outer edge of the rotor, then again on the outer edge of the contact area where the wheel makes contact. If the second measurement shows some run out, the hub is bent. That is solved by replacing the wheel bearing assembly. If there is negligible run out at the center of the rotor, (and on the wheel), but you find some on the outer edge of the rotor, the rotor is warped. "Thickness variation" around the rotor will be felt as a brake pedal pulsation when the brakes are applied. It is more common to find the lateral run out, which basically means the friction surface is turned or tipped relative to the center mounting section. That will not cause the brake pedal to pulsate, but it will cause the brake caliper to be pushed to one side, then the other side, once per wheel revolution. That is easier to feel in the steering wheel at higher speeds because the mass is being shoved around faster.
That lateral run out on the rotor is commonly caused by improper wheel mounting procedures. Years ago we slammed the lug nuts on with powerful air tools. Today that has a real good chance of causing warped rotors and damaged wheel studs. Without looking up the specs for your car, a common torque for front-wheel-drive cars is 95 foot pounds for steel wheels, and 85 foot pounds for aluminum wheels. There are four reasons for using a click-type torque wrench for this. A small person will be able to get the nuts loose to change a flat tire. The wheel will not work loose. The studs and wheel will not be damaged. And finally, the clamping forces will be even and equal all the way around.
When lug nuts are over-tightened, the mating surface between the nut and wheel will be distorted or chewed up. Those mating surfaces must match perfectly and make full contact to insure the nut won't work loose. Over-tightening also peels the threads, then that metal gets caught, often to the point of making it impossible to screw the nut off. Of course it's the person trying to get the wheel off who gets blamed for the damage, but it was caused by the previous person who didn't use a torque wrench.
When the lug nuts are not tightened evenly, the clamping forces are unequal. The heating and cooling cycles the rotor goes through leads to warping. That can be aggravated if a hot rotor is hit with water from a puddle. The solution for that is to machine the rotor as long as it will still be at least the minimum legal thickness. Every shop has a book that lists those specs. They will tell you if the rotor is thick enough to machine, but you must remember that we're not just truing up a little thickness variation. (That could involve as little as.006" being removed). We have to remove enough from one side to make it parallel to the mounting face, and the same amount from the side. Overall we'll be removing more than what we were able to measure. If that puts the final measurement below the published legal minimum thickness, no reputable mechanic will risk his reputation, and a potential lawsuit, on that.
This type of wobble has another common cause if a front brake job was just performed. This affects GM front-wheel-drive cars more than most others, but it can happen on any car. Once you remove the rotor and look at the hub, you'll see one or three access holes. Water sprays up there, and you'll find corresponding round spots of rust on the back side of the rotor. Those rust spots must be scraped off, along with the rust and scale at the outer edge right next to where the edge of the hub makes contact. That is to insure the rotor sits squarely on the brake lathe. If that rust is ignored, the rotor can mount crooked, and a warp will be machined into it.
It is common for do-it-yourselfers to slap a set of new pads on the car with no regard to preparing the caliper or rotor. If the rotor is reinstalled in a different orientation, that rust spot will be caught between the rotor and hub. That will instantly cause the lateral run out. You'll find that when measuring run out at the wheel lip. Even if the rotor is never removed, it is held in place by the wheel and lug nuts. Mounting new tires or replacing brake pads removes the clamping force on the rotor. If some rust or scale breaks free, it can get caught between the rotor and hub. After a lot of frustration, the rotor may finally be replaced out of desperation. If the scale falls out at the same time, it is assumed the rotor was at fault.
There IS a way a worn ball joint can cause a wobble, but it is not common because it would need to be worn so badly, that it finally fell apart first. That is real common on Ford front-wheel-drive cars. We usually find a worn ball joint allows the ball to move up and down in the socket when we pry on the control arm. That can cause a clunking noise over bumps, but these on your car are not "load-carrying", so little wear takes place. It's the sideways movement that is more serious, but less common. That movement allows the wheel to move out of alignment. If you look at the geometry of the steering system on your car, from the front, you'll see the outer tie rod end's ball and socket is higher up from the road surface than is the lower ball joint. The upper strut mount doesn't move, but if the lower ball joint allows the bottom of the wheel to move out 1/2", (exaggerated greatly, for clarity), it will be necessary for the outer tie rod to move out roughly 3/8" to maintain the correct "toe" setting. "Toe" is the direction the wheel is steering when the steering wheel is straight. Toe is set during an alignment. It can not be changed to accommodate a worn and moving ball joint.
To say this a different way, if a worn left ball joint lets the bottom of the wheel move out a little, that wheel will also turn to the left a little. (This varies with different steering system designs on different car models). When the wheel is turned away from the center of the car, the tire tread will walk off to that side, but only until the sidewall can't flex any further, then it will snap back, then start all over again. THAT is what you can feel as a wobble. Most of the time that will just show up as a bad tire wear pattern on the "leading edge" of the tire. To exaggerate for clarity, imagine the wheel turned to the left a little more, ... And a little more, ... And even more, until the wheel is at 90 degrees to the car. Now it's easy to visualize the leading edge is the inner edge of the tread. That is the edge that will wear, but is important to note that this incorrect toe wear always affects both tires equally, and on the same inner or outer edges. (There's a different cause of tire wear that only affects one tire by itself).
To identify toe wear, run your fingertips around the edge of the tread, first on the inner edge, then the outer edge. One way you'll feel high and low spots in the blocks of rubber. The other way your fingers will catch on the sharp high spots. Most of the time this type of wear is "read" by the alignment specialist, then he specifically looks harder to find what caused it. If the worn ball joint wasn't caught during the pre-alignment inspection, it would be discovered when toe can not be set to stay in specs. That is the final pair of adjustments in any alignment. Once set, the steering wheel is jostled a little, then the settings are given a final check. Toe will be found to have changed if there's still a worn part that was overlooked.
You can also get a wobble from a worn inner CV joint housing, but this is not common on GM vehicles. Also, it almost always occurs at lower speeds, up to about 35 mph, and only when under load. This typically occurs when turning out of a parking lot and accelerating up to speed. It can cause a pretty hard steering wheel oscillation, but the clue is it goes away as soon as you stop accelerating.
Hope that gives you some more ideas of things to check.
Tuesday, March 21st, 2017 AT 5:13 PM
