Light front end, steering and seat wobble

SAMMY12

MEMBER
1996 FORD THUNDERBIRD
3.8L
6 CYL
2WD
AUTOMATIC
100,050 MILES

Just had upper control arms replaced as well as sway bar links both sides front. Car has new brakes and appears rotors have been turned. Front left wheel drags when jacked up and binds a little in one or two spots making me think the rotor is possibly warped a little. Guy I bought it from said check the wheel bearing lock nut, so maybe had a wheel bearing replaced and got too tight. Anyway After changing those parts and aligning the front end all seems good until about 65-70 mph when I start to notice a small wobble/shake then upon braking you can feel it get a little worse. I am thinking possibly one of three things but not sure. Wheel bearing too tight and already wearing out, 2.) Warped rotor which would explain the fact that is it more pronounced when braking, or 3.) Wheel balance? I just bought the car from someone else, and of course they told me nothing was wrong with it, but obviously knew it had a shake, noise and was in need of some repairs.

1 Reply

CARADIODOC

MECHANIC
33,737 POSTS

You won't feel an over-tightened wheel bearing. If you have the bolt-on hub and bearing assembly, that requires a very high torque on the spindle nut and it has to be set with a click-type torque wrench. 180 foot pounds is a common value. If you have the older tapered inner and outer bearings, over-tightening those will cause the rollers to squeeze the grease out and not let it stay in there to do its job. That will lead to early bearing failure, but again, you won't feel that.

The intermittent binding you're describing is common and normal, even with newly-machined rotors. If you can still spin the wheel freely by hand, you won't feel any pulsation in the brake pedal.

What is more likely, since you only feel this at higher speeds, is the rotors are warped in a different way. "Thickness variation" is what you'd feel when spinning the wheel by hand, and it would make the brake pedal pulse up and down once per wheel revolution. "Lateral run out" is where the two sides of the rotor are perfectly parallel all the way around, but the center hub is cocked slightly in relation to the braking surface. That makes the rotor move sideways back and forth with each wheel revolution, and that makes the caliper do the same thing. At lower speeds you won't feel that because it doesn't take much effort to cause that movement, but at higher speeds the force of shoving the caliper side-to-side makes it tug on the steering linkage.

You didn't say if you're feeling this in the brake pedal or the steering wheel. The clue to lateral run out is you feel it in the steering wheel. And you may not feel it in the brake pedal. Thickness variation is felt in the brake pedal first and even at low speeds, and it can tug on the steering linkage too.

Even new rotors commonly warp, especially those made in China. There's nothing wrong with their quality, but here, we cast parts from cast iron, then set them aside for 90 days to "age" before they get their final machining. In China, they cast 'em, pack 'em and ship 'em, then they age on your car. A little warping can be expected, and a light machining later will take care of that. If you demand new rotors under warranty, you'll just have the same problem again with those.

Even domestic rotors can warp depending on how long they stand with one spot under the hot pads, if they get splashed with water when they're hot, and things like that.

An often-overlooked cause of a vibration is a warp that is accidentally machined in during routine brake service or improper cleaning. There's three access holes in the front hub when a sealed bolt-on bearing is used. Water splashes through those holes and little round rust spots form on the back side of the rotor. If those spots aren't cleaned off, the rotor will mount on the brake lather crooked and a warp will be machined in.

Related to that, if a rotor is just removed, then reinstalled in a different orientation, those rust spots will get sandwiched in between the rotor and hub and prevent it from sitting squarely on the hub. The rotor will wobble and so will the wheel.

All of these things can be found with a micrometer and a dial indicator. The dial indicator is for looking for lateral run out but it works best with front-wheel-drive cars. On rear-wheel-drive cars you have to spin the front wheels by hand, and that leads to inaccurate readings from pushing the wheel sideways.