Run out

Lateral run out is when the hub or rotor's friction surface moves left and right as it rotates. If the hub is bent, it will cause the run out be be seen on the rotor and the wheel. If the rotor is warped, that will not be seen on the wheel, but it may be felt in the brake pedal if it is bad enough. The center of the rotor is sandwiched between the hub and wheel, so you will not see the run out in the wheel.

It is extremely rare to find a bent hub, but to check for that, use a dial indicator while rotating the hub by hand. If that is okay, but you find excessive run out on the rotor's friction surface, check again on the center plate. If you find no run out on the center plate, the rotor is warped and must be machined if it will end up still above the published legal thickness limit, otherwise it must be replaced. If you find run out on the rotor's center plate, but not the hub, there is rust or scale wedged between those two parts. This is actually pretty common on GM products, especially front-wheel-drive cars. There are one or three access holes in the hub to get to the bearing assembly mounting bolts. Water splashes up through those holes and causes round spots of rust to form on the back side of the rotor's center plate. If the rotor is removed, then reinstalled in a different orientation, those rust spots will prevent the rotor from sitting squarely against the hub. Those spots must be scraped off before a rotor is machined. If they are not, the rotor will not sit squarely on the brake lathe, and a warp will be machined in.

The same thing can happen if debris gets stuck behind the wheel. You will find run out on the wheel lip, but not on the rotor's center plate. This often occurs with cast aluminum wheels. Chunks break off from corrosion, then if they are not noticed, can get caught between the wheel and rotor.

The issue is if you can feel it. In the 1990's, one truck model was available new with cast wheels from an aftermarket supplier. A lot of owners complained of a slight steering wheel vibration. That was traced to excessive run out on those wheels. The limit where it was believed you could not feel a vibration was 0.045". Anything over that, we were supposed to replace the wheel under warranty. We could check lateral run out on the truck, but if that was okay, we had to dismount the tire, then check for "radial" run out on the tire balancer. For that, the dial indicator was placed standing up, where the tire's sidewall would be, at the bead seat on both sides. That means the measurement was taken at four places, both wheel lips, in both directions. If any one of those was over 0.045", the wheel got replaced.

Radial run out makes the axle jump up and down, similar to an out-of-balance tire. Both radial and lateral run out can be compensated for on the tire balancer, but because the imbalance and the weight added to offset it are at different distances from the center, those two weights only offset each other equally at the one speed the wheel balancer spun the wheel at. That is about forty five to fifty mph. You may still feel the imbalance at faster or slower speeds. Also, most balancers do not consider that a high spot on the tire forces the axle upward when it comes around and hits the road surface. For that, we have "road force" wheel balancers that spin the tire and wheel very slowly while looking for these irregularities. Some of those problems are solved or reduced by simply spinning the tire on the wheel, then re-balancing it.

If the run out is caused by the hub or rotor, 0.045" at the wheel would equate to around 0.006 to 0.008" just outside the wheel studs, on the rotor's outer surface. I should have mentioned that at least three lug nuts need to be installed to insure the rotor is being held tightly to the hub, unless you have "captive" rotors that require unbolting the wheel bearing to get at the mounting bolts on the back side. Rotors that slip on and off easily have to be held on snugly to get an accurate run out measurement. Vehicles with cast wheels use lug nuts with pretty chrome covers that could limit how far those nuts go on when there is no wheel there. You may need to stack some washers under the nuts, or find standard nuts to use temporarily.

Scrub the rotor's wheel contact area with a wire brush so debris does not make the dial indicator jump around. It is very common to find no run out or as little as 0.001". If the hub is bent or there is debris caught behind the rotor, you are going to find way more than that. It is very rare that you have to guess whether you have too much run out. Measuring this works best when running a front-wheel-drive vehicle in gear, on a hoist. For your car, you must be careful when turning the hub that you do not put pressure on the steering system that turns the spindle left or right. Most dial indicators have a heavy metal base that sits on the ground or on a block. If you have one with a vise grip pliers or a magnetic base, those can be attached to the brake caliper's mounting knuckle, then there will not be any error introduced from turning the steering system.