The term “hub cleaning” is a bit of a misnomer in that we are not removing grease or dirt from the hub. Instead, we are descaling it or removing rust and particle buildup that has become embedded, or through corrosion become part of the hub face. Before entering a discussion of this subject you must clearly understand why this area is of concern. As many brake trainers and brake companies will tell you, the No. 1 cause of brake comebacks is pedal pulsation. Tied for No. 1 or right up there with the top two or three cause is brake noise, something that will not be addressed in this article.

Pedal pulsation is a situation in which when the brake pedal is applied, an up-and-down movement is felt and possibly accompanied by shaking of the steering wheel. In other words, the brake pedal itself is “pulsating.”  Although this is an acknowledged fact and something that is perceived by many, the true cause is not known or understood by consumers. Simply stating that replacement of the rotor with a “cheap import” rotor is not the real reason for pedal pulsation.

Let’s get back to the basics. First, as shown in Figure 1, the rotor has a shiny spot on it. This shiny spot or area is there because the rotor is not running parallel or at a right angle to the spindle centerline. When the rotor turns, if the brakes are lightly applied or the pads are retracted from previous application, this high spot or area of the rotor that is not running parallel contacts the pad, causing the wear pattern shown. What causes this type of wear? This is a situation of the rotor, because it’s not running parallel throughout its whole 360° of travel, contacting the pads sooner than the other areas. This means that with light braking there is more pressure put on this area and when the pads retract there is a longer wear surface on this area. When you apply the brakes when such a pattern is present, the pads engage the rotor over most of the surface, but when they hit this high spot you get some kickback to the brake pedal.

Other situations can also develop that cause pedal pulsation, but let’s look at what is commonly stated. Use a micrometer to measure the rotor in six, eight or 10 positions to see if there is any thickness variation. This is a valid check and there are specifications for it, but thickness variation in itself may not cause pedal pulsation. What causes thickness variation? Thickness variation can be caused by the manufacturing of the rotor, the turning of the rotor or a situation in which a repeated area of wear like that shown in the first photo occurs over a wide range of the rotor. You may have very slight thickness variation or major thickness variation. I have personally seen as much as 0.0020 inches of thickness variation on some rotors. Note that thickness variation can also be caused by a buildup of transfer layer on the rotor so don’t automatically assume that it’s a hub-related problem.

Let’s look at the root cause. Just as when you go to the doctor’s office, the first thing they do, other than ask how you are today, is take your weight and blood pressure. Rotors and hubs must also have their “weight” and “blood pressure” taken to establish their usability in the overall mechanical repair operation of the vehicle. If a hub has runout, the rotor will also have runout because of the influence of the hat and the accompanying disc friction material. This runout can cause a situation that can lead to what is commonly referred to as rotor warpage. Continued use may cause thickness variation, but not always. When you apply the brakes when a rotor disc has runout you’ll feel that amount of runout kicking back through the brake pedal. So, let’s look at what causes this runout.

You can have a situation where there is no rust and scale between the hub face and the rotor hat or any corrosion or scale buildup in the inside of the rotor hat, yet have rotor runout. This can happen because the hub face has rotor runout. Measuring rotor runout on a hub face is extremely difficult, as you are trying to put a dial indicator into the very narrow area between the outside of the studs and the edge of the hub face. This is extremely difficult to set up for, and because of the tapered hole through which the stud has been pressed, it is difficult to measure. Measuring on the inside of the stud is even more difficult. The fact is that if a hub has runout beyond specifications it’s bad and will create a brake-pedal-pulsation problem. How do you fix it? You install one of the aftermarket shims between the rotor and the hub. This requires precision dial indicating to determine. It must then be correctly positioned. Or you replace the hub-and-bearing assembly.

If your hub face has rust and scale on it, the most common problem, you can eliminate any possibility of a good rotor having runout by removing that rust and scale. It’s easier said than done. Many of the hub face cleaning tools such as the one shown in Figure 2 work fairly well, but they don’t do as good a job as possible cleaning the area between the circular cleaning area and the lip on the hub face and they don’t do that good of a job cleaning the outside edge between the studs. In other words, the tool works. It’s just not ideal.

In past years a hub-cleaning bead-blaster-type tool was available in the marketplace. It has recently been redesigned and improved. Shown in Figure 3, this tool has several advantages over other methods. First, it has a tent-type item that fits over the hub face. It fits over the hub and knuckle assembly and captures any blasting dust. Second, by blasting, it removes only the rust and scale and doesn’t remove metal. It does an excellent job of cleaning all surfaces. It doesn’t really take any longer than using the conventional over-the-stud-type tool and does a much better job. After using this tool it’s just a matter of taking some low-pressure air and blowing off the system to remove any remaining grit. If you’re concerned about the wheel studs having any grit on them, clean them with air pressure or wash them down with brake cleaner, as tests have proved that it does not damage the threads in any way.

What do you do when you get a brake job in? You should figure in as a professional technician that hubs must be cleaned, no matter how good they look. If rotors are being replaced you don’t want to put in induced air by leaving any amount of scale built up on the hub. Build about 10 minutes of cleaning per wheel into your overall brake process and use the best cleaning method you have. Caliper brackets must also be cleaned. This is the basis of an entirely different article.

After cleaning the hub, install your slip-on rotor and use proper adapters. Tighten down the lug nuts to torque. Set up a dial indicator and while rotating the rotor see if there is any runout. If there is runout beyond specifications, you can remove the rotor, reposition it and re-measure. If you still have runout you either have problems because of the rotor or the hub and you may find that if you can quickly and easily set up some type of hub measurement, it’s the smart thing to do before replacing rotors. Again, time must be built in for this to be part of the total job.

Think of the amount of time you’ve spent on comebacks for pulsating brake pedals or for rotors that have been replaced as “defective” because of pulsating brake pedals. Correctly and completely diagnosing brakes and understanding how to service them is every bit as complicated and important as for any other part of the vehicle. You may think that fuel injection and fuel pressure is difficult, but at least you have something there that you can measure and diagnostic trees and charts to follow. Try finding an intelligent diagnostic chart and testing procedures for rotor runout that applies in the real world. Very few exist.
In the future Undercar Digest will be diving into the area of rotor transfer layer buildup, rotor runout, thickness variation, hub-face scale buildup and other related issues that are causing this No. 1 or No. 2 comeback area of brake service. Stay tuned for more information.