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Problem:

High speed pulsation, usually above 35mph, occurring shortly after brake job or tire service is performed (usually between 3,000 and 6,000 miles).

Cause:

One or more of the rotors installed with excessive runout. Generic specification being used by domestic OEMs is .002”. Excessive runout causes disc thickness variation (DTV) over time. The average for most vehicles is 3,000 to 6,000 miles. This number can vary up or down based on the variables involved.

Figure 28.1 Jacking

Rust on the hub’s mating surface is a leading cause of runout. The rust can form to a point where it actually pushes the rotor away from the hub even with the wheel bolted on (See Figure 28.1). This process has been termed “jacking”. It works much the same way as a tree root under a side walk. There are tremendous forces involved as the rust “grows” between the rotor and hub.

Solution:

Cleaning of the rotor and hub’s mating surfaces is a critical part of the brake job. The hub to rotor mating surface must be free of rust or runout induced DTV can occur shortly after the brake job. As little as .001” of rust at the outside edge of the hub will result in .002” to .004” of runout.

The method used to clean the hub will depend on the severity of the rust buildup. The hub’s mating surface can be a difficult surface to clean due to the wheel studs. The area between the wheel stud and hub centering flange is the most difficult area to gain access to. Here are the best methods to clean the hub’s mating surface.

Method 1 – Mild rust build up:

1. Use an angle grinder equipped with a scotch brite disc to clean the majority of the surface area. Get as close to the studs as possible and change the disc when needed.

2. Finish the process by using the tool shown in Figure 28.2 to clean the area around the studs. This tool fits over the wheel stud to allow easy cleaning of the hard to reach area of the hub. Figure 28.3 shows the finished result.

Figure 28.2 OTC Tool

Figure 28.3 OTC Tool Hub

Method 2 – Severe rust buildup:

The hub shown in Figure 28.4 will not be able to be cleaned effectively using the steps outlined above. The end result would look something like Figure 28.5. The OTC tool does NOT work on mild to heavy rust. It has a tendency to polish the rust instead of removing it. The most effective method is to use the steps below:

Figure 28.3 Rusty Hub

Figure 28.5 Polished Rust

1. Abrasive blasting is the most effective method for rust removal. There is a specialized blast cabinet available that allows cleaning of the hub while it is still on the vehicle. The tool uses a drawstring boot to allow the unit to be used on the vehicle as shown in Figure 28.6.

Figure 28.6 Rust Buster

2. Figure 28.7 shows the finished product. This is the same hub shown in Figure 28.4. This hub will not be the source of rust induced runout. While there are other methods that can be used to clean rusty hubs the method described above are the most effective. Other methods can be time consuming, yield a lower quality job and may not result in all of the rust being removed.

Figure 28.7 Finished Product

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