Occasionally opportunities present themselves unexpectedly. As I was preparing to start this month’s Technically Speaking article, I received a phone call asking if I would be interested in some input or assistance from a acknowledge industry expert in the brake field. I’m never one to turn down an opportunity to listen to another’s point of view, especially when it’s an experienced professional opinion. So, I put these questions to Bob Peters, chief engineer, friction material engineering of Akebono Brake Corporation. Here are the results:

Enjoy this professional perspective to the questions and hopefully your knowledge base of what to do regarding rotor washing is expanded.

Q. If a manufacturer says its rotors can be used right out of the box, does that mean that the coating used does not cause any change in coefficient of friction with new pads compared to a non-coated rotor?

A. Typically rotors have a zinc-based coating (which appears somewhere between dull gray and silver in color) or are packed with a rust preventive oil. When a rotor is labelled as “ready to use” that implies that has a zinc-rich coating has been applied as opposed to an oil. These coatings are designed such that they are easily worn away during the first couple of normal (no high-speed or heavy-decels are required) brake applications.

Of course, if the rotor is smudged with grease during the install, the grease should be removed with brake cleaner and wiped with a lint free cloth. Cleanliness is very important. Friction output during the first few stops should not be greatly affected by the removal of the coating, but if there is any concern, a few normal stops is all it takes to remove it.

Q. If the supplier of the rotor doesn’t say anything about being used right out of the box without washing, is washing recommended and why?

A. Of course, follow the instructions if there are any, but cleaning a rotor with brake cleaner on the wear surfaces is a good practice if the instructions are vague or non-existent. Again, wipe dry with a lint-free cloth. Brake cleaner is a very effective solvent – we haven’t seen any oil-based rust preventive that brake cleaner does not dissolve.

Q. If a rotor is washed, what substance can or should be used? At one time a company in Florida sold a special soap/substance to wash rotors with that stated that the rotor would not surface rust using that product. Does soap or other products cause or allow surface rust to easily/quickly happen after washing?

A. In our shop, we use brake cleaner almost exclusively. The use of water, especially cleaners that use a mild acid or base (a soap) are apt to accelerate corrosion of the rotor. That is why we do not use water-based cleaners.

Q. Obviously some rotors are coated with an aluminum-type of coating, which should not be washed. What happens to the rotor finish and initial braking ability if someone uses a solvent-based product in an attempt to clean off such coating?

A. Brake cleaner does not dissolve this type of coating, or at least not very quickly, so the use of brake cleaner does not normally cause any issue. Our technicians will normally squirt some brake cleaner on both inner and outer surfaces of a coated rotor prior to running a test to make sure that any grease, brake fluid, or oil is removed from the rotor. We find that it’s the cleanliness of the rotor that makes the biggest difference to output immediately following a pad and rotor swap.

Q. If a rotor is coated with any one of several anti-rust coatings that are petroleum-based products, what is the best product to clean it properly? Does it then require a soap-and-water final wash?

A. I may sound like a broken record here, but our shop uses an aerosol brake cleaner to remove any grease, brake fluid or oil that has found its way onto the braking surface of the rotor. Brake pads normally need to wear in slightly in order to stabilize in output, which takes approximately 100 stops or so. That may sound like a lot of time to wait; however, the friction level doesn’t change drastically, so normally the driver doesn’t notice the difference.

Initial rotor surface finish – the measure of roughness – can also affect initial output of the brake. Remember, the rotor surface is as important as the brake pad in developing friction. A rotor that is very rough will typically generate lower output than a smoother rotor.