Brake Rotors - Part 3

Performance Friction calls them discs. Wilwood calls them rotors. Not even the brake firms can agree on the correct nomenclature. Whatever we call them you’re going to have to replace them at some point.

Brake discs wear out. We use friction to stop our cars and we remove material from our brake discs at every corner. You see the result of all this friction on your wheels when you park in the paddock. You call it brake dust.

There are no rules about how long a brake disc should last. It all depends on how aggressive your brake pads are and your braking technique. Some people wear discs out in a single weekend and others manage to get several weekends out of their discs.

At some point though everyone is going to need a replacement. You’re just going to have to be very careful since we have a huge number of offshore rotors flooding the market. Not all brake discs are created equal.

Size and Shape: The first step is to look back in your records and see what you currently have on your car. Who did you purchase them from? Do they still carry them? If you were happy with them just purchase the same disc from the same company.

If a company can reduce the amount of iron used in manufacturing a brake disc they can make more money. One way to reduce the amount of iron in the disc is to put fewer vanes in the disc, or just make the vanes thinner. The problem is when the mass of the brake disc is reduced so is the disc’s ability to handle heat,

Your brake disc is really a heat sink. Heat is stored in the cast iron as the brake pads create friction. The vanes of the disc then direct air through the casting venting the heat to the atmosphere.

Besides the size of the vanes there can be differences in the specific iron alloy that is used in a brake disc. While (Fe) iron in itself is a base molecule element on the periodic table not all cast iron alloys are the same. The iron alloy used by Wilwood in their Spec 37 rotor series is a different mix than the alloy used in their lighter weight racing and performance rotors.

Lighter weight brake discs can contribute to better handling since they reduce unsprung weight. On the other hand a certain amount of mass is necessary to absorb the heat manufactured from the friction forces. The engineers at Wilwood point out that they’re constantly looking for the optimum point between heat management and unsprung weight.

Slotted or Drilled: Over the years just about everything has been tried. This all started when a gas from the brake pads would form between the brake pad and the disc. That gas would limit the friction. It was called out gassing. It doesn’t happen any more.

Brembo though feels that both drilled and slotted rotors continuously clean and refresh the pad surface, shed water and debris, prevent gas build up, and increase pad bite. Brembo suggests that drilled discs offer improved ventilation and slotted discs offer increased durability.

New Brake Discs: There are a variety of opinions about how to treat your new discs. Everyone agrees that you have to carefully bring new brake discs up to temperature and then let them cool in a systematic fashion. If you do this properly you’ll also create a layer of brake pad material on the disc and the brake pad will make contact with similar material.

If you simply drive out on the track and run hot laps right away the pad material will glaze over. This is especially important if you change brake pad brands. Your new brake pad will be different from the old material on your brake disc. The two different types of pad compounds may not be compatible.

In some cases brake discs are delivered with a thin zinc coating to prevent corrosion. Prior to beginning the bedding procedure, this plating should be removed from the braking surfaces by driving the car slowly (under 30mph) and performing very light brake applications in order to remove the plating without generating heat.  With too much heat this plating can be deposited unevenly on the disc, and impregnated into the pad. Once a visual inspection of the braking surfaces confirms that the plating has been evenly removed you can begin the normal bedding procedure.

There is temperature sensing paint on this rotor

Frozen Discs: They call it cryogenics. It has to do with very low temperatures and how materials behave when they’re subjected to extreme temperatures. The people who do this usually use a computer-controlled process over a 24-hour period where the cryo machine gradually cools the rotors down to -300º F then slowly brings them up to over +300º F. The discs are then allowed to cool to room temperature. The idea is that this will improve the life of the rotor by making subtle changes in the crystal structure. A cryogenically treated brake disc becomes more resilient to the stresses caused by the constant heating and cooling. This is all very difficult to prove in the real world. Almost all of the testimony is anecdotal. Most of what we find is “my rotors lasted twice as long.”

Carl Bush at Wilwood said they did some testing with cyro-trated brake discs a few years back. They found that “the process had no measurable affect on performance or durability.” They came to the conclusion that it “would be less expensive to simply replace the rotors with new parts when needed.”

Carl Bush also points out that not all cast iron it the same. Cast iron made from different mixes will “respond differently to cyro-treatment.” He goes on to point out that almost all of the data that has been presented deals with low temperature braking. At this point Wilwood feels it’s better to work with the metallurgy of the cast iron formulas. In fact the cast iron mix is rather different for racing rotors than it is for lower temperature applications.

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