20th Annual Technical Forum: Brake Friction

20th Annual Technical Forum: Brake Friction

Q: Can you mix different kinds of friction materials on the same wheel?

 A. Yes. Some original equipment and aftermarket brake suppliers are now using different types of friction materials for the inner and outer brake pads on the same wheel. By using different friction materials for the inner and outer brake pads, they can take advantage of the different performance characteristics of different friction materials to improve overall brake performance and customer satisfaction.

Semi-metallic friction materials, for example, can handle high braking temperatures better than nonasbestos organic (NAO) and ceramic compounds and have good fade resistance. But semi-metallic pads may require more brake pressure when the brakes are cold, and increase rotor wear and noise due to their harder consistency.

Ceramic friction materials are typically quieter than semi-metallic linings, resist wear better than most NAO friction materials and produce little visible brake dust. Many ceramic compounds are also rotor friendly and reduce rotor wear. But ceramics can’t handle as much heat as semi-metallics can under hard braking conditions.

Back in the 1970s, GM had some passenger car disc brake applications that used semi-metallic pads on the inside and asbestos pads on the outside. When asbestos was phased out and nonasbestos friction materials became commonplace, this practice was discontinued — only to reappear again recently as a way of optimizing brake performance by combining different friction materials.

Ceramic friction materials have been used as original equipment on a majority of cars for many years, but recently GM and some other car makers have gone back to harder semi-metallic linings on some applications to extend pad life and reduce stopping distances. The trade-off has been reduced rotor life in some cases.

One way around this has been to mix different friction materials on the same wheel, which is what some OEM and aftermarket brake suppliers are now doing.

A semi-metallic pad or friction material with a slightly higher coefficient of friction may be used on the inside of the rotor with a softer and less aggressive ceramic or non-asbestos organic pad on the outside.

Under no circumstances should different types of friction materials be used on opposite sides of a vehicle (same axle). Any significant difference in friction coefficients side-to-side (right to left or vice versa) can cause uneven braking and brake pull. Since brake pads are normally replaced in matched axle sets (both front wheels, both rear wheels or all four wheels), this is usually not a concern.

But it can be if a customer is replacing only the brake pads on one wheel because the pads were contaminated by brake fluid from a leaky caliper. The same issue can arise if a customer is replacing a leaky or frozen caliper with a loaded caliper that comes preassembled with new pads. The new pads in the loaded caliper should match the friction characteristics of the old pads on the opposite wheel, otherwise the pads on both sides need to be changed to keep braking effort equal

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