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17th Annual Technical Forum: Brake Friction


8/10/2009
By Larry Carley

Counterman magazine presents 15 technical and sales topics in an easy-to-read question-and-answer format for the magazine's annual Technical Forum. This article appeared in the August 2009 issue.
 
Q. When should brake pads be replaced?
A. The time to replace brake pads is when they are worn down to minimum thickness specifications, which for many applications is when the remaining pad thickness is about 1/8th inch or less. But it may also be necessary to replace pads sooner if the friction material is cracked, flaking or separating from the pad backing plate, or if the pad has been contaminated with brake fluid or grease, or if the pads are noisy, or if the pads need to be upgraded to something better because the OEM pads are not delivering the kind of brake performance that satisfies the vehicle owner.

There is no specified mileage interval for inspecting or replacing brake pads because pad life depends on the type of driving that’s done, how often and how hard the brakes are applied, the size and weight of the vehicle and the type of friction material in the pads.

Stop-and-go city driving, aggressive driving and riding the brakes accelerate pad wear. Larger, heavier vehicles such as full-size SUVs tend to wear down their brake pads much faster than smaller, lighter vehicles.
Every time the brakes are applied, the pads rub against the rotors and wear a little. After 40,000 to 60,000 miles, the front disc brake pads on many vehicles may be worn down to minimum thickness specifications and need to be replaced. Rear disc brake pads, or the shoes inside rear brake drums, typically last about twice as long as the front brakes because they handle less of the braking effort.

In replacing friction, Ken Selinger, director of friction products for Federal-Mogul, recommends following the OEM manufacturers’ lead by choosing “like for like” friction materials. “Often, choosing friction materials that are different from what was OE can lead to perceptible changes in vehicle braking performance,” he said. “Simply put, look for reputable brands with a long track record of success because they model themselves after the OEM friction specifications. Ultimately, this approach helps to greatly simplify the selection process and delivers credibility, success and desirable repeat business.”

Q. Why do some replacement brake pads cost more than others?
A. There are a variety of reasons for price differences in the marketplace. Most “premium” grade pads incorporate features as chamfers, slots and built-in shims to control noise and vibrations. These features increase manufacturing costs.

Chamfers and slots change the loading on the surface of the pads and the frequency at which they vibrate to reduce noise. Some manufacturers also use a “layered” construction when they mold their pads with different friction materials sandwiched together to control noise and performance. Special surface coatings may be used to improve pad seating and reduce noise. Some premium pads may are be “preburnished” with additional heat treatments at the factory to eliminate many of the problems that can occur if new pads are not broken in correctly.
Another reason for price differences is that the friction materials used in premium pads are usually superior to those used in value priced “economy” pads or “standard” grade pads, even when the different product lines come from the same manufacturer.

Premium pads are generally engineered to perform better in almost every way, including longevity, stopping power, fade resistance, pedal feel and noise control. Product pricing can also vary depending on how a pad supplier positions their brand in the marketplace.














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