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4/23/2012 8:00:00 AM Write a Letter to the Editor Write a letter to the Editor

Opinion: New Stopping Distances, New Technology

The performance issue isn’t really with the air disc itself; it’s when the air disc is mated with the old reliable North American air-actuated drum brake.

We operate in an extremely competitive market that drives us to expect a return, which means, frankly, that the low-cost drum brake is going to be around for a while. When there is a mix of air discs and air drum brakes, there is the definite potential for a compatibility problem — not just from the brake designs but also the friction materials they typically use. As it heats up, the drum brake “fades,” or loses torque, and the air disc brake doesn’t.

Drum brake “fade” happens for two reasons — the drum expands as it heats, actually moving away from the brake lining and making the stroke of the air chamber longer and therefore less effective. Also, the type of lining used on drum brakes usually has a resin base that will start burning off at the surface when the brake gets too hot, around 600 degrees Fahrenheit. As a result, the drum brake essentially will try to push part of the braking workload off onto other brakes on the vehicle.

The difference with the disc brake is that as it heats up, it maintains its brake torque output for the exact opposite of the reasons a drum brake will “fade.” First, as the disc rotor heats up, it actually moves toward the friction material, making the air chamber no less efficient. Second, the lining or friction material pad usually is made primarily of metallic particles that will continue to produce friction to the melting point of the metals, around 1,500 degrees Fahrenheit.

When mated with a drum brake, the air disc will naturally accept the workload the drum brake is pushing onto it, and, in a worse-case situation, prematurely wear out the pads and/or crack the rotor.

All this is, of course, amplified if the drum brake is poorly maintained — and especially if it has inadequately performing aftermarket linings.

We’re still facing some challenges, clearly, but brakes are as critical as they’ve ever been. Selecting and maintaining the brake systems correctly is the key both to safe operation and to the lowest possible operating costs.

TMD Friction of North America makes brake friction materials for the automotive and commercial vehicle industry. It has offices and manufacturing facilities in Troy, Mich., and Queretaro, Mexico.

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