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Fact and Friction: What Goes Into Your Brakes


There is no use firing up the engine of your car or truck and going somewhere if you can't stop when you get there. That may lead you to believe that the brakes are more important than the engine--though that was not always the case.

At first, most cars' top speed was less than 10 mph, so stopping usually meant withdrawing engine power. But as speeds increased, brakes became necessary and were patterned after those on horse-drawn wagons.

At the turn of the century, brakes were lined with friction materials such as wood and leather. Though leather did a good job of stopping the vehicle, it wore out quickly. Neither wood nor leather was much good when wet. Sometimes, drivers would look for something to hit in order to stop.

As brakes evolved, leather gave way to camel hair and cotton cloth impregnated with asphalt and rubber, but these linings occasionally caught fire. Then, late in the first decade of the 1900s, something that worked very well came along: asbestos.

Brakes operate by changing kinetic energy (the energy of motion) into heat energy. Touch your brakes after use, and you will be convinced. Heat, however, can wreak havoc with the brake system by wearing out the linings and damaging the iron drums and rotors. If too much heat builds up, it can even boil the brake fluid.

That made asbestos an excellent material for brake linings. It has good friction properties, dissipates heat well, provides good pedal feel and is cheap and durable. But it was blamed for causing lung disease and banned in the mid-'80s.

Though the dust created as brakes wore was not generally considered a health hazard to the motoring public, it was a significant workplace hazard for brake technicians and the people who worked in auto manufacturing plants.

The last couple of decades have seen a lot of changes in brake friction materials. The material that brake manufacturers attach to the brake shoes or disc backing plates is much like a cake batter with a variety of ingredients:

* Friction materials. They provide the needed grip.

* Friction modifiers. With too much friction, the brakes would grab, so materials such as cashew shell oils, walnut shells and graphite are mixed with powdered metals such as zinc, brass and aluminum. In general, these are called semi-metallic brakes.

* Binders. These are the adhesive, the glue, that holds everything together. Phenolic resins are the most common. By the way, the resins are what makes that acrid odor if the brakes are overused and overheated.

If you have ever followed a truck down a long hill, you will never forget the smell.

* Fillers. The substances, such as rubber chips, that manufacturers add for a variety of purposes, including pedal feel and noise reduction.

* Curing agents. They ensure that everything cooks up perfectly in manufacturing.

Most brakes today have semi-metallic linings that offer good braking when cold or hot; resist "fade," or loss of braking; and give long life. Semi-metallics' main ingredient often is powdered iron or steel fibers (like chopped steel wool).

One of the biggest problems with semi-metallics: They are dirty. They leave that dark dust--brake crumbs--all over those nice alloy and chrome wheels, especially on wheels with disc brakes. And semi-mets tend to make noise.

As the quest continues for better brake friction material, the latest development is ceramic linings. Ceramic disc pads, first used as original equipment in the mid- '80s, are becoming more popular; some are available to retrofit vehicles that didn't originally come with them.

Ceramic pads with copper fibers fade even less than semi-mets at high temperatures, cool faster and wear longer. They also are quieter. And they don't create that dark dust that looks so awful on your wheels; the dust is much lighter in color and has less tendency to stick to wheels.


Bob Weber is an ASE-certified master automobile technician, having recertified every five years since 1978. His column, Motormouth, appears in the Chicago Tribune. Contact him at MMTribune

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