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Automobile Brakes
brakes basics BRAKES BASICS

The brakes on today's cars and trucks have come a long ways from those that brought the vehicles of a generation ago to a halt. The basics of hydraulics haven't changed, but friction materials have as have many of the components in today's brake system. Composite rotors have replaced one-piece cast iron rotors, new master cylinder and caliper designs have been introduced, and electronic antilock brakes and traction control are standard equipment or optional on most new cars and trucks.

To better understand the brakes on today's vehicles, we need to consider how brake systems evolved. Prior to the 1920s, all brake systems were mechanically operated rather than hydraulic. Rods or cabled connected to the brake pedal were used to tighten band brakes around the flywheel, driveshaft or a wheel drum, or to rotate a cam or wedge that pushed a set of brake shoes outward against the inside of a drum. So on most vehicles, only the rear drive wheels had brakes. The few vehicles that did have front brakes used a hollow flexible steel "Borden" cable to operate the brake drums. Four wheel braking was better than two wheel braking, but required a complex arrangement of equalizer yokes and pulleys to even out the braking forces so the rear wheels wouldn't lock up and skid when the brakes were applied.

One of the major problems that had to be solved with early brake systems was finding a friction material that would provide adequate friction (but not too much friction), would wear well brake caliperand also be capable of withstanding high temperatures without charring. Brake linings made of cotton webbing were tried but didn't work very well because they burned easily if overheated. Inventors tried various materials ranging from walrus hide to concoctions made of camel hair and ground up walnut shells.

In 1908, the friction material that was to become the basis for all future brake linings was introduced by Herbert Frood, asbestos. The first asbestos linings were made of cloth woven from asbestos yarn and reinforced with brass wire. The material was impregnated with a resin to add strength and increase its frictional properties. Four years later, Frood made brake linings that were pressed into shape by dies. This improved the wear characteristics of the linings and reduced the amount of time needed for break-in. And in 1921, he introduced the first molded brake linings made of chopped asbestos -- a change which significantly lowered manufacturing costs. The types of materials that are used in brake linings today are covered in the section on brake linings.

At the same time the friction material problem was being addressed, automotive engineers were trying to perfect the brake system itself. The first drum brakes with linings located inside the drum that expanded outward rather than rubbing on the outside of the drum made their appearance on the 1902 Renault. The shoes were forced outward by a mechanical cam connected to a lever and cable. The internal drum brake was found to require less braking effort than external drum band brakes because the shoes has a "self-energizing" effect that tended to pull the shoes tighter and increase friction.

These early drum brakes were made of sheet metal rather than cast iron. Though lightweight they proved to be noisy and easily scored. They also couldn't take a lot of heat, which caused them to warp and bellmouth. Brake drums made of cast iron proved to be much better. Cast iron provided a better friction surface which was more resistant to wear and scoring, and it was a stronger material which reduced the problems of distortion and bellmouthing. The added thickness and rigidity of the casting also helped it dampen noise as well as absorb and dissipate more heat. Buick introduced aluminum drums with cast iron liners in 1957. Aluminum is lighter weight and cools better than cast iron. But aluminum's high cost has limited aluminum drum applications.

The invention of hydraulic brakes in 1918 by Malcom Loughead proved to be a real breakthrough because hydraulics allowed the brakes to be applied by fluid pressure. Because fluids are incompressible, force applied by a piston to a fluid will be transmitted equally to pistons located at each wheel brake. This eliminated the rods, levers and cables that were previously needed to work the brakes, which greatly simplified the job of providing balanced four-wheel braking. Hydraulics also reduced the amount of pedal effort required to stop the vehicle, which made for easier braking and safer driving. In 1920, Duesenberg became the first production vehicle to offer hydraulic brakes. Chrysler was the next in 1924, and soon all the vehicle manufacturers offered hydraulic brakes.

The next big innovation in brakes came with the addition of power-assisted braking. Cadillac was the first to offer vacuum assisted power brakes in 1930. The original "Master-Vac" power brake booster that became the predecessor to today's vacuum boosters was patented in the 1950s by Bendix. A popular option in the 1960s, it became standard equipment on most vehicles by the mid-1970s partly because everybody wanted it and partly because disc brakes were becoming more common. Today power brakes are standard equipment on nearly all cars and light trucks.

disk brake pads Good as drum brakes were, discs proved to be even better. As vehicles became heavier and more powerful, tire and wheel sizes were downsized. This left less room and air cooling for the brake drums. Another limitation of the drum brake design was that it tended to trap water when driven through puddles. The trapped water would then act like a lubricant and prevent the brakes from stopping the vehicle.

Using a disc rather than a drum offered several advantages. One was that it wouldn't trap water, making it a safer brake for wet weather driving. Another was that it could safely handle higher temperatures without fading. The flat faces of the rotor presented a more open surface for air cooling, and the addition of fins between the rotor faces (vented rotor) increased its ability to absorb and dissipate heat even more. A third advantage was that disc brakes could be made more compact to save weight and space. And disc brakes were self-adjusting which eliminated the need for self-adjusters or periodic adjustment. On the down side, disc brakes were more expensive to manufacture because of the added cost of the caliper assembly. Adding a parking brake to a disc brake was also more complicated because it required either a locking caliper or a "mini-drum" inside the rotor. And because there was no self-energizing effect when the brakes were applied, disc brakes required more pedal effort (which meant power brakes were usually required). Yet in spite of these drawbacks, disc brakes eventually replaced drums up front because their performance advantages outweighed their disadvantages.

The first disc brake for an automotive application was invented by Elmer Sperry for an electric car in 1898. The car had disc brakes on the front wheels, and used an electromagnet to press a friction pad against one side of the disc. The first mass production application of a disc brake system that was similar to what we use today was on the 1949 Crosley. In 1956, Triumph and Jaguar offered disc brakes as standard equipment, and by the early 1960s disc brakes were used on many European cars. But until the early 1970s, disc brakes weren't used much on domestic cars (Corvette being one exception) because of their higher cost.

When disc brakes were adopted, most vehicles only used them up front where they provided the most good. Using drums in the rear saved cost and allowed the use of a simpler (less expensive) parking brake. Even today, most domestic as well as many import vehicles still use the front disc/rear drum arrangement because it provides the best compromise between cost and performance. Four wheel disc brake systems tend to be limited to sporty models and high end luxury sports sedans. For more information, see the section on DISK BRAKES.

The next major improvement in the evolution of today's brakes came with the arrival of electronic antilock brake systems (ABS). The addition of ABS to a brake system adds a significant margin of safety by minimizing wheel lockup and skidding when braking on wet or slick surfaces or when stopping suddenly on dry pavement. ABS allows you to maintain steering control when braking hard so you can steer your way around obstacles and out of trouble. It also helps to keep the vehicle straight and stable during a panic stop, which reduces the chance of getting sideways. That's why ABS has become standard or optional on most vehicles today.

The origins of ABS date back to shortly after World War Two when ABS was originally developed for heavy aircraft. The first "modern" electronic ABS system for automotive use was invented by the Robert Bosch Corp. and offered on certain European Mercedes-Benz and BMW models in 1978. Other European vehicle manufacturers soon followed suit and the option quickly grew in popularity. In 1985, the first ABS-equipped BMWs and Mercedes produced for the U.S. market came ashore, starting the rush to ABS on this side of the Atlantic Ocean. That same year, Ford become the first domestic vehicle manufacturer to offer an ABS system, with General Motors jumping on the ABS bandwagon the following year. By 1990 25% of all new cars and light trucks were equipped with ABS as either standard or optional equipment, and by 1995 ABS was available on over 90 of all new vehicles.

As ABS evolved, traction control capabilities were added. This allowed the ABS system to monitor and prevent wheel spin when accelerating on wet or slippery surfaces, too. Traction control proved to be especially helpful on performance cars with wide tires which were notorious for poor wet weather traction.

Traction control works something like an electronic limited slip differential. When the wheel speed sensors detect wheel spin, the brakes are applied to the wheel with the least traction so engine power will go to the wheel with the most traction. If both wheels are spinning, both are braked to maintain traction. Some traction control systems also employ various "power reduction" strategies to reduce engine power when the drive wheels start to spin. The system may momentarily reduce engine power by retarding spark timing, disabling one or more fuel injectors or reducing the throttle opening

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