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Disc Versus Drum Brakes

Most Army wheeled vehicles are adapted from commercial models. However, the Army is not keeping up with the best commercial automotive technology in one important area: the brakes.

The Army has a fleet of vehicles that numbers approximately 500,000. Combat vehicles operate on both unpaved roads and paved highways. Normally, light trucks, heavy-haul trucks, and high-mobility, multipurpose wheeled vehicles use paved concrete or asphalt roads for everyday duties. Nearly all of the engines, transmissions, differentials, and brake systems used in Army vehicles are commecially produced.

Why are so many commercial systems integrated into Army vehicles? The reason is that Army vehicles are adaptations of commercially marketed vehicles. Army light trucks are modified civilian vehicles, heavy-haul dump trucks are modified commercial dump trucks, and semi-tractors and -trailers are modified over-the-road rigs—the same type we see moving freight down our Nation’s highways. They have the same frame designs, the same driveline designs, and the same brake systems. In fact, the number of vehicle manufacturers is limited, and many use all of the same components.

It would be neither efficient nor effective for the Army to design, create, and test new engines or transmissions from scratch, and the costs would be prohibitive. Minimizing risks to personnel is a priority, and tests on vehicle equipment previously conducted by industry provide data and engineering information that can be used to reduce the danger of injuries or death. System upgrades contribute to risk reduction, but occasionally state-of-the-art advances in the private sector are overlooked. The best example is the use of technologically superior disc brakes over traditional drum brakes.

European and Domestic Use

Most European heavy over-the-road trucks use disc brakes on all axles, with over 1.5 million disc brakes produced in Europe. U.S. commercial truck manufacturers entered the disc brake market with front-axle discs, but they were not promoted in the industry. (I worked on Mack Truck disc brakes in the 1970s.) Four-wheel disc brakes have been available for American-made light trucks for several years, and they have demonstrated superior stopping efficiency. FWD/Seagrave, Pierce Manufacturing, Spartan Motors, and Sutphen use air discs on fire, emergency, and rescue vehicles. Motor Coach Industries and Prevost Car order them installed on intercity coaches.

Currently, the Army uses disc brakes only on M998-series high-mobility, multipurpose wheeled vehicles, with four-wheel hydraulic discs, and a limited number of commercial light trucks adapted for military use, with front disc brakes.

Drum Brakes

Drum brakes operate using two steel shoes with a friction material bonded or riveted onto the faces of the shoes. The two shoes are mounted at the outer end of an axle on a backing plate. When hydraulic or air pressure is applied to the shoes, they expand inside the brake drum, which is attached to the wheel and tire. The force of the shoes against the inside of the drum produces friction and stops the rotation of the drum-wheel-tire unit, thus stopping the vehicle.

This is a satisfactory system for some applications, such as light-duty vehicles. However, the larger and heavier the vehicle, the larger and heavier the drum brake unit must be to dissipate the enormous heat generated in stopping a large vehicle. This means a large vehicle needs large shoes, a massive (150 pounds or more) cast-iron drum, heavier gauge steel backing plates, and large supporting operation activators.

The shoes also must be enclosed in a drum, which greatly reduces or eliminates the air circulation needed to remove heat buildup. The drum sheds heat only from the surface area on its outer circumference. When the shoes get wet, their ability to brake weakens (a phenomenon known as “brake fade”) because the drum cannot sling water out of the unit; the water is trapped and held inside the drum by centrifugal force as the drum rotates until it is evaporated by heat. Air drum brakes have approximately 20 internal moveable parts and another 20 external parts for operation.

To maintain air drum brakes on a heavy rig, time must be allotted to inspect material wear of the shoes caused by friction. The maintainer must crawl under the vehicle to see the wear. If all 40 parts are mechanically acceptable, the brake is adjusted using one wrench while the wheel is raised off the ground. If the problem diagnosed requires that the brake be torn down, the wheel must be raised; if it is a dual wheel, a wheel jack must be used to remove the duals and drum—a 600-pound package—before the tear-down can be started.

Disc Brakes

Disc brakes were developed in England in the 1890s and patented in 1902. They were extensively used on fighter planes during World War II and were adapted to automobiles in the 1940s and 1950s. By 1970, front disc brakes were installed on most American cars. In the late 1970s, light American trucks and a limited number of heavy trucks used front axle discs. Today, use of disc brakes has become common in the U.S. automotive industry.

Heavy truck air disc brakes have many advantages over drum brakes. Disc brakes operate with a large cast-iron rotor attached to the vehicle spindle or axle that rotates. A steel wheel and tire are bolted to the rotor. A stationary caliper is mounted to the axle housing, with two steel pads made of friction material bonded or riveted to one side of the pads. The pads, operated by an internal cylinder, float outward when air or hydraulic pressure is applied. This forces the pads against the rotor, stopping the vehicle. Disc rotors are not enclosed, so they sling water when wet and thus dry quickly with minimum brake fade. They do not pull the vehicle to one side, as drums can do; all stopping is in a straight line.

Disc brakes dissipate heat quickly because the rotors and pads are open and the rotors have large internal ventilation air passages. Because discs run cooler than drums, less heat is transferred to tires, which increases tire longevity. Disc brakes are 30 to 40 percent lighter in weight than drum brakes, which also reduces tire wear and, with less rebound over potholes and rough roads, keeps tires in contact with the road more effectively. Consistent tire-to-road contact produces consistent steering efficiency.

Disc brakes have approximately one-fourth the number of parts of drum brakes. They require inspection, but not adjusting, and that inspection can be performed without lifting the axle. Discs last four times longer than drums, making them the more cost-effective alternative. The most important vehicle functions are braking and steering. All-wheel disc brakes immediately improve both. They decrease stopping distances by up to 50 percent and, with less brake weight, reduce the unsprung weight of axles and wheel bounce, thereby maximizing tire-to-road contact and increasing steering control. [“Unsprung weight” is the weight of all components of a vehicle that are not supported by the vehicle’s springs. These components include wheels and tires. The lower the weight of these components, the better they can handle bumps and potholes in the road.]

I believe that future tactical military vehicles should be ordered with all-axle disc brakes. Whether current brake systems are hydraulic- or air-operated, a retrofit developmental program could be initiated to fit tactical vehicles and trailers with all-axle disc brakes. Several major brake manufacturing corporations are able to provide the necessary expertise and components to reach this objective.

Requiring disc brakes on Army vehicles would reduce the long-term logistics footprint by helping the Army to reduce brake maintenance time, extend brake replacement intervals, increase tire life, and consume fewer brake replacement parts. If a single vehicle component can provide all these benefits, I believe that serious consideration must be given to including that system on all Army vehicles.

Louis J. Gorenc is an equipment specialist with the Combat Vehicle Evaluation Team at the Integrated Logistics Support Center, Tank-automotive and Armaments Life Cycle Management Command, at Warren, Michigan. He has a bachelor’s degree in criminal justice administration from Concordia College in Michigan and worked as a heavy equipment mechanic for 25 years.