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Stopping Points

By on May 1, 2008
RoadKing Mag

Did you know that the brakes that stop your 80,000 pound rig are directly descended from wood blocks? That’s what was used to stop or slow down horse-drawn wagons in the earliest days of commercial transportation. Wood wheels had iron bands as tires to protect the wheels from rocks and ruts. The metal presented a perfect surface to apply a clamping force to stop the wheel turning. That’s wheel, singular. In those days, “trucks” had one wheel braking.

The driver used arm power and leverage to apply the block to the iron tire. Later, pivot points were changed and foot power applied the force. Drivers thought their clamping the wheel slowed the wagon, but what they were really doing was changing kinetic energy to heat.

That’s how brakes actually work. Energy cannot be created or destroyed. It can, however, be converted and stored. Whether an ancient horse drawn wagon or a modern tractor-trailer, it takes energy to get the vehicle rolling. It may be measured as horse power or horsepower, but the process applies energy to a body at rest to move it. In motion, it retains it as kinetic energy. To slow or stop the vehicle, kinetic energy is converted to something else, usually heat. In the case of our wagon, the wood is the friction material rubbing the iron tire and heating it. Any heat quickly dissipates to the surrounding air.

The exact same principle applies to today’s big rigs. Instead of a wood block, we have sophisticated, highly engineered blocks of long-lasting friction materials. Instead of an iron tire, we have massive brake drums for the friction materials, the brake linings, to rub against. The iron, sometimes cast into a steel shell to save weight and add strength, absorbs and dissipates the intense heat generated when slowing that much mass.

Instead of a simple lever to apply the friction material, truck brakes have become highly engineered, complex devices. The first drums were on drive shafts. Friction was applied by tightening a leather strap around the outside of the single drum. Soon, automobiles and trucks had internal mechanisms to wedge linings, then compounded with heat-resistant asbestos (now a known carcinogen), against the insides of the drums. Linings were mounted on shoes or tables, removable metal supports to hold the linings (brake shoes) in place and to support them against the drums. The mechanism evolved as foot brakes took over, and the pedals pulled wires that activated these more efficient mechanical brakes.

The need to apply brakes to all four wheels led to the use of hydraulic fluid. Liquids are not compressible, and when under pressure, they apply that pressure equally in all directions. If the pedal pushes down on a one square inch piston with 10 pounds of force, a 10 pound-per-square-inch pressure is applied throughout the system. A two square inch cylinder in each brake will apply 20 pounds of force (10 psi times 2 square inches) at each corner, for 80 pounds total force.

That’s fine for cars, but not for big trucks. The principle of converting pressure into force applies, but modern era trucks use compressed air. The engine powers a compressor that builds up between 100 and 120 psi.

Instead of the brake pedal pulling a cable or pushing on hydraulic fluid, it actuates a valve that releases stored compressed air. The air acts on chambers at each brake. Drive wheel and trailer brake chambers are usually 30 sq. in, so releasing air to charge the system at 10 psi results in 300 pounds, and a 100 psi panic stop applies 3,000 pounds of force to each brake lever. Further force multiplication takes place when the slack adjusters, acting as levers, transfer the forces to cams inside the brake mechanism. The cams force the shoes against the drums with as much as 10,000 pounds of force.

Forces like that can easily lock brakes, especially if tires’ friction with the road is reduced. Snow, ice, rain and even wet leaves can suddenly lessen friction, locking a brake and causing a jackknife or trailer swing. In the 1970s, NHTSA regulations required all heavy trucks to have anti-lock brakes (ABS).

Technology, however, was not sufficiently advanced, and after legal action was brought by the trucking industry, NHTSA was forced to withdraw the regulation, but work continued on ABS. By the time Federal Motor Carrier Safety Standard (FMVSS) 121, dealing with air brakes, again required it, ABS technology had developed and today, the vast majority of trucks on the road have it.

The sophisticated electronics that were developed for ABS also enabled a great deal of safety technology that we’ll cover in a future issue. NHTSA also announced that they will propose changes in required stopping distance. While reductions of 20 percent, or even 30 percent can be met using drum brake technology, the addition of a 75 mph requirement will probably require the adoption of air disc brakes. They are now standard equipment in Europe. We’ll discuss that further when NHTSA issues the requirement.

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