18 Wheeler Braking Systems

Commercial motor carrier brake systems are a complex system of several units designed to stop fast-moving and heavy tractor-trailer trucks. A failure of these brake systems creates dangerous road conditions, which pose a threat to all drivers on the road. In a 2007 study sponsored by the U.S. Department of Transportation, roughly 29% of all large truck crashes studied involved some problem with a truck’s brake system.

Federal regulations from the Federal Motor Carrier Safety Administration impose restrictions and duties on commercial motor carriers, including the type of brakes to use and requiring regular inspection of braking systems. However, violations of these Federal regulations do occur, posing a danger to everyone on the road.

Federal regulations on commercial trucks are strict and require a commercial truck: (1) to develop a certain braking force that is at least equal to the percentage of its gross weight; (2) to be able to decelerate to a stop from 20 miles per hour at not less than a rate specific to its size; and 3) to stop form 20 miles per hour in a specified distance, measured from the point at which movement of the service brake pedal or control begins.

Trucking companies and drivers may follow a course of action that cause brakes to fail, all in the name of keeping transportation costs low. Some trucking companies and drivers deliberately unhook or de-power the front brakes on trucks and rely only upon the brakes of the trailer and downshifting to stop or slow the vehicle. This is done to minimize the expense of tire, brake wear, and replacement costs; but it greatly reduces the ability of the truck to come to an abrupt stop and this course of conduct endangers all on the road. Additionally, trucking companies may load their trailers improperly, either not evenly distributing the load or carrying too much in their trailers. When trucking companies improperly load their trailers, brakes may overheat and malfunction.

Federal regulations require trucking companies to keep maintenance records demonstrating that the truck maintenance has been performed according to regulations. Additionally, every driver is required to perform and complete a daily inspection report of the condition of the tractor and trailer equipment. These required inspections may include: 1) checking the brake shoes to ensure they function properly and do not have missing or broken mechanical components; 2) checking the loose brake components; and 3) listening for air leaks in the brake chamber, which would indicate problems with the brake system. Failure to perform these inspections and maintenance greatly increases the possibility of the brakes failing and creating dangerous conditions for drivers on the road.

There are three types of brake systems that a tractor-trailer truck will have: service, parking and emergency brake systems. Service brakes are the primary mechanism designed to stop a motor vehicle. Service brakes work by applying pressure to the brake pedal either through air pressure or by hydraulic pressure or by electricity. In commercial motor carriers that have a split service brake system, when there is a partial failure of a portion of the brake system, the remaining portions of the service brake system will continue to operate and must be able to stop a vehicle at 60 miles per hour within a specified distance.

Air brakes, as used in service brake systems, use air as a medium for transmitting pressure or force from the driver control to the service brake. When attempting to stop, the braking system uses pressurized air to trigger the braking mechanism – pushing pressurized air against the lining of the drum and disc when the driver steps on the brake pedal. Hydraulic brakes, also used in service brake systems, utilize hydraulic fluid as a medium for transmitting force from a service brake control to the service brake.

Tractor-trailer trucks also use a parking brake system, which must be used only during parking. The parking brake system is held in the applied position by energy that is different than the air pressure, hydraulic pressure, or electric energy that is used in the service brake system. Typically, these brake systems utilize heavy-duty springs. While many of these springs use compressed air to activate, they do not use the actual air pressure to stop the vehicle.

The third type of brake is the emergency braking system, which is the mechanism designed to stop a motor vehicle after a failure of the service brake or parking brake systems. With emergency brake systems, if the tractor-trailer truck loses air pressure, then the emergency brake system activates the spring brake (used in the parking brake system) as a means of stopping the truck. In the event of a total brake failure, a driver is able to use these brakes manually to stop the truck and avoid a collision.

Since 1998 semi-trucks also come with anti-locking brake systems which keeps the wheels from locking up and causing the driver to lose control of the vehicle. Anti-lock brake systems automatically control the degree of rotational wheel slip during braking. Each of the braking systems comes with its own variations and potential problems if the brake systems fail.