Air brakes (service brakes) are the primary means of slowing and/or stopping big trucks such as mine. In conventional systems, braking is performed with compressed air in lines leading to mechanisms on the insides of drums at the ends of each tractor and trailer axle. When I step on my truck’s brake pedal the compressed air forces brake shoes to press against the insides of the drums. A compressor, pressure governor and air tanks maintain a supply of compressed air.
On later model tractors, disk brakes have begun to replace drum brakes. In disk systems, braking is performed when compressed air forces pads to clamp disks at the ends of each axle.
A very important aid in controlling speed and slowing the truck is the engine compression release brake (often called the “Jake brake” after its manufacturer Jacobs Manufacturing Company). When activated it alters the operation of the engine valves to convert the engine from a power producer to a power absorbing air compressor that acts as a drag on the speed of the truck. Its effect is much like that of turbofan engine thrust reversers deployed on an airliner when it lands. It helps control my truck’s speed down grades and at highway exit ramps and can help me bring my truck to a stop.
A disadvantage of engine brakes is the noise they make—if unmuffled it can be a loud, harsh, pounding growl. My truck and many others are muffled and thus quieter. Some states permit use of engine brakes only if muffled. Muffled or not, most cities and towns prohibit engine brake use within municipal limits, which is not a concern at surface street speeds if there are no steep downhill grades. Though not common, there are surface street grades on which engine brakes are important in controlling heavily loaded trucks. Cities and towns may also prohibit engine brake use on their downhill sections of freeways and other highways and on certain exits. To take noise restricted exits, I may slow my approach even more than usual.
There are also downhill grades on the open road (typically on interstates or major highways with two or more lanes in each direction, in sparse traffic, usually in the countryside), with engine brake noise restrictions requested by residents living along the highway. In such areas, descending truck drivers have to gear down enough to use service brakes alone without overheating them. These constraints can create risky situations for truck drivers who have to control their speeds under heavy loads. Overheating brakes from overuse is dangerous as well as damaging to brake parts and tires. Even when engine brakes are permitted, you can often smell hot brakes at the bottom of grades.
Steep exit ramps off downhill grades compound speed control requirements. Warning signs and posted reduced speed limits prepare drivers for these exits. It becomes doubly important that personal vehicle drivers not cut in front of trucks taking them. For these exits, it would seem that muffled engine brakes, at least, should be permitted.
Parking and Emergency Brakes
By law my truck and other big trucks are also equipped with a parking and emergency brake system activated by mechanical force. When parking, I release air pressure with knobs on the dash that in turn frees powerful springs to set the brakes. Similarly in the event of a severe air leak, this system releases its “open” hold on these springs to put the brakes on.
The point to remember is that while these systems work well with such a heavy vehicle, at highway speeds it can still take up to 500 feet or more to stop a truck in good weather and road conditions. That is why I advise you to try to stay well ahead of any truck, preferably a tenth of a mile or more, on the open road.
With a lighter amount of freight, a truck will obviously stop in a shorter distance than with a heavily loaded trailer. But with at least some freight weighing down its trailer tandems, a truck may also stop in a shorter distance than with no freight. This is because an empty trailer’s tires may skid more readily in hard braking, in spite of the trailer’s anti-lock braking system.
On the open highway, try to stay a tenth of a mile (500 feet) or more ahead of any truck.
Do not cut in front of a truck to reach an interstate, freeway or other highway exit that the driver is not taking, or worse, that the driver is taking.