Heavy duty truck disc brakes are very different from those on a car. They’re really air disc brakes which are far more powerful. However, in the US, they’re not particularly common with only 5% of trucks actually carrying them. This is in spite of the fact that they are safer and reduce stopping distances by up to 30%, resulting in far fewer accidents.
In the US, the vast majority of trucks use traditional drum brakes. There’s no regulation that demands the use of air disc brakes. This is in direct contrast to Europe where around 80% of trucks are equipped with disc brakes. There is a shift to truck disc brakes in the US, but progress is very gradual and will take a number of years before the transition is complete due to the large expense involved.
How Brakes Work
Brakes are intended to slow and stop a vehicle. Drum and disc brakes work in different ways, but with exactly the same result. Disc brakes are more efficient which is why they’re more popular in cars. However, they also cost significantly more. Car brakes work on a hydraulic system centered in the master cylinder and operate by forcing fluid through the braking system.
Truck brakes, however, work on compressed air so the system can never run out of fluid. The system has to keep up the supply of compressed air, direct the flow of air and also apply pressure to turn it into a force that’s used directly on the brakes. The biggest problem with truck brakes is air lag; there’s usually a brief gap between pushing down on the pedal and the brakes actually grabbing.
For safety reasons, many trucks have a double air brake system. This is for optimum safety so if one system fails, the other takes over so there’s no danger of the brakes failing. The brake chamber in the system contains a spring which has around 2,500 pounds of pressure. The airflow holds this back but if the pressure drops below 60 pounds per square inch, a low-pressure light appears. If there’s not enough pressure in the system, the emergency brakes will deploy.
Truck Disc Brakes
The main type of brake usually found in a truck is called the S-Cam brake. Both disc and drum brakes use air in much the same manner. In both cases, air pressure works on both the slack adjuster and the brake chamber. In truck disc brakes, however, the S-Cam is replaced by a power screw. The air pressure on the brake chamber and the slack adjuster actually turns the power screw. This, in turn, grabs the rotor (also known as the disc) between the brake lining pads on the calipers.
Benefits of Truck Disc Brakes
Perhaps the greatest benefit of truck disc brakes is their ability to stop over a much shorter area which cuts at least 30% off of stopping distances. The air pressure is always constant so coming to a complete stop is much easier. Brake fade can be a big problem with drum brakes but with disc brakes, it’s virtually eliminated. There is no increase in brake lining wear with truck disc brakes.