Understanding the Compression Brake System

If you’ve spent any time driving a lorry, you’ll know that the feel of the vehicle is different to that of conventional vehicles. It’s not just about wheelbase, engine size and vehicle weight, however: parts of the mechanism are actually qualitatively different from those of more common road vehicles. One of the key distinctions between your average vehicle and those specifically designed for haulage work is in the brakes.

The brakes used in HGVs are known as compression brakes, while those used elsewhere tend to be hydraulic brakes.

What Are the Similarities?

Compression and hydraulic brakes have a number of similarities. Most notable is, of course, the end result: both of them are designed to reduce the velocity of a vehicle in motion. When you look at the “business end” of either system (i.e. the part closest to the wheels), both the brakes used in haulage work and those used in conventional vehicles have similar mechanisms - drums, shoes and pads are all present and perform the same functions.

There are some similarities at the front-end too: both kinds of brakes are similar from a driver’s perspective, operating a lever in order to ultimately apply friction to the wheels and hinder their rotation. However, the two systems have a number of key differences.

What Are the Differences?

From an external, “black box” perspective, the differences you’ll notice are the slower response time of compression brakes to hydraulic brakes: as a driver, when you hit a hydraulic brake, you’ll feel the effect on the car instantaneously. However, as any haulage worker will confirm, hitting the brakes on an HGV takes a little under a second to even begin to slow the vehicle. Additionally, compression brakes tend to make a loud sound of escaping air when used.

The two mechanisms are also very different. Hydraulic brakes transmit the motion of the lever through a hydraulic system. As the motion is transmitted through incompressible fluid (brake fluid), these brakes are very responsive. Compression brakes, however, use compressed air: essentially, during the compression stroke of the engine’s pistons, air is allowed into the cylinders and is compressed by the downstroke. This compressed air is used to turn the camshaft and ultimately apply friction to the wheels, through either a power screw or an S-cam and a drum.

The lack of brake fluid means that there is no fluid to replace in compression brakes, and the kind of brakes that use a power screw (air disc brakes) are almost as responsive as hydraulic brakes, reducing stopping distance by 40% when compared to drum brakes. However, due to traditional dominance of drum brakes in the industry, some countries have been slower to adopt disc brakes for haulage work than others. In Europe, for example, the vast majority of lorries operate on disc brakes, while in North America only about one in twenty trucks runs on disc brakes.

If your lorry is still running on drum brakes, you might want to consider upgrading: the shorter stopping distance is well worth having. In any case, improvements are being made all the time, and there’s no way of knowing how long it’ll be before a new braking system comes along.