What Is a Truck Retarder?

Disc brakes, drum brakes, brake pads, and calipers are terms that most drivers are familiar with. Drivers use these components every day without fail.

But did you know that large vehicles like trucks and semi-trailers have an additional braking system built into their systems? These are called retarders.

What Is a Retarder In Trucking?

A retarder is a type of supplemental braking system that aims to reduce wear on the disc and drum brakes during a long descent一a situation that is most likely to cause the brakes to overheat.

By using retarders, the friction brakes won’t wear out as fast and their service life can be extended.

Retarders aren’t intended to serve as primary braking systems because their effectiveness decreases as the vehicle slows down. They only help vehicles slow down and prevent the vehicle from running away when traversing downward.

Retarders only provide additional braking assistance as the final braking activities are still done by the vehicle’s conventional brakes.

Classification of Retarders In Trucks

Retarders for trucks are generally classified into three types一hydraulic, electric, and engine compression.

Hydraulic Retarders

A hydraulic retarder relies on the viscous drag between dynamic and static vanes in a fluid-filled chamber to reduce vehicle speed.

Compared to engine brakes, a hydraulic one is generally more quiet and often inaudible when the engine is running.

A basic retarded uses vanes that are attached to the transmission driveshaft. Some hydraulic retarders are driven separately using drive shaft gears.

When retardation is required, hydraulic fluid is pumped into the chamber, creating viscous drag that will eventually slow the vehicle down.

During this process, the oil or water pumped into the chamber heats up, so it’ll be circulated throughout the cooling system to dissipate heat.

Hydraulic retardation can be regulated by adjusting the chamber’s fill level.

Electric Retarders

An electric retarder uses electromagnetic induction to generate stopping power. Electric retarders are often mounted to axles, transmissions, and drivelines.

This type uses a rotor and stator attached to the vehicle’s chassis. There is, however, no contact point between the two parts, and no hydraulic fluid is needed.

Instead of using hydraulic fluid, an electric retarder draws power from the vehicle’s battery to produce a magnetic field that will move the rotor. This process produces eddy currents in the rotor, which then produces an opposing magnetic field to the stator.

As a result, the opposing magnetic fields slow the rotor down as well as the components attached to it.

Unlike a hydraulic retarder that relies on the engine’s cooling system, an electric retarder incorporates internal vanes to kickstart its cooling process.

Aside from trucks, hybrid vehicles also utilize electric retarders to assist mechanical brakes. The only difference is that a retarder on a hybrid vehicle can also help recycle the energy during braking.

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Engine Compression Retarders

An engine compression retarder relies on compression strokes to absorb energy from a vehicle in motion.

In a typical combustion engine, the piston compresses the air in the cylinder to use in the next power stroke, which can take a significant amount of energy. The same energy can be drawn from another cylinder that’s in its power stroke.

An engine compression retarder uses the energy produced during the compression stroke of each piston since the other cylinders aren’t generating power between power stroke intervals.

As the piston approaches the top of the compression stroke, the retarder opens the exhaust valves for that piston, venting the absorbed energy from the vehicle’s motion.

Essentially, this type of retarder absorbs energy during the compression stroke and releases the same energy through the exhaust valves as the piston completes its compression stroke.

The Jake Brake is the most common example of an engine compression retarder.

A Jake Brake is typically found on large diesel-powered trucks and semi-trailers, releasing compressed air via exhaust valves to supply additional stopping support when braking.

Jake Brakes have been around for quite some time. Invented in 1957 by Clessie Lyle Cummins, these were intended to improve semi-highway safety and reduce overheating and fires in trucks when descending long stretches of road.

One downside to using Jake Brakes is that they are extremely loud, making it illegal in certain areas around the US.

How to Use a Retarder

Retarder operation can vary, depending on the type of vehicle and retarder. Still, all retarders generally apply the same principles, so similarities can be expected regardless of their type.

Most trucks, tractors, and semi-trailers will usually have a retarder lever located on the right side of the steering wheel, as well as a button that turns it on.

Once the retarder is on, the driver can regulate braking power by moving the lever up and down.

Keep in mind, however, that retarder operation relies heavily on vehicle speed, so make sure the engine RPM isn’t falling below 1,600 revolutions when you’re using it.

To deactivate the retarder, you can press the same button or use the accelerator pedal.

Precautions When Using a Retarder

Retarders are intended to extend the service life of your brakes while providing additional braking support. However, there might be some cases where using the retarder can do more harm than good.

For example, using a retarder during winter or whenever you’re driving on slippery surfaces can cause the vehicle to move at a higher speed instead of slowing down, resulting in composition breakage.

The Importance of Retarders In Trucks

Retarders are extremely important in large trucks mainly because they can help reduce brake lag or the amount of time it takes for the brakes to fully activate and stop the vehicle from moving.

Also, activating the retarders can keep the temperature on the brakes relatively low.