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2026

Mountain roads are complicated and vehicle speed is slow. How should the auxiliary brake be used?

What truck equipment is the most popular nowadays? Without a doubt, it is the hydraulic retarder. From a few hundred test-installed trucks in 2009 to tens of thousands of units sold annually today, it is clear that it has been recognized by the market. Especially now that the technology is very mature, the hydraulic retarder has become the preferred configuration for many long-haul trucks.

Under normal circumstances, after toggling the retarder control handle, the ECU will control the proportioning valve to inject gas into the retarder oil reservoir shell. The gas will push the retarder oil into the working chamber (the higher the gas pressure, the more oil enters the working chamber). The rotor spins at high speed, throwing the oil onto the stator, generating a reaction force, that is, a braking torque, which in turn serves to reduce the vehicle speed (the more oil, the greater the reaction).

Is the hydraulic retarder really useful on highways? Recently, friends in WeChat groups have discussed this topic. There is no denying that on long downhill sections, the effect of the hydraulic retarder is obvious. So, on national highways with poorer road conditions and more curves, does it encounter awkward situations?

▎Discussion: With many curves and steep slopes, how exactly should the hydraulic retarder be used on national highways?

注：液缓不会抱死车轮

In the previous WeChat group, discussions about whether the hydraulic retarder is good to use on national highways were basically divided into two different viewpoints. One is that on mountain roads with low-speed turns and downhill, if the braking force of the hydraulic retarder is too strong, it is prone to causing understeer, so it is not recommended to turn it on. The other viewpoint is that by setting the cruise gear to a low-speed mode, one can pass through turns and downhill at low speed, and in emergency situations, braking can be applied to control the vehicle.

Car speed can be fast or slow, and slopes can be high or low. The operation varies under different conditions. Here we do not judge the correctness of any viewpoint, but we can think about problems through some principles of hydraulic retarders.

Normally, after toggling the retarder control handle, the ECU will inject gas into the retarder oil tank housing by controlling the opening of the proportional valve. The gas will press the oil into the working chamber of the retarder (the higher the air pressure, the more oil enters the working chamber). The rotor spins at high speed, flinging oil onto the stator and generating a reactive force, that is, a braking torque, which helps reduce the vehicle speed (the more oil, the greater the reaction).

In summary, based on some information, it usually takes about 1.5 seconds for the hydraulic retarder to enter working mode. In short, as one member in the group noted, when driving on winding mountain roads with not very good conditions, entering and exiting turns can cause significant changes in vehicle speed. The hydraulic retarder requires some reaction time and has difficulty adapting to the speed changes.

Under general conditions with mild slopes and relatively low speeds, it is recommended that truck drivers can use engine compression braking on downhill sections to appropriately control speed. If the national road conditions are good and the speed variation is not significant, deceleration can be achieved through a hydraulic retarder.

When the vehicle is on a long downhill with a steep slope, the combined effect of the hydraulic retarder and engine compression braking can achieve better deceleration. I believe all truck drivers can see this clearly.

The hydraulic retarder is a great piece of equipment, but theory alone is far from enough; it only works well when used properly.

▎Wheel Lock-up: The hydraulic retarder will automatically disengage

In the past, truck drivers often discussed whether excessive retarder braking torque might cause wheel lock-up while decelerating. Of course not; this issue was already taken into account when the hydraulic retarder was designed.

ABS Coordination: In practical use, when the vehicle's wheels lock up, the ABS will function to prevent locking. For safety reasons, when the ABS alarms, the control system will disengage all auxiliary brakes, such as the retarder.

Additionally, some truck drivers are concerned that when the hydraulic retarder is engaged, vehicles behind may find it difficult to notice that the vehicle in front has started slowing down, which could easily lead to rear-end collisions. In fact, there is no need to worry about this. The ECU control panel of the hydraulic retarder is directly linked to the brake lights. When the retarder is activated, the brake lights will illuminate normally, serving to warn vehicles behind.

● Editor's Note:

Finally, I want to remind truck drivers that the hydraulic retarder is an auxiliary brake and cannot directly replace the main brake. The retarder mainly serves to slow down the vehicle and can help maintain a consistent speed. When stopping, the main brake must be used.

“Not pressing the brake pedal, constant speed downhill.” This is a phrase frequently mentioned by many truck drivers after installing a hydraulic retarder. Without an emergency situation, the combined effect of the hydraulic retarder and engine braking can indeed allow for not pressing the brake pedal. However, for many drivers with limited experience, trying to achieve “not pressing the brake pedal” is actually unsafe. All fuel-saving practices must be carried out with safety as the top priority.

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