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2026

The mountain road is tricky and the car speed is slow. How is the best way to use the auxiliary brake?

What's the hottest truck equipment these days? No doubt, it's been the hydraulic retarder. From a few hundred test vehicles in 2009 to tens of thousands sold annually now, it's clear that the market has recognized it. Especially with the technology being so well-developed today, the hydraulic retarder has become the go-to choice for many long-haul trucks.

In general, after you toggle the retarder control handle, the ECU will control the proportional valve to pump gas into the retarder oil reservoir. The gas pushes the oil into the working chamber (the higher the gas pressure, the more oil enters the chamber). The rotor spins at high speed, throwing the oil onto the stator, generating a reaction force, which is the braking torque, and this helps to reduce the vehicle's speed (the more oil, the stronger the reaction).

So, is a hydraulic retarder really useful on highways? Recently, friends in a WeChat group have been discussing this topic. There's no denying it, on long downhill stretches, the effect of a hydraulic retarder is obvious. But what about on national roads with rough conditions and lots of curves—will it run into awkward situations?

▎Discussion: Lots of curves and steep slopes – how exactly should you use a hydraulic retarder on national roads?

注：液缓不会抱死车轮

In previous WeChat group discussions, opinions on whether using engine braking on national highways is good basically fall into two different views. One view is that on mountain roads with low-speed turns and downhill, if the engine braking force is too strong, it’s easy to cause understeer, so it’s not recommended to turn it on. The other view is that if you set the cruise control to low-speed mode, you can go through turns and downhill at low speed, and in emergencies, you can brake to control it.

Car speeds vary, slopes are higher or lower, and the way you handle things changes depending on the situation. We're not here to judge whether someone's opinion is right or wrong, but we can think about the issues using some principles of hydraulic retarders.

Usually, after you move the retarder control handle, the ECU will control the proportional valve opening to send gas into the retarder oil housing. The gas pushes the oil into the working chamber (the higher the gas pressure, the more oil enters the chamber). The rotor spins quickly, splashing the oil onto the stator, creating a reactive force—that is, braking torque—which helps slow down the vehicle (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 get into working mode. Simply put, as one trucker in the group mentioned, when driving on mountain roads with rough conditions, entering and exiting corners causes significant changes in speed, and the hydraulic retarder needs some reaction time, making it hard to keep up with the speed changes.

In situations where the slope isn't steep and the vehicle speed is relatively slow, it's recommended that drivers use engine compression braking to appropriately control speed on downhill sections. If the national road conditions are good and the speed variation is minimal, you can achieve deceleration using a hydraulic retarder.

When driving on a long, steep downhill, the combined effect of the hydraulic retarder and engine compression braking can produce even better deceleration. I'm sure all drivers can see this for themselves.

A hydraulic retarder is a great piece of equipment, but theory alone isn't enough—you truly need to use it properly for it to be effective.

▎Wheel lock-up? The hydraulic retarder disengages automatically

In the past, many drivers would discuss whether a hydraulic retarder with too much braking torque could cause the wheels to lock up while slowing down. Of course not; this issue was already taken into account in the initial design of the hydraulic retarder.

ABS Coordination: In actual use, when a vehicle’s wheels lock up, the ABS provides anti-lock functionality. For safety reasons, when the ABS alarm goes off, the control system will deactivate all auxiliary braking, such as the retarder.

Also, some drivers worry that when the hydraulic retarder is on, vehicles behind might have difficulty noticing that the car in front has started slowing down, which could lead to rear-end collisions. In fact, there’s no need to worry about this. The ECU control panel of the hydraulic retarder directly links to the brake lights, so when the retarder is on, the brake lights will illuminate normally, warning vehicles behind.

● Editor’s Note:

Finally, a reminder to all drivers: the hydraulic retarder is an auxiliary brake and cannot directly replace the main brake. The retarder’s role is mainly to slow the vehicle down and help maintain a constant speed; a main brake must be used when stopping.

"No need to press the brake pedal, coast downhill at constant speed." This is something many drivers mention after installing a hydraulic retarder. If there’s no emergency, using the hydraulic retarder along with engine braking can indeed allow you to go without touching the brake pedal. However, for many less experienced drivers, trying too hard to achieve "no brake pedal" driving can actually be unsafe. Any fuel-saving approach must always be implemented with safety as the top priority.

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Assisted braking isn't foolproof; remember to use caution or avoid it in certain situations.

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