Articulated lorry on a steep descent with fading brakes
You are driving a three-axle articulated lorry with a fully loaded trailer, gross combination mass about 38 tonnes, down a continuous 8% descent that runs for 4 km. The road is dry, the speed limit is 60 km/h, and there is a slow vehicle lane on the right. After 2 km you notice the service brake pedal feels softer and the vehicle is decelerating less for the same pedal effort. Your speed has crept from 45 km/h to 62 km/h. The engine retarder is switched on at its middle setting and the exhaust brake is working. There is a lay-by 300 m ahead. Traffic behind is light.
1. What is happening, and what should you do in the next 300 m?
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Diagnosis: Soft pedal plus reduced retardation after sustained braking on a long descent points to brake fade - heat build-up reducing friction, with possible fluid/vapour effects. The 62 km/h speed on a 60 km/h limit descent confirms braking performance is degrading.
Actions within 300 m:
- Stop braking continuously. Release the service brake and let the retarder/exhaust brake and engine braking (select a low gear) do the work.
- Select a lower gear now - engine braking is your most reliable retardation when the service brakes are hot.
- Use the lay-by to stop safely, out of the traffic lane, and let the brakes cool.
- Do not ride the brakes harder to hold speed - that adds heat and worsens fade.
- Warn following traffic if you slow sharply (hazards only if appropriate and legal).
After stopping: let brakes cool naturally, inspect for smoke, fluid leaks or a low/dropping pedal, and do not continue the descent until you are satisfied braking is restored. Report the defect.
2. Why is holding a low gear and using the retarder better than continuous service braking on this descent?
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Continuous service braking converts the vehicle's potential energy into heat in the brakes faster than airflow can remove it, so drum/disc temperature climbs until friction drops (fade) and, in the worst case, brake fluid boils. Engine braking and a retarder absorb energy through the driveline and engine, spreading the load and keeping service brakes cool and available for emergencies.
A useful discipline is brake firmly and briefly, then release to let airflow cool the brakes, rather than light continuous pressure. Combined with a gear low enough that the engine holds the descent speed, this keeps you in control and preserves a reserve of braking for a genuine emergency.