Brake fade on an 8% descent with three cars behind
You are descending a mountain road in western Norway with a 42-tonne combination: a 6x4 tractor unit and a three-axle semi-trailer loaded with 27 tonnes of palleted bricks. At the top a sign gives the gradient as 8% for the next 6 km. It is October, the air is 4 °C, the road is wet with fallen leaves in the bends, and the forecast is for the temperature to drop below zero at the bottom of the valley. The unit has a retarder with four positions, an engine brake and standard drum brakes on the tractor and trailer. You selected a gear that holds about 45 km/h on the retarder's third position. Two kilometres down you notice the service brake pedal travels further than it did, a hot smell, and the combination accelerating past 60 km/h although the retarder is still in position three. Three cars are close behind and the gradient drops to 3% in about 500 m, where a gravel lay-by is marked.
1. Explain what has happened to the brakes and set out, in order, what you do over the next 500 m.
Read the worked answer
What has happened The drums are overheated. Continuous light braking keeps the linings in contact with the drum and generates heat faster than airflow can remove it, so the lining loses friction and the pedal goes long. A long pedal can also mean brake fluid boiling or a leak, and either way the service brake can no longer be trusted. The retarder alone in position three is no longer holding 42 tonnes on 8%.
The next 500 m, in order
- Do not pump the pedal and do not coast or select neutral. Select the lowest gear that keeps the engine below its limit and put the retarder to maximum, so the engine and retarder, not the drums, do the work.
- If you must shed speed, brake firmly in one application and then release, giving the drums air rather than dragging them.
- Signal right, hold your line and let the three cars past when it is safe.
- Stop in the lay-by, park on level ground, apply the parking brake and chock the wheels.
At the lay-by Let the drums cool for 20 to 30 minutes. Never pour water on them. Check the hubs from a safe distance and look for a dragging brake, a leaking cylinder or a damaged drum before deciding whether to continue.
2. Why is a low gear plus retarder better than riding the service brake down, and what does the 8% sign tell you to do before you commit to the descent?
Read the worked answer
Where the energy goes A retarder and engine brake turn the combination's potential energy into heat in the retarder and exhaust, which are continuously cooled. The wheel brakes have a fixed heat sink: a few hard applications from speed, and that is it. Riding them down a long descent means the heat sink is never allowed to recover, and the brakes are gone exactly when you need them.
What the sign tells you 8% for 6 km is a warning to prepare, not a piece of scenery. Before the descent:
- Test the brakes at low speed while you still have an escape.
- Let the air build to full pressure and listen for leaks with the engine off.
- Choose the gear that holds the combination on the retarder alone, and check it holds on the steepest part, not just the first bend.
- Note where the lay-bys and check-brake signs are.
If the gear you need does not hold the load, you turn back or take another road.
3. The retarder acts on the drive axle only. What risk does that create on this wet, leaf-covered surface with 27 tonnes behind, and how do you use it?
Read the worked answer
The risk Retarding at the drive axle means the braking force enters the combination at the wheels that must also pass it forward through the fifth wheel to the trailer. On wet leaves or black ice the drive wheels can lose grip or lock, the trailer keeps pushing, and the combination folds at the coupling. The front axle keeps rolling, so you may not feel the loss of grip until the trailer is already steering you.
How to use it
- Drop to retarder position two on wet leaves, and lower still or off on ice; high setting equals high braking torque equals wheel lock.
- Take a higher gear than you would on dry tarmac, so the retarder applies less torque.
- Do not combine retarder and service brake in a bend, and keep steering inputs smooth.
- Watch the temperature drop into the valley; wet roads turning to ice is the moment to reduce the retarder setting and let ABS help with gentle service braking instead.
The trailer's EBS helps but does not remove the risk.