Braking and Stopping with Multiple Trailers
Domain 4 covers what changes when you must stop two or three trailers instead of one, and what does not. The exam tests brake lag, stopping distance, wheel-lock behaviour, and correct use of the trailer hand valve and retarders.
More weight, more lag
Air takes time to reach each trailer's relay valve, so brakes at the back of the combination apply after the tractor's. With triples that delay is longer than with a single trailer, and it adds directly to stopping distance. Total stopping distance is perception plus reaction plus brake lag plus braking distance; adding trailers and weight raises the last two.
Technique that keeps the set straight
- Brake early and progressively. A smooth squeeze lets the trailer brakes come in without a surge.
- If a trailer starts to swing out, release the brakes so it can roll and regain traction, and steer in the direction it is sliding.
- Never use the trailer hand valve to stop or to park. It applies only the trailer brakes, so the trailers can lock and swing around the tractor, and it will not hold a parked rig.
- Retarders help on dry pavement. Switch them off on wet, icy, or snow-covered roads and when trailers are empty, because they can lock the drive wheels and start a jackknife.
- On a downgrade, select the gear before the crest and use light, steady braking. Overheated drums on three trailers leave very little braking.
Worked example
A driver descending a grade with a light load in the rear trailer slows using the hand valve. The light trailer's wheels lock easily and it swings toward the shoulder. What went wrong: the hand valve applied only trailer brakes, and an empty trailer locks at far lower pressure than a loaded one. The correction on the grade is to release the hand valve, use the foot brake with the transmission in a low gear, and accept a slower descent so all axles share the work and the set stays straight.
Traps that cost marks
- Assuming more axles always means shorter stops. Brake lag and weight usually make stops longer.
- Using the hand valve as a parking brake.
- Skipping the low-speed test that shows each trailer's brakes actually apply.
- Riding the brakes down a long grade until they fade.