Braking with Multiple Trailers
More trailers, longer stops
A multi-trailer combination has more mass and more brake chambers, but it does not stop in a shorter distance - it stops in a longer one. Brake timing matters: the tractor brakes apply first, then the trailer brakes, and with several trailers the lag adds up. Brake lag is the delay between pressing the pedal and the trailer brakes actually engaging; at highway speed this delay translates into many feet of travel.
Concepts to name
- Brake lag: the time for air to travel through the lines to the rear trailer; longer combinations have more lag.
- Stopping distance: increases with weight and speed; total stopping distance includes perception, reaction, and brake lag.
- Brake fade: overheating brakes lose effectiveness on long downgrades; use engine braking and a lower gear.
- Spring brakes: apply automatically when air pressure drops; know the warning threshold.
- Antilock brakes (ABS): prevent wheel lockup; they do not shorten stopping distance on all surfaces but preserve steering control.
Common traps
Questions often claim ABS shortens stopping distance on every surface - it does not. Another trap is the belief that more trailers mean more braking force and therefore shorter stops. A third is ignoring brake lag when following distance is calculated.
Rule of thumb
Add at least one second of following distance for every trailer behind the tractor, and always descend a grade in a gear low enough that you rarely need the brakes.