Braking and Stopping with Multiple Trailers
This domain tests stopping a combination whose brakes do not act at the same instant. Stopping distance is perception, reaction, brake lag, and braking distance, and brake lag grows with each added unit because air must travel further and pass more relay valves.
Core rules
- Increase following distance. Use one second per ten feet of vehicle length below 40 mph, plus one second above 40. A 65-foot triple at highway speed needs roughly 7 seconds.
- Brake early and smoothly. Hard braking on a long combination invites trailer swing and jackknife.
- On long downgrades, use stab braking or a retarder where permitted; do not ride the brakes.
- Use the trailer hand valve only to test trailer brakes, never to park or to stop.
- If pressure falls toward about 45 psi, the tractor protection valve shuts off the trailer and the trailer spring brakes apply. Loss of air stops you, but the trailer braking alone can bend the combination.
- After crossing deep water, test brakes at low speed.
- Brake before a curve, not in it, and stop on straight, level ground where possible.
Worked example
You drive a 70-foot double at 55 mph in rain. Base following distance is 7 seconds for length; rain adds margin, so hold 9 to 10 seconds. At 55 mph, a second is about 80 feet, so you want roughly 700 to 800 feet of clear space - far beyond what feels natural in a car.
Traps that cost marks
Assuming the last trailer's brakes apply as fast as the tractor's; using the hand valve to hold a parked combination; braking in a curve; leaving brake adjustment unchecked on the dolly.