Braking Technique, Stopping Distance, and Skid Control
This domain tests smooth braking, the parts of total stopping distance, air-brake lag, and skid control with and without ABS. Expect questions that compare speeds, braking methods, and the correct response to wheel lock.
Stopping distance
- Total stopping distance = perception distance + reaction distance + braking distance.
- Perception and reaction happen before the brakes apply; at 55 mph a vehicle covers about 81 feet per second.
- Air-brake lag adds more distance because pressure must travel to the chambers.
- Braking distance increases with the square of speed; doubling speed can roughly quadruple braking distance.
Braking technique
- Apply the service brake firmly and hold it; do not fan or pump normal air brakes.
- Before a long downgrade, select a lower gear and use engine braking; avoid riding the brakes.
- Controlled braking: apply as hard as possible without locking wheels, release to regain steering, then reapply.
- Stab braking: full application, release when wheels lock, reapply when they roll; use only when appropriate.
ABS and skids
- With ABS, apply firm steady pressure and steer; do not pump the brake.
- ABS may not shorten stopping distance on gravel or loose snow, but it helps keep steering control.
- If wheels lock without ABS, release the brake enough to regain rolling and steer in the direction you want to go.
Worked example
A truck stops in 90 feet of braking distance at 30 mph under good traction. At 60 mph, braking distance is about four times 90, or 360 feet, not 180 feet. Then add perception, reaction, and air-brake lag. This is why speed control before the downgrade matters more than hard braking at the bottom.
Traps that cost marks
- Pumping ABS brakes and losing steering control.
- Assuming ABS always shortens stopping distance on loose gravel.
- Riding the brakes down a long grade instead of selecting a lower gear.
- Forgetting air-brake lag when estimating total stopping distance.