An articulated vehicle equipped with standard S-cam air brakes is traveling at 55 mph on a dry, level highway when the driver identifies a road obstruction and performs a full-effort stop. In evaluating the specific physical components comprising total stopping distance, which statement accurately analyzes the role and timing of mechanical air brake lag?
- ABrake lag distance occurs concurrently during driver reaction time, meaning total stopping distance is calculated solely by adding perception distance to actual vehicle braking distance.
- Mechanical brake lag adds approximately one-half second (about 32 feet at 55 mph) to stopping distance after pedal application before compressed air builds sufficient pressure to actuate the brake shoes.Answer
- CEngaging the engine retarder completely eliminates air brake lag by utilizing driveline resistance to lock the drive wheels prior to service brake application.
- DProperly adjusted automatic slack adjusters eliminate mechanical brake lag entirely, allowing heavy air brake systems to match the instantaneous physical response of hydraulic brakes.
Answer
Mechanical brake lag adds approximately one-half second (about 32 feet at 55 mph) to stopping distance after pedal application before compressed air builds sufficient pressure to actuate the brake shoes.
Total stopping distance in commercial motor vehicles with air brakes consists of four distinct stages: perception distance, reaction distance, brake lag distance, and effective braking distance. Because air takes roughly one-half second to travel through the air lines and fill the brake chambers, the vehicle travels approximately 32 additional feet at 55 mph before the brakes actually begin slowing the vehicle.
Step-by-Step Solution
Key Concept
Total Stopping Distance Breakdown and Air Brake Lag Mechanics
Estimated Time:1m 30s