7.1 Braking Fundamentals: Heat, Brake Lag, Stopping Distance, and Fade

Braking Fundamentals: Heat, Brake Lag, Stopping Distance, and Fade

Quick Answer: Brakes stop a vehicle by turning its energy of motion into heat, which the drums or rotors must shed to the air. Brake failure happens when more heat is absorbed than can be dissipated, usually because the vehicle is driven too fast for conditions. Alberta's Commercial Driver's Guide breaks total stopping distance into four parts: perception (about 3/4 second), reaction (about 3/4 second), brake lag (about 4/10 second for air brakes), and braking distance. Longer vehicles have more lag. Heavier, faster, and downhill vehicles need much more distance.

Brakes are heat machines

Moving vehicles have energy of motion. To stop, the brakes rub a stationary lining against a rotating drum or rotor, and that friction converts the energy into heat. The heat must then be dissipated to the atmosphere by the drums or rotors. The course's key warning:

Brake failure arises when more heat is absorbed by the drums or rotors than can be dissipated. This is caused by driving too fast for conditions.

Everything in this chapter follows from that idea. More weight, more speed, and steeper grades all mean more energy and more heat. Speed is the one the driver controls.

Brake lag: the air brake delay

With hydraulic brakes, the fluid is always in the lines, so pressing the pedal applies the brakes almost immediately. Air brakes are different. When you press the pedal, air must flow through the valves and lines and fill the chambers before the brakes apply. The Commercial Driver's Guide puts this lag time at about 4/10 of a second. Lag time distance is how far the vehicle travels during that delay.

What increases lag:

• Vehicle length. The longer the vehicle, the more brake lag.
• Line layout and fittings. Bends and couplers slow the airflow.

Relay valves near the axles (Section 3.2) exist largely to cut this lag.

The four parts of total stopping distance

The Commercial Driver's Guide defines total stopping distance as the sum of four distances:

1. Perception distance. Perception time is how long it takes to see a situation and realize you need to stop. For the average person it is about three-quarters of a second. It is often longer for inexperienced, distracted, tired, or unwell drivers.
2. Reaction distance. Reaction time is how long it takes to physically react, by releasing the accelerator and pressing the brake pedal. For the average driver it is about three-quarters of a second, and slower if you are tired or have used alcohol or drugs.
3. Lag time distance. This is how far the vehicle travels during the about 4/10 of a second air brake lag.
4. Braking distance. This is how far the vehicle travels once the brakes are working. It depends on the force applied to the brakes, the condition of the linings and drums, tire traction, and vehicle weight and speed.

Worked illustration

The table uses the guide's perception, reaction, and lag times. To show scale, it assumes a steady braking deceleration of about 4.5 m/s². That figure is an illustration, not an Alberta standard: real braking distances vary with load, brakes, tires, and road.

Two lessons stand out. First, at 100 km/h you travel about 11 m before the brakes even begin to work, just from lag. Second, going from 50 to 100 km/h doubles the perception, reaction, and lag distances but quadruples the braking distance.

Weight, speed, and grade

• Weight. Increasing mass increases the braking needed. Kinetic energy is proportional to mass, so a loaded vehicle's brakes must absorb far more energy than an empty one's. The guide gives the example of a 66-passenger bus: at 45 kg per passenger, the load is about 3,000 kg (6,600 lb) on top of the bus's own weight, and it takes dramatically longer to stop than a car.
• Speed. Kinetic energy is proportional to the square of speed. Doubling your speed means four times the energy to turn into heat, and about four times the braking distance.
• Grade. A vehicle going downhill needs a longer stopping distance than on a level road, because gravity keeps pulling it forward. Uphill, it stops in a shorter distance.

Large vehicles take longer to stop than passenger cars. That sounds obvious, but new commercial drivers must plan for it: they need more space and more time for every manoeuvre.

Brake fade

Brake fade is a loss of braking effectiveness while the brakes are in use. The Alberta course explains that it comes partly from component expansion and partly from reduced friction between the lining and the drum:

• Expansion. Hot drums grow in diameter, away from the linings, so the pushrod must travel farther for the same application. An out-of-adjustment brake can run out of stroke (Section 6.2).
• Reduced friction. Overheated linings grip the drum less.
• Water. Wet linings and drums also brake poorly until they dry. Light applications while moving slowly help dry them.

A faded brake gives less stopping power even with the pedal pressed harder. It also leaves the remaining brakes to do more work, so they heat and fade in turn. The cure is prevention: correct speed, the right gear, properly adjusted brakes, and the downgrade techniques in Section 7.2.

Brake balance

The Commercial Driver's Guide stresses that brakes must be properly adjusted, because otherwise some brakes work harder than others, which could cause a skid. Overusing the trailer brakes (hand valve) wears them more than the tractor brakes and unbalances the combination, which could lead to a jackknife. Balanced, properly adjusted brakes share the heat and keep the vehicle straight.