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

Alberta Air Brake Q Endorsement

Track your progress as you read

Finished sections get a check mark.

1 Chapter 1: Alberta Q Endorsement Program and Air Brake Fundamentals

1.1 Alberta Q Endorsement Path, Course Requirements, and Knowledge Test Logistics 1.2 Licensing Scope, Farm Vehicle Exceptions, and the Technician Route 1.3 Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours

2 Chapter 2: The Supply Circuit: Compressor, Governor, Tanks, and Air Dryers

2.1 Air Compressors, Drive Systems, and Governor Operation 2.2 Supply and Service Tanks, Contamination, and Draining 2.3 Air Dryers, Air Brake Antifreeze, Safety Valves, and Check Valves

3 Chapter 3: Service Circuits, Control Valves, Gauges, and Low-Air Warnings

3.1 Dual Air Systems: Primary and Secondary Service Circuits 3.2 Brake Chambers, Control Valves, Quick Release Valves, and Relay Valves 3.3 Air Pressure Gauges, Low-Air Warnings, and the Driver's Response

4 Chapter 4: The Park Circuit and Tractor/Trailer Circuits

4.1 Spring Parking Brakes, the Park Control Valve, Compounding, and Caging 4.2 The Tractor Protection System, Trailer Supply Valve, and Glad Hands 4.3 Trailer Service Control, the Trailer Hand Valve, and Air Park Brake Trailers

5 Chapter 5: Air System Failures and Emergency Braking

5.1 Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits 5.2 Trailer Air System Failures: Service Line, Supply Line, Reversed Lines, and Trailer Tanks

6 Chapter 6: Foundation Brakes and Brake Adjustment

6.1 Foundation Brakes: S-Cam Drum, Disc, and Wedge Brakes 6.2 Brake Stroke, Adjustment Limits, and Stroke Indicators 6.3 Adjusting Manual Slack Adjusters and Maintaining Automatic Slack Adjusters

7 Chapter 7: Braking Fundamentals, Driving Techniques, and Brake Management Systems

7.1 Braking Fundamentals: Heat, Brake Lag, Stopping Distance, and Fade 7.2 Downgrades, Auxiliary Retarders, Bobtailing, and Winter Braking 7.3 Antilock Brakes, Automatic Traction Control, and Stability Control

8 Chapter 8: Vehicle Inspections and the Air Brake Practical Test

8.1 Trip Inspection Law, Major and Minor Defects, and Roadside Enforcement 8.2 Inspection Set-Up and Recognizing Air Brake Defects 8.3 Trip Air Brake Inspection Steps 1–3: Tractor Protection and Park Control Checks 8.4 Trip Air Brake Inspection Steps 4–6: Supply Circuit, Leak Test, Brake Adjustment, and Scoring

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

Key Takeaways Explain This Topic AI

Brakes convert the energy of motion into heat, which the drums or rotors must dissipate; brake failure results when more heat is absorbed than can be dissipated, usually from driving too fast for conditions.

Total stopping distance is perception distance plus reaction distance plus brake lag distance plus braking distance; perception and reaction each take about three-quarters of a second.

Air brake lag, the time for air to flow through the system and apply the brakes, is about 4/10 of a second; hydraulic brakes apply almost immediately because the fluid is always in the lines.

Brake lag increases with vehicle length and with line layout and fittings: a body job has the least, and a triple trailer combination, with five pairs of glad hands and up to 35 m of line, has the most.

Kinetic energy rises with weight and with the square of speed; downhill stops take longer and uphill stops shorter than on level ground.

Last updated: September 2026

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.

Vehicle Lag
Body job (straight truck, components close together) Least affected
Tractor with one trailer More

Triple trailer combination Most: control air for the last trailer passes through five pairs of glad hand couplers , each set with two 90-degree bends, over distances up to 35 metres (114.8 feet)

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:

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.

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.

Lag time distance. This is how far the vehicle travels during the about 4/10 of a second air brake lag.

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.

Speed Perception (0.75 s) Reaction (0.75 s) Lag (0.4 s) Braking (illustrative) Total
50 km/h (13.9 m/s) 10.4 m 10.4 m 5.6 m 21.4 m about 48 m
80 km/h (22.2 m/s) 16.7 m 16.7 m 8.9 m 54.9 m about 97 m
100 km/h (27.8 m/s) 20.8 m 20.8 m 11.1 m 85.7 m about 139 m

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.