Braking Systems, Brake Fade, Stopping Distances, and Air Brake Delay

Braking System Types on Heavy Rigid Vehicles

Heavy rigid vehicles may use several braking technologies, often in combination. Understanding how each works — and how each can fail — is essential for safe operation and for the Victorian HR knowledge test.

Comparison of Heavy Vehicle Braking Systems

Brake Fade on Descents

Brake fade is the loss of braking effectiveness that occurs when brake friction material overheats. The friction coefficient of brake linings drops sharply above a certain temperature (typically around 300–400°C for drum brakes). On long downhill sections, continuous light braking can cause drums to heat up until the brakes produce almost no retardation.

Symptoms of brake fade include:

• The brake pedal feels normal but the vehicle does not slow as expected
• Increasing pedal travel with diminishing response
• A burning smell from the brake area
• In severe cases, brake smoke

Preventing brake fade

The key strategy is to reduce reliance on the service brakes on descents:

1. Select a low gear before the descent so the engine provides braking through compression (engine braking).
2. Use an auxiliary retarder (exhaust brake or engine brake) to provide continuous deceleration without heating the service brakes.
3. Use the service brakes in firm, short applications rather than dragging them continuously. Brake hard to reduce speed by 10–15 km/h, then release to let the drums cool, and repeat as needed. This is called snub braking and is far more effective at managing brake temperature than constant light pressure.
4. Never exceed the speed at which the selected gear can hold the vehicle. If speed builds up beyond what the gear can manage, you will need heavy braking — which causes fade.

Empty vs Loaded Braking

A common misconception is that an empty vehicle is easier to stop safely. In reality, braking dynamics change significantly with load:

• Loaded vehicle: More mass means more kinetic energy to dissipate, so the vehicle needs a longer stopping distance at the same speed. However, the extra weight also increases the downward force on the tyres, providing more traction (grip) and allowing harder brake application before the wheels lock.
• Empty vehicle: Less mass means less kinetic energy, so theoretically shorter stopping distance. But the reduced weight on the driving and braking axles means less traction. The same brake force that merely slows a loaded truck can lock the wheels of an empty one. Once wheels lock, the vehicle loses directional control and the braking distance actually increases (a sliding tyre has less grip than a rolling tyre under threshold braking).

This is particularly dangerous when a driver is accustomed to a loaded vehicle and then drives empty: the brakes feel much more sensitive, and a panic application can lock the wheels. Drivers must adjust pedal pressure based on load — lighter pressure when empty.

Air Brake Delay

Air brake systems have an inherent lag between the moment the driver presses the brake pedal and the moment the brake chambers at the rear of the vehicle (or on the trailer) actually apply. This delay exists because:

• Air must travel through the supply and control lines from the foot valve to the relay valve and then to the brake chambers.
• The longer the air lines (long vehicle or vehicle plus trailer), the greater the volume that must be pressurised before the brakes move.
• Air is compressible, unlike hydraulic fluid which is effectively instantaneous.

Typical air brake delay is 0.5 to 1 second from pedal to full brake application at the rearmost axle. At 80 km/h (22.2 m/s), a 1-second delay means the vehicle travels approximately 22 metres before the brakes even begin to work. This must be factored into following distances and approach speeds.

Compensating for air brake delay

• Maintain a Crash Avoidance Space (discussed in the next section) that accounts for the delay — do not follow other vehicles at the same distance a car would.
• Begin brake applications earlier than you would in a light vehicle, particularly when approaching intersections or stopped traffic.
• On vehicles with trailers, ensure the trailer brake valve (load-sensing valve or relay emergency valve) is correctly adjusted so the trailer brakes apply slightly before the towing vehicle's brakes — this prevents the trailer pushing the towing vehicle (jackknife risk).

Stopping Distances

The total stopping distance for a heavy vehicle is the sum of:

1. Reaction distance: Distance travelled during the driver's reaction time (typically 0.75–1.5 seconds). At 80 km/h, a 1-second reaction = ~22 m.
2. Air brake lag distance: Distance travelled during air brake delay (~0.5–1 s). At 80 km/h, ~11–22 m.
3. Braking distance: Distance travelled from when brakes apply to when the vehicle stops.

Stopping distance increases with the square of speed. If speed doubles, braking distance increases fourfold. A heavy rigid vehicle at 100 km/h needs roughly four times the braking distance of the same vehicle at 50 km/h — before adding the extra reaction and air brake lag distance from the higher speed.

Because a heavy vehicle's service brakes, load and retarder all interact, the safest approach is to always maintain speed margins that leave plenty of room. The Victorian 100 km/h cap for heavy vehicles (over 4.5t GVM) is not just a legal limit — it is a safety limit reflecting the long stopping distances at highway speeds.