5.2 Downgrade Control and Brake Fade

Downgrade Control and Brake Fade

Slowing and controlling a heavy vehicle on a long, steep downgrade is one of the most demanding tasks a commercial driver faces. The massive weight of a loaded truck or bus creates tremendous kinetic energy as gravity pulls the vehicle downhill. If a driver relies solely on the service brakes to control this speed, the brakes will quickly overheat, leading to a catastrophic condition known as brake fade. To operate safely, drivers must understand the mechanics of brake heat, how to select the proper gear, and how to use the snubbing technique to manage speed.

The Mechanics of Brake Heat and Brake Fade

Drum brakes work by pressing friction linings (brake shoes) against the inside of a rotating metal drum. This friction converts the kinetic energy of the rotating wheels into thermal energy (heat). Under normal stopping conditions, the heat generated is absorbed by the drum and dissipated into the surrounding air. However, on long, steep downgrades, continuous or prolonged brake application generates heat much faster than it can be dissipated.

When drum brakes reach extreme temperatures (typically exceeding 250°C to 300°C), they suffer from brake fade, which occurs due to two distinct physical processes:

1. Brake Drum Expansion: As the cast-iron brake drum heats up, it expands outward, away from the brake shoes. This expansion increases the internal diameter of the drum. Consequently, the brake shoes must travel a greater distance outward to make contact with the drum surface. The slack adjuster and pushrod must stroke further. If the pushrod reaches its travel limit (bottoms out in the chamber), no additional braking force can be applied, even if air pressure is increased to the maximum. If the brakes are slightly out of adjustment, even minor drum expansion can cause total brake failure.
2. Lining Friction Loss (Glazing): The friction lining materials are designed to operate within specific temperature ranges. When overheated, the bonding resins in the linings can vaporize or melt, forming a slick, glazed surface. This reduces the coefficient of friction between the shoe and the drum. In extreme cases, the vaporized resins form a layer of hot gas between the lining and the drum, effectively acting as a lubricant. This prevents the shoes from gripping the drum, even under high application pressure.

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Downgrade Speed Control and Engine Management

To prevent brake fade, a driver must never rely solely on the service brakes to control speed on a downgrade. Instead, the driver must utilize the engine's braking effect to resist gravity. This is called engine compression braking or retardation.

Pre-Selection of Gear

The driver must select a low gear before starting down the hill. A good rule of thumb is to use a gear that is lower than the gear required to climb the same hill. If the hill is unfamiliar, err on the side of caution and select a very low gear.

Changing gears on a steep downgrade, especially in vehicles with manual transmissions, is extremely dangerous. If the driver misses a gear change, the transmission can become stuck in neutral (known as being 'locked out' of gear), leaving the driver with no engine braking. In this situation, the driver is forced to rely entirely on the service brakes, which will quickly overheat, fade, and fail, leading to a runaway vehicle.

Auxiliary Retarders

Many commercial vehicles are equipped with auxiliary retarders, such as engine compression brakes (often called Jake Brakes), exhaust retarders, hydraulic retarders, or electromagnetic retarders. These devices use engine compression or magnetic resistance to slow the drive wheels, saving the service brakes for stopping.

However, retarders only apply braking force to the drive axle. On slippery surfaces (rain, snow, ice), the aggressive braking force of a retarder can lock the drive wheels and cause a jackknife or skid. Therefore, retarders must be turned off or deactivated on slippery roads.

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The Snubbing Technique

When descending a grade in the correct low gear, the vehicle's speed may still slowly increase due to gravity. To control this speed without overheating the brakes, the driver must use the snubbing technique (sometimes called interval braking). The technique is executed as follows:

1. Determine the Safe Speed: Based on road signs, vehicle weight, weather, and traffic, determine the maximum safe speed for the descent (e.g., 60 km/h).
2. Let Speed Rise: Allow the vehicle to roll in gear until its speed reaches the safe speed.
3. Apply Brakes Firmly: Apply the service brakes firmly and smoothly, using an application pressure of approximately 100 to 135 kPa (15 to 20 psi).
4. Reduce Speed by 10 km/h: Hold the brake application until the vehicle's speed is reduced to approximately 10 km/h below the safe speed (e.g., from 60 km/h down to 50 km/h). This brake application should take about 3 to 5 seconds.
5. Release Brakes Fully: Release the brakes completely. This allows the drums and linings a period of time to cool as the vehicle rolls and air circulates around them.
6. Repeat the Cycle: Once the brakes are released, the vehicle will begin to accelerate again under the force of gravity. When the speed climbs back to the safe speed, repeat the process.

Why Riding the Brakes is Dangerous

Riding the brakes—applying constant, light pressure on the pedal to maintain a constant speed—is the primary cause of runaway trucks. Light pressure keeps the brake linings in continuous contact with the drums. Because there is no release phase, the heat builds up continuously, with no opportunity for cooling. The drums expand, the linings glaze, and within a short distance, the brakes fade completely, leading to a runaway vehicle. Snubbing, by contrast, alternates between short, firm applications (generating heat briefly but slowing the vehicle effectively) and long release intervals (allowing airflow to cool the drums and linings).