6.3 Brake Fade, Auxiliary Retarders, and Non-Synchromesh Transmissions

Brake Fade, Auxiliary Retarders, and Non-Synchromesh Transmissions

Key Focus: Heavy vehicle kinetic energy increases with the square of speed and directly with vehicle mass. On steep descents, relying solely on foundation friction brakes causes catastrophic brake fade. Drivers must master auxiliary retarding systems and proper gear selection in non-synchromesh transmissions to control heavy vehicles safely.

Controlling a 40- to 68-tonne commercial vehicle on extended downhill gradients (such as Mount Ousley, Bulli Pass, or the Moonbi Ranges) requires a thorough understanding of thermal brake fade, engine retarding devices, and transmission mechanics. A driver who mismanages these systems risks runaway vehicle disaster.

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The Physics and Mechanics of Brake Fade

Friction foundation brakes (drum brakes and disc brakes) convert the immense kinetic energy of a moving vehicle into thermal energy (heat).

What Causes Brake Fade?

1. Thermal Saturation: When foundation service brakes are applied continuously or repeatedly during a long descent, temperatures within the brake drums and linings soar beyond 300°C to 500°C.
2. Lining Glazing & Chemical Breakdown: At elevated temperatures, the bonding resins in brake lining friction compounds melt and vaporise, forming a microscopic gas layer and hard glass-like glaze across the friction surfaces. This dramatically drops the coefficient of friction (μ).
3. Drum Thermal Expansion: Intense heat causes the cast-iron brake drum to expand outwards and increase in diameter. The brake shoes must travel significantly further to contact the drum surface. If the brake pushrod stroke is at or near its adjustment limit, the brake chamber pushrod will bottom out, resulting in zero clamping force regardless of pedal pressure.

Prevention: Snub Braking & Gear Selection

• Select the Correct Low Gear Early: Before starting down a steep hill, the driver must downshift to a low gear—the same gear (or lower) required to climb the same gradient.
• Snub Braking Technique: Allow the engine and auxiliary retarder to control speed. If vehicle speed climbs above the safe threshold (e.g., 40 km/h), apply firm service brake pressure for 3 to 5 seconds to reduce speed by 8–10 km/h, then release completely. This brief, firm braking cycle gives the drums and linings maximum time to dissipate heat into ambient air.

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Auxiliary Braking Devices (Retarders)

Auxiliary retarders provide continuous, non-friction retarding power, absorbing vehicle kinetic energy through the powertrain without generating heat in the wheel brakes.

[Exhaust Brake]          ──► Butterfly valve in exhaust creates exhaust manifold backpressure.
[Engine Compression]     ──► 'Jake Brake' opens exhaust valves near TDC, releasing compressed air.
[Hydraulic Retarder]     ──► Automatic transmission fluid churned between rotor and stator vanes.
[Electric Retarder]      ──► Electromagnetic eddy currents create opposing drag on driveshaft.

1. Exhaust Brakes

• Operation: A pneumatic or electric butterfly valve closes inside the exhaust manifold pipe. This restricts exhaust gas expulsion, turning the engine cylinders into backpressure air pumps that resist piston upward movement.
• Characteristics: Quiet, reliable, and provides moderate retarding power; ideal for light-to-medium rigid trucks.

2. Engine Compression Brakes ('Jake Brakes')

• Operation: Electronically controlled hydraulic solenoids open the engine exhaust valves near Top Dead Center (TDC) of the compression stroke. The air compressed during the upward stroke is released into the exhaust manifold rather than pushing the piston back down.
• Characteristics: Produces substantial retarding force (often 300–600+ retarding horsepower on large 15-litre diesel engines). Drivers can select multiple retarding stages (Low / Med / High or 2/4/6 cylinders).
• Noise Restrictions: Jake brakes produce a loud staccato bark. In built-up urban zones with NO ENGINE BRAKES signage, drivers must disengage engine compression brakes unless in an emergency.

3. Driveline Retarders (Hydraulic & Electromagnetic)

• Hydraulic Retarders (Voith / ZF Intarder): Integrated into the transmission, oil is forced between rotating and stationary vaned discs, creating intense viscous fluid drag.
• Electromagnetic Retarders (Telma): Mounted on the propeller driveshaft, magnetic coils generate powerful eddy currents that resist driveshaft rotation without physical contact.

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Wet & Slippery Road Retarder Hazards

While auxiliary retarders are invaluable safety tools on dry mountain descents, they present an extreme hazard on wet, icy, or unsealed roads.

The Mechanism of a Retarder Jackknife

• Auxiliary retarders deliver 100% of their braking torque exclusively to the drive wheels (the steer and trailer wheels freewheel with zero retardation).
• On a slippery or low-traction surface (wet bitumen, spilled diesel, black ice, loose gravel), high retarding torque can easily exceed the available tyre-to-road friction coefficient.
• The drive tyres will break traction, spin down, and lock up. This immediately causes the rear of the prime mover to step sideways, initiating a violent and unrecoverable combination jackknife.
• Golden Rule: When road surfaces are wet or slippery, switch auxiliary retarders OFF or set them to their lowest setting, relying instead on balanced, ABS-monitored all-wheel service braking.

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Comparison Table: Auxiliary Retarders vs Service Brakes

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Non-Synchromesh ('Roadranger') Transmissions & Clutch Brakes

Heavy combination trucks frequently utilize non-synchromesh (constant-mesh) manual gearboxes (e.g., Eaton Fuller 13-speed or 18-speed Roadranger transmissions). These gearboxes do not have brass synchroniser cones; gear engagement requires the driver to match the rotational speeds of the input and output shafts precisely.

The Double-Declutching Procedure

To change gears smoothly without grinding teeth:

Upshifting Sequence (Accelerating):
1. Clutch IN ──► 2. Shift to Neutral ──► 3. Clutch OUT ──► 4. Engine RPM drops ──► 5. Clutch IN ──► 6. Engage Gear ──► 7. Clutch OUT

Downshifting Sequence (Decelerating / Climbing):
1. Clutch IN ──► 2. Shift to Neutral ──► 3. Clutch OUT ──► 4. BLIP Throttle (Rev match) ──► 5. Clutch IN ──► 6. Engage Gear ──► 7. Clutch OUT

Purpose and Correct Operation of the Clutch Brake

• What it is: A small friction disc located on the transmission input shaft between the clutch release bearing and the transmission front bearing cover.
• How it engages: It is activated only during the final 25 mm (1 inch) of clutch pedal travel (when the pedal is pushed completely flat against the floor).
• Correct Usage: Used strictly when the vehicle is stationary to stop the spinning input shaft, allowing the driver to engage first gear or reverse from neutral without grinding.
• Destructive Misuse: Depressing the clutch pedal to the floor while the vehicle is moving forces the clutch brake against a rapidly spinning shaft, burning out and destroying the clutch brake in seconds.

Missed Shifts on Descents

If a driver attempts a downshift on a steep downhill descent and misses the rev match, the gearbox will become stuck in neutral. Because the vehicle accelerates rapidly under gravity, the engine cannot rev high enough to match shaft speed. The driver loses all engine compression and auxiliary braking, resulting in a runaway truck.

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Operational Transport Scenario

Scenario: A truck driver is descending Mount Ousley (a steep 7-8 km descent near Wollongong) in a 42.5-tonne semi-trailer. Sudden light rain begins falling, creating slippery, greasy road conditions on the descent.

1. Pre-Descent Preparation: Before cresting the summit, the driver reduces speed to 40 km/h and selects 4th gear low range in their 18-speed Roadranger transmission.
2. Retarder Management: Recognizing the slick road surface, the driver switches the 3-stage engine compression brake from 'High' (6 cylinders) down to 'Low' (2 cylinders) to prevent the tandem drive tyres from losing traction and initiating a drive-axle jackknife.
3. Descent Execution: The vehicle descends at a controlled 35–40 km/h under low engine compression retarding. Whenever speed creeps toward 45 km/h, the driver uses smooth snub braking on the foot brake for 4 seconds, bringing speed down to 35 km/h. The foundation brakes remain cool and fully operational for the entire descent.

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Common Exam Traps & Pitfalls

• Trap 1: Retarders on Wet Roads: Exam questions regularly test auxiliary retarder behavior in rain. Never use high engine retarder settings on wet roads; it locks the drive wheels and triggers jackknifes.
• Trap 2: Clutch Brake Engagement at Speed: Pushing the clutch pedal fully to the floor while shifting gears on the open road will instantly destroy the clutch brake. It is strictly for stationary gear selection.
• Trap 3: Brake Fade Cause: Brake fade is caused by excessive heat buildup that glazes friction linings and expands drums—not by hydraulic or air leaks.