8.5 Gearboxes, Double-declutching and Auxiliary Brakes

Know the gearbox before using it

The handbook describes synchromesh, automatic and non-synchromesh gearboxes. The driver needs the correct licence conditions and operating skill for the vehicle. A truck with a familiar class label may have an unfamiliar transmission, range selector or retarder. Review the controls while parked and receive instruction before entering traffic.

A synchromesh gearbox uses synchronisers to help match the rotating components for a selected ratio. It operates broadly like a conventional manual car gearbox, but the vehicle’s weight, ratios and clutch system can differ. Do not force the lever when engine and road speeds are unsuitable. Resistance or grinding is a reason to correct the technique or investigate a fault, not apply more force.

Non-synchromesh transmission

A non-synchromesh, or crash, gearbox does not provide the same synchronising assistance. The driver must match speeds. The name does not mean crashing the gears is acceptable. Poor matching can damage the transmission and distract the driver while speed and road position change.

The handbook states that double-declutching is essential while learning this gearbox. The gear lever moves first to neutral, then to the desired gear, with the clutch pedal released between those movements. An experienced instructor should teach the exact pedal travel, timing and engine-speed matching for the vehicle. A written sequence supports understanding but cannot replace supervised practical training.

Understand the double-declutch sequence

For a typical instructed manual change, depress the clutch to leave the current gear and move to neutral. Release the clutch in neutral so the relevant components can be matched. Adjust engine speed as required for the intended ratio, depress the clutch again, select that ratio and release smoothly. The exact technique depends on the vehicle and instruction.

An upshift generally requires a lower engine speed for the same road speed in the higher ratio. A downshift generally requires a higher engine speed for the same road speed in the lower ratio. This explains the speed-matching task; it is not an instruction to use a universal throttle blip or fixed revolutions-per-minute target on every truck.

The handbook does not recommend double-declutching for synchromesh gearboxes and warns of possible long-term damage. Use the technique specified for the gearbox rather than assume the most complicated method is always best. Never practise an unfamiliar change in a hazardous descent or congested intersection.

Range, split and mode controls

Heavy gearboxes can have range or split selectors that change the available ratio. Learn the pattern and when each control can be used. A lever position may represent a different ratio depending on the selected range. Wrong selection can result in engine overspeed, loss of useful drive or an unexpected change in retarding effect.

Automatic and automated systems can offer manual selection or downhill modes. They do not remove the driver’s responsibility to choose a suitable speed and understand whether the system will hold or change a ratio. Follow manufacturer guidance. Do not assume every system behaves identically when the accelerator is released or the retarder is activated.

Gear selection and safe movement

Select a ratio appropriate to load, gradient and the next manoeuvre. A loaded uphill start can need a lower ratio than an empty departure. A curve or intersection should be approached in a gear allowing smooth control without a rushed change midway. Railway crossings should be planned to avoid stopping or unnecessary shifting on the tracks.

Avoid coasting in neutral. It removes useful engine control and can make re-engagement difficult. Do not use the clutch to hold the vehicle on a hill or rest a foot on it during travel. Use the proper brake and starting procedure. If a gear is missed, maintain observation and follow the trained recovery procedure rather than force an unsuitable ratio.

How auxiliary braking relates to gears

An exhaust brake restricts exhaust flow to create retarding effect. An engine brake changes engine operation to absorb energy. Electric, magnetic or hydraulic retarders absorb energy through their particular system. Their effectiveness and limits depend on engine or driveline speed, temperature and design.

These devices supplement the service brake and generally cannot complete the final stop. A suitable gear can make them more effective on a descent, but excessive retarding demand through the drive wheels can cause a skid on a slippery road. Noise also matters in built-up areas; use equipment responsibly without compromising control. Learn the settings and warnings for the actual system.

Key decisions

Use the transmission technique and controls the vehicle requires, select ratios before demanding manoeuvres and retain safe observation throughout a change. Gearbox skill and licence conditions must both be suitable.

Official sources

Official handbook, gearboxes and auxiliary brakes, pages 57–59; gearbox licence conditions. Sources checked 10 October 2026.