Double Clutching
A manual transmission shifting technique where the clutch is engaged and disengaged in neutral to match gear speeds.
Double clutching is a manual transmission driving technique where the driver depresses the clutch pedal twice during a single gear change. Historically, double clutching was mandatory on older trucks and cars equipped with unsynchronized manual transmissions to prevent gear grinding. On modern synchronized gearboxes, it is rarely necessary but is still practiced to reduce wear on synchronizer rings.
The shift sequence begins by depressing the clutch and shifting from the current gear into neutral. The driver then releases the clutch pedal, taps the accelerator (rev-matching) to bring the input shaft speed up to match the output shaft speed for the target gear, depresses the clutch again, and shifts into the next gear. This manual speed matching takes the load off the transmission synchros.
If a manual transmission has worn synchronizers, shifting gears can become difficult and result in loud metallic grinding. Double clutching allows a driver to continue operating a vehicle with bad synchros without grinding the gear dog teeth. However, if shifting is persistently rough, it indicates the synchronizer rings or clutch assembly require service.