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Double clutching, sometimes called double declutching, is a method of shifting gears with a manual transmission. Instead of pushing in the clutch once and shifting to another gear, the driver first shifts the transmission into neutral, and then to the next gear, pressing the clutch with each change. The technique, once widely used for automobiles with standard transmissions, is now limited mostly to commercial trucks and specialty autos, like race cars, that do not have synchronized manual transmissions.
A clutch is a device that allows power to be transmitted from one part of a machine to another. In practical terms, a vehicle clutch is used to allow the power of the engine to be transmitted to the transmission, or gearbox. As an engine is given more gas, it is necessary to change gear ratios to match the engine's revolutions per minute (rpms) with that required by the driveshaft to move the car at a certain speed. Pressing the clutch pedal in a standard or manual transmission vehicle disengages it from the engine, allowing the engine to turn without transferring power to the wheels. Once the gear selector is shifted to a new gear, the clutch is released to allow the engine to provide power again. In most manual vehicles, the clutch is operated by pushing down on the left pedal.
Double clutching adds another step to the basic act of changing gears. Rather than going straight from one gear to the next, the driver shifts first to neutral and the clutch is released. This allows the engine to speed up or slow down to the necessary rpms so the transition into the next gear is smoother. The driver then presses the clutch and shifts into the target gear, and the clutch is released again, putting the car back into gear. It only takes a split second for the engine speed to match the speed of the gears, so the entire action is done very quickly in a fluid sequence.
Benefits of Double Clutching
By shifting into neutral before changing gears, double clutching can help save wear and tear on the gears and gear selector. If the engine and transmission are not moving at the same speeds, the teeth on the gear selector will not mesh with the gear smoothly, causing them to grind. Most modern cars are equipped with devices called synchronizers that help match the speed of the transmission with that of the engine, which effectively eliminates the need for double clutching. Some large trucks, however, have so many gears that synchronizers are inefficient; in addition, race-car drivers can get more power out of the engine without them.
Double clutching can aid high-performance driving because it can give the driver more control of a vehicle as he or she slows to go through a turn. Without the maneuver, a vehicle has a tendency to fishtail when going around a sharp turn and decelerating from a high speed, because jamming on the brakes tends to abruptly throw the weight of the vehicle forward. By downshifting, a driver can also slow the vehicle with the engine, taking some pressure off the brakes. This can be particularly important for heavily laden commercial trucks descending steep downgrades, where burning out brakes can create a serious hazard.
Sometimes the driver of a regular manual transmission car may want to make use of double clutching. Downshifting, shifting into a lower gear, has advantages in slippery conditions; rather than brake and skid, a car with a manual transmission can make use of the engine to slow down, making a skid less likely. Even with synchronizers, a driver trying to shift from fifth gear into second could have some trouble. In this case, double clutching by pausing in neutral and giving the engine a little gas to match the rpm of the engine and transmission can allow such a drastic shift.
Some driving purists maintain that double clutching is the proper way to shift and makes for a much smoother ride. It may eliminate part of the jumpiness sometimes felt in a manual transmission car, and it may decrease the stress on the gearbox. Others counter that double clutching puts more wear on the clutch, since it is used twice as often as in normal shifting; each time the clutch is engaged, friction causes it to wear down. Either way, many people find that double clutching is a skill worth having for emergency downshift situations and can be learned with a little practice.