Double clutch

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A double clutch (also called a double declutch) is a driving procedure primarily used for vehicles with an unsynchronized manual transmission.

In a gearbox with neutral between each gear, a typical shift actually involves two gear changes, once into neutral, and again into the next gear. During any shift, disconnecting drive components via a clutch properly unloads the engine and transmission of undue pressure applied by the opposing components. Fully utilizing the clutch for each shift out of, and then into each gear is double clutching. Due to the absence of a neutral spacing, double clutching is ill-advised for sequential gear changes, as in a fully sequential gearbox such as a sportbike.

Obviously simply not releasing the clutch pedal while in neutral such as in a typical shift is much more efficient than double clutching and second only to sequential or non clutch shifts. However, excess wear can take place on the separated clutch plates anytime the engine and transmission vary drive loads. Double clutching can minimize this clutch plate wear when extended durations occur between matching engine and transmission drive pressure, such as at a stop light.

Before the introduction of transmission synchronizers (in the 1920s) and helical cut gears, double clutching was a technique required to prevent damage to an automobile's gear system. Due to the difficulty and most often unnecessary redundancy involved in learning the technique, coupled with the advent of synchronized gearing systems, it has largely fallen into disuse. However, drivers of large trucks often use the double clutching technique when unable to keep the transmission unloaded during shifting, as large vehicles are usually equipped with older, simpler and more durable unsynchronized gearboxes.

The purpose of the double-clutch technique is to aid in matching the rotational speed of the input shaft being driven by the engine to the rotational speed of the gear you wish to select (directly connected to rotating wheels). When the speeds are matched, the gear will engage smoothly and no clutch is required. If the speeds are not matched, the dog teeth on the collar will "crash" or grate as they attempt to fit into the holes on the desired gear. A modern synchromesh gearbox accomplishes this synchronization more efficiently. However, when the engine speed is significantly different than the transmission speed, the desired gear is often unengageable even in a fully synchronized gearbox. An example is trying to shift into a gear while traveling outside the gear's speed or directional range, such as reverse while moving forward.

Double clutching, although time consuming, eases gear selection when an extended delay or variance exists between engine and transmission speeds. When shifting up on a non-synchroniser equipped vehicle, the clutch pedal is pressed, the throttle is released, and the gearbox is shifted into neutral. The clutch pedal is then released. As the engine idles with no load, the RPM will decrease until they are at a level suitable for shifting into the next gear. The driver then depresses the clutch again and shifts into the next gear. The whole manoeuvre can, with practice, take no more than a fraction of a second, and the result is a very smooth gear change.

Conversely, in order to downshift, engine RPM must be increased while the gearbox is in neutral and the clutch is either engaged or disengaged. This requires the driver to shift into neutral, apply throttle to bring the RPM up to a suitable speed, and finally shift into gear. This operation can be very difficult to master, as it requires the driver to gauge the speed of the vehicle and throttle accurately. Double clutching occurs if the clutch pedal is released while matching engine speeds in neutral and again engaged prior to shifting into the next gear.

A related downshifting technique is called heel-and-toe, during which the brake and accelerator pedal are pressed by the right foot while the clutch pedal is pressed by the left foot. This aids both slowing the vehicle while at the same time accelerating the engine for a matched downshift. Note that Heel-and-Toe should be used with any type of gearbox when simultaneous braking and downshifting is necessary. Though difficult, mastering the Heel-and-Toe technique in conjunction with necessary clutching is essential for high performance driving (e.g., rally racing) because driving as fast as possible is only achieved when in the optimal gear regardless of the simultaneous braking, accelerating, and clutching required for shifts. Left foot braking while accelerating the engine with the right foot to accommodate downshifting in a clutchless situation accomplishes the same feat.

The purpose of the heel-toe-double-clutch is to downshift into the correct gear for exiting the corner while not wasting speed or time during or after the turn while placing the least wear and tear on the entire drivetrain. This is typically seen when avoiding a shift through each gear such as when the vehicle is slowing faster than one prefers to downshift, such as a rapid speed drop from the highest to lowest gear. Often racers will simply clutch into neutral and wait for the vehicle speed to match the desired gear and then clutch again into that gear instead of shifting through every gear in between.

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