Double clutch
A double clutch (also called a double declutch) is a driving procedure primarily used for vehicles with an unsynchronized manual transmission.
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 involved in learning the technique, and because of the advent of synchronized gearing systems, it has largely fallen into disuse. However, drivers of large trucks still use the double clutching technique, as those vehicles are usually equipped with older, more efficient and more durable, unsynchronized gearboxes.
The purpose of the double-clutch technique is to match 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). Once the speeds are matched, the gear will engage smoothly. 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 automatically.
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.
However, in order to downshift, engine RPM must be increased while the gearbox is in neutral and the clutch is engaged. This requires the driver to shift into neutral, release the clutch pedal, apply throttle to bring the RPM up to a suitable speed, depress the clutch pedal again, 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 accurately.
A related 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. Note that Heel-and-Toe can be used with any downshift clutch operation, not just with double-clutching. Though difficult, mastering Heel-and-Toe in conjunction with double clutching is essential for high performance driving (e.g., Rally racing) because taking a corner turn as fast as possible is essential to any race. 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, which also places less wear and tear on the entire drivetrain.
External links
- Video of heel-toe braking in conjunction with double-clutch downshifts in a racecar. (.WMV)
- What's All This Double-Clutching Stuff, Anyhow? (another detailed description and reasons for double clutching)
- Heel and Toe Double Clutching Technique