Double Clutch explained

Double declutch explained

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http://en.wikipedia.org/wiki/Manual_transmission#Unsynchronized_transmission

A double clutch (also called a double declutch) is a driving procedure used for vehicles with an unsynchronized manual transmission.
Before the introduction of synchronizers (1920s/30s) and helical cut gears, double declutching was 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 synchonized gearing systems it has largely fallen into disuse. However, drivers of large trucks still use double clutching, as those vehicles are usually equipped with the older, more efficient, and more durable unsynchronized gearboxes.
The purpose of the double clutch is to match the speed of the rotating parts of the gearbox for the gear you wish to select to the speed of the input shaft being driven by the engine. 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 double-clutched vehicle, the clutch pedal is pressed lightly 2 or 3 cms, the throttle is released, and gearbox shifted into neutral. The clutch pedal is then released. As the engine idles with no load, the rpms 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 maneuver 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 revs must be increased usually 300 or 400 rpm)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 revs up to a suitable speed, (eg 1000revs up to 1400 revs) depress the clutch again, keeping the revs up, 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 and is often conducted as cars in front slow down, however this is overcome by
1. not changing down to stop in traffic.
2. planning ahead (system of Vehicle Control) when approaching a hazard, corner etc.
cheers
Keith