Editing Double-clutching (technique) (section)

==Theory==
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 the driver wishes to select. The output shaft in the transmission is directly driven by the rotating wheels, and each gear set is a different ratio, so in a five-speed transmission in fourth gear, one will have three lower gearsets not engaged spinning at three different, and faster, rates and one higher gear spinning at a ''slower'' rate than the input shaft. In order to shift down, the fourth gear has to be disengaged, leaving no gears connected to the input shaft. This is ''neutral'', and the input shaft and gears all need to be accelerated so the speed of the output shaft and the lower gear the operator wishes to select match speeds long enough for the [[dog clutch]] to lock them together. 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 "clash" 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 from the transmission speed, the desired gear can often not be engaged 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 accidentally into first from near the top of the second, or intentionally from reverse to forward gear while still moving at speed. Double clutching, although (slightly) time-consuming, ease gear selection when an extended delay or variance exists between engine and transmission speeds, and reduces wear on the synchronizers (or baulk rings), which are brass cone clutches themselves, and wear very slightly each time they are used to equalize the transmission revs with the output revs.