Double de-clutching

Double clutching (also called double de-clutching) is a method of shifting gears used primarily for vehicles with an unsynchronized manual transmission, such as commercial trucks and specialty vehicles. While double clutching is not necessary in a vehicle that has a synchronized manual transmission, the technique can be advantageous for smoothly downshifting in order to accelerate.

With this method, instead of pushing the clutch in once and shifting directly to another gear, the driver first engages the transmission in neutral before shifting to the next gear. The clutch is pressed and released with each change.

The double clutching technique involves the following steps:

In a gearbox with neutral between gears, 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 unloads the engine and transmission of the force from the opposing components. Using the clutch for each shift out of, and then into, each gear is double (de)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 typical motorcycle.

Before the introduction of transmission synchronizers (in the 1920s), double clutching was a technique required to prevent damage to an automobile's gearing during shifts. 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 the driver wishes 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 "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 travelling outside the gear's speed or directional range, such as accidentally into 1st from near the top of 2nd, or intentionally from reverse to a forward gear whilst still moving at speed.

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Wikipedia