Double clutch (technique)

The double clutching technique involves the following steps:

• The accelerator (throttle) is released, the clutch pedal is pressed, and the gearbox is shifted into neutral.
• The clutch pedal is then released, the driver matches the engine speed to the gear speed either using the throttle (accelerator) (when changing to a lower gear) or waiting for engine speed to decrease (when changing to a higher gear) until they are at a level suitable for shifting into the next gear (although double clutching is almost always used for downshifting only).
• At the moment when the revs between input shaft (i.e. engine revs) and gear are closely matched, the driver then presses the clutch again and shifts into the next gear. The result should be a very smooth gear change.

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 clutching or declutching. Due to the absence of a neutral spacing, double clutching is impossible for sequential gear changes, as in a fully sequential gearbox used in most motorcycles and racecars.

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 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 (or were) usually equipped with older, simpler, and more durable unsynchronized ("crash") 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. 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, 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 travelling outside the gear's speed or directional range, such as accidentally into first from near the top of second, or intentionally from reverse to a forward gear whilst still moving at speed.

Double clutching, although (slightly) time consuming, eases 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.

One can "rev match" the same way without declutching the first time to take the transmission out of gear; as long as it is not under load. A transmission can usually be pulled out of gear and into neutral without significant difficulty, and then the operator can match revs, clutch and engage gear in the same manner as previously described. The first declutching is simply to ease taking the transmission out of gear. It is also possible to rev match and shift into a lower gear without using the clutch at all, but unless one is very skilled and practiced, this will generally take a lot of material off the synchronizers or grind gears excessively until it gets into gear (with unsynchronized transmissions). Provided one correctly matches the revs, a transmission can be shifted into gear quite easily without any clutch, but this is usually best left for emergencies only for the average automobile driver.

Although double clutching is a testing requirement when obtaining a commercial driver's license, most experienced truckers learn to shift gears without using the clutch. This is known as floating gears or float shifting, with the clutch needed only during starting and stopping.

Conversely, in order to shift down, engine speed must be increased while the gearbox is in neutral and the clutch is engaged. This requires the driver to slow the vehicle sufficiently, 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 to the intended gear accurately; vehicle weight and road gradient are important factors as they influence the vehicle's acceleration or deceleration during the shift.

Sometimes, truck drivers use the engine brake to help match the engine speed to the gear. The most common situation is with a loaded vehicle which has no split gears or half gears in the lower range, from gears 1–4. In this case, it is especially difficult and sometimes impossible to get from 1 to 2, and sometimes even from 2 to 3 while starting on a hill. The problem is that by the time the engine speed has dropped sufficiently to enable a shift into the higher gear, the vehicle will have slowed down too much or possibly even stopped, making the shift impossible. The engine brakes, which on some models can be set to different intensities (retarding variable numbers of engine cylinders) enable a shift by dropping the engine speed quickly enough to catch the higher gear before the vehicle has decelerated too much. This technique, sometimes called "jake shifting", requires high skill and much practice shifting without the clutch, and is usually not recommended among truck drivers because mistakes can cause damage to the transmission. Some authorities prohibit jake shifting.

• 10 Speed Shifting TIPS, 2018 – Truck Driver Students, YouTube, 29 September 2016 (double clutch, finding gears, slow down gears on stopped vehicles)
• What's All This Double-Clutching Stuff, Anyhow? (another detailed description and reasons for double clutching)