Double-clutching is a method of shifting gears that requires the driver to press the clutch pedal twice when shifting gears. It is used to help synchronize the speed of the engine and the transmission and to help reduce wear on the clutch.

Double clutching (technique)

Gear shifting method
This article is about the driving technique. For other uses, see Double clutch (disambiguation).
not to be confused with double clutch transmission.
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double clutch (also called double trip outside the United States) is a shifting method used primarily for vehicles with unsynchronized manual transmission, such as commercial trucks and special vehicles. Although dual clutching is not necessary on a vehicle that has a synchromesh manual transmission, the technique can be advantageous for smooth shifting for acceleration and, when done correctly, prevents wear on the synchronizers that normally equalize the input and output speeds of the transmission. transmission to allow downshifting.

With this method, instead of pushing the clutch engaging once and shifting directly into another gear, the driver first shifts the transmission into neutral before shifting to the next gear. The clutch is pressed and released with each shift. A related technique of downshifting/rpm matching is heel and toe displacementwhere the throttle is blepped (i.e. momentarily open during downshift) by the driver’s heel during braking.

History

Before the introduction of transmission synchronizers in the 1920s, double clutching was a necessary technique to prevent damage to an automobile’s gears during shifts. Due to the difficulty and, most of the time, unnecessary redundancy involved in the technique, together with the advent of synchronized gear systems, it has fallen into disuse in light vehicles. but many tractor units are still supplied with a non-synchronized gearbox that requires the use of the double-clutch technique.

technique

In an unsynchronized gearbox with neutral between gears, a typical shift actually involves two gear changes, once in neutral and again in the target gear. During any shift, disengaging the drive components through a clutch releases power from the opposing engine and transmission components. Using the clutch for each gear change and then inward, each gear is double clutch or disengage. Due to the absence of a neutral spacing, the double clutch is impossible for sequential gear changes, as with a fully sequential gearbox used in motorcycles and racing cars.

The double clutch technique involves the following steps:

  • The accelerator (accelerator) is released, the clutch pedal is pressed and the gearbox is placed in neutral.
  • The clutch pedal is then released, the driver adjusts the engine speed to the gear speed using the accelerator (when shifting to a lower gear) or waiting for the engine speed to decrease (when shifting to a higher gear) to a adequate level to shift to the next gear.
  • The moment the revolutions between the input shaft (i.e. engine revolutions) and the gear are close, the driver presses the clutch again, shifts to the next gear and releases the clutch. The result should be a smooth gear shift.

Although double clutching is a test requirement when obtaining a commercial driver’s license in some jurisdictions, many truck drivers learn to shift gears without using the clutch. This is known as Floating Gears or Floating Change, with the clutch needed only during starting and stopping. However, this is not recommended by unsynchronized gearbox manufacturers such as Eaton as it will usually cause additional wear on the gears.

Dual clutching can be difficult to master as it requires the driver to gauge the vehicle’s speed and accurately accelerate into the intended gear; vehicle weight and road grade are important factors as they influence vehicle acceleration or deceleration when shifting gears.

Truck drivers sometimes use engine braking to help match engine speed to gear. The most common situation is with a loaded vehicle that does not have split gears or half gears in the lower gear, from gears 1 to 4. In this case, it is especially difficult and sometimes impossible to change from 1 to 2, and sometimes even from 2 to 3 starting from a slope. The problem is that by the time the engine speed drops enough to allow a shift into a higher gear, the vehicle will have slowed down too much or possibly even stalled, making shifting impossible. The engine brakes, which on some models can be adjusted to different strengths (slowing down varying numbers of engine cylinders) allow for a shift by slowing the engine down quickly enough to shift into the highest gear before the vehicle slows down too much. This technique, sometimes referred to as “jake shifting,” requires a lot of skill and practice shifting gears without the clutch and is generally not recommended among truck drivers because mistakes can cause damage to the transmission. According to the National Highway Traffic Safety Administration (NHTSA), municipalities across the United States have banned the use of engine compression brakes because of noise emission, and as a result, shifting gears has been prohibited.

Theory

The purpose of the dual-clutch technique is to help match the rotational speed of the engine-driven input shaft to the rotational speed of the gear the driver wishes to select. The output shaft in the transmission is driven directly by the spinning wheels, and each gearset has a different ratio; rates and a higher gear turning in a Slower rate than the input shaft. To downshift, fourth gear must be disengaged, leaving no gear connected to the input shaft. That’s it neutraland the input shaft and gears need to be accelerated so that the output shaft speed and the lower gear the operator wants to select match the speeds long enough for the dog clutch to lock them together. When the speeds are matched, the gear engages smoothly and no clutch is needed. If the speeds are not equal, the dog teeth in the collar will “bump” or squeak as they try to fit into the desired gear holes. Modern synchronize gearbox performs this synchronization more efficiently. However, when the engine speed is significantly different from the transmission speed, often the desired gear may not be engaged, even on a fully synchronized gearbox. An example is trying to shift into a gear while traveling outside the speed or directional range of the gear, such as accidentally shifting into first gear near the start of second gear, or intentionally shifting into reverse into a forward gear while still moving at speed.

The dual clutch, although (slightly) time consuming, makes gear selection easier when there is an extended delay or variation between engine and transmission speeds and reduces wear on the synchronizers (or baulk rings), which are the cone clutches themselves. brass, and they wear very slightly each time they are used to equalize the transmission revs with the output revs.

References


Source: Double clutching (technique)
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