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Concept
Non-synchronous transmission
Summary
A non-synchronous transmission, also called a crash gearbox, is a form of manual transmission based on gears that do not use synchronizing mechanisms. They require the driver to manually synchronize the transmission's input speed (engine RPM) and output speed (driveshaft speed).
Non-synchronous transmissions are found primarily in various types of industrial machinery; such as tractors and semi-tractors. Non-synchronous manual transmissions are also found on motorcycles, in the form of constant-mesh sequential manual transmissions. Prior to the 1950s and 1960s, most cars used constant-mesh (and also sliding-mesh) but non-synchronous transmissions.
Most early automobiles were rear-engined, using a single-speed transmission and belt-drive to power the rear wheels. In 1891, the French Panhard et Levassor automobile used a three-speed manual transmission and is considered to have set the template for multi-speed manual transmissions in motor vehicles. This transmission used a sliding-gear design without any form of speed synchronization, causing frequent grinding of the gear teeth during gear shifts.
The Panhard design was refined over the years by other manufacturers to include "constant-mesh" gears (instead of sliding gears). The first usage of synchromesh was by Cadillac in 1928.
Trained drivers of vehicles with non-synchronous transmissions sometimes use the techniques listed below. If improperly implemented, these techniques can cause damage to the vehicle or the loss of control of the vehicle.
Double-clutching: releasing the clutch in neutral to synchronize the speeds of the shafts within the transmission
Float shifting: shifting without using the clutch
In big rigs and semi-trucks, the driver may have to complete 24 or more gear changes when accelerating from a standstill to .
Unlike any other type of transmission, non-synchronous transmissions often have a clutch brake mechanism, which is usually activated by pressing the clutch pedal all the way to the floor or pressing a button on the top of the gear lever.
Official source
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