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 synchronise the transmission's input speed (engine RPM) and output speed (driveshaft speed).
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 synchronise the speeds of the shafts within the transmission
- Float shifting: shifting without using the clutch
In large heavy haul trucks, the driver may have to complete 24 or more gear changes when accelerating from a standstill to 70 mph (113 km/h).
Unlike any other type of transmission, non-synchronous transmissions often have a clutch brake mechanism, which is usually activated by the clutch pedal is pressed all the way to the floor or pressing a button on the top of the gear lever. The purpose of the clutch brake is to slow down (or stop) the rotation of the transmission's input shaft, which assists in shifting the transmission into neutral or first gear when the vehicle is at a standstill. The clutch brake not only slows or stops the idle gear axis, but can also prevent shifting into gear until the clutch pedal is released a few inches off the floor. In order to shift into gear, the clutch must be half way off the floor, otherwise the clutch brake will prevent the transmission from being shifted into or out of gear.
Comparison of transmissions
Any transmission that requires the driver to manually synchronize the engine speed with the speed of the driveshaft is non-synchronous. Non-synchronous transmissions are mostly used in trucks, large industrial machines and power take-offs.
Most manual transmissions in modern passenger vehicles are fitted with synchromesh to equalize the shaft speeds within the transmission, so they are synchronous transmissions. All automatic transmissions have synchronizing mechanisms, and semi-automatic transmissions that use dog clutches typically have cone-and-collar synchronizing mechanisms.
|Automatic / Semi-automatic|
- "Sliding Gearbox - 1891 Panhard et Levassor". www.youtube.com. Sporty 40. Retrieved 4 January 2020.
- "The 1891 Panhard et Levassor". www.themotormuseuminminiature.co.uk. Retrieved 4 January 2020.
- "1902 Panhard and Levassor". www.howstuffworks.com. 7 December 2007. Retrieved 4 January 2020.
- "Synchromesh Gear Box- How's That Work?". www.hooniverse.com. 3 November 2014. Retrieved 8 January 2020.
- 2005 Commercial Driver's License Manual (PDF). Federal Motor Carrier Safety Administration New Hampshire Dept. of Motor Vehicles. p. Section 13, page 3. Archived from the original (PDF) on 2012-05-21.
'Double clutch if vehicle is equipped with non-synchronized transmission
- "What is a Clutch Brake?". www.itstillruns.com. Retrieved 15 January 2020.
- "Clutch Brakes". www.amsautomotive.com. Retrieved 15 January 2020.
- "6-10 Bolt Mechanical Power Takeoff". www.thomasnet.com. Retrieved 2007-07-16.
- "Synchronizers; graphic illustration of how they work". www.howstuffworks.com. Retrieved 2007-07-18.