page_title: Manual transmission
section_title: Synchromesh
hierarchical_section_title: Manual transmission / Internals / Synchromesh
caption_attribution_description: English: synchronization of the Volkswagen Golf 1600 4th Series Italiano: sincronizzatore della volkswagen golf 1600 4° serie
context_page_description: A manual transmission, also known as a manual gearbox, a standard transmission, stick shift, gearbox, or clutch, is a type of transmission used in motor vehicle applications. It uses a driver-operated clutch, usually engaged and disengaged by a foot pedal or hand lever, for regulating power and torque transfer from the engine to the transmission; and a gear selector that can be operated by hand.
  A conventional 5- or 6-speed manual transmission is often the standard equipment in a modern base model vehicle, with 5- speed being common on lower-end vehicles and commercial vehicles. Higher-end vehicles, such as sports cars and luxury cars are often usually equipped with a 6-speed transmission for the base model. Automatic transmissions are commonly used instead of manual transmissions; common types of automatic transmissions are the hydraulic automatic transmission, automated manual transmission, dual-clutch transmission and the continuously variable transmission. The number of forward gear ratios is often expressed for automatic transmissions as well.
context_section_description: In order to provide smooth gearshifts without requiring the driver to manually match the engine revs for each gearshift, most modern passenger car transmissions use 'synchromesh' (also called 'synchronizer rings') on the forward gears. These devices automatically match the speed of the input shaft with that of the gear being selected, thus removing the need for the driver to use techniques such as double clutching. The synchromesh transmission was invented in 1919 by Earl Avery Thompson and first used on production cars by Cadillac in 1928.
  The need for synchromesh in a constant-mesh transmission is because the dog clutches require the input shaft speed to match that of the gear being selected, otherwise, the dog teeth will fail to engage and a loud grinding sound will be heard as they clatter together. Therefore, to speed up or slow down the input shaft as required, cone-shaped brass synchronizer rings are attached to each gear. When the driver moves the gearshift lever towards the next gear, these synchronizer rings press on the cone-shaped sleeve on the dog collar so that the friction forces can reduce the difference in rotational speeds. Once these speeds are equalized, the dog clutch can engage and thus the new gear is now in use. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed. Many transmissions do not include synchromesh on the reverse gear (see Reverse gear section below).
  The synchromesh system must also prevent the collar from bridging the locking rings while the speeds are still being synchronized. This is achieved through 'blocker rings' (also called 'baulk rings'). The synchro ring rotates slightly because of the frictional torque from the cone clutch. In this position, the dog clutch is prevented from engaging. Once the speeds are synchronized, friction on the blocker ring is relieved and the blocker ring twists slightly, bringing into alignment certain grooves or notches that allow the dog clutch to fall into the engagement.
  Common metals for synchronizer rings are brass and steel, and are produced either by forging or sheet metal shaping. The latter involves the stamping the piece out of a sheet metal strip and then machining to obtain the exact shape required. The rings are sometimes coated with anti-wear linings (also called 'friction linings') made from molybdenum, iron, bronze or carbon (with the latter usually reserved for high-performance transmissions due to their high cost).
  Mechanical wear of the synchronizer rings and sleeves can cause the synchromesh system to become ineffective over time. These rings and sleeves have to overcome the momentum of the entire input shaft and clutch disk during each gearshift (and also the momentum and power of the engine, if the driver attempts a gearshift without fully disengaging the clutch). Larger differences in speed between the input shaft and the gear require higher friction forces from the synchromesh components, potentially increasing their wear rate.
caption: Synchronizer rings
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