The Way a Syncromesh Manual Transmission Works

  Modern gearboxes are constant syncromesh, in other words, all input and drive gears are always in mesh. Only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, while the others are allowed to rotate freely. This greatly reduces the skill required to shift gears. Most modern cars are fitted with a synchronized gear box, although it is entirely possible to construct a constant mesh gearbox without a synchromesh, as found in a motorcycle, for example. In a constant mesh gearbox, the transmission gears are always in mesh and rotating, but the gears are not rigidly connected to the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism for any individual gear consists of a collar, or dog collar, on the shaft which is able to slide sideways so that teeth, or dogs, on its inner surface bridge two circular rings with teeth on their outer circumference, one attached to the gear, one to the shaft, one collar typically serves for two gears, sliding in one direction selects one transmission speed, in the other direction selects the other. When the rings are bridged by the collar, that particular gear is rotationally locked to the shaft and determines the output speed of the syncromesh transmission.

  In a synchromesh transmission, to correctly match the speed of the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone shaped brass clutch attached to the gear, which brings the speeds to match prior to the collar locking into place. The collar is prevented from bridging the locking rings when the speeds are mismatched by synchromesh rings, also called blocker rings or balk rings. The gearshift lever manipulates the collars using a set of linkages, so arranged so that one collar may be permitted to lock only one gear at any one time, when shifting gears, the locking collar from one gear is disengaged and that of another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.

  The modern cone system was developed by Porsche and introduced in the 1952, cone synchronizers were called Porsche type for many years after this. In the early 1950s only the second to third shift was synchromesh in most cars, requiring only a single synchromesh and a simple linkage, drivers' manuals in cars suggested that if the driver needed to shift from second to first, it was best to come to a complete stop then shift into first and start up again. With continuing sophistication of mechanical development, however, a fully synchromesh transmission with three speeds, then four speeds, and then five speeds, became universal by the 1980s. Many modern manual transmission cars, especially sports cars, now offer six speeds.

Reverse gear, however, is usually not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears while moving into reverse is not required. There are vehicles that are exceptions to this rule, but they are european and don't pertain to this article which is mainly directed to American cars.