Synchromesh: Continuation from our Helical Gear post yesterday.
Conyiniuing my converstion with Brian of GotTransmissions.com, I explained that Constant-mesh gearboxes are quieter and slicker, but you still have to match the spinning speeds before you could engage the chosen gear. The final improvement, and the one which removes the need for double-clutching, is synchromesh.
Synchromesh is a refinement of the part of the constant-mesh gearbox where the difficulty lies: matching the little teeth on the inside of the selector hubs with the little teeth on the side of the main drive gears. In its simplest form there is a tapered section on the hub side of the main gear and a matching, grooved, tapered, bronze ring called a blocking or braking ring on the gear side of the hub. As the hub approaches the gear the bronze ring rubs against the taper, and friction spins the gear up. Too fast and the selector teeth will clash, too slow and there won’t be enough movement between the teeth on the hub and the gear, also causing a gear clash.
Synchromesh started to be fitted to cars in the late 1930s, but was by no means universal until the late 1950s. Even then it was not uncommon for first and/or second gears not to be synchromeshed.
Overdrives: The system described above is a four-seed system. Most gearboxes are typically four speed. However most modern cars and trucks use 5 or 6 speed standard transmissions now. Fifth or sixth gear is typically an overdrive ratio. Overdrives work much the same as a non overdrive in terms of function.
Overdrive boosts fuel mileage by lowering RPM’s of the engine. Lowering engine speed gives the engine longer life. It is not an excuse for less maintenance.
As a matter of interest you will need to research what fluid your particular standard transmission uses when it is serviced or maintained.
Read our other standard transmission posts on GoTransmissions.com blog for further theory of function and internal descriptions of what is going on inside your manual transmission.
