All the gears except those needed for reverse are constantly in mesh. The gears on the output shaft revolve freely around it, while those on the layshaft are fixed. No drive is being transmitted.
In first gear, the smallest gear on the layshaft (with the fewest teeth) is locked to it, passing drive through the largest gear on the mainshaft, giving high torque and low speed for a standing start.
In second gear, the difference in diameter of the gears on the two shafts is reduced, resulting in increased road speed and lower torque increase. The ratio is ideal for climbing very steep hills.
In fourth gear, the input shaft and mainshaft are locked together, providing 'direct drive': one revolution of the propellor shaft for each revolution of the crankshaft. There is no increase in torque.
For reversing, an idler gear is interposed between gears on the two shafts, causing the mainshaft to reverse direction. Reverse gear is usually not synchronised.
Synchronising the gears
The synchromesh device is a ring with teeth on the inside that is mounted on a toothed hub which is splined to the shaft.
When the driver selects a gear, matching cone-shaped friction surfaces on the hub and the gear transmit drive, from the turning gear through the hub to the shaft, synchronising the speeds of the two shafts.
With further movement of the gear lever, the ring moves along the hub for a short distance, until its teeth mesh with bevelled dog teeth on the side of the gear, so that splined hub and gear are locked together.
Modern designs also include a baulk ring, interposed between the friction surfaces. The baulk ring also has dog teeth; it is made of softer metal and is a looser fit on the shaft than the hub.
The baulk ring must be located precisely on the side of the hub, by means of lugs or 'fingers', before its teeth will line up with those on the ring.
In the time it takes to locate itself, the speeds of the shafts have been synchronised, so that the driver cannot make any teeth clash, and the synchromesh is said to be 'unbeatable'.
