The Manual Gearbox

The layout shown is the in-line layshaft (countershaft) gearbox used in many rear-wheel-drive cars. The input shaft and mainshaft are coaxial: the mainshaft's front end runs in a pilot bearing inside the input pinion, so the two can turn at different speeds. A constant-mesh head pair (20 driving 34 teeth here, 1.70 : 1) drives the layshaft cluster, which is one rigid piece. Each layshaft gear is permanently meshed with a free-running gear on the mainshaft. Front-wheel-drive transaxles usually omit the layshaft: their input shaft carries the driving gears and a parallel output shaft carries the free gears and drives the final drive directly, but the selection principle is identical.

An indirect ratio is the product of two reductions: i = (z_head,driven / z_pinion) × (z_main / z_lay). With the illustrative tooth counts, 1st is 34/20 × 37/17 = 3.70 : 1 and 2nd is 34/20 × 30/24 = 2.13 : 1. Every head and speed pair has the same tooth sum (54) and the same module, so all pairs share one centre distance; real gearboxes achieve the same thing with profile shift and slightly different modules. Forward gears are helical (18° helix here) for smooth, quiet tooth engagement; the axial thrust this creates is taken by the shaft bearings.

Selection is by synchronisers. Each hub is splined to the mainshaft and carries a sliding sleeve (collar). As the selector fork moves the sleeve towards a gear, spring-loaded struts push a brass baulk ring onto a friction cone on that gear. Cone friction accelerates or slows the gear and everything geared to it (the layshaft, the clutch disc and input shaft) until it matches mainshaft speed. Until then, chamfered blocking teeth on the ring stop the sleeve going further. Once speeds match the ring indexes, the sleeve's internal splines slide over the gear's dog teeth, and torque passes through those dogs, not through the cone. With the clutch released, only the disc, input shaft and layshaft inertia has to be synchronised, which is why gear changes need the pedal pressed.

4th here is direct drive: the 3–4 sleeve locks the input pinion's dog teeth straight to the mainshaft, so no tooth mesh carries load and the ratio is exactly 1 : 1, which is also the most efficient path. 5th is an overdrive (34/20 × 17/37 = 0.78 : 1) for lower engine speed when cruising. Reverse uses a straight-cut idler that slides into mesh between a layshaft gear and a gear on the mainshaft. The third external mesh reverses the direction, and the idler's own tooth count cancels out of the ratio (34/20 × 33/16 = 3.51 : 1). Reverse is commonly unsynchronised, so the shafts should be stationary before it is selected.

The selector mechanism converts lever movement into rail movement: sideways movement chooses a rail, fore-aft movement slides it. Detents hold each rail in place and an interlock prevents two rails moving at once, so two ratios can never be engaged together, which would lock the gearbox. All tooth counts, ratios and efficiencies in the lab are illustrative values chosen for teaching, not the data of a production gearbox.