How the Transmission Works.

Taken from a very clear explanation of the workings of the Cubs gearbox from  Princeton site which further details student group restoration of the 200cc gem. Larn yersel motorcycle mechanics! It’s basically three shafts, two of which spin on the same center, and a series of fixed or sliding gears that interlock to achieve different drive ratios transmitting power from the engine (via the clutch) to the back wheel. Great stuff!

A detailed look into the Triumph TigerCub 4-Gear Transmission System.

The transmission consists of two interlocking shafts with four gears each — the number of teeth on each gear is shown above.

The two shafts are called the mainshaft (shown on the top) and the layshaft (on the bottom). In the TigerCub’s transmission, the mainshaft consists of two parts: the input shaft, which brings power from the engine, and the output shaft, which rides on a bearing concentric to the input shaft but outside of it, and which transmits power to the rear wheel. In the picture above, the input shaft is the smaller, copper-colored cylinder in the upper-right corner, and the output shaft is the larger, silver-colored cylinder to its immediate left.

The input shaft is tied to the first and third gears on the mainshaft (M1 & M3)–that is, it co-rotates with them at the same angular velocity. M2 spins freely unless anchored to the mainshaft by M3, and M4 co-rotates with the output shaft. L2 and L4 co-rotate with the layshaft, and L1 and L3 spin freely unless anchored to the layshaft by L2.

This picture shows the transmission in neutral–that is, the input is disengaged from the output, so the wheel receives no torque despite the engine’s running. M1 and M3 spin with the input shaft, but since M3 is not engaged with either M2 or M4, neither of those two rotates with the mainshaft. M1 and M3 drive L1 and L3, but since L2 is engaged with neither of those gears, they spin freely and the layshaft does not itself rotate. Thus the output via M4 is unaffected by the input.

In shifting from neutral to first gear, L2 is moved to the left and engages with L1. This allows M1, co-rotating with the input, to drive the layshaft via L1. The spinning layshaft then drive M4 via L4, giving the gear ratio shown above.

Input => M1 => L1 => Layshaft => L4 => M4 => Output

From first gear, L2 shifts to the right and engages with L3, driven by M3 in the middle position. L3 is smaller than L1, and M3 larger than M1, so a smaller gear ratio results.

Input => M3 => L3 => Layshaft => L4 => M4 => Output

In third gear, L2 shifts back to its middle position, unengaged with either L1 or L3, while M3 shifts to the left and engages with M2. M2, co-rotating with the input, drives L2.

Input => M2 => L2 => Layshaft => L4 => M4 => Output

In fourth gear, L2 remains in the middle, and M3 shifts to the right to engage directly with M4, tying the input immediately to the output. As in neutral, the layshaft is not utilized.

Input => M3 => M4 => Output

( Based on the position of the two shifting gears (M3 on the mainshaft and L2 on the layshaft), the gear ratio of the transmission is different.  The forks control the position of the two shifting gears by sliding them left and right along the shaft.  The forks are crescent shaped and fit into slots on the side of the shifting gears so that they are able to move the gears left and right but do not effect the rotation.

Side view of fork engaging gear shoulder.

On top of each of the forks is a roller that fits into a pathway on the shifting plate. As the shifting plate changes positions, the fort-rollers follow the path, thus changing the position of the two forks. It is this motion that allows the transmission to change gears based on the position of the shifting gears.