The transmission

If you've been reading my posts in order then you will by now be familiar with the myriad of gears in the transmission system. I was having trouble getting my head around how it works and how it interfaces with the pedals so this is my attempt to sort it all out. If you're familiar with this stuff then stop here- there will be nothing new and probably some stuff that's wrong.

The clutch

The clutch consists of the clutch base splined onto the crankshaft, the clutch plates located in the clutch basket, the clutch basket itself which bears a drive pinion molded into its top and the spring/release assembly located above the basket. The crankshaft is splined onto the clutch base located in the left hand case. This means the clutch base turns with the crank. The clutch plates fit around the clutch base, but the plates aren't simple discs. They have dog-ear projections. On the metal plate, the projections point towards the centre and on the friction plates they point away from the centre. The clutch base has dog-ear indentations into which the ears of the metal plates will fit. The clutch base and metal plate will therefore both turn whenever the crank is turning. Sandwiched above and below the metal plate is a pair of cork lined friction plates. These have dog ear projections on their outer rim which fit into slots around the edge of the clutch basket. The clutch basket can therefore only rotate if the friction plates are forced against the metal plate such that they are all dragged round together. This is achieved by means of the spring located above the clutch basket and compressed against it by the crankshaft end nut. This forces the basket down and thus pushes the friction plates against the metal plate. Under these circumstances as the crank turns the clutch centre also turns and as the plates are forced together drive is transmitted to the clutch basket and thus to the drive gear molded into the top.

The clutch is released by means of the clutch cup which covers the spring and locates underneath it using a special circlip. The cup can be raised by a cable-operated lever which raises the cup so also the clip in its base. This pulls p on the base of the spring, relieving its pressure on the clutch basket. This allows the plates within to separate. The metal plate then rotates between the friction plates which remain stationary and drive is disconnected.


The gearbox.

The gearbox consists of all the gears and gear change mechanisms  and interfaces with the  pedal system. The first point to realise is that all gears (including those of the pedal system) rotate all the time that the motor is running and the clutch is engaged whether the bike is moving or not. Drive is connected or disconnected to the transmission shaft and its chain sprocket by means of a sliding dog on the transmission shaft. This has three dog teeth on either side and slips along coarse splines on the transmission shaft. When the dog is slipped against  the desired gear the three dog teeth engage with those on the gear itself. Rotating the gear thus rotates the dog and as this is splined to the mainshaft, the shaft is also rotated. Slipping the dog thus locks the selected gear to the mainshaft whilst the unused gear is is left free to rotate idly on the shaft.

Top gear.

The crank rotates the clutch basket and the 15t drive pinion molded on top. This meshes with the large pedal gear (80t) which rotates but as it has more teeth it does so more slowly than the motor by a factor of 15/80. Gearing ratio is calculated by dividing the tooth number if the DRIVEN gear by that if the DRIVING, so in this case 80/15 or 6:1. However in top gear the 80t is dogged directly to the main shaft and this speed transmitted directly to the chain sprocket, although the difference in size of engine and rear wheel sprockets does reduce wheel rpm still further.

For first gear it's necessary to allow more powered cycles per wheel revolution: this means that road speed is reduced for the same motor rpm. This is achieved by means of two features molded into the rear of the 80t gear. The first is a smaller (18t) gear which  obviously turns whenever the large gear turns. This is meshed to the double gear column (24t/17t) on the layshaft. Both these layshaft gears thus rotate whenever the 80t gear is turning. The lower layshaft gear is 17t and this is meshed permanently to a 24t gear that rotates at the base of the main shaft. Consequently all these gears are in motion whenever the 80t gear is turning.  To enable gear change the two gears on the main shaft (that is the 80t double gear at the top and the 24t gear at the bottom) each carry 3 tooth dogs. The main shaft between the two is splined and a sliding dog can slip up and down this shaft, moved by the gear change fork. When the sliding dog is at the top it engages with the dog behind the 80t double gear. Since the sliding dog is splined to the main shaft this means that rotation of the 80t gear rotates also the sliding dog and this is transmitted to the main shaft through the splines. Although all other gears are turning the 24t at the base of the main shaft simply rotates idly on the shaft. When the dog slides to the bottom of the main shaft it engages with the dog on the 24t gear. Again this means that rotation of the 24t will now drive the main shaft through the splines. The 80t gear now acts as simply an intermediary in power transmission; It rotates idly on the main shaft transmitting no thrust to it. Instead power is transmitted to the 24t top layshaft gear. This turns the whole layshaft column, including the 17t  gear at its base. This is meshed with the 24t at the base of the main shaft so this is also driven, and since it's now dogged to the mainshaft this is also rotated. This chain of gears reduces engine speed by  a ratio of 24/18 and again by 24/17. Combining these factors gives a  first gear total reduction of 1.88:1 compared with 1:1 in second.