63 Tiger Cub Motorcycle

FRS 106, Michael Littman – Spring 2011


Gears, Shifter, Kick Starter, Flywheel

Team – Margaret, Scott

An artistic view of our transmission.

What is a transmission?

The word transmission means the action of passing something on to another place.

In an engine, it is where the gears are located and do their important task. The power from the engine has to be transferred to the tires. The transmission transfers the power from the engine, to the shaft, then to the tires. Transmissions use gears and a clutch to convert the speed of the power source into torque. The gears within the transmission are used to maintain optimum engine performance. They balance torque versus speed.  The gears turn engine speed (rpm) into torque (pounds/feet). For example, a lower gear would be preferred to go up a steep hill or on a dirt path, because more tire torque would be wanted although speed is sacrificed. Going down a highway, a higher gear would be used because there is less resistance to make movement, so less torque is needed, and a faster speed is obtained.

Transmission Overview

This is a picture taken from the manual that depicts the main components of the transmission. The parts of the picture will be broken down and explained in further depth later on this page. The photo should help the basic visualization and understanding of parts that make up the process that occurs within the transmission, providing a better understanding on how all the parts come together in the end.

Number of Teeth on each Gear

The picture here shows the number of teeth on each gear connected to the mainshaft or layshaft. The changing of the ratio between the number of teeth on these two shafts is what determines the speed the motorcycle will be able to achieve within the particular gear ratio.

The shifter forks move the internal gears with 20 teeth back and forth, depending on the which gear (1,2,3,4) the engine is in. Those small internal gears then engage with the larger ones next to them, determining the actual output ratio to the chain sprocket. Power is transferred from the main shaft gear into those on the layshaft, which affects the ratio, and then back to the mainshaft and finally to the output sprocket.


How the Shifter Works

When the rider moves the shifter lever to change gears, the lever is connected directly to the gear change quadrant. The gear change quadrant then moves the shifter plate, which in turn pulls the shifter forks apart or pushes them together. The moving of the shifter forks also moves the gears on the mainshaft and layshaft. Moving the gears on these two shafts changes the ratio of RPM’s between the engine and tire, thus giving the rider more torque and less speed or more speed but less torque.

Learning from Pictures

Taking apart the engine was very helpful to our group for learning about how the transmission works. Most of us had never

participated in a dismantling of an engine; so we learned directly how to take apart and rebuild this engine. Pictured above is a shot into the transmission side of the Tiger Cub engine. In the upper left quadrant is the camshaft. In the upper right quadrant we can see the oil pump. Then, in the bottom half, we can see the entire transmission mechanism, including the shifter plate, gears, and gear change quardrant. When putting back our own engine, we often referred back to pictures we had taken, like this one, to help with reassemblely.

The TRANSformation

Here is the change in appearance of the transmission. Shown on the left is a picture of before we started or touched anything, and on the right is a picture of our transmission cleaned and with some worn parts replaced. Before we started working on the transmission, the transmission was coated with a gritty grime, and much scrubbing and scraping of the parts and cover was needed to restore the transmission to “like new” condition. Some other things we did within the transmission include making and putting in new gaskets and other foam paddings, and replacing worns parts, such as the metal square that engages the kick start.

Problem Solving

When a group of new motorcycle mechanics takes apart, refinishes, and puts back together a bike for the first time, they are bound to have made a few mistakes. Parts and processes then must be double and triple checked to be able to determine or locate even the smallest mistake.  In problem determination, first a hypothesis of what the problem could be and potential causes are made, then checked and tested in an attempt to solve the problem. Here, the motorcycle is show taken apart, for there was speculation as to whether the oil pump was working, since no oil was flowing into the oil tank.

Modern Transmissions

Seen here is a standard transmission used in modern cars today. The stick shift, shown in gray, sits at the driver’s right hand and when moved back and forth, engages the green purple and blue shifter forks that move the gears back and forth to change the gear (1,2,3, 4, reverse).


Shown here is a more modern, yet still fairly basic gear assembly. While the general set up is still similar – with sifter forks and a main and lay shaft – the gears themselves have angled teeth. This is a design improvement, because such angles decrease the possibility of slippage between gears as they can now only spin in one direction.
Lastly, we have a BMW automatic, hybrid transmission. The simple idea of a lay shaft and main shaft have been substituted, with a main shaft and smaller gears surrounding it. This set up is more complex and designed for high performance. There are also many metal coils (left hand side of the picture) which is how the engine uses both gas and electric power to improve fuel efficiency.