63 Tiger Cub Motorcycle

FRS 106, Michael Littman – Spring 2011

Bottom End

Team – Elliot, Margaret, Scott, Katie


The bottom end houses the transmission, crankshaft, flywheel, oil pump, kick start, and clutch.


The crankshaft could be said to be the heart of an engine. In an internal combustion engine, it connects the piston rods and the flywheel. Its purpose is to convert the vertical motion of the piston into the rotational motion for the flywheel. The pistons (of the top end) are connected to the flywheel via a connecting rod (see figure below).

Crankshaft and Flywheel Exploded View

Connecting Rod



The flywheel is essentially just a large, massive disc with a high moment of inertia. It has a high kinetic once it’s moving, and is used to store kinetic energy in the engine. The combustion of gases in the cylinder transforms chemical energy into the linear kinetic energy of the piston, pushing the piston down and rotating the flywheel. In a four stroke engine, this explosion happens once for every two complete trips of the pistonfrom top dead center to the bottom of its trajectory (two full revolutions of the flywheel). In other words, the combustion of gas pushes the piston down once, and before another round of combustion occurs, the piston travels back up to the top of the cylinder to top dead center to emit exhaust, back down to the bottom to allow fuel/air to enter, and to top dead center again to compress the fuel mixture. Of these four steps, only one is powered directly by the combustion of gases in the engine. The movement of the piston in the other three steps is powered by the flywheel. As the piston is pushed down by the combustion, its linear kinetic energy is transferred into rotational kinetic energy in the flywheel. Because the flywheel has such a high moment of inertia, it has a high amount of kinetic energy. This kinetic energy is in turn transferred to the piston in the other three phases in order to move it down, up, down in the cylinder. Were it not for the flywheel, the piston would be pushed down in the cylinder once, and would have no way to return to the top.

Flywheel and Crankshaft Assembly

Piston and Flywheel in Motion

Oil Pump

The oil pump is responsible, simply put, for pumping oil through the engine. The lubrication of the engine is essential as it reduces friction between moving parts, thus reducing heat in parts such as the pistons, bearings, and shafts. It receives rotational energy from the crankshaft and converts this to linear energy for its pumping action via a gear. The other side of the oil pump is connected to the oil line which brings oil to the rest of the vehicle. In this system, oil is drawn out by the oil pump and through the oil filter. The flow made by the pump allows the oil to be distributed throughout the engine.

Kick Start and Gear Shifter

The kick start and gear shifter are two pedals that sit on the outside of the bottom and serve two very different functions

The kick start is how a driver would start the motorcycle. By “kicking” down on the pedal, the shaft of the kick start, which has a gear on it, turns the main shaft of the transmission. Once the crankshaft begins to rotate, the engine starts (if everything is working).

The gear shift is how the transmission is able to switch between gears and change gear ratio of the motorcycle. By changing the gear ratio, the motorcycle is able to different functions, such as higher top speeds (higher gears) or neutral. The gear shift pedal is attached to a shifter plate inside the engine and when the clutch is engaged, the pedal can be “kicked” up or down to switch between gears (see Transmission section for more details).

Does not include top plate so that inside may be seen.