Sports Cycle

The Belly of the Beast : The Bottom End

I like to compare motorcycle components to the human body to make it easier to envision. The bottom end is the entrails of a bike. This is where all the commotion in the engine is converted into forward motion. The two clam-shell-like metal halves that surround the bottom end are called cases. All motorcycle engine bottom ends contain at least one crankshaft (on certain designs, two crankshafts occupy the space within the engine cases, but these are rare, especially in the United States).

" The bottom end of a bike refers to the bottom part of the engine, where the crankshaft and (usually) the transmission reside. The top end refers to the upper part of the engine, which contains the pistons, cylinders, and valve gear. "

The crankshaft is connected via connecting rods to the pistons, which are the slugs moving up and down within the cylinders (the hollow shafts in the top end inside which internal combustion occurs). The connecting rods attach to the crankshaft on eccentric journals, or metal shafts, and their up-and-down movement is converted into a circular motion through the design of these journals.

As you’ll see in the next chapter, the design of the crankshaft does much to give the engine its character; for example, it’s the design of the crankshaft that gives Harley- Davidsons their characteristic rumbling sound (it’s what also makes Harley engines vibrate like washing machines with unbalanced loads).

The Transmission

On most modern motorcycles, the circular motion created by the crankshaft setup is transmitted to the transmission through a series of gears or by a chain. For all practical purposes, you can think of the transmission of a modern motorcycle as part of the bottom end. The crankshaft and the gearset usually reside inside the same cavity within the cases. This is called a unitized transmission, and since the 1950s and 1960s, it has been a part of just about every motorcycle engine designed.

" A unitized transmission (often referred to as a “unit transmission”) is a transmission that is an integral part. "

One notable exception is Harley-Davidson, the only company still manufacturing motorcycles that use separate engines and transmissions, connected by a primary drive. The primary, as it is called, consists of a sprocket attached to the crankshaft that rides outside the engine cases, in a case of its own (called the primary case). This case is connected to the transmission, which is located behind the engine, via a primary chain.

Harley’s primary drive system presents added maintenance chores: The primary chain must be periodically adjusted, a rather difficult process that creates more opportunities for things to go wrong. The exposed components are also more vulnerable to damage than components protected within the cases. No one knows for certain why Harley continues to use this archaic setup, but if I had to guess, I’d say it’s purely for looks.

" A primary drive is a drive chain connecting the engine’s crankshaft to its transmission. "

The transmission is an integral part of the bottom end on most motorcycles, but it is really a separate system. Transmission design varies from bike to bike, but most share some basic characteristics. Each transmission contains a gearset, which, as you might have guessed from the name, is a set of gears. Designers use complex mathematical equations between the diameter of these gears and a variety of other parts on the motorcycle to give a bike certain characteristics.

These gears are engaged by shifting a lever with your left foot, which moves forks inside the transmission, which in turn move the gears within. If you have any trouble with the transmission, the odds are it will involve those forks.