How Does The Crankshaft Drive The Clutch And Gearbox On A Motorcycle? | Cycle World

How Does The Crankshaft Drive The Clutch And Gearbox On A Motorcycle?

Tracing engine power production from the pistons to the rear wheel

This is a partially disassembled Honda CBR600RR engine. I am going to show you several things about this assembly. First of all, I have put the crankshaft into the engine, and it is able to turn. That crankshaft is driven by four pistons. The big end of the connecting rod that I am holding fits onto one of the four crankpins.

Power from the crankshaft goes out a gear, which is part of the shaft and cut in one piece. It drives a larger gear on the clutch. This combination is called the primary drive. The secondary drive is from the engine to the rear wheel. Once the power gets to the clutch, it spins the upper shaft in the gearbox.


Power in neutral goes nowhere. In order to go somewhere, you engage one of the six gear pairs. The power goes through the two gears that are engaged. The other five pairs freewheel. That is not as loss-making a process as it sounds because the gears are spinning in air and oil is only sprayed onto them sparingly.

How does power get from an engine, which is producing a series of combustion thumps—two per crankshaft revolution—to the rear wheel, which we want to revolve smoothly? To make that process happen, we need something springy in the drive. That something—those somethings, if you will—is in the clutch basket.

The business of absorbing the bumps of combustion and turning that into smooth rotation is the job of six springs. The gear drives the clutch basket through those springs. So when the engine fires and the crankshaft suddenly accelerates, the springs smooth it out and the pressure of combustion dies away. The springs then expand and send power to the rear wheel.