A 125 cc engine with its covers off. Start it, pull the clutch, shift gears and click any part.
Drag to orbit, scroll or pinch to zoom, right-drag to pan. Click a part.
Every gear pair has the same 49-tooth sum at module 2.0, which is what lets all five pairs sit on two shafts 49 mm apart and stay in constant mesh. Road speed assumes a 2.75-18 rear tyre with a rolling circumference of 1.95 m.
Teeth. Each gear pair is a small gear on the mainshaft (driven by the clutch) meshing with a bigger one on the countershaft. Gear ratio = countershaft teeth ÷ mainshaft teeth: 36 ÷ 13 = 2.769, so in 1st the mainshaft turns 2.77 times for each turn of the countershaft. Overall ratio multiplies three steps: primary drive (67 ÷ 18 = 3.722) × gear ratio × final drive (43 ÷ 14 = 3.071). In 1st that is 31.7 engine turns for one turn of the rear wheel, which multiplies torque by the same amount for pulling away. Fixed, splined, free: a fixed gear is part of its shaft; a splined gear turns with the shaft but can slide sideways; a free gear spins on the shaft until a sliding gear's dogs lock it. Locked by names the sliding gear that does that locking.
Only one dog clutch is ever engaged. Every other gear spins freely on its shaft while staying in mesh.
The shift drum has curved grooves. Each press of the gear pedal turns the drum one step (the star and its spring-loaded stopper hold it there), and the grooves push the three forks sideways. A fork slides one splined gear until its dogs, the square lugs on its side, drop into the holes of the free gear next to it. Only one pair of dogs is engaged at a time, so only one gear pair carries the power. Neutral sits between 1st and 2nd, which is why the pattern is 1-N-2-3-4-5.
The trains that feed the crank in and take power out of it.
The primary drive slows the crank down 3.7 times before the clutch. The cam drive turns the camshaft at half crank speed, because each valve opens once every two turns in a four-stroke. The balancer spins a weight opposite the piston to cancel some vibration. The electric starter needs a big reduction (13.8 : 1) so a small motor can turn the engine over; its one-way roller clutch lets go once the engine runs. The final drive sprockets set how fast the rear wheel turns for a given gearbox speed: try changing them below.
Change the sprockets, the tyre and the engine speed to see the overall ratio and road speed in each gear. A smaller front or bigger rear sprocket gives more pull and less top speed; boda boda riders often fit a bigger rear sprocket for heavy loads and hills.
Everything on this stand is one engine: a 125 cc air-cooled single-cylinder four-stroke of the unit-construction type that powers most commuter and boda boda motorcycles, where crankshaft, generator, both starters, clutch and the five-speed constant-mesh gearbox share one crankcase. The cases and covers are removed so every gear, chain, fork and roller moves in sync with the crank. Start it, pull the clutch, shift through the box and click any part for its job and tooth count.
Every tooth count, ratio, drum position and valve timing behind the model. In the workbook, change a tooth count and the ratios recalculate.
Gear and parts data (Excel) Engine specification (JSON) Gearbox chapter Engine in the Overview
Moto Zero is an openly licensed motorcycle technology programme by Transitions Lab. The mechanism follows a typical Honda-pattern 125 cc commuter engine; dimensions and tooth counts are representative rather than taken from one manufacturer's drawings. Clearances in the clutch and dog gaps are exaggerated so the movement is visible.