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Engineering
How Motorcycle Gears Work
Manual transmission, clutch, and gear ratios
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Inside most motorcycles, a constant mesh gearbox keeps all gear pairs engaged while sliding dog clutches lock one selected gear to a shaft. The clutch briefly disconnects engine torque so the shift drum and shift forks can move the dogs into the next position. Power flows from the crankshaft through the clutch, into the input shaft, across the selected gear pair to the output shaft, then through the final drive chain to the rear wheel. The total gear reduction equals the selected transmission ratio multiplied by the primary drive ratio and final drive ratio.
Key Facts
- Gear ratio = driven gear teeth / driving gear teeth.
- Wheel torque = engine torque x total gear reduction x drivetrain efficiency.
- Total reduction = primary ratio x selected gear ratio x final drive ratio.
- Speed tradeoff: higher gear reduction increases wheel torque but decreases wheel speed for a given engine rpm.
- Engine rpm is proportional to vehicle speed x total gear reduction when clutch slip is zero.
- Power is approximately conserved through gears: P = τω, with some loss to friction and heat.
Vocabulary
- Gear ratio
- A gear ratio compares the number of teeth or rotational speeds between two meshing gears and determines how torque and speed change.
- Clutch
- A clutch is a friction coupling that connects or disconnects engine power from the transmission input shaft.
- Constant mesh gearbox
- A constant mesh gearbox keeps gear pairs permanently meshed while dog clutches select which gear is locked to a shaft.
- Shift drum
- A shift drum is a rotating cam mechanism that moves shift forks to select different gears.
- Final drive
- The final drive is the chain, belt, or shaft system that transfers torque from the transmission output to the rear wheel.
Common Mistakes to Avoid
- Thinking first gear makes the engine stronger is wrong because the engine torque does not increase inside the engine. The gear ratio multiplies torque at the rear wheel while reducing wheel speed.
- Ignoring the final drive ratio is wrong because rear sprocket and front sprocket sizes strongly affect total reduction. A transmission gear ratio alone does not determine acceleration or cruising rpm.
- Assuming gears slide in and out of mesh is wrong for most motorcycle transmissions. The gear teeth usually stay meshed, and dog clutches lock the chosen gear to the shaft.
- Shifting without reducing torque load can be wrong because loaded dog teeth resist movement and can wear or clash. The clutch or a quickshifter reduces torque so the dogs can engage cleanly.
Practice Questions
- 1 A motorcycle has a primary ratio of 1.8, a first gear ratio of 2.5, and a final drive ratio of 3.0. What is the total reduction in first gear?
- 2 An engine produces 60 N m of torque. If the total reduction in second gear is 10.0 and drivetrain efficiency is 90%, what torque reaches the rear wheel?
- 3 Explain why a motorcycle in a lower gear accelerates harder but reaches a lower maximum road speed at the same engine rpm than it does in a higher gear.