A MotoGP chain drive transfers power from the engine and gearbox to the rear wheel through a front sprocket, chain, and rear sprocket. This final drive is the last gearing stage before the tire pushes on the track, so it strongly affects acceleration, top speed, and throttle response. Teams tune the sprocket sizes for each circuit because different tracks reward different compromises.
A short, tight track may need stronger drive out of corners, while a long track may require more top speed on the straight.
Key Facts
- Final drive ratio = rear sprocket teeth / front sprocket teeth
- Wheel torque after final drive = countershaft torque × final drive ratio, ignoring losses
- Higher final drive ratio gives more rear wheel torque but lower speed for the same engine rpm
- Lower final drive ratio gives less rear wheel torque but higher speed for the same engine rpm
- Chain speed is the same along both sprockets, but sprocket angular speeds differ with tooth count
- Approximate wheel rpm = engine rpm / (gearbox ratio × final drive ratio)
Vocabulary
- Final drive ratio
- The ratio between the rear sprocket tooth count and the front sprocket tooth count that sets the last torque and speed change before the rear wheel.
- Front sprocket
- The small sprocket on the gearbox output shaft that pulls the chain from the engine side.
- Rear sprocket
- The larger sprocket attached to the rear wheel hub that receives chain force and turns the wheel.
- Chain tension
- The pulling force in the chain that transmits torque between the front and rear sprockets.
- Torque multiplication
- The increase in wheel torque caused by using gear ratios that trade rotational speed for turning force.
Common Mistakes to Avoid
- Thinking a bigger rear sprocket always makes the motorcycle faster is wrong because it improves acceleration but can reduce top speed by reaching maximum engine rpm sooner.
- Ignoring the front sprocket is wrong because changing one tooth at the front often has a large effect on the final drive ratio.
- Using sprocket tooth counts without forming a ratio is wrong because the important quantity is rear teeth divided by front teeth, not either number alone.
- Assuming final gearing changes engine power is wrong because gearing changes torque and speed at the wheel while engine power output is mainly set by the engine and throttle conditions.
Practice Questions
- 1 A MotoGP bike uses a 16 tooth front sprocket and a 40 tooth rear sprocket. Calculate the final drive ratio.
- 2 A bike has 90 N m of torque at the gearbox output shaft and a final drive ratio of 2.50. Ignoring losses, calculate the torque delivered to the rear wheel hub.
- 3 A team changes from a 40 tooth rear sprocket to a 42 tooth rear sprocket while keeping the same front sprocket. Explain how this change affects acceleration out of corners and maximum speed on a long straight.