km/hAny speed you know, e.g. at 5,000 rpm in top gear.
Result
Change in final drive (positive = shorter, more acceleration)7.14 %
Speed at the same rpm93.3 km/h
How it is calculated, with your numbers
- Current final drive
fd1 = r1 / f145 / 15 = 3.000 - New final drive
fd2 = r2 / f245 / 14 = 3.214 - Change in final drive (positive = shorter, more acceleration)
change = (fd2 / fd1 − 1) × 100(3.21429 / 3 − 1) × 100 = 7.14 % - Speed at the same rpm
v2 = v1 × fd1 / fd2100 × 3 / 3.21429 = 93.3 km/h
Working uses the metric units the formula is written in.
- f1 Current front teeth
- r1 Current rear teeth
- f2 New front teeth
- r2 New rear teeth
- v1 Speed at an rpm with current gearing (km/h)
- fd1 Current final drive
- fd2 New final drive
- change Change in final drive (positive = shorter, more acceleration)
- v2 Speed at the same rpm
What it means
The final drive ratio is rear teeth divided by front teeth. A smaller front or bigger rear sprocket raises it ("shorter" gearing): quicker acceleration and more rpm at any speed, lower top speed in each gear. One tooth at the front changes the ratio far more than one tooth at the rear.
A different final drive also makes the speedometer read differently on bikes that take speed from the gearbox output, and it changes the chain length needed.
Assumptions and limits
- Same tyre and rpm before and after.
Sources
- Foale, T., Motorcycle Handling and Chassis Design: The Art and Science, 2002.
Related calculators
Chain length calculator (links)How many links a chain needs for your sprockets and axle position.Road speed in gear calculatorSpeed at any rpm from primary, gear and final drive ratios and tyre size.Track gearing calculatorThe rear sprocket that puts the redline exactly at the end of a circuit's longest straight.