km/hFrom your data logger or lap timer.
rpmUsually just below the limiter.
mm
Result
Ideal rear sprocket teeth64.2
Nearest whole sprocket64
How it is calculated, with your numbers
- Overall reduction needed
need = rpm × circ / (vmax × 1000000 / 60)14,000 × 1,980 / (285 × 1000000 / 60) = 5.836 - Ideal rear sprocket teeth
rear = need / (primary × top) × front5.83579 / (1.652 × 0.88) × 16 = 64.2 - Nearest whole sprocket
rearr = round(need / (primary × top) × front)round(5.83579 / (1.652 × 0.88) × 16) = 64
Working uses the metric units the formula is written in.
- vmax Top speed reached on the straight (km/h)
- rpm Target rpm at that speed in top gear (rpm)
- primary Primary reduction ratio
- top Top gear ratio
- front Front sprocket teeth
- circ Rear tyre rolling circumference (mm)
- need Overall reduction needed
- rear Ideal rear sprocket teeth
- rearr Nearest whole sprocket
What it means
Gearing for a circuit is a trade-off: too short and you hit the limiter before the braking marker, too tall and the engine never reaches peak power in top gear. Start from the speed you actually reach on the longest straight and the rpm you want there; the calculator gives the overall reduction that achieves it and turns it into a rear sprocket.
Wind direction and a better drive out of the last corner change the top speed, so many riders gear for slightly below the limiter.
Assumptions and limits
- Uses the measured top speed; a gearing change can itself change it slightly.
Sources
- Foale, T., Motorcycle Handling and Chassis Design: The Art and Science, 2002.