rpm
mmMeasure it: mark the tyre, roll the bike one turn with you aboard, measure the distance.
Result
Road speed212.5 km/h
How it is calculated, with your numbers
- Overall reduction
overall = primary × gear × rear / front1.652 × 1 × 45 / 15 = 4.956 - Wheel speed
wheel = rpm / overall9,000 / 4.956 = 1,816 rpm - Road speed
speed = wheel × circ / 1000000 × 601,815.98 × 1,950 / 1000000 × 60 = 212.5 km/h
Working uses the metric units the formula is written in.
- rpm Engine speed (rpm)
- primary Primary reduction ratio
- gear Gear ratio
- front Front sprocket or pulley teeth
- rear Rear sprocket or pulley teeth
- circ Rear tyre rolling circumference (mm)
- overall Overall reduction
- wheel Wheel speed
- speed Road speed
What it means
The engine's speed is cut three times before it reaches the wheel: by the primary drive (crank to clutch), by the gearbox, and by the final drive (chain sprockets, or belt pulleys on most Harley-Davidsons). Multiply the three ratios to get the overall reduction, divide the rpm by it, and multiply wheel revolutions by the tyre's rolling circumference.
Primary and gear ratios are printed in the service manual and often in the maker's spec sheet.
Assumptions and limits
- No clutch or tyre slip.
- Rolling circumference under load is a few percent less than the unloaded tyre's; measuring it is best.
Sources
- Foale, T., Motorcycle Handling and Chassis Design: The Art and Science, 2002.
- Gillespie, T. D., Fundamentals of Vehicle Dynamics, SAE International, 1992.