Mini Bike Jackshaft
Calculator
Calculate the combined reduction of a two-chain mini bike jackshaft drive and see how both sprocket pairs determine theoretical speed.
Your setup
Your results
CalculatedReductions multiply through consecutive stages; they do not add.
Assumes rigidly connected jackshaft sprockets and no slip. Converter and transmission stages are excluded from this two-chain worksheet.
How the calculation works
Follow the formula and keep the input units consistent at every step.
R₁ = jackIn / front; R₂ = rear / jackOut; R = R₁ × R₂; v = RPM × πD / (60R)- Identify the engine-to-jackshaft driving and driven sprockets.
- Divide axle teeth by jackshaft output teeth for stage two.
- Multiply both ratios before calculating wheel speed.
A Worked Example
A 12-tooth engine driver feeding 18 teeth, then a 12-tooth jackshaft output feeding 60 teeth, gives 1.5 × 5 = 7.5:1. At 3,600 rpm and 13 inches, speed is 18.56 MPH.
What Your Result Means
Reductions multiply through consecutive stages; they do not add.
Assumptions & Limitations
Assumes rigidly connected jackshaft sprockets and no slip. Converter and transmission stages are excluded from this two-chain worksheet.
Common Mistakes
Do not treat a free-running idler as a compound jackshaft. An idler alone does not change the overall tooth ratio.
Questions about this calculation
Which values should I enter?
Identify the engine-to-jackshaft driving and driven sprockets. Do not treat a free-running idler as a compound jackshaft. An idler alone does not change the overall tooth ratio.
How should I interpret the result’s precision?
Displayed results are rounded for readability. Intermediate calculations retain floating-point precision; extra displayed decimals do not improve your input accuracy.
Can I use metric units?
Yes. Select Metric above the calculator to convert your inputs. Results always state their units. Tire-size designations keep their conventional mm and inch units.
What are the main limitations?
Assumes rigidly connected jackshaft sprockets and no slip. Converter and transmission stages are excluded from this two-chain worksheet.
Method, sources & relevant guides
Angular speed and tangential speed. Gear reductions follow tooth engagement and conservation of rotation count.
OpenStax — University Physics: rotation with constant angular accelerationMethod documentation updated October 18, 2026. Independent professional review has not been claimed. Read the calculation and review policy.