| Metric | Setup A | Setup B |
|---|---|---|
| Chaincase ratio (bottom ÷ top) | — | — |
| Clutch reduction (secondary ÷ primary) | — | — |
| Overall reduction (chaincase × clutch) | — | — |
| Jackshaft RPM (engine ÷ clutch) | — | — |
| Driver RPM (engine ÷ overall) | — | — |
| Track travel per driver rev (with growth) | — | — |
| Estimated speed (with slip) | — | — |
| RPM needed for target speed | — | — |
| Driver torque estimate (with efficiency) | — | — |
| Tractive force estimate | — | — |
| Time | RPM | Slip % | Growth % | Eff % | Overall A | Speed A | Overall B | Speed B | Δ Speed |
|---|---|---|---|---|---|---|---|---|---|
| Run a calculation to populate history. | |||||||||
| RPM | Speed A | Speed B | Δ (B−A) |
|---|---|---|---|
| Calculate to generate sweep results. | |||
| Scenario | RPM | Top/Bottom | Clutch (P,S) | Slip | Growth | Travel method | Estimated speed |
|---|---|---|---|---|---|---|---|
| Stock trail | 8200 | 18 / 45 | 1.00, 1.80 | 3% | 0% | Pitch×Teeth | ~ 67 mph |
| Gear up | 8200 | 19 / 45 | 1.00, 1.80 | 3% | 0% | Pitch×Teeth | ~ 71 mph |
| Power estimate | 8200 | 18 / 47 | 1.00, 1.95 | 6% | 1% | Diameter | ~ 54 mph |
Formula used
This tool estimates track-driven speed from ratios and an effective driver travel distance. It also estimates torque at the driver when engine power or torque is provided.
- Chaincase ratio = BottomTeeth ÷ TopTeeth
- Clutch reduction = SecondaryRatio ÷ PrimaryRatio
- Overall reduction = ChaincaseRatio × ClutchReduction
- Driver RPM = EngineRPM ÷ OverallReduction
- Travel per rev = (Pitch × DriverTeeth) or (π × Diameter), then multiplied by (1 + Growth%)
- Speed = DriverRPM × Travel × 60 converted to mph/km/h, then multiplied by (1 − Slip%)
- Torque from hp (lb·ft) = HP × 5252 ÷ RPM
- Driver torque ≈ EngineTorque × OverallReduction × Efficiency%
- Tractive force ≈ DriverTorque ÷ DriverRadius
How to use this calculator
- Enter your engine RPM, slip, and optional growth.
- Fill Setup A with sprockets and clutch ratios.
- Choose Pitch×Teeth or Diameter for driver travel.
- Optionally fill Setup B to compare a change.
- Use Target speed to estimate required RPM.
- Use the RPM sweep to compare speeds across a range.
- Export your session using CSV or PDF.
- Real speed depends on clutch tuning, belt condition, and traction.
- Torque and force are estimates, not dyno measurements.
- Always test changes in safe conditions.
Why gearing changes feel dramatic
Changing sprockets or clutching can transform how your sled pulls and tops out. This calculator turns teeth counts, clutch ratios, and driver travel into clear numbers you can compare before you wrench. It is useful for trail tuning, lake runs, and hill pulls.
Overall reduction and ratio math
Overall reduction is the main value to watch. It combines chaincase ratio (bottom ÷ top) and clutch reduction (secondary ÷ primary). With 18/45 sprockets and 1.80 ÷ 1.00 clutching, overall becomes 2.500 × 1.800 = 4.500. If you gear up to 19/45, chaincase becomes 2.368 and overall drops to 4.263.
RPM to driver speed relationship
At a given engine RPM, driver RPM equals engineRPM ÷ overall. Using 8200 RPM and 4.500 overall gives about 1822 driver RPM. With 4.263 overall, driver RPM rises to about 1924. That change usually increases top speed, while reducing torque multiplication at the driver and track.
Track travel: pitch or diameter
Track travel per driver revolution can be estimated two ways. The pitch method uses pitch × driver teeth. With 2.52 inch pitch and 8 teeth, travel is 20.16 inches per rev. The diameter method uses π × effective driver diameter, which can better reflect worn, lifted, or aftermarket drivers. Pick the method that matches what you can measure reliably.
Growth and slip adjustments
High speed loads can stretch the track slightly, so the tool includes track growth. A 0.5% growth raises 20.16 inches to 20.26 inches per rev. Slip is applied after speed is computed; 3% slip reduces a 70.0 mph ideal estimate to 67.9 mph. If snow is loose, try 5–8% slip to stay realistic.
Torque, efficiency, and tractive force
Power delivery is not free, so efficiency matters for force estimates. If efficiency is 95% and engine torque is 102 lb·ft, driver torque for a 4.500 overall setup is roughly 102 × 4.500 × 0.95 = 436 lb·ft. Divide by driver radius to estimate tractive force. With a 7.20 inch diameter driver, radius is 3.60 inches or 0.30 ft, giving about 1450 lbf before traction limits.
Sweep tables and planning a swap
Use the RPM sweep to see how setups separate across the band, such as 6000–9000 RPM in 500 RPM steps. The target-speed field back-solves required RPM for each setup, which helps spot over-gearing when RPM demand is too high. Finally, the helper suggests a top or bottom tooth count to hit a desired overall ratio, then you can verify chain fit and clearance.
FAQs
1. What does overall reduction mean?
Overall reduction is the total ratio from engine to driver. It equals chaincase ratio (bottom ÷ top) multiplied by clutch reduction (secondary ÷ primary). Higher overall reduction gives more pull, but lowers speed at the same RPM.
2. Why is my real speed lower than the estimate?
Real speed drops due to track slip, snow drag, belt slip, and wind. Increase the slip percent until the calculated speed matches your GPS on similar snow. Also check track tension and clutch calibration.
3. Should I use pitch or diameter travel?
Use pitch × teeth when your pitch and tooth count are known and stock. Use diameter when you have an effective driver diameter from measurement or aftermarket parts. Diameter can better reflect wear, lifts, and track growth effects.
4. How do I gear for deeper snow or towing?
Aim for a higher overall reduction by using a smaller top sprocket, a larger bottom sprocket, or higher clutch reduction. That increases driver torque and helps keep RPM in the powerband under load, at the cost of top speed.
5. How is torque calculated in this tool?
If you enter torque, it is used directly. If you enter horsepower, torque is derived using HP × 5252 ÷ RPM. Driver torque is then estimated as engine torque × overall reduction × efficiency percentage.
6. What is the target-ratio helper for?
The helper suggests a sprocket tooth count that matches a desired overall ratio using your Setup A clutch ratios. Use it for quick planning, then verify available sprockets, chain length, and clearance before ordering parts.