Jackshaft Gear Ratio Calculator
Enter sprocket teeth, tire size, torque, chain pitch, and center distances. The calculator returns ratios, RPM, speed, torque, chain length, and drivetrain checks.
Ratio and Speed Graph
The graph compares stage ratios and shows expected speed growth across the RPM range.
Example Data Table
| Setup | Driver | Jackshaft Input | Jackshaft Output | Driven | Overall Ratio | Use Case |
|---|---|---|---|---|---|---|
| Light kart | 12 | 36 | 14 | 60 | 12.86:1 | Strong low speed pull |
| Mini bike | 11 | 33 | 15 | 54 | 10.80:1 | Balanced trail setup |
| Utility machine | 10 | 40 | 12 | 72 | 24.00:1 | Heavy load reduction |
| Speed setup | 14 | 28 | 18 | 48 | 5.33:1 | Higher final speed |
Formula Used
Primary Ratio = Jackshaft Input Teeth ÷ Engine Sprocket Teeth
Secondary Ratio = Driven Sprocket Teeth ÷ Jackshaft Output Teeth
Overall Ratio = Primary Ratio × Secondary Ratio
Jackshaft RPM = Engine RPM ÷ Primary RatioAxle RPM = Engine RPM ÷ Overall Ratio
Speed MPH = Axle RPM × Tire Diameter × π ÷ 1056
Wheel Torque = Engine Torque × Overall Ratio × Efficiency
Links = 2C + (T1 + T2) ÷ 2 + (T2 - T1)² ÷ (4π²C)C = Center Distance ÷ Chain Pitch
How to Use This Calculator
- Enter the engine sprocket and jackshaft input sprocket teeth.
- Enter the jackshaft output sprocket and final driven sprocket teeth.
- Add engine RPM, torque, tire diameter, and efficiency.
- Enter chain pitch and both shaft center distances.
- Add a target speed to compare your selected gearing.
- Press the calculate button to view results above the form.
- Use the CSV or PDF buttons to save the calculation.
Jackshaft Gear Ratio Planning Guide
What a Jackshaft Does
A jackshaft is a short intermediate shaft. It lets one drive source use two chain or belt stages. This layout is common on karts, minibikes, compact machines, conveyors, and custom engineering builds. The main purpose is ratio control. A small change in one sprocket can make a large change at the wheel or final shaft.
Why Two Stages Matter
This calculator separates the system into a primary stage and a secondary stage. The first stage runs from the engine or motor to the jackshaft input sprocket. The second stage runs from the jackshaft output sprocket to the axle or driven sprocket. Multiplying both stage ratios gives the total reduction. A larger total ratio gives more pulling force. It also lowers final speed. A smaller ratio gives more speed. It also reduces launch torque.
Speed, Chain, and Safety Checks
Jackshaft speed is important. Bearings, hubs, keys, and sprockets all have limits. The tool compares the estimated jackshaft RPM with your selected safe limit. It also estimates chain speed for both stages. High chain speed may need better alignment, lubrication, guarding, and quality chain. Chain length estimates help you plan center distance and link count before buying parts.
Torque and Target Speed
Torque estimates are useful during early design. The calculator multiplies engine torque by the total ratio and efficiency. This gives estimated axle torque. The tractive force value converts that torque into tire force. It helps compare hill climbing, hauling, and acceleration potential. Real machines may differ because clutches, belts, tires, weight, and surface grip change the result.
Use the target speed comparison when choosing sprockets. Enter your desired top speed. The tool estimates the ideal overall ratio and shows the percentage difference from your current layout. A positive difference means your setup is shorter than the target. A negative difference means it is taller.
Final Design Advice
Always treat the output as a design guide. Confirm shaft strength, key size, chain rating, guard clearance, and brake capacity before operation. Test at low speed first. Listen for chain noise, vibration, or heat. Good alignment and correct tension often matter as much as the numbers. Document each sprocket set after testing. Small records make future changes easier, especially when tire size, clutch engagement, or motor speed changes later.
FAQs
1. What is a jackshaft gear ratio?
A jackshaft gear ratio is the combined reduction or speed change made by two sprocket stages. It uses one stage from the engine to the jackshaft and another from the jackshaft to the final driven sprocket.
2. How do I calculate overall jackshaft ratio?
Divide jackshaft input teeth by engine sprocket teeth. Then divide driven sprocket teeth by jackshaft output teeth. Multiply both answers to get the overall ratio.
3. Does a higher ratio mean more torque?
Yes. A higher reduction ratio increases axle torque and launch force. It also lowers final speed. This is useful for hills, heavy loads, and slower machines.
4. Does a lower ratio increase top speed?
Usually yes. A lower overall ratio lets the axle turn faster at the same engine RPM. However, the engine still needs enough torque to pull the load.
5. Why is jackshaft RPM important?
Jackshaft RPM affects bearing life, shaft safety, sprocket balance, and chain speed. Excessive RPM can cause heat, vibration, wear, or failure if parts are not rated for it.
6. Is the chain length result exact?
It is an engineering estimate based on sprocket teeth, pitch, and center distance. Real chain length may need adjustment for tensioners, slots, wear, or frame tolerances.
7. What efficiency value should I use?
Clean chain drives often use 90% to 98%. Older, dirty, misaligned, or heavily loaded systems may be lower. Use a conservative value for safer torque planning.
8. Can this calculator be used for belts?
The ratio and RPM logic can help with belt stages. Chain length results are for chain pitch, so belt length should be checked with belt-specific formulas.