Advanced Motor Sizing Form
Example Data Table
| Build type | Total mass | Speed | Grade | Gear ratio | Typical motor range |
|---|---|---|---|---|---|
| Light city conversion | 900 kg | 80 km/h | 5% | 8:1 | 35 to 55 kW |
| Small road car | 1500 kg | 110 km/h | 8% | 9:1 | 80 to 130 kW |
| Utility vehicle | 2200 kg | 100 km/h | 10% | 10:1 | 130 to 220 kW |
| Performance build | 1700 kg | 160 km/h | 6% | 7.5:1 | 180 to 350 kW |
Formula Used
Grade angle: θ = atan(grade percent ÷ 100)
Aerodynamic drag: Fdrag = 0.5 × ρ × Cd × A × Vair²
Rolling resistance: Froll = m × g × Crr × cos(θ)
Grade force: Fgrade = m × g × sin(θ)
Acceleration force: Faccel = m × (target speed ÷ acceleration time)
Total force: Ftotal = Faccel + Froll + Fgrade + Fdrag
Wheel power: Pwheel = Ftotal × vehicle speed
Motor power: Pmotor = Pwheel ÷ drivetrain efficiency
Wheel torque: Twheel = Ftotal × tire radius
Motor torque: Tmotor = Twheel ÷ (gear ratio × drivetrain efficiency)
Motor rpm: rpm = vehicle speed ÷ tire circumference × 60 × gear ratio
How to Use This Calculator
Enter the curb mass and payload. Choose the correct mass unit first.
Add your target speed and desired acceleration time. Use realistic values.
Enter grade, drag coefficient, frontal area, and rolling resistance.
Add tire radius, gear ratio, drivetrain efficiency, and rpm limit.
Press the calculate button. Read the result above the form.
Use the recommended peak motor size for short bursts.
Use continuous cruise power for heat and reliability planning.
Download the CSV or PDF report for later comparison.
Choosing the Right EV Motor Size
An electric vehicle motor must match the real job. A small motor may overheat. A large motor may waste cost and space. Good sizing starts with vehicle weight, tire radius, target speed, and acceleration time. It also needs road grade, drag area, rolling resistance, and drivetrain efficiency.
Power Demand
Motor power is not only about top speed. It is the power needed to push the vehicle through air, climb slopes, overcome tire losses, and accelerate mass. Drag rises quickly with speed. A small speed increase can require much more power. Hills also change the result. A ten percent grade needs far more force than level driving. This calculator separates each force. That makes weak points easier to see.
Torque and Gear Ratio
Torque moves the vehicle from rest. Wheel torque depends on total road force and tire radius. Motor torque depends on gear reduction. A higher gear ratio lowers motor torque demand, but it raises motor rpm. A low ratio may protect rpm, yet it can demand more motor torque. The best choice balances launch feel, top speed, controller limits, and efficiency.
Continuous and Peak Ratings
EV motors usually have peak and continuous ratings. Peak power helps during acceleration and short hill climbs. Continuous power matters for steady cruising, long hills, heat control, and reliability. Use the safety factor when choosing a motor. Heat, battery sag, old bearings, soft tires, wind, and extra cargo can all increase demand.
Practical Design Notes
The result is a planning estimate. Real builds also need controller current limits, battery discharge rating, cooling, axle strength, and traction. Tire diameter changes rpm and torque. Battery voltage affects current. High current needs larger cables and better protection. Check the recommended motor rpm against the motor limit. If rpm is too high, reduce gear ratio or choose another motor. If torque is too high, increase reduction or select a stronger motor. Always test with conservative settings first. Use certified parts where safety matters.
Data Quality Matters
Use measured weights when possible. Guessing low can understate power needs. Enter a realistic headwind for open roads. For city vehicles, compare several speeds and grades. Save reports for later changes. Then review results together carefully.
FAQs
What does this EV motor size calculator estimate?
It estimates peak power, continuous power, wheel torque, motor torque, rpm, battery current, and road force. It helps compare drivetrain choices before buying parts.
Should I choose peak power or continuous power?
Use peak power for acceleration and short climbs. Use continuous power for long cruising, heat control, and reliability. A good motor must satisfy both needs.
Why does road grade matter so much?
Grade adds a direct climbing force. Heavy vehicles on steep roads need much more torque and power than the same vehicle on level ground.
How does gear ratio affect motor size?
A higher gear ratio lowers required motor torque. It also raises motor rpm. The correct ratio balances launch torque, top speed, efficiency, and rpm limits.
Why is tire radius included?
Tire radius affects wheel torque and rpm. Larger tires need more torque for the same road force and reduce motor rpm at a given gear ratio.
What safety factor should I use?
A 20% to 30% factor is common for early planning. Use more margin for hills, towing, hot weather, heavy payloads, or uncertain input data.
Can this replace professional EV design?
No. It is a planning tool. Final design should check controller limits, battery discharge rating, cooling, fusing, cables, mounts, brakes, and legal rules.
Why is my rpm status above the limit?
Your gear ratio, tire radius, speed target, or motor rpm limit may not match. Reduce gear ratio, use larger tires, or choose a higher speed motor.