Sizing a motor for a vehicle
Working from vehicle targets — acceleration, top speed, range — back to motor specifications.
- Explain sizing a motor for a vehicle at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
Motor sizing starts not with the motor at all, but with vehicle-level targets: 0-100 km/h time, top speed, gradeability and range — the motor spec is derived from those, not the other way round.
In depth
Gear Ratio Calculator
Match a reduction ratio to top speed and launch torque.
ratio = motor rpm × wheel circumference ÷ (road speed × 60)The reduction gear trades speed for torque. Choosing it is a compromise between the top speed the motor can reach and the tractive force available at low speed.
Example: 16,000 rpm, 2.05 m circumference, 200 km/h target → ratio ≈ 9.8:1.
- Most EVs use a single-speed reduction between 7:1 and 11:1.
- A taller ratio raises top speed but reduces gradeability and launch force.
- Two-speed gearboxes exist mainly on heavy or performance vehicles.
Torque Calculator
Torque available at the shaft and at the wheel for a given power.
T (Nm) = P (kW) × 9550 ÷ n (rpm) ; T_wheel = T × gear ratioTorque is the inverse of the power equation. A single-speed reduction gear multiplies motor torque by its ratio while dividing speed by the same factor.
Example: 150 kW at 4,500 rpm → 318 Nm at the shaft; through a 9:1 reduction that is 2,865 Nm at the wheels.
- Wheel torque divided by rolling radius gives tractive force in newtons.
- Traction, not motor torque, usually limits launch acceleration.
- Gear losses of 2–4% should be subtracted from wheel torque.
Working the sizing chain
A target 0-100 km/h time and vehicle mass set the peak tractive force needed, which combined with wheel radius and gear ratio sets peak motor torque. A target top speed sets the required motor speed at that ratio. Peak power then falls out of torque and speed together, and continuous power is set by what the cooling system and thermal mass can sustain.
Lab: Balance efficiency and performance
Trade gearing, mass and drive mode against consumption and acceleration.
Acceleration needs wheel force; efficiency dislikes everything that produces it. Shorter gearing raises launch force but drops the motor into a less efficient high-frequency region at cruise. Mass hurts both acceleration and consumption; aerodynamic drag only matters at speed.
Key takeaways
- Motor sizing is derived backward from vehicle-level performance targets.
- Peak torque, peak power and continuous power each answer a different sizing question.
- Tyre traction, not motor capability, often sets the real launch acceleration limit.
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