Many e-bike motor suppliers provide torque data for their assist motors, but for this data to be truly useful in comparing assist motors, it must be measured consistently across different e-bikes.
However, some motor manufacturers measure the torque at the motor’s output gear, while others measure it at the output of the bike’s gear system (on the driven wheel). The latter measurement is only valid in the lowest or lightest gear position.
Additionally, different wheel sizes make a direct comparison of torque invalid when trying to quantify the level of assist provided by a specific motor solution. For example, a torque of 100 Nm provides 40% more assist power on a bike with 0.5-meter diameter, 20-inch (47-406) wheels compared to an e-bike with regular-sized 28-inch wheels (40-622).
The conclusion is clear: differences in torque measurement methods and the way torque translates into propulsion force make it impossible to compare “motor torque” on equal terms alone.
To properly compare data from different motor suppliers and wheel sizes, you need to convert torque into propulsion force, as this better reflects the actual assist power helping to propel the vehicle. This allows for a direct and fair comparison on equal terms.
To accurately compare assist power, you need to know:
- The (peak) motor torque in Newton-meters (Nm) as measured at the bike’s wheel, not elsewhere.
- The wheel diameter in meters (m).
The maximum assist force can then be calculated using the formula:
F = torque / radius
or equivalently:
F = torque * 2 / diameter
Examples:
For the FRIKAR, which provides 57 Nm of peak torque per motor and has a wheel diameter of 0.5 m:
Fassist(max) = 57 Nm * 2 / 0.5 m = 228 N
For a regular e-bike with 60 Nm of torque at the rear wheel and a wheel diameter of 0.7 m:
Fassist(max) = 60 Nm * 2 / 0.7 m = 171 N
Final comment:
On the FRIKAR, the 57 Nm is the maximum assist we can provide from the assist motor. Since we also have an electronic gear that provides power from the cyclist to the wheels, using the same type of motor as the assist motor, we can deliver a total of 456 N of propulsion force.
With a total vehicle mass of 210 kg and gravity of 9.81 m/s², the maximum theoretical hill the Frikar can climb is 23.2%. However, factoring in friction and air drag, we estimate that the maximum hill you should be able to climb with a fully loaded FRIKAR is 16%.
More about torque on ebike motors.