Torque Vs Wheel size


Motor torque and what the rider feels can be drastically different from one another and depends on several factors. Motor torque is in the form on Newton-Meters, which represents the amount of force that is applied by a rotating member at exactly 1m from the axis. Now, on a bicycle tyre, the distance from the centre of the wheel to the tire tread is our distance, and for a 26” bike is not 1 meter but a mere 330mm! That means that the resulting force of a motor that is rated to 100Nm would be exerting 303 Newtons at the road on a 26” wheel and 100 Newtons if the wheel radius was 1m (hypothetical of course).

This is why a motor laced to a 20” wheel will feel like it has more acceleration than a motor laced to a 26” wheel, because it does! More force can be produced as the level arm (wheel size) is smaller in the 20” models. Now this sounds like a win, right? Smaller wheel means more acceleration, where’s the problem? The problem is in the motors RPM Constant (KV). The KV of a motor represents the RPM the motor will rotate at with each applied volt.  Therefore, if a system is set at 36V, the max RPM of the motor is a constant value. Reducing the size of the wheel also means that the wheel must rotate faster to be travelling at the same linear velocity!

In the above diagram, each wheel is laced to the same motor that has a Maximum Torque of 100Nm, a KV of 2.77 and voltage of 36V. The wheel on the left produces 200 Newtons of force at its wheel because the distance from the rotating axis is half that of the 100 Newton wheel. However, the left wheel is required to spin twice as fast as its big wheel brother at 100RPM as opposed to 50RPM, to be travelling at a linear velocity of 18.84kph.
For this motor having a KV of 2.77, the max RPM of the motor at 36V will be:

36V x 2.77KV = 100RPM

Therefore, the left motor is at its max RPM but only at 18.84KPh because of the small radius wheel! However as the right motor is twice the diameter, it will be able to reach a maximum linear speed of 37.67KPH due to its larger wheel radius. So choosing a wheel size that is appropriate for your application and need, as well as a motor with KV rating that is ideal is an important step to selecting the right motor for you!