It goes without saying that one of the most important aspects of an ebike is its motor. “But wait,” you say, “aren’t they all much the same?”
No, they are not. Here are the things you need to look out for when considering the motor on your ebike:
- Is the motor from a reputable brand? The most reputable brands of motors in ebikes in New Zealand are Bosch, Brose, Shimano, Yamaha and Bafang. These are the brands that were reviewed by Consumer Reports NZ. You can read their review here.
- How many watts is the motor? This is actually much less important than the torque (turning force) rating of the motor. Most European motor brands are 250 watts, because that is the legal limit in Europe. In New Zealand, the legal limit is 300 watts.
- What is the motor’s torque (turning force)? This is measured in newton-metres. The motors on some ebikes have as little as 50 newton-metres of torque, while others have as much as 80 newton-metres or in some cases, slightly more. According to Claudia Wasko, VP & GM for Bosch EBike Systems in the Americas, “rated power (wattage) is not that meaningful, you’ve got to look at the newton-metres.”
In what situations does having more torque really matter?
The primary situation where more torque will make a big difference is when you’re riding steep, windy off-road trails.
What’s the torque difference between mid-drive and rear-hub drive motors?
Rear-hub motors drive the back wheel directly, so regardless of what gear the chain drive is in, the back wheel will always have whatever the torque rating of the motor is, when at full assist. So a rear-hub motor with 45 newton-metres of torque will deliver 45 newton-metres of turning force on full assist.
The torque of a mid-drive motor is transferred to the back wheel through the chain, which has variable gearing. So, let’s say the mid-drive bike has a 42 tooth chainring on front, and the derailleur is set to run the chain on a 32 tooth sprocket on the back cluster. And, let’s say the motor has 80nm of torque. In that setting, the turning force delivered to the wheel will be 80 x 32/42 = 61 newton-metres of torque. A mid-drive motor that has 50 newton-metres of torque with the same gearing as above would deliver 50 x 32/42 = 38 newton-metres of torque. If the 50 newton-metre motor was on a bike that had a 36 tooth sprocket on the back, then the turning force delivered to the motor in that gear would be 43 newton-metres of torque.
The moral of the story
If you’re going to be riding timber trails, rail trails and other such less-challenging trails, then don’t concern yourself too much about torque. (Do however think about your battery size. See our blog “How far can you ride on a charge.”)
However if you’re going to be riding in steeper back-country with lots of sharp turns, and you’d like to maximise the assistance available from your mid-drive motor then find out the following:
- How many newton-metres of torque is the motor rated for?
- How many teeth in the front chainring? (Usual range is 38-48)
- How many teeth in the largest rear sprocket (usual range is 32-36)
- Calculate the torque your motor will deliver as follows: Torque rating of motor (e.g. 80nm) x teeth on largest rear sprocket/teeth on front sprocket (e.g. 32/42).
