Watts Up With E-Bikes

 

 

in more energy transferred into motion. Voltage pushes amps to cause the motor to turn and do work. 

WHAT’S A WATT?

The amount of work the electricity does is measured by watts. However, some energy is lost through heat. Poorly made motors can consume a lot of energy and not do much work. As such, just because a motor has a high wattage does not necessarily mean it will perform well. The motor could just sit there and get hot. Further, you may not need a lot of wattage to propel your ride. Typical wattage ratings are 250 watts to 750 watts. (Over 750 watts is not considered a bicycle in most jurisdictions; it’s a moped or motorcycle.) Some of the newest e-bikes are coming out with lower, not higher, wattage. Now, you might find a very lightweight bike with a smaller battery and motor that performs nearly as well as a similar older and heavier bike. 

Even Tesla cars run on 18650 or 21700 batteries, up to 7,000 of them! Photo courtesy of Tién Nguyen

 

WHAT ABOUT TORQUE?

This is twisting force. A motor’s torque is a good measurement to compare its ability to twist the wheels of the bike. Many bikes now list their torque spec. Typical levels are 50 N/m to 120 N/m (that’s 37–89 foot-pounds). Unfortunately, like many specs, there is no universal standard measurement. Torque is leverage, and that leverage ratio can be changed through gearing. That’s why center-drive motors have lower wattage ratings; shifting changes the way the torque is applied. Torque also changes as the rpm change. As a result, it’s hard to compare one motor’s performance to another using only torque as a guide. More is not necessarily better. After about 120 N/m of torque, the motor puts tremendous stress on the drivetrain. Go above 120 N/m and you really have a motorcycle. As with anything, it’s best to take a ride and then decide.

WHAT ABOUT WATT/AMP-HOUR?

As the name suggests, it’s the amount of energy in a battery as delivered over time. For example, a 10-Ah battery will deliver about 1 amp of current for 10 hours. Similarly, a 500-Wh battery will deliver…

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