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How do you compare the torque of a mid-drive and hub motor?
Posted May 21,2024
When it comes to electric bicycles, choosing the right motor is crucial for performance. Van Raam, a company specializing in e-bikes, primarily uses hub motors. However, mid-drive motors are also prevalent in the cycling world. This article will explain how to compare the torque of these two types of motors, providing insights with an example comparison between the torque of the Easy Rider tricycle with a hub motor and two types of mid-drive motors.
What is Motor Torque?
Torque, measured in Newton meters (Nm), is a force that causes rotation. In the context of e-bike motors, torque determines how much power the motor can deliver to move the bike. For example, a motor with 100 Nm torque and a 1-meter stick can lift a 10 kg weight at the stick's tip.
Hub Motors vs. Mid-Drive Motors
A. Hub Motors
Hub motors, like those used by Van Raam, are built into the wheel. They have a reputation for longevity and reliability. The torque of a hub motor is measured at the motor shaft, which directly drives the wheel.
B. Mid-Drive Motors
Mid-drive motors are located at the bike's crankshaft and transmit power to the rear wheel via the chain. The torque of a mid-drive motor is measured at the crankshaft, making direct comparison with hub motors complex without adjustments for gear ratios.
How to Compare Motor Torque
To accurately compare the torque of hub motors and mid-drive motors, you need to convert the motor torque to the torque at the driven wheel, considering gear ratios.
A. For Hub Motors
1. Direct Drive: The torque at the wheel is equal to the motor's torque.
2. Chain Drive: Adjust the motor's torque by the gear ratio between the motor's sprocket and the wheel's sprocket.
B. For Mid-Drive Motors
1. Consider the gear ratio between the crankshaft and the rear wheel.
2. Account for the impact of the bike's gear hub and shifting, which can reduce the effective torque.
Easy Rider Tricycle Comparison
The following comparison uses torque graphs to illustrate differences between the Easy Rider tricycle's hub motor and two mid-drive motors.
A. Mid-Drive Motor with 85 Nm Torque:
1. Delivers strong initial torque.
2. As speed increases, torque decreases significantly, falling below the hub motor's torque.
This reduction is due to gear shifting at a pedal speed of 40 RPM, common among Uniqualis's users.
B. Mid-Drive Motor with 48 Nm Torque:
1. Initially stronger than a hub motor across the torque range but only briefly.
2. Provides more power up to 5 km/h but less than the Easy Rider’s hub motor beyond that speed.
C. Hub Motor on Easy Rider:
1. Consistently delivers strong torque, especially at speeds above 6 km/h.
2. Offers a more stable and reliable performance without the significant drop-offs seen in mid-drive motors.
Conclusion
When comparing the torque of mid-drive and hub motors, it's essential to consider the torque at the driven wheel and the gear ratios involved. Hub motors, like those in Uniqualis's Easy Rider tricycles, often provide more stable and consistent torque at higher speeds compared to mid-drive motors, which show stronger initial torque but decrease significantly as speed increases. Thus, for sustained performance, particularly at higher speeds, hub motors can offer superior torque and reliability.
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