Note: this article only applies to e-bikes up to 250W
I want to build some lightweight e-bikes and I've figured out which motor is best suited to achieve this. The ebike will also be used for the revolutionary Maxun One solar bike. In this article, I want to share my research work. In this technical article, I conduct a comprehensive performance analysis of the Xiongda 2-speed motor and compare it to both 250W hub motors and mid-drive motors. My extensive simulations and testing confirm that the Xiongda motor performs just as well as mid-drive motors that are considerably more expensive.
Mid drive motors use a torque sensor together with software to provide a natural cycling feeling, which many people like. But I prefer a throttle only because when driving on my solar bike, the Maxun One, I don't have to pedal at all most of the time.
I’m a big opponent of derailleur gears on ebikes. Only road bikes need a derailleur because of the low mechanical losses. But for an e-bike this doesn't matter. Because a hub gear is maintenance-friendly it is much better choice for an ebike. The only disadvantage is that with using a hub gear it is not possible anymore to use a hub motor in the rear wheel.
Now, we are going to look at the pros and cons of the following ebike motors:
Note that Xiongda is the only manufacturer.
Mid drive motor
Things to know about hub motors
It is important to know that, from an electromechanical viewpoint, all 250W well-built mid-drive motors have more or less the same efficiency characteristics. That is because all use the same motor type: a permanent magnetic motor.
To get high efficiency, a permanent magnetic motor must have a high speed, see this article: Permanent magnet DC electric motor tuning
When driving uphill, the speed is low but the required power is high. A hub motor has a low-speed uphill and that is why the efficiency is very bad here. The speed of a mid-drive motor is constant at all cycling speeds, that's why a mid-drive motor performs better.
I want to use a standard carbon frame because is low weight. Therefore, the mid-drive motor has to be mounted on a common bottom bracket.
There are only two motor available: the Tongsheng TSDZ2 and the Bafang BBS02
However, these motors have some serious drawbacks:
I have a lot of experience with the Cute Q-85SX hub motor, which is used on my Maxun One solarbike. The motor is, lightweight (1.76kg), cheap and very durable. The only problem is that the motor has a low efficiency uphill, which causes high losses. To overcome this, you always have to pedal too uphill. If only the lowest weight is the main argument, then a hub motor is the best choice. But since the ebike needs to be powerful uphill, I can't use a normal hub motor.
When considering the pros and cons of all motors, the Xiongda 2-speed hub motor comes out on top. Extensive simulations have shown that it performs as well as a 250W mid drive motor. The weight of 3.2kg is quite high, but this can be compensated with using a lightweight hollow ISIS bottom bracket, which is not possible with the mid drive motors TSDZ2 and BBS02.
A nice-looking bicycle needs a carbon frame and a carbon front fork too. The dropout distance of a front fork is standard 100mm. But the 100mm wide Xiongda 2-speed motor is made for v-brakes only, while disc brakes are the standard nowadays. Xiongda has a disc brake version of the 2-speed motor but this is unusable because its dropout is 110mm and such front forks don’t exist in carbon.
The Xiongda 2-speed hub motor allows me to build a very lightweight ebike. Any common carbon frame can be used which makes the ebike nice looking. As the simulations shows, the motor has almost as good uphill performance as a mid-drive motor.
With a standard 100mm carbon front fork, no disc brake is possible, but this is actually not a big disadvantage, I can live with this.
It is remarkable that the Xiongda 2-speed motor has a fill factor of almost 100%, not a single winding can be added anymore. That significantly increases the motor's efficiency.
The motor is with 3.2kg very heavy. I think, it is possible to reduce the weight. Some parts are made of steel and may be replaced by aluminum or titanium.
For example, the black plate between the two planetary gears is now made of steel. The diameter and thickness are resp. 94mm and 7mm. In steel, this part weighs about 350g and in aluminum 125g. That already gives a saving of 225g. If we continue in this way, perhaps more than 500g can be saved and the motor will weigh less than 2.7kg.