Motors compared

Details: Last Updated: 18 February 2020

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Which e-bike mid-drive motor is the best?

In this article, mid-drive motors of 250W and maximal 3.5kg are compared from a mechanical viewpoint.

Mid-drive motors compared from a mechanical viewpoint

Mid drive motor	Net weight [kg]	Power [W]	Torque [Nm]	Double freewheel	Gear ratio	Gear stages	3rd party battery	Throttle control	Conversion kit
Bafang M500	3.3	250/500	95	Y		3			Y
Polini E-P3	2.9	250	70	Y					N
Bosch Active Line	3	250	40	N	14.2	1	N	N	N
Brose Magnesium	2.9	250	90	Y	27	3 belt drive	N		N
Shimano Steps	2.9	250	70			3	N		N
Yamaha PW-X2	3.1	250	80	Y	47.7	2	Y with hack	N	N
Kervelo Quartz 12	4 incl. gear	250	70						N
Metz	2.8	250	85						N
OLI Sport 85	3.5	250	83						N
Sachs RS	3.5	250	110						N

Who can help complete the missing data in the list?

Why compare mid-drive motors on mechanics?

Ebike mid-drive motors are often compared with each other, but this relates always to behavior and performance and these properties are mainly determined by software. With another software or motor controller, mid-drive motors can behave completely different.
Most motors use pedal assist control and this is in particular very software dependent. The differences in driving experience, and the many tests you find in magazines, are mainly caused by the pedal-assist software and not by the motor itself.

All mid-drive motors have about the same characteristic

It is important to know that, from a mechanical viewpoint, all well-built mid-drive motors of 250W are more or less the same. That is because all use the same motor type: a permanent magnetic motor. By nature, all small permanent magnetic motors have equal characteristics. From a mechanical viewpoint, the only substantial difference in performance is determined by the gear ratio. In practice, the maximum motor power will be limited by the motor controller and the software, but in this article, we focus on mechanical properties.

The best motor power graph

Here is the motor power graph of the Yamaha PW-SE. This mid-drive motor has one of the largest gear ratios, and therefore the highest efficiency. I have measured the motor parameters and made the motor graph, see the article about the Yamaha PW-SE.

It is very important to know that it is almost impossible to develop a motor with a much higher efficiency. This is due to the laws of physics, there is no better conductive material than copper and there are hardly any better permanent magnets than Neodymium.

This graph with motor curves circulates on the internet, but it is not correct, compare the yamaha with my graph above:

Maximum motor power

Note that the power of a permanent magnetic motor is a relative term. A 250W motor can easily deliver a much more power, for example 500W, and that occurs in practice, when accelerating and going uphill. The official motor power is the power that can be delivered continuously without overheating or otherwise being overloaded. Of course, this is never exactly 250W.

Why using a mid-drive motor?

For a high efficiency, a permanent magnetic motor must have a high speed, see this article.
When driving uphill, the speed is low but the required power is high. A hub motor has a low speed uphill and thus a very poor efficiency. The speed of a mid-drive motor depends on the cadence and therefore remains approximately constant.

What is the best mid-drive motor?

For my Maxun One solarbike, I’m looking for the best mid-drive motor that is available at the moment. I want to have maximum performance combined with minimum weight. Because I want to use speed control by throttle, instead of the standard pedal assist control, I have to use an external motor controller. Thus, because of the external motor controller, only the mechanical properties of the motor are of interest. That is why I have established a number of essential criteria that the motor must met.

Double freewheels

This is also called zero cadence support.
With many mid-drive motors, such as the Bosch, the bottom bracket always rotates when the motor is on. That is a disadvantage, it is handy to be able to drive away without having to pedal. This requires a motor with two internal freewheels; also called one-way clutches. It is also nice to be able to drive just by turning the pedals around without any force.

3rd party battery

With most motor systems, only a battery of the own brand can be used. This is a big disadvantage and actually prohibited, see this article:
https://www.avdweb.nl/solar-bike/batteries/koppelverkoop
Note that my Maxun One solar bike needs a special LiFePO4 battery because normal lithium batteries cannot withstand the high charging current of the PV panels.
To my knowledge, Yamaha is the only brand where you can use your own battery. Although the motor is protected against this, it can be hacked easily, see here:https://www.avdweb.nl/solar-bike/batteries/battery-hack
By the way, you can use your own battery on any motor you want, with the use of an external motor controller.

Throttle control / power-on-demand

With throttle control you can direct control the motor power, as opposed to pedal assist, where the motor power depends on the pedal force. Since my solar bike drives entirely by solar energy, I don't necessarily have to pedal. That is why I want to use only throttle control. But, due to the legal rules, the pedals must be turned around to enable motor assist.

Low weight

Why does an ebike have to be lightweight? For riding on a flat road, a few kilos more or less don’t matter much. But a lightweight ebike is useful when you have to walk up a staircase, for example at a train station without a lift. An ebike of 25kg is unmanageable, but a 15kg ebike can easily be lifted with one hand, so every kilo of weight saving is important.

Power

There are 250W motors wherein the same type is also available in more power. For instance, the 250W Bafang M500 is the same as the Bafang M600 with 500W power. The power of the M500 is just limited to 250W due to legal regulations. The advantage of these motors is that they are mechanically over-dimensioned when using just 250W. The table then shows 250/500.

Gear ratio

For a high efficiency, the internal motor must have a high motor speed, see this article:
https://www.avdweb.nl/solar-bike/hub-motor/permanent-magnet-dc-hub-motor-tuning
For a high motor speed, a high gear ratio is required. That is why the gear ratio GR is important. The motor speed is cadence x GR.

Gear stages

Generally, the efficiency of a gearbox decreases with the number of gear stages. A small housing requires gearwheels with small diameters, but this has the disadvantage that more gear stages are needed.
For example: The Bafang M500 has three gear stages with small gear wheels.
The Bosch Active Line has just one gear stage, so the gear efficiency is high. But the gear ratio is low, and a very small motor gear has to be used. This has impact on the maximum allowed output torque.
The motor with the best gearbox is the Yamaha PW-X2, it has a large housing that allows large gear wheels. It needs just 2 gear stages. The gear ratio is the highest of all mid-drive motors, so we can assume that the Yamaha PW-X2 has the highest motor efficiency.

Conversion kit

Most motors such as Bosch, Yamaha etc. cannot be purchased separately by private individuals. Bafang is the only brand that sells motors to them.

Crank system

All mid-drive motors currently have an ISIS crank system. The advantage of the standard ISIS system is the ability to mount lightweight cranks.

The best mid-drive motor

These mid-drive motors are the best in my opinion:
Polini E-P3
Yamaha PW-X2

The most important requirements are met:

Double freewheels
High gear ratio
Low weight

Unfortunately, all are not available separately by private individuals.

New mid-drive motors

There are some new motors since 2019:

Metz
OLI Sport 85
Sachs RS
Kervelo Quartz 12

Kervelo Quartz 12

The Kervelo Quartz 12 is very special, it has a built-in hub gear, that saves a lot of weight. I don't know anything about mechanics yet, but I think this might be the best mid-drive motors at the moment…