5 Reasons Mid-drives are the Best Electric Bike Motor
Are you looking to buy an ebike and confused by all the options? Not sure which is the best electric bike motor?
The simple answer is: Avoid any ebike with a hub motor … front hub or rear hub … with just one exception.
Mid-Drive Motors Use the Gears
Scientific American found that bicycles are the most efficient form of locomotion for any species on this planet. A key feature of a bike is its gears.
Gears let the rider pedal at their optimal rate regardless of whether they are racing on the flat or climbing a steep hill.
Imagine cycling up a hill in the wrong gear. Not only are you pedaling too slow, but quickly you burn out your legs as you overwork your leg muscles. In contrast, think of trying to go fast on the flat in the wrong gear. Your legs can’t move that quickly. This is why gears are critical.
Mid-drive electric motors take advantage of a bike’s existing gearing systems. Hub motors do not.
When you start on a hill, to achieve the same torque, you downshift. Because a mid-drive motor adds power to the chain, this power is scaled up or scaled down via the gears just like how your own human power is geared.
In contrast, inside the typical hub motor there is only one gear. Even if there are gears visible on the rear wheel, these visible gears solely affect the power provided by your pedaling – the power delivered via the chain. The rear hub motor itself does not benefit from these gears.
So, to compensate for a hill, a hub motor will draw more power.
There are two reasons why drawing more power is a problem.
First, motors and batteries have an optimal power output. Drawing power outside this range generates a lot of heat. Not only does this waste battery power, but when demand is too high, hub motors either burn out or automatically shut down – neither of which do you want mid-hill. (Go to 12:30 on the below video.)
Second, UK law limits the continuous power output of ebike motors to 250W. For comparison, an average rider might maintain 165W of power output, but a professional cyclist produces around 430W.
Yes, an electric motor is like having a second person pedaling, but neither of you is a professional cyclist. A steep hill remains a challenge.
Johnny Nerd Out demonstrated the difference gearing makes on an identical bike with virtually identical motors from the same manufacturer.
The side by side demonstration starts at 03:21
Rideability: Torque Sensors
UK Law requires that electric bikes only provide power when the rider is pedaling. There are two ways for your bike to detect that you are pedaling: cadence sensors and torque sensors.
Cadence sensors (which are standard for front and rear hubs) detect when the pedals change position.
Torque sensors (which are standard for mid-drive motors) detect the actual pressure you apply down on the pedals.
The first issue with cadence sensors is that they only engage the motor after you have rotated the pedals a half or full turn. When starting from a standing stop, that first turn is the hardest and your ebike will provide no assistance.
The second issue is that cadence sensors work like a light switch – the motor is either on or off. If you are pedaling and the motor is on, the same amount of power is supplied whether you are struggling on a hill or cruising along on the flat.
The demonstration of on/off power assist starts at 01:55
To increase or decrease the amount of power supplied, you have to manually adjust the power setting. The result is that cadence sensors can feel jerky, laggy and counterintuitive.
Torque sensors detect the amount of force you are applying to the pedals themselves and scale up power accordingly – resulting in a more natural ride with more natural power assistance.
Changing a Flat Tyre on a Hub Drive is a Pain
All bikes are attractive to thieves. Given their value, the motor and battery on an electric bike are particularly attractive.
Front and rear hub motors are easier to steal as they attach to a wheel and not the frame. To compensate, manufacturers of hub motor ebikes add security to make the relevant wheel harder to remove.
Now imagine getting a flat tire on the wheel with the hub motor. These security features make the wheel harder to remove and the flat harder to repair.
The hub on the affected wheel also requires power. So, extra wires connect to the hub, which will need to be disconnected. The wheel itself is heavier than a standard wheel.
All these issues make repairing a flat very difficult on the road and a complex operation at home.
The attached video shows an engineer at an electric bike company demonstrating how to replace a flat tire on his company’s electric bike. After watching even part of this video, it will become clear that repairing a flat while out on a ride is impossible.
Changing a flat tyre with a rear hub ebike motor. Complications start at 01:27
The description on the article is: “Don’t spend $100+ at a shop doing this job when the tube is literally $10.” That is the reality. Common repairs and adjustments can require you to take the bike to a specialist shop.
Mid-drive motors replace the pedals, crank, and front gears of a traditional bike. Rear hub motors replace a much larger and more specialised set of components, requiring specialist attention when problems arise.
Wear and Tear
Mid-drive motors add power at the crank, where you pedal. This means that extra power (and more importantly, dramatic changes in power) are applied to the chain – causing it to wear faster.
With a mid-drive motor, you will want to monitor your chain for wear and replace it if it starts getting slack.
Hub motors have a larger hub compared to a traditional bike meaning that the spokes are shorter and connect at a sharper angle relative to the rim. Both these factors make the spokes weaker.
The extra weight of the wheel (the unsprung weight) accentuates the impact of riding over cobbles or bouncing off potholes.
Hub motors add power (and dramatic changes in power) to the wheel itself. All of this results in extra wear on the spokes.
Johnny Nerd Out worked in an ebike repair shop and found that hub motors regularly work spokes loose and then break them.
Discussion of spoke damage starts at 06:01
Replacing a chain costs around £15 and could be done at home. The cost of getting a rear wheel rebuilt with a hub motor is substantial and that is if you can find a bike shop that will even do it.
Balance – Why Front Hubs are Problematic
The more weight you have over the front wheel, the “heavier the steering” – meaning the harder it is to steer. This is one reason why racers lean forward aggressively – to slow down the steering, which can be dangerous at higher speeds.
At slower speeds, responsive steering is required to maintain balance.
One issue with front hub motors is they add a lot of weight to the front wheel, making the bike less manoeuvrable and making the bike harder to balance at slower speeds.
Even when pushing your ebike, front hub motors add further problems. The extra weight can make the bike noticeably more difficult to mount a kerb, let alone carry the bike upstairs.
Even getting an ebike onto the bike rack of your car can be surprisingly cumbersome when the bike is out of balance – because of the weight of either a front hub or rear hub motor.
Hub Motors: One Good Thing
The one good thing about hub motors … especially front hub motors … is that the components are less expensive – meaning that the bike is more affordable.
If you lock your bike up in public during the day (and thus it is vulnerable to theft) and if you mostly use your bike for short trips on the flat, then a front or rear hub electric bicycle may be sufficient for your needs.
Front hub motors have the added benefit of easy maintenance given that the electric motor does not interfere with any of the traditional complexity at the pedals and crank … or at the rear gears.
Mid-drives: Manufacturers' Choice
The top two manufacturers of motors for electric bikes (representing 52% of the market) are Bosch and Shimano. They only make mid-drive motors. Other major manufacturers (Brose and Yamaha) also only makes mid-drives.
The third place manufacturer (Bafang) makes front and rear hub motors. All their front hub motors are flagged as being suitable for city riding.
Their rear hub motors are flagged as being suitable for city, touring, and road rides but not cargo or off-road riding.