What Is A Pedal-Electric Bicycle Mid-Motor vs. A Hub Motor?

Find out the differences between pedelec mid-motors versus hub motors.

Stromer ST5 Electric Bike with Hub Motor on white background.
Stromer SYNO Sport motor with 850 watts (USA 650 watts) and a torque of 48 Newton meters. The Stromer ST5 Electric Bike with Hub Motor.Stromer

There are two types of electric drive systems for electric-assist bicycles: hub motors or mid-motors. As the name implies, hub motors place the electric motor in the front or rear wheel hub to help you move the bicycle down the road. A mid-motor puts an electric motor and gears right on the pedal shaft and boosts your pedal power directly, transmitting it through a chain or belt to the rear wheel.

Early electric bicycles from 2000 or so mainly used hub motors on the front or rear wheel. Many of these came from Taiwanese or Chinese suppliers. Some were direct-drive hubs; others were geared hubs which often used injected-molded-and-not-particularly-shock-resistant plastic gears. The quality varied, but, with their hand-wound stators and somewhat lackluster engineering, most didn’t—to be kind—meet high reliability standards. If those early eBikes weren’t the hallmark of reliability, most bicycle shops—who were new to electric vehicles as well—were also struggling to deal with the new technical challenges. As you can imagine, this didn’t always make for happy customers.

Super 73 Hub Motor close up.
Super 73 Hub Motor close up.Mark Hoyer

This all changed in 2010 when Bosch introduced its first mid-motor design at the Eurobike show. Bosch packaged a small, high-speed and high-production automotive motor (rumored to be from a power-steering application) along with an elaborate reduction system to convert its thousands of rpm into something a bicycle could use into one compact mechanical package. A bicycle manufacturer had only to produce a frame the mid-motor bolted into. Bosch, one of the largest auto suppliers in the world, certainly knew how to make high-quality parts. It also had a big power-tool business, one that had repair centers in all major European cities.

The Bosch mid-motors were not only more reliable than the hub motors that preceded them, but they also could be taken out as a unit and shipped to a nearby Bosch service hub if they had an issue. The Bosch mid-motor was a roaring success, and Bosch became the biggest supplier in the eBike business, building up toward $1 billion in sales. Bosch's success has since attracted a bevy of competitors, many themselves German auto suppliers like Brose, Oechsler, and ZF, all which have recently introduced mid-motors of their own. Yamaha is also a big mid-motor maker that builds its own bikes and supplies other makers. Another supplier is bicycle components giant Shimano.

Super 73 with Hub Motor.
Super 73 with Hub Motor.Mark Hoyer

But does that mean the mid-motor is the end point of eBike design? While many people will tell you that a modern eBike must have a mid-motor, all you have to do is to ride something like a Swiss Stromer ST2 to get an opposing viewpoint. I have yet to ride an eBike (at least a legal one) that has the immediacy and performance of a Stromer with its rear hub motor. As to technical issues against using a hub motor, they add significant weight to the wheel they are mounted in. Mid-motors have the advantage of not adding any weight to the wheels, which on a high-performance electric-assist mountain bike means the suspension will work better following bumps and provide more control and a better ride.

That advantage goes away for rigid commuters and road bikes. The mid-motor bikes tend to have a lot of motor-induced drag when their assist speed is exceeded, more an issue for the 15-mph maximum assist in the European Union than in the US. And it’s clear that due to their gearing and clutch complexity, mid-motors have a complexity and price disadvantage compared to a good direct-drive hub motor.

Yamaha mid motor
Yamaha mid motorYamaha

Also, some advanced features—yet to be seen in production eBikes—can more easily be utilized with hub motors. An example is that a powerful rear hub motor can replace the rear brake with an inexpensively implemented motor/ABS system, something that eBike and motorcycle maker Erik Buell has patented for his Fuell Flow e-motorcycle, and that will surely be seen on eBikes in the future. And reliable Asian suppliers have emerged for hub motors, companies like Taiwan's TDCM and China's Bafang. There will be more.

So consider this a battle not yet won, and no premature victory should be declared for mid-motors. The mid-motor, in its current form, has a compelling advantage for long-travel e-mountain bikes, but much less of one for commuters and cruisers. The real victory has already been won for eBike owners, with the improvements to function and reliability that have come to both drivetrain types.