by John Duprey
There are a few different types of e-bikes systems on the market today. Besides mid-drive and hub motors, there are friction drive systems that have a small wheel that rides on the bike's tire to propel the bike. There is also a pusher trailer design that does just that. Mid-drive systems are quickly catching up in numbers, while the majority of e-bikes in use today have hub motors. Let's have a look at some of the differences between these two predominant types of e-bikes.
A mid-drive motor replaces the bottom bracket and chain ring (gear) where your crank arms and pedals are attached. It doesn't allow for a front derailleur so care must be taken to chose a chain ring smaller than or similar in size as the bikes previous largest chain ring. If you plan to climb hills often, than a smaller chain ring is recommended. If you plan to ride mostly flat terrain, a larger chain ring will provide maximum speeds. Mid-drive motors are geared internally. Additionally they use the bikes chain and rear wheel gearing. Most bikes have between 8 and 11 gears at the rear wheel. The addition of the motor more than eliminates the need for the gearing a front derailleur would provide. The rider pedals normally and changes gears just like they would without a motor. When riding slower or climbing a hill, you shift down to a lower gearing, and when going fast you shift into a higher gear, as you would riding a motorcycle or driving a manual shift car. Just like riding a motorcycle, in order to minimize wear and tear on the transmission, it's best to momentarily reduce the power when shifting gears. With a mid-drive motor and a low gear selection, a quality e-bike can climb almost any hill you're willing to point the bike up. Mid-Drive motors allow the motor to spin independent of the crank arms. Should the rider wish, they can pedal the bike as normal without assist from the motor.
There are two types of hub motors, direct-drive and geared motors. Hub motors can be found on either front, back, or both wheels of a bike. Generally front hub motors tend to be low powered (200-350 watts). Over powering and spinning the front wheel would mean a loss of control of the e-bike. A direct-drive hub motor was first patented in the US by Ogden Bolton in 1895. The design has evolved since and most modern e-Bikes use more reliable brush-less motors, although there are still a few older e-bikes around using brushed motors. A direct-drive motor has no gearing, thus one revolution of the motor is equal to one revolution of the wheel. Direct-drive motors have to be large in diameter in order to provide a sufficient amount of torque. They are not good for hill climbing and tend to bog down when climbing. A direct-drive motor creates drag when pedaling without power and tends to be larger and heavier than a geared hub motor. A geared hub motor allows the motor to run at a higher rpm than the wheel is turning. The gearing allows the motor to be smaller and lighter than the direct drive motor. It also provides better climbing performance over a direct drive motor but of course doesn't have the flexibility and range of gearing that a mid-drive motor offers. Hub motors don't have the high torque outputs of a mid-drive motor. For example; our 1000 watt mid-drive motor produces a 160 Nm of torque compared BionX or Pedego 500 watt hub motors at 50 Nm. (at the time of this writing they don't offer 1000 watt motors or don't disclose the torque rating, so I am using the 500 watt motor for comparison) The higher the torque number the more powerful the motor.
One difference between mid-drive motors and hub motors is regenerative braking (regen). This is a method of using the electric motor to recharge the battery while slowing the vehicle. Some hub motors have this feature, while mid-drive motors do not. In my experience driving a 2015 Nissan Leaf electric car to the top of Pikes Peak back down, I was able to recover 49% of the indicated battery level that had been used to climb the peak. Why not 100%? The Leaf converted potential energy (battery charge) into kinetic energy (motion) and back again to potential energy. The laws of physics come into play here. Energy gets lost in every step along the way. Inefficiencies in the motor acting as a generator, the charging circuits, the wiring, and the battery, combine to leave us well short of the energy we started with. The peak climb is an optimal example since it is one long steady climb followed by one steady descent. In normal stop and go city driving the recovered energy would be less. An e-bike is less efficient in recovery since a considerable amount of energy is lost to wind resistance, and the kinetic energy carried by the bicycle and cyclist (mass x velocity) is minimal to begin with. Recreational cyclists often tend to pedal a bit, then coast, pedal more and coast, which doesn't work if regen is slowing the bike whenever you stop pedaling. If one were to put an e-bike up on a stand and pedal hard it would take 5+ hours of pedaling hard to recharge a battery. It is so much easier to plug the battery in at a coffee shop or at work to recharge. Though with a pedal assist range of 80 miles, battery capacity is probably not an issue.
The most efficient way to save energy is to not use it to begin with since the laws of physics prevent one from recovering much of it for re-use. If it takes less energy to get you rolling and maintain your speed, that's a win! If you coast while not using any energy, that's another win! If you keep moving, avoiding the brakes, anticipating stops and coasting, that's a win! A drive system that uses less energy to get and keep you rolling will be more efficient than one that uses more energy to get you rolling but recovers only a little of that energy when you stop.
A cyclist will generally pedal at a cadence (rpm) of 45-75. This is a narrow rpm that the mid-drive motor is generally operating at and so it can be optimized for peak efficiency to match the cyclists cadence. By changing gears to accommodate the cyclists comfortable cadence the motor will be operating at it's optimum efficiency as well.
A hub motor will be operating at a speed based based on the rpm of the wheel which will vary considerably between stopped to whatever is your top speed and may rarely be operating within it's efficiency range.
A mid-drive motor places the motor's weight low and at the center of the bike which is the optimum point for best handling. If the bike has suspension, a mid-drive motor is supported and cushioned by that suspension.
A hub motor places it's weight in the center of the wheel where it must absorb all the shock and vibration generated by the ground track. Since the spokes must be shorter to accommodate the motor there is less ability for the wheel to mute the shock and vibration. This can have an undesirable affect on the handling of the bike.
The most common bike maintenance is fixing flats. I seem to get them all too often. I use good tires and put a liquid sealant in the tubes, but I still get an occasional flat. And thus I carry a pump, patch kit, and a spare tube whenever I ride. A flat with a mid-drive motor is just like any normal bike.
With a hub motor it's a different story. A high-powered hub motor has a torque arm to prevent the axle from spinning. Picture the coaster brake you once had on a single speed bike. A spun axle on a hub motor will likely break the electrical wires going into the motor, short circuiting the power wires, and possibly damaging other electronics. The frame can be damaged as well. The torque arm helps to prevent this from happening. Fixing a flat on an e-bike with a hub motor requires removing additional parts, more tools, and disconnecting the wires which enter the motor through the center of the axle. Time consuming for sure, and complicated enough to make some riders refer to a mechanic for this normally minor repair.