Electric Bicycle Mid Drive Motors


It wasn’t long ago that anyone who was looking knew you had a mid-drive motor on your electric bike. The coffee can hanging off the bottom bracket was a dead give away. Today though, mid-drive motors have become more elegant in their design. And with the growing number of mid-drive system manufacturers designing systems for use by bicycle manufacturers and DIYers, mid-drives are showing up on more and more electric bikes. Part of the appeal of mid-drive systems is their natural feel.


There’s no feeling of being pushed or pulled like can happen with hub-drive systems. They just feel like you’ve got some seriously strong quads and hams giving you enough energy to climb hills that would normally have you huffing, a touch of extra speed when wanted, or the ability to travel farther while expending the same amount of energy.


Nestled into the bottom bracket, these motors use the bike’s gears and derailleur system to exponentially increase the torque supplied by the motor to the rear wheel. This makes mid-drive systems especially effective when used to climb large hills. The motor can stay in its most effective RPM range and by shifting gears up or down, the appropriate ratio can be selected so the motor can run longer without overheating. 


Mid-drive systems use a variety of sensors to help control motor output. Depending on the complexity of the system and the manufacturer, there could be a torque sensor that reads the amount of pressure applied to the cranks, a crank speed sensor that monitors cadence, and a bicycle speed sensor that senses road speed. E-bike manufacturers may use all or any combination of these sensors to help the system provide optimum performance. The most high tech of these systems even offer onboard diagnostics that make sure each sensor is working properly.  By being located in the bottom bracket, the added weight of the motor is low and centralized in the bike. This keeps the center of gravity as low as possible and feels more natural. This is especially important on bikes, like mountain bikes, that are used for technical off-road riding. The centralized weight also makes loading and carrying these 2-wheelers on bike racks easier. 


Like hub motors, some mid-drive systems offer regenerative braking. Depending on the amount of regen, riders can feel resistance, so there’s no traditional freewheeling. This makes pedaling without assistance from the motor tougher, and some cyclists don’t like the slight dragging feeling. Regen comes at a cost too. Systems that offer this feature require more complicated controllers to keep the battery from being over charged. 


To keep the rider informed of system info, manufacturers of mid-drive systems offer a variety of computers. From simple systems that tell speed and battery charge only to high-tech models that track biometric data like pulse and target heart rates as well as syncing with smart phones or the cloud to provide turn-by-turn navigation, firmware updates, system configuration and troubleshooting. Most of these are mounted on the handlebar on removeable brackets (good for preventing theft and to help protect from weather), but some are molded into custom housings on the handlebar or frame. 


As with all engineering projects, there are some tradeoffs. Because mid-drive systems use the bike’s gears, the cyclist must have an understanding of how to shift gears. These gear changes can put extra stress on the chain and gear teeth and can result in clunky shifting and added wear on drivertain components because the electric motor is always engaged. Some of the more sophisticated systems, especially the Bosch system, come with shift sensing technology designed to mitigate this potential problem. Other manufacturers have introduced constantly variable gears and automatic shifting hubs to provide smooth shifting under load. 


For DIYers, the installation of mid-drive systems is more complex when compared to hub motors. Installation requires a pretty robust understanding of bicycle mechanics because the cranks, bottom bracket, chain rings and chain must be removed and reinstalled properly. Choosing a correctly sized system for a specific bottom bracket requires precise measurements and understanding of the types of bottom brackets on the market. Always check with the mid-drive system manufacturer to make sure everything is compatible. 


Mid-drive systems usually cost more. Often they require specialized bottom-bracket designs, which means additional development costs by frame designers. But as more manufacturers enter the market, prices are coming down on the hardware making mid-drive equipped models prices more in line with hub motor electric bikes. 


This article originally appeared in the Electric Bike Buyer’s Guide included in the Q3 2015 issue of Electric Car Insider magazine. The Buyer’s Guide features 30 electric bicycle profiles. Each full-page profile includes multiple photos and important specifications so buyers can make informed buying decisions. The most current print issue of the magazine is available on hundreds of independent newsstands in the U.S and Canada including Barnes & Noble, Hastings and Chapters Indigo. Never miss an issue by subscribing today.

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