January 19, 2021
What are Hub Motors?
Hub motors were the first type of drive systems for bicycles to be patented and they continue to be popular today. Instead of trying to integrate a motor into the bicycle drivetrain (complimenting the gears and chain that the rider uses) hub motors stay completely separate. Electricity is run through copper wires to create electromagnets which repel traditional rare Earth magnets and create force that rotates the hub forward (and sometimes backwards). In the early days brushed motors were used because they are inexpensive and require less sophisticated control systems but the brushes wear out over time and require replacement. These days, nearly every hub motor (geared or gearless is brushess and uses direct current DC).
Hub motors for bicycles are usually positioned at the middle of a wheel and when the bike is powered off they function much like a traditional hub (connecting the tire, rim and spokes to the axle. Spokes are flexible and light weight, they absorb some shock when riding but can come out of true over time. Regular bicycle maintenance is still required with an electric bike and one downside to hub motor designs is that they add additional weight to the wheel and require extra wires to deliver electricity and communications about operation. This means that truing wheels and repairing flats requires more effort.
So hub motors take the place of regular light weight hubs, connecting the wheel to the bicycle axle. As they receive electricity and spin, the bike is propelled forward and some of this energy is exerted into the frame at the dropouts. Usually the sturdiest place to mount a hub motor is in the rear wheel because these dropouts are reinforced and have four legs connecting them to the rest of the frame instead of just two on the front fork. These four arms consist of two seat stays and two chain stays.What is a Mid-Drive Motor?
As we transition into mid-drive systems, imagine this scenario. A rider with a hub motor driven electric bike approaches a very steep hill, stops completely and then uses a twist throttle to power forward. The motor is likely going to struggle because it is designed for relatively flat surfaces and provides “peakey” output as mentioned before. So the motor groans and slowly pushes the rider forward. Without pedaling along, most hub motor designs just cannot carry an average sized passenger up a steep incline from rest. This is where we get into the benefits of a mid-drive system.
Pictured below is a 2020 Yamaha PW-ST Centerdrive mid drive motor system on a Haibike ebike:
Climbing is where mid-drive motors really shine. Unlike a hub motor, this design lives at or near the bottom bracket (the point where the crank arms attach through the frame for pedaling) and drives the chain forward instead of the wheel itself. Mid drive systems benefit from many of the same mechanical drivetrain systems as the rider (the use of gears for climbing or going fast) and reduce unsprung weight. This is the optimal setup for efficiency (extending ride distance) and climbing.
Imagine a full suspension electric mountain bike with a mid drive motor. The front and rear wheels can rebound quickly and efficiently because their mass is lower and the overall weight of the bike itself is connected to the main section of the frame making it feel fluid. When approaching a large hill, the rider can shift into a low gear providing mechanical advantage making it “easier” to pedal and climb. Just as the rider benefits in this scenario so too does the motor. While the overall speed of the bike is reduced for climbing, neither the rider, nor motor will be over exerted thanks to the gears. In this scenario the rear dropouts also endure less strain because the weight and force of the motor are spread out and connected to the major tubing of the bottom bracket.
Gears are both a blessing and a curse with mid drive systems because now instead of just the rider exerting force into the system (the chain, rear cassette cog teeth and derailleur), the motor is as well. If you’ve ever changed gears when pedaling hard, you may remember the awful sounds and sensations of mashing, crunching or grinding. The teeth used to pull the chain and the derailleur arms used to move the chain from one sprocket to another are sensitive, requiring a certain finesse to activate properly. Doing so will extend the life of your bike and help you to avoid tune-ups and cassette replacement.
To shift optimally I suggest gaining speed then relaxing the force being applied to the pedals, shifting, then maintaining a gentle cadence until the chain is correctly aligned before exerting more force. This may be difficult on a pedelec system where the rider relaxes their input but the motor continues to pull hard. In this kind of situation it may be ideal to shift on a flat surface where little force is required to keep the bike moving forward. The problem here is that often shifting is done when encountering hills or starting from rest; just as more force is required to maintain speed. Long story short, be gentle when shifting and recognize that mid-drive motors pull the same chain that you do as a rider.
The final advantage with mid-drives is that the motor weight is located centrally on the frame in addition to being low. This is going to provide stability when riding or lifting the ebike for transport or storage. One quick downside is that the motor may be vulnerable to rocks and logs on the trail with a mid-drive because it’s not surrounded by spokes and a wheel like hub motors are. In my experience, most mid-drives are protected with some plastic or metal casing that can take a few hard hits before suffering damage.
One other drive system that sort of splits the difference between mid drive and hub drive is the shaft drive. It works like an automobile, positioning the motor more towards the center of the bike while driving the rear wheel with a shaft. These are not very common today, perhaps because they require extremely customized frames that are not symmetrical. While a rear wheel drive truck often has a drive shaft extending from the motor (under the hood) back to the rear wheels that is located centrally under the body, a bicycle has to use one of the chain stay arms to support the shaft. This makes it look a bit awkward and perhaps difficult to service as well.
In summary, the drive system you choose will impact the overall weight and weight-distribution of your electric bike. It will provide more or less efficiency for riding fast, climbing or navigating bumps and it will cost more or less depending on how customized the frame is and whether it offers regeneration and special sensors for shifting gears. I lean towards geared hub motors for affordable light weight around-town transportation, direct drive hub motors for quieter riding, increased power and regenerative braking and mid drive motors for mountain biking or lots of hill climbing.