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. In some cases a front mounted hub motor is preferred because it allows an internally geared hub or continuously variable transmission CVT hub such as the NuVinci to be used in the rear. Some of the newest electric bikes like the Smart ebike combine an internally geared hub (that the cyclist pedals) with a hub motor and are able to put both in the rear wheel. Now that you’ve got an understanding for what a hub motor is… let’s talk about the benefits, drawbacks and ride quality. Hub motors tend to be peaky vs. torquey meaning they operate best at medium and high speeds. This makes them zippy and satisfying no matter which gear you’re pedaling in but less efficient over a range of speeds, especially slower ones. For example, if you’re starting from rest going up a hill and try to accelerate with a hub motor, it may struggle and even shut itself off. Here’s a short video I shot with a smaller 250 watt hub motor climbing a hill to demonstrate this. A big drawback to hub motors is that they position weight further out towards the end of the bike (either the front or back) which reduces balance. This can play a role when jumping a bike or taking it off road. Additionally motor weight is built into the wheel which increases unsprung weight, that is weight that cannot be “sprung” as a part of the main frame. This can be a tricky concept so I’ve made a short video to help clarify. In short, suspension can perform better if the elements it is suspending are lighter because they do not have to deal with as much inertia. Ultimately, hub motors allow the rider can choose whichever gear they want, pedal at any cadence desired and apply varying force with each stride without playing a material role in how the motor functions. I love hub designs for this reason, there’s nothing pulling on your chain, making it difficult or jarring to change gears… but while the motor is always ready to go, it’s not always performing optimally (for either power or speed). Read More
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