E-Bike Motor Breakdown
As with standard pedal bikes, before choosing a new bike, it’s important to first ask yourself some key questions: what type of riding will you be doing and what type of rider are you? Oh yeah, you should also think about what type of motor you want. E-bike motors can be easily broken into three categories, so to answer that last question, let’s first sort out the choices you have.
WHAT’S THE HUBBUB ABOUT?
The first is the direct-drive hub motor. They replace the bicycle hub of either the front or rear wheel. In direct-drive hub motors, the entire hub basically is the motor. Electric motors have two main parts: a part that rotates (called the “rotor”) and a part that doesn’t rotate (called the “stator”). A direct-drive hub motor’s axle is solidly bolted to the bike, and the stator is built on the axle inside the case. The moving part of the hub motor, the rotor, is built inside the rim of the motor case. Basically, electric current flowing through the coils of wire in the stator creates magnetic force that makes the rotor move.
Depending on the way the direct-drive motor is designed and built, it can be a “torque” low-rpm motor or a “speed” motor that operates at a little higher rpm for more speed but has less torque. Direct-drive motors are usually silent. The main problem with hub motors is that all the turning force is generated right at the center of the wheel. The motor is pulling huge amounts of power at low speeds and with high loads. Going up a steep hill with a load on the bike, the motor is nearly stalled and can’t generate much torque. Huge power in and not much torque out means the motor is getting really hot. Pulling a really steep hill can heat a motor up to the point of “letting the smoke out of it” and ruining the motor.
Geared hub motors may look identical to a direct-drive motor where the hub is the motor, but the geared hub motors have a smaller motor inside that turns internal gears so that the motor spins faster than the wheel. This allows for more efficient power use at low speeds and under heavy loads. Geared hubs may be a little heavier, but when comparing similar power output, they are usually lighter. They may also have a lower-rated power output because the motor is smaller. They are generally noisier (with a mild gear sound) and certainly have more moving parts than a direct-drive motor. The other advantage to geared hubs is that they often have an internal “freewheel,” which lets you pedal the bike with the motor off without having to turn the non-working motor against magnetic resistance.
Aftermarket hub motors that bolt onto a standard bike are growing in popularity because they’re the easiest to install. Buy a hub motor laced into a wheel and bolt it in place—done like dinner. Companies like Currie Technologies and BionX can sell you a complete kit that anyone with a set of common tools can install in an afternoon and be rolling. For relatively short trips on fairly level terrain, they’re a good option.
MID-DRIVE MADNESS
The mid-drive design seems to be enjoying the biggest surge in popularity. There are several advantages to a mid drive where the drive system is frame-mounted and the power from the motor goes through the bike’s chain to the rear wheel. As the name implies, typically, these motors mount in the center of the frame, either above or behind the bottom bracket and crankset, or actually taking the place of the standard bottom bracket. The mid drive is more complex than a hub motor, requires some specialized parts, and is naturally more expensive. These come in fabricated designs like the Stokemonkey, aftermarket units like the Bafang 8fun that replace the crank in the standard bottom bracket, or the latest generation like the Bosch, where the frame must be designed for the drive from the beginning with a motor mount rather than a bottom bracket. Kits like the Stokemonkey are complicated, require skill to mount and must be set up correctly, but you can use your bike’s gears to match the power output to the load and terrain. The Bafang is a little involved to mount, and like the Stokemonkey, you must find a mounting spot for the controller. Probably the biggest game-changer in electric bikes is the new Bosch (and similar) mid-drive system that’s built to fit a special frame. It has a geared motor integrated with the crankset, and although it’s a small motor, it’s a mid-drive setup so it has gears.
At the moment, Bosch can’t make enough units to keep up with the white-hot demand, which is leading to a lot of frustration, particularly in North America where delivery has been delayed by months. We may look back on this moment in five years and say, “That was when everybody started using one system.” I wouldn’t be surprised if the Bosch turned out to be “The Lord of the (Chain) Rings”—one system to rule them all: commuters, cargo bikes, tourers, and mountain bikes. Dedicated mid drives like the Bosch can’t be retrofitted to your existing bicycle frame.
THE FINAL DRAW
Where a hub motor is essentially “high gear all the time,” a mid drive has as many gears as your bike does. That lets the motor spin fast enough to be in the optimum rpm range, even though you may be in ultra-low gear crawling up the hill with four bags of charcoal and three cases of beer strapped to the rack.
The actual electric motor design is the same in all three types of drives. The direct drive is a large, low-rpm motor. The actual motor part of the geared hub motor is smaller than the hub shell. It is designed to turn higher rpm, and the gearing multiplies the motor’s torque. The Stokemonkey mid-drive kit is essentially a modified brushless hub motor. Instead of the motor casing turning, it’s mounted on a bracket. The axle turns a small gear, connected by a short chain to a sprocket on the crankset or a jackshaft. Commercial mid drives like the Bosch also use a higher-rpm motor with gear reduction.
With either type of hub motor, you don’t want the wheel speed to drop. If you lug the motor down too low in the (wheel) rpm range you lose assist, waste power and make heat. That means that the rider has to make up the difference to keep speed up on hills.
Mid-drive systems allow you to utilize the gears of the cassette to help the engine stay in an efficient rpm range. If you want a mid drive to perform well, you match your pedaling cadence to the best output of the motor. Obviously, matching cadence is easier to do for technical riding.
Hub motors can deliver amazing performance, and for general use and commuting they work fine. The harder you push your bike and the more extreme the climbs, the better the mid drives are. If you are serious about mountain biking off-road, a mid drive has amazing handling advantages, with the weight of the drive centralized.
New ideas are hitting the street every day, like the Copenhagen Wheel, developed at MIT, and Electron Wheel, scheduled to hit the U.S. market by summer. It’s innovative packaging, but it is still an all-in-one-package rear hub motor. Remember that the absolute best e-bike is the one you’ll ride. Make sure it will do what you want a bike to do. Spend some time thinking about how you might use an e-bike—not only in the ways you use a bike now, but how you might use it if suddenly biking was easier and more fun! E-biking can be fun, healthy and green. Make choices that will fit your life, and you’ll find yourself thinking up all kinds of excuses to get on the bike and go for a ride.