Fundamentally speaking, electric motors translate electrical energy into mechanical energy. E-bikes use brushless DC motors, or BLDC motors, meaning they don’t use brushes to alternate the direction of current flowing to the motor, as older electric motors did. Those brushes made the motors less efficient and tended to wear out over time, so brushless motors have been the standard for more than a decade.
Open up a BLDC motor and you’ll see a bunch of wires wound around a circular series of poles. That’s the stator; it becomes an electromagnet when the motor controller draws current from the battery into the wires. You’ll also see a circular series of permanent magnets, either directly inside or outside the stator. The orientation of the magnets relative to the stator depends on the type of BLDC motor, but either way, that’s the rotor.
Grasping the interaction between the rotor and the stator is crucial to understanding how e-bike motors work. When current runs through the stator’s electromagnets in a circular sequence, those electromagnets repel and attract the permanent magnets on the rotor, causing it to spin. The stator is attached to a shaft. On a mid-drive motor, the shaft spins to generate torque, and that torque gives you pedaling assistance via a small chainring connected to the shaft. On hub motors, the shaft becomes the axle and therefore doesn’t spin. Instead, the rotor itself spins, causing the entire motor (hub) to spin, thus creating torque to spin the front or rear wheel.
In addition to the motor, all e-bikes have motor controllers and batteries. The controllers modulate the amount of power flowing to the motor, which uses your input to transfer the desired amount of current from the battery into the motor. “What makes an e-bike an e-bike is the experience of how power is being doled out,” Lemire-Elmore says. Pedal-assisted e-bikes might use a speed (a.k.a. cadence) sensor, which regulates e-assist by detecting the rider’s pedaling cadence, or torque sensors, which sense how much torque the rider is putting into the pedals. Some e-bikes have throttles that allow you to use the motor independent of your pedaling, although regional laws define where you can and cannot use throttle-equipped e-bikes.
Despite sharing the same basic tech, the motors you’ll see on today’s e-bikes come in three basic variants. Mid-drive motors are positioned at the center of the bike’s frame, where you’d normally find the bottom bracket. Hub-driven e-bikes have motors within the front or rear hub, and there are two types of hub motors. Direct-drive hub motors, apart from their bearings, have no moving parts: The motor just spins around the axle, which is secured to the frame’s dropout. Geared hub motors use a series of planetary gears to lower the motor’s RPM and increase its torque output. You’ll also find aftermarket e-bike kits that allow you to equip a standard bike with a mid-drive or hub motor, and among aftermarket kits, there are friction drives, which use a spinning wheel that contacts the rear tire to create propulsion.
Mid-drive motors are located between an e-bike’s cranks. An electric motor generates torque that spins a shaft that’s connected to a chainring. The motor is therefore supplementing your pedaling power within the bike’s chain-drive, rather than adding an additional power source. There’s also a gear-reduction system within the motor pack. Bosch mid-drive motors spin hundreds of times per minute—much faster than you could pedal—so the motor’s internal gearing reduces the RPMs at the shaft, therefore optimizing the system’s performance to a rider-friendly cadence of 50 to 80 RPM, Bosch’s Weinert says. All but the lowest-end mid-drive systems include gear sensors that cut the power to the motor while you’re changing gears to avoid breaking the chain while the bike isn’t in gear.
Direct-Drive Hub Motors
Direct-drive hub motors are the simplest e-bike motors. The motor’s shaft becomes the rear axle. Because the shaft is fixed in place, the motor (a.k.a. the hub) spins around the shaft, propelling you forward. Direct-drive motors tend to be larger in diameter than geared hub motors, Grin Technologies’ Lemire-Elmore says, because bigger hubs mean increased leverage and higher torque outputs, which is needed to supply adequate power at lower RPMs. Direct-drive e-bikes can also generate electrical energy during braking in a process called regenerative braking. “Motors are perfectly bidirectional,” Lemire-Elmore says. “They can go forward and backward with equal efficiency.” When you squeeze the brakes, a cutoff switch tells the motor controller to become a generator, and the resistance generates electrical energy. The energy regained from regenerative braking is minimal—YouTuber Tom Stanton found an average range increase of 3.5 percent with his regenerative system, although energy gains increase on hilly routes—but the primary benefit is brake-saving stopping power on long descents, as the braking energy is absorbed electronically rather than through friction.
Geared Hub Motors
Geared hub motors operate like direct-drive hub motors, except that within the hub, there’s an electric motor that spins at a much higher speed. That motor’s shaft connects to a series of planetary gears that connect to the hub, spinning the hub at a lower speed. This method generates more torque, but less top-end speed. Geared hub motors tend to be smaller in diameter than direct-drive motors because they don’t need as as large of a motor to generate the same amount of torque on the wheel, but the planetary gears also make the hubs wider. The motors also include a freewheel: That means there’s no potential for regenerative braking, but they’ll coast freely instead of creating minor drag when they’re not under power, which makes geared hub motor-equipped e-bikes ride more like traditional bicycles.
Friction-driven e-bikes seem archaic compared to those with contemporary hub motors and mid-drive systems, but the low-cost design has merits for cyclists who want to convert a traditional bike with minimal effort. A bolt-on motor drives a small wheel that contacts the tire, usually below the chainstays or above the seat stays, although some kits attach to the fork’s brake mount. The motor’s wheel spins the tire, driving you forward. The friction means increased tire wear, but the upside is that the kits are easily interchangeable between bikes. You won’t find friction drives on new e-bikes because they tend to be cumbersome and less efficient, but all-in-one kits likethis one from Alizeti are among the easiest ways to electrify a standard bike.
If you’re technically inclined and don’t mind getting your hands dirty, you can retrofit almost any bike with a hub motor or mid-drive system. Choose the motor, method of pedal assist, and battery size to fit your needs with aftermarket e-bike kits. The Bafang G310 geared hub motor is a favorite among e-bike manufacturers, for instance, and the entire DIY kit costs between $405 and $1,056, depending on your selection of components (and not including the battery). For less work, there’s the Copenhagen Wheel, a direct-drive hub motor and rim that slides right into your rear dropouts.
Deciding between a hub-driven or a mid-drive e-bike means evaluating your priorities in a bicycle. With that in mind, these are the pros and cons of each design.
Mid-Drive Pros and Cons
Generally speaking, mid-drives climb steep hills more efficiently than hub-driven e-bikes because they can use the bike’s existing geared drivetrain to take advantage of higher gear reduction for low-speed climbing, rather than supplementing it as an additional non-geared power source. (The efficiency disadvantage happens when a hub motor isn’t spinning at its optimal RPM—a powerful geared hub motor should be just as efficient as a mid-drive.) Their centered position on the bike also creates a more balanced ride. That, combined with climbing advantage, makes them the go-to motor for e-mountain bikes. Changing tires on mid-drive e-bikes is easier because there’s no wiring between the frame and the hub, and that allows users to run any wheelset.
The downside of adding a mid-drive motor to a chain-driven bike is increased chain wear. Respectable e-bike manufacturers won’t skimp on chain quality, but the added torque means you might be replacing chains more often. Mid-drives are also more expensive because they contain more mechanical components and higher gear reduction, which drives up cost.
Hub-Drive Pros and Cons
Because hub motors operate outside a bike’s chain drive, they don’t wear down chains and cogs like mid-drives can. They’re also cheaper because they’re mass-produced in much larger quantities and don’t require manufacturers to alter a frame to fit a specific motor.
Hub motors, especially direct-drives, don’t climb as efficiently as mid-drives. “If you’re cruising uphill at low speed and the motor is spinning at low speed as well, you’re turning a lot of that power into heat rather than forward motion,” Weinert says. The higher wattage required by direct-drive hub motors means bigger motors and batteries, which adds weight. Weight distribution isn’t as centered, either, although the effect on the bike’s handling depends on the weight of the motor. Lastly, changing tires can be tedious because you’ll need to disconnect the wires that power and control the hub motor.
If you’re considering a hub-driven e-bike, find out whether the motor is geared or direct-drive. Each design has its pros and cons.
Generally speaking, geared motors are better for low-speed, high-torque applications, and direct-drive motors are better for high-speed uses. “[Geared motors] can be half the weight of a direct-drive motor that has the same torque,” Lemire-Elmore says, because of the geared motor’s higher internal RPM. However, because they’re geared down for torque, geared motors struggle to achieve the same top speed as direct-drive systems, which can handle higher speeds and more power without becoming overstressed. Geared motors coast with less resistance than direct-drive motors, although the added coasting resistance of a direct-drive motor is minimal; it’s equivalent to adding another set of tires, says Lemire-Elmore.
Direct-drive motors tend to be bigger and heavier because they require more magnetic material to generate low-speed torque, but that added power and mechanical simplicity helps them operate well at higher speeds. They also tend to be quieter than geared motors, although newer geared motors with helical-cut gears (rather than straight-cut gears) are nearly inaudible as well. Direct-drives can also benefit from modest added range and decreased brake wear due to regenerative braking.
Attempting to compare e-bike power ratings is a great way to lose your sanity. That’s because “rated power,” the metric some manufacturers use, doesn’t equal a motor’s actual power output or maximum potential power output. “The actual power output of a motor depends entirely on how heavily it is loaded in a given situation and the maximum electrical power that the controller lets flow into the motor,” Lemire-Elmore says. “It has little to nothing to do with a rating anywhere.”
The power rating might indicate how much power you’re getting for a specific amount of time, although there’s no universal standard for peak or rated power duration. “That could be 10 seconds or 30 seconds,” Weinert says. “Some motors quote peak power at 750 watts, but you may only be able to get that for 1 to 2 seconds.”
Here’s how to parse manufacturer jargon. “Power” is a measure of how quickly work is being done. Torque, a metric listed by some manufacturers, is a rotational measurement of force. To determine a motor’s power in watts, you have to know how fast it’s spinning: Torque multiplied by rotational speed equals power. A motor’s power output therefore peaks at a specific amount of revolutions per minute, and even if you knew the RPMs for peak power (good luck getting that figure), you wouldn’t be doing that math midride.
You can get an idea of how much maximum power you’ll actually feel if a manufacturer lists an e-bike battery’s voltage and (continuous) amperage from the motor controller. That’s a better indicator than the motor rating because ratings are arbitrary, but with regards to electrical energy, you can multiply volts by amps to get watts. For instance, the Juiced Bikes CrossCurrent X is rated at 750 watts, a.k.a. 1 horsepower. The battery is rated at 52 volts and the motor controller delivers 20 amps of current. Therefore, 52V x 20A = 1,040W, but you’re not going to feel 1,040 watts because BLDC motors aren’t 100 percent efficient. “It’s probably 75 percent efficient [at that higher power level],” Lemire-Elmore says of the Bafang motor. If the motor is 75 percent efficient, the math says you’ll feel a maximum of 780 watts of peak power, which is pretty close to the 750-watt motor rating. By comparison, the Blix Bikes Vika Travel folding e-bikehas a motor rated at 250 (continuous) watts, yet the battery is rated at 36 volts and the motor controller lists 18 amps. Even if the motor loses 25 percent of input power to inefficiency, the theoretical maximum output power should be 486 watts, which is almost double the 250-watt rating. Crucially, Blix notes the bike’s 250 watts are continuous, while Juiced Bikes doesn’t say how long its 750-watt figure can be sustained.
Torque is less subjective. If a manufacturer lists an e-bike’s peak or sustained torque in newton metres, go with that. Better yet, percentages of support (as Bosch lists) tell you how much help the motor is giving you at a given level of e-assist. Otherwise, if you’re dying to know how much power your bike can produce for a sustained period of time, we’d recommend reaching out to the manufacturer and asking for the meaning of the bike’s power rating before you buy.
There are a few more things to know about e-bikes that affect your long-term riding experience. Here’s what else you should note.
E-bikes use sensors to determine pedal-assist levels based on rider input. There are speed sensors, a.k.a. cadence sensors, which dole out e-assist based on the cadence of your pedaling. Blix Bikes’ Malmberg says the sensors are affordable, low-maintenance, and provide a relaxed riding experience that many cyclists appreciate. “If you want to go faster, pedal faster, not harder,” Malmberg says. Speeding up is therefore as simple as increasing your cadence, no matter how much effort you’re putting in. Speed sensors are common on hub-driven e-bikes.
Torque sensors, by contrast, determine the proper amount of motor torque by measuring how much torque you’re applying to the pedals. To go faster, you must pedal harder. The experience is more akin to riding a traditional bike. Torque sensors are popular in mid-drive bikes, especially e-mountain bikes, because they offer riders more control over the application of e-assist: You don’t want tons of power all at once when negotiating a tricky section of trail.
To Throttle or Not
Some e-bikes come with throttles that allow riders to access the bike’s e-assist without pedaling. Throttles are a matter of rider preference, although they become especially useful on hub-driven bikes if your drivetrain breaks down midride. They’re also a matter of legality: Some states define e-bikes by classes. A class 1 e-bike has only pedal-assist and tops out at 20 mph, a class 2 e-bike has pedal-assist and a throttle and tops out at 20 mph, and a class 3 e-bike has pedal-assist that can top out at 28 mph. Whether class 3 e-bikes can have throttles depends on who you ask: Aventon’s Pace 500 has throttle-assist up to 20 mph and pedal-assist up to 28 mph. In other words, check your local laws before buying an e-bike with a throttle (or one with e-assist that exceeds 20 mph).
Quality Issues and Warranties
As the price of an e-bike decreases, it’s increasingly important to check its warranty information before you buy. (It’s always a good idea, actually.) Here’s one reason: Lower-end e-bikes might not have thermal rollback, a feature that measures the motor’s internal temperature to keep it from overheating. Think of it like the rev limiter on a car’s internal combustion engine. “[Cheap e-bike companies] hedge their bets that most people aren’t trying to climb over a mountain pass with the motor on full power,” Lemire-Elmore says. “Say you’re pulling a trailer uphill with two kids, the system could self-destruct.”
When a motor gets too hot, the protective enamel surrounding the stator wires can melt off. Put simply, too much sustained, low-speed climbing can fry a motor without thermal rollback, and its absence from a bike is not something manufacturers will readily disclose (although new e-bikes without thermal rollback typically have motors that can handle more power than manufacturers spec them at). Still, there’s lots of documentation around the Internet of e-bike motors overheating. That’s just one of many things that can go wrong with a motor, battery, or motor controller, so it’s crucial to know what you’re getting into before you buy.
We hope you’re now better-equipped to buy the right e-bike for you. If you have additional questions that we haven’t covered, drop them in the comments and we’ll do our best to update this article with all the relevant information you need to know about e-bike motors.