Electric bike motors


Everything you wanted to know about electric bike motors - mechanics, pedal assist, hub motors vs mid drives vs friction motors but were afraid to ask!

Electric bikes have become more popular than ever and are now available at many price points. If you’re thinking about buying an electric bike (even if it’s not from us), chances are you have some questions about the electric motor that propels you over the hills and straightaways delivering that rush of adrenaline to your heart! So we distilled the hell out of the subject into an article designed to better help you understand the different motors which are available and the way each interacts with machine and rider.
We hope you’ll walk away with a better understanding of the technology, if only to satisfy your curiosities about those awesome machines that seem to be multiplying throughout our streets and pathways.

How electric bike motors work

Fundamentally speaking, electric motors translate electrical energy into mechanical energy. Electric 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. The contact brushes on the older versions made the motors less efficient and tended to wear out over time, so brushless motors have become the new standard.
picture of an electric bike motor inside

Brushless electric motors use permanent magnets and electromagnets to turn electrical energy into mechanical energy. Open up a brushless 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 sprocket 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 of the electric bike.

 

 How electric motors work in concert with the bike 

In addition to the motor, all Electric 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 separates many electric bikes is the method of power distribution to the motor. Pedal-assisted Electric bikes may use a speed (cadence) sensor, which regulates the electrical assist by detecting the rider’s pedaling cadence, or torque sensors, which sense how much force the rider is putting onto the pedals. An Unconquered Custom Electric Cycle has a throttle which allows you to accelerate rotational speed of the motor independent of your pedaling. Different classifications of ebikes link

 

Different types of electric bike motors

Despite sharing the same basic technology, the motors you’ll see on today’s Electric bikes come in four basic types. Friction, mid-drive, direct-drive hub, and geared hub. Friction drive motors (mainly found in aftermarket kits) use a spinning wheel which turns against, and is in contact with the rear tire to create propulsion. Mid-drive motors are positioned at the center of the bike’s frame, where you’d normally find the bottom bracket. Hub-driven Electric bikes have motors within the front or rear hub, and there are two types of hub motors. Direct-drive hub motors, which, 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 torque output. In the following article we will explain each in detail and explain why we champion one technology within our builds here at Unconquered Custom Electric Cycles.

 

1) Friction drive electric bike motors

Okay, spoiler alert – these motors are crap (sorry, but not sorry) Friction-driven electric bikes seem archaic compared to those with contemporary hub motors and mid-drive systems, and for good reason. However, the low-cost design has merits for cyclists who want to convert a traditional bike with minimal effort and minimal cost. 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, and decreased motor life expectancy, component failure and all the disadvantages of a terrible design. But the upside is that the kits are easily interchangeable between bikes. You won’t find friction drives on an Unconquered Custom mainly because they tend to be cumbersome, less efficient, un reliable, and waaaay too much maintenance. But it has to be said that the all-in-one aftermarket kits may still be among the easiest ways to electrify a standard bike.

Why we don’t use friction electric bike motors

Because they are crap. Next!

 

2) Mid-drive electric bike motors

exploded view of a mid drive electric bike motor

 

Mid-drive motors are located within the center of an electric bike’s crankset. An electric motor generates torque that spins a shaft that’s which is connected to a chainring (or sprocket). The motor is completely supplemental to your pedaling power within the bike’s chain-drive, rather than adding an additional power source. No full throttle with these babies. There’s also a gear-reduction system within the motor pack which is sometimes times made from plastic. Mid-drive electric motors can spin hundreds of times per minute—much faster than you could pedal—so the motor must have internal gearing which reduces the RPMs at the shaft, therefore optimizing the system’s performance to a rider-friendly cadence of 50 to 80 revolutions per minute. Mid drive motors must also come equipped with 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. (Slightly over engineered) And don’t forget all of the torque generated from these motors is transferred directly to your chain increasing chain wear. Respectable electric bike manufacturers will not skimp on chain quality, but the added torque means you may find yourself replacing chains more often. Mid-drives are also more expensive on the aftermarket because they contain more mechanical components and higher gear reduction, driving up their cost.

Why we don’t use mid drive motors within our electric bike builds

Proponents of mid drive electric motor technology will tell you that the centered position on the frame creates a more balanced ride (lower center of gravity etc). And it is true, that changing tires on mid-drive electric bike is easier because there’s no wiring between the frame and the hub, so this allows riders to run any wheelset. What they won’t tell you, is that many times bikes will require frame alterations for the mid drive motor to fit. Also, unless a mid-drive motor is spinning at absolute optimal RPM there is no technical argument for an efficiency advantage over a geared hub motor. And the cons of the over engineering, too many moving parts, too many low quality components, zero opportunity for regenerative braking, unnecessary chain stress, frame alterations and no capability for throttle only or (full throttle) acceleration disqualify them for use within Unconquered Custom Electric builds

3) Direct-drive hub electric bike motors

cross sectional view of a direct hub drive electric bike motor

 

See a deconstructed direct-drive motor above. The hub and rotor (the left item with the magnets) spin around the stator (the center item with the wiring). Direct-drive motors like this one tend to be tall and narrow.
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 (the hub) spins around the shaft, propelling you forward. Direct-drive motors tend to be larger in diameter than geared hub motors because bigger hubs mean increased leverage and higher torque outputs, which is needed to supply adequate power at lower RPMs. Direct-drive electric bike motors can also generate electrical energy during braking in a process called regenerative braking. These motors are perfectly bidirectional, and they can go forward and backward with equal efficiency. When you squeeze the brakes, a cut-off switch tells the motor controller to become a generator, and the resistance generates electrical energy. The energy regained from regenerative braking is minimal an average of less than 2% and this advantage is quickly countered by the larger size and weight over say, a geared hub motor. The real primary advantage regenerative braking contributes is the brake-saving stopping power it lends on long descents, as the braking energy is absorbed electronically rather than through friction.

Why we don’t use direct drive hub motors at Unconquered Custom Electric Cycles

Direct-drive hub motors, don’t climb as efficiently as geared hub motors. If you’re cruising uphill at low speed and the motor is spinning at low speed as well, you can bet that a lot of that power is dissipating as heat rather than creating forward motion. The higher wattage required by direct-drive hub motors also means bigger motors and batteries, which adds weight. Direct-drive hub motors also tend to be bigger and heavier because they require more magnetic material to generate low-speed torque. They used to be considered quieter than their geared hub counterparts, but new geared motors with helical-cut gears (rather than straight-cut gears) used within Unconquered E Cycle builds are almost completely inaudible during operation.

4) Geared hub electric bike motors

a deconstructed view of a geared drive hub electric bike motor

 

A deconstructed geared hub motor. The planetary gears (second from left) slow down the speed of the hub to the desired operational speed and create additional torque.

Geared hub motors operate like direct-drive hub motors, except that within the hub, there is 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 desired torque, and delivers a feeling of quick acceleration and power to riders of all sizes when you twist the throttle. Geared hub motors tend to be smaller in diameter, and lighter than direct-drive hub motors because they don’t need a large footprint to generate the same amount of torque on the wheel, these motors also include a freewheel which greatly reduces drag during coasting situations, cancelling out any advantage a direct drive motor gets from regenerative braking. Geared hub electric bike motors are much better than their direct drive counterparts for high-torque applications. Also geared hub motors can be half the weight of a direct-drive motor yet produce the same torque because of the geared motor’s higher internal RPM. Because geared hub motors operate outside a bike’s chain drive, they don’t wear down chains and cogs like mid-drives motors can. They also don’t require frame alterations for the motor to fit like the mid drives. Lastly, they are able to be configured to run exclusively under throttle control which makes them perfect for the Unconquered Custom Electric Cycle application. SOLD!!

Power ratings on an electric bike motor

Attempting to compare electric bike power ratings is a great way to lose your mind. That’s because “rated power,” the metric most 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. 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,” I.e. 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 better understand manufacturer speak. “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 measuring), you wouldn’t be doing that calculus during your ride. You can get an idea of how much maximum power you’ll actually feel if a manufacturer lists an electric 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 geared motor we lace into the rear hub of an Unconquered Custom 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 = 1,040 Watts However, the motor is probably 75 percent efficient [at that power level]. 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 our 750-watt motor rating. 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 tell you how much help the motor is giving you at a given level of electrical 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 electric bike’s power rating before you buy.

 

To Throttle or not to throttle

Some e-bikes come with throttles that allow riders to access the bike’s electric assist without pedaling. Throttles are a matter of rider preference, although they become especially useful on hub-driven electric bikes if your drivetrain breaks down halfway through your journey. It should also be said that when it comes to throttles most of our competitors offer only a weak either/or solution meant to satisfy the classification system which governs these vehicles throughout the US.
Unconquered Customs offers a fully programmable (on the fly) one size fits all solution to satisfy any local electric bike classification rule within the United States and Canada. Speed regulation on an Unconquered Custom is a self-imposed feature meant to help the rider conform to any legal class for on-road driving.
At Unconquered Customs we realize that, much like stepping on the pedal of a car, our customers love to twist the throttle , and simply experience the pure joy of motorcycling the way it was intended to be. In fact we have built an entire electric vehicle company on that exact premise. When operating your Unconquered Custom Electric Cycle on private property or a closed circuit race track, Full throttle (motorcycle) mode can be selected from the handlebar mounted controller. This assist mode works similar to the Class 3 operation, however, the 28 mph speed limit is removed and the system’s power levels are increased to the highest levels possible allowing the bike to reach its true top speeds.

When in full throttle (motorcycle) mode, an Unconquered Custom's top speed is not governed.

In this mode, the maximum speed may be much faster than 28 mph depending on many factors such as tire pressure, wind, road surface, inclination and ambient outdoor temperature. The final top speed depends solely on the power available to the motor. 

  Full throttle mode a requires a certain level of responsibility. When you know you have access to safe and private road or path surface and want to see exactly what your bike is capable of, use full throttle mode. In this mode, each Unconquered Custom Electric Cycle can go even faster. If you want a bike that goes this fast, then an Unconquered Custom is definitely for you! Go have fun – remember its your god given American right to do so!

Just don’t take it on any public streets without switching to a power-limiting mode and remember to always wear a helmet!

In conclusion

We sincerely hope that you found this article on electric bike motors informative and insightful. Hopefully you are now better equipped to select the motor/ bike configuration which is right for you! If you have any further questions on the subject, please feel free to fill out the Questions? form below and book a chat with an Unconquered Rep. 

Or, Design an Unconquered Custom electric bike that suits your style! Click on the button below and get started!

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