Here’s how to decide what Kv value motor will work best for your electric bike project. In general, a low Kv motor provides more torque but requires a higher voltage. Getting optimum performance out of your top-tier electric bike motor will depend on the other components such as the battery and controller.

Key Takeaways:

* Kv value is the rpm the motor will turn when given 1 volt of current with no external load

* The best Kv motor for your e-bike depends on speed, gearing, and the battery voltage available

* A higher voltage system will generally be better for range and battery life, given the same power

Types of E-Bike Motor

The most popular E-Bike motor choices are either hub motors placed within the front or rear wheel of the bicycle, or mid-drive motors placed near the crank and using gears for reduction. There are geared and direct-drive hub motors available for electric bikes. Some e-Bike kits may use off-the-shelf RC motors. These are generally higher RPM brushless DC motors that must be geared down to work well on an electric bike. Brushless motors such as common RC motors often list specs including diameter, kV rating, and watts.

Before you choose a motor based on kV rating, you’ll need to know the voltage of your battery and controller system, plus the gear ratio and the desired power output.

Kv rating isn’t a measure of motor power, but it determines the power curves of the motor and thus how it will perform in a given application, compared to another motor with similar peak output.

Voltage and Maximum Power

To determine what types of motors will work best on your e-bike, you’ll want to think about the speed and load requirements as well as the battery capacity, battery management, controller, and gearing. To pick the best Kv rating, you’ll want what works best with the system as a whole. You’ll need to know what power rating you need, what voltage your battery and controller can supply, and what range and speed range you need. For example, a 250-Watt pedal-assist setup will need a different kV motor than a 1200-Watt commuter e-Bike. You can determine the desired motor RPM based on how fast you want to go and your wheel size or what diameter wheel and tire setup you’re using. A common bicycle tire with 26-inch wheels will be turning about 259 rpm at 20 mph, whereas a 20-inch tire going the same speed will turn at 336 RPM.

Voltage vs Motor RPM

The voltage of the input current controls the motor speed of a typical electric bike motor. Motor RPM depends on the voltage and the winding constant, which is often given in Kv values. A higher Kv brushless motor, which will have fewer windings but thicker wire, will spin faster at a given voltage. A lower Kv motor, with more windings, will require higher voltage current flow in order to reach the same rpm. In other words, more voltage per RPM. So if you know the speed you need your electric motor to run, based on wheel speed and gearing, and you know the battery voltage you’ll be able to provide, you can figure out what Kv value works for your speed range.

A motor with a high KV value that’s too small won’t be able to provide enough torque to get your bike moving, while a low Kv motor will require high voltage to get it turning at its most efficient speed range.

Torque and Watts

Watts are a measure of power, or how much work a motor can do in a given amount of time. Motors with the same output rating in watts can have different motor rpm and torque characteristics, however. Power equals torque times RPM, so a low torque motor spinning at a higher motor speed can have the same output as a high torque motor spinning at a lower RPM.

Input vs Output Wattage

Electric RC motors are often advertised by maximum power input or peak output watts. These are different values and may be confusing, as neither figure reflects the actual sustained performance of the motor. Power output will be at most about 80-95% of the input in watts. This figure depends on the efficiency of the motor, which in turn depends on the resistance of the motor windings, the air gap between rotor and stator, and the heat build-up in the magnets and copper wires.

STAT:

The E-Bike market is expected to generate over $20 billion in global revenue between 2020 and 2023.

44 US states have a separate legal classification for E-Bikes

US Electric bike sales grew by 145% from 2019 to 2020

Sources:

https://www.statista.com/statistics/674381/size-global-market-electric-bicycles/

https://cyclingindustry.news/e-bike-sales-3-7m-17m-2030-industry-experts/

https://en.wikipedia.org/wiki/Electric_bicycle_laws

Peter Campbell (1996). Permanent Magnet Materials and Their Application. Cambridge University Press. p. 172. ISBN 978-0-521-56688-9.

Brushless Motor Kv Constant Explained. Learningrc.com (2015-07-29). Retrieved on 2019-12-26.

What Kv Motor Does Your Electric Bike Need FAQ

How much battery range do I need?

To figure out the needed battery size, you'll want to plot out the length of your commute or other common errands and be sure to take into account the effect of hills, ambient temperature, and traffic. A 2-mile commute in San Francisco will probably use more electricity than a 2-mile commute on flat ground. Also, the efficiency of Li-ion cells goes down when it's cold out.

How do I increase the range of my electric bike?

A well-designed controller with throttles and pedal-assist can greatly increase the range of an electric bicycle. You can also increase range by using a wide voltage range controller that lets you take advantage of regen, sending current back into the system when decelerating or maintaining a constant speed going downhill.

Are 250 Watts enough for an e-Bike?

For an e-Bike that's mainly a pedal assist, 250 watts is useful and allows for small, unobtrusive motor and downtube battery components, but if you don't want to pedal, or if you carry cargo or go up steep hills, you may want more power.

Ryan Mcbride

Ryan McBride is a writer based in Los Angeles. He has contributed work to magazines and sites including Gayot, Spy, Paper, Ladygunn, 3AM and the Crab Creek Review. He writes consumer reviews, essays, science and tech journalism as well as literary and art criticism.

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