Understanding E-Bike Terms

Torque

Torque is a measurement used to describe the rotational force applied by a motor. Newton meters (N-m) or pound-feet (lb-ft) are used to measure torque. Most current available E-bike motors fall in the range of 20 to 80Nm. 

Higher torque motors are better suited for steeper climbs and riders demanding faster acceleration. Riders who are primarily on flatter terrain with less need for quick acceleration will have less need for a high-torque motor. Torque is most valuable in situations with frequent starting/stopping and where elevation changes often. Torque is an integral factor in speed however factors like drive type (mid-drive vs. hub drive), weight, and gearing will all play a considerable role in a bike's power and speed. 

Torque and power are similar yet they are not the same. Torque is simply the measure of twisting force that causes rotation whereas power is the result of torque and rotational speed.

Electric motors produce torque through the process of a magnetic field from a stator (stationary part) interacting with a rotor (rotational part). The more current that can flow through the stator windings the more torque will be produced. This increased current flow can come at the cost of extra weight penalties and increased power consumption. 

Torque is essential in getting a bike moving and maintaining speed through resistance. Riders that start and stop quickly and frequently or ride in locations with changing elevations will benefit most from more torque. 

Upsides to higher torque: Faster acceleration and better climbing capabilities. 

Downsides to higher torque:  Increased weight, increased wear on drivetrain parts, reduced efficiency, and higher cost. 

Power

Power describes the rate at which a motor transfers energy. Power is an essential factor in how fast an e-bike may go and how well it will carry speed. Power is also necessary for overcoming resistance from steep climbs, wind, or weight. An increase in power can increase speed but power does not directly translate to a bike's top speed. Other factors of resistance and efficiency are essential in determining how fast an e-bike will move. 

Power (W) = Torque (Nm) x Rotational Speed (RPM) / 9549(Converts speed to Watts)

E-bike power is generally measured in Watts(W) and power ranges usually fall between 250W and 750W. E-bike motors with power ranges between 500-750W are typically considered high-power motors and will be equipped with higher-powered batteries. These bikes will be faster and carry speed better. E-bike motors falling between 200-350W are usually considered low-powered motors. Lower-powered motors can be optimal for stronger riders or those who primarily stick to flatter terrain over shorter distances.

Power will vary largely depending on drive types. When comparing power between mid-drive and hub drive designs, a mid-drive motor can output power better because its design allows it to work in parallel with the bike gearing to optimize torque. 

It is essential to consider that many countries have laws that restrict e-bikes and require them to fall under specific limits for speed and power. In the United states e-bikes are categorized under classes 1,2 and 3. Europe has similar restrictions with slightly different categorization. These classes all fall under their own specific limitations of power, speed, and features.

Upsides of higher power: Faster speeds, better climbing, and hauling capabilities.

Downsides to higher power: Increased energy consumption, increased weight, increased cost, subject to legal limitations to motor power.  

E-Bike Classes

E-bikes have blurred the lines between motorcycles and bicycles and resultingly, governments worldwide had to establish distinctions between e-bikes and motor vehicles. 

In the United States e-bikes are not considered motor vehicles; however, there are currently three classes that legally categorize e-bikes, each with its specific limitations. 

Class 1: Pedal assist only up to 20mph

Class 2: Pedal assist and or throttle up to 20mph

Class 3: Pedal assist and optional throttle up to 28mph 

Many European countries also have restrictions in place to limit e-bike speed and power. 

Pedelecs: Under EU legislation e-bikes must be governed at 25km/h with pedal assist and have less than 250 watts of power. E-bikes under this category are referred to as pedelecs and are classified as normal bicycles. 

S-pedelecs: Higher-powered e-bikes classified as L1e-A motorized bikes may have up to 1000 watts of power. These e-bikes are referred to as S-pedelecs and may have top speeds with pedal assist up to 45km/h. S-pedelecs are considered motor vehicles and usually require insurance and registration to be used on public roads. 

Motor Drive Types

Mid Drive

Mid-drive motors feature a motor that is mounted in the middle of the bike between the pedals. These motors are usually integrated into the bottom bracket assembly. From this location, the motor connects to the chain and powers the drivetrain. Higher-end E-bikes will use mid-drive motors as they tend to be better balanced and more stable. In many cases, a mid-drive motor position allows for a lower center of gravity and a more confidence-inspiring feel compared to hub-drive motors. Mid-drive motors provide a more natural power transmission similar to pedaling due to motor placement. Mid-drive motor placement allows for the motor to assist in pedaling while also utilizing the bike's drivetrain. By using the bike's gears, the electric motor in a mid drive design can remain in an optimal torque band regardless of the bikes speed. The advantage being more power to the ground and the ability to achieve higher speeds compared to mid-drive designs. 

Mid-drive motors allow for better functioning frame suspension designs, a low balanced center of gravity, improved climbing and top speed, and an overall better-performing platform than other drive types. 

Upsides to Mid Drive Motors: More balanced feel, compatibility with higher quality drive trains, and lower center of gravity.

Downsides to Mid Drive: Higher cost, require specific crank and chainrings. 

Hub Drive

Hub drive motors are designed with the motor located within the hub/axle of the rear wheel. Hub drives are often more affordable than mid-drive designs because they can be simpler to design and build. In a hub drive design, a motor powers the back wheel which propels the bike forward. Bikes with this design can be cheaper however the hub drive system is not as effective at distributing power. Hub drives are limited in performance because the motor must rotate the wheel to create power compared to mid-drive designs where the motors can utilize the drivetrain to more effectively transmit power. 

Hub drive motors can be designed to be small and compact and can easily be integrated into more portable bike designs like folding bikes. 

Hub drives are well suited for riders looking for more affordable options and have less need for the increased power and responsiveness of a mid-drive e-bike. 

Upsides to hub drive motors: Affordable, simple to maintain, and portable. 

Downsides to hub drive motors: Heavy, less performance.

Batteries 

Most available e-bikes use rechargeable lithium-ion batteries. Power is measured in Watt-hours and is an essential factor in how much power the motor will receive and how long the battery will hold a charge. Battery power usually falls between 300-1000 Watt-Hours(Wh). Larger-capacity batteries are best suited for riders looking to cover large distances with less concern for weight. Larger batteries will require more material and will be heavier but they will be able to provide larger amounts of power to the motor for longer periods. Watt hours measure the amount of energy stored in a battery over time and is calculated by multiplying the power(watts) by time(hours). 

Upsides to larger battery capacity: More power to the motor, increased range. 

Downsides to larger battery capacity: More weight, increased cost.