What is the Relationship Between Torque and Speed on an Ebike?

The power output of an electric motor is a good indicator of how well an electric bike will perform. However, it is also a good idea to consider the torque rating. This is because there is a strong correlation between torque and speed. Torque determines how well an electric motor accelerates, and speed determines the maximum speed of the motor.

When purchasing an electric bike, choosing one with the right torque and speed will help meet your specific needs and riding preferences. Read on to learn more about the relationship between torque and speed, and how to choose the right ebike for you.

Understanding the Basics of an Electric Motor

An electric motor converts the electrical energy of a battery into mechanical energy, which it achieves by applying principles from electromagnetism.

An electric motor has two sets of electromagnets – one static, one moving. The static magnets, also known as the stator, are attached to coils. They are energized to create a magnetic field and spin the moving magnets, also known as the rotor, to generate torque. This torque is used to move a shaft.

The two most common motors are mid-drive vs hub motors for ebikes. On a rear-hub motor, the rotor connects to a shaft and rotates the rear wheel axle, moving the ebike forward. On a mid-drive motor, the stator connects to a shaft that drives the cranks.

Speed

Speed is a measure of distance traveled over time. The higher your speed is, the greater the distance you travel over time. The maximum speed of an electric bike is determined by the power output of the motor, which is measured in wattages, or watts. The average wattage of an electric bike is 250w to 750w, while the peak wattage is the maximum wattage that the motor can reach in a short burst.

Torque

Torque is a measure of force that can cause an object to rotate on its axis. On an electric bike, torque measures the force you apply to the pedals or cranks.

The more torque you produce, the greater the rotational power of the motor – i.e. the faster you accelerate. Torque is calculated by multiplying force by distance. The average torque range on an electric bike is from 35Nm to 80Nm or more, and the term Nm stands for Newton-meter.

The equation to calculate torque is:

Torque = Power / Speed

It is also worth noting the difference between torque and RPM (Revolutions Per Minute). Torque is the rotational force that the engine generates, while RPM is the number of times the engine’s crankshaft makes a full rotation each minute.

What is the Relationship Between Speed and Torque?

Torque plays a significant role in the acceleration of an electric bike. Generally, the more torque you have, the faster your electric bike will accelerate. This is especially the case when you are starting from a complete stop.

The faster you go on an electric bike, the torque you generate will decline. This is because speed and torque are inversely proportional to each other. To put it another way, the rotational force on an electric bike is at its lowest when you are at your highest speed.Image source: National Instruments Corporation

When riding at maximum speed on an electric bike, the electric motor does most of the hard work to maintain your speed. Alternatively, when riding at a moderate pace, the torque and power output of the electric motor will be about the same.

Four Factors That Govern How Much Torque Is Required

Four factors determine how much torque and RPM you need to gain momentum on an electric bike. Here is a detailed breakdown of each contributing factor:

1. Rolling Resistance

Rolling resistance is the level of friction, also known as the opposing force, that a vehicle must overcome to generate momentum. This friction comes from the rolling motion that occurs between the tires and the surface the vehicle is moving on. The rolling resistance you incur depends on the tire material and surface roughness. The higher the rolling resistance, the more torque you need to overcome that resistance.

2. Grade Resistance

Grade resistance is the gravitational force that pulls a vehicle back when climbing an inclined surface. The steeper the incline or hill is, the more torque you need to overcome that incline.

3. Acceleration Force

Acceleration force helps a vehicle reach a predefined speed from rest within a specified period. The amount of torque you have in an electric motor will have a direct impact on the level of acceleration force. The more torque you have, the less time it will take to reach a predefined speed.

4. Load

The weight of the rider and the electric bike will determine how much torque is required to generate momentum. The heavier the rider and the electric bike, the more torque you need to accelerate and maintain speed.

How Does Gear Ratio Affect Speed and Torque?

There is a strong correlation between the gear you are in and how fast you accelerate and reach top speed. The lower the gear, the easier it is to pedal. Riding at a lower gear is also beneficial when you are using the electric pedal assist or throttle feature.

Starting at a lower gear means the electric motor does not have to work as hard to generate torque and force the tires to rotate. This is especially true at a complete stop or climbing a steep incline.

The Balancing Act: Torque vs Speed in Ebikes

You should now understand the relationship between torque and speed. Here is how to choose an electric bike that strikes the right balance between both.

Why They Can’t Both Be Maxed Out?

There are a few reasons not to get an electric bike with the highest torque and speed.

First, greater torque requires a heavier, bulkier motor to generate enough force. This can negatively impact the weight and ride quality of your electric bike. For instance, a heavier electric bike is often harder to maneuver as you must anticipate corners in advance. More powerful electric motors also require a larger ebike battery, which adds further weight.

Too much torque can be a disadvantage when you ride off-road. If you are not careful, the rear tire may lose traction and cause you to spin out or fall over.

How to Choose the Right Balance for Your Needs

Do you live or work in an area with lots of hills? Do you haul lots of cargo like groceries, camping supplies, or goods for delivery? If you said yes to either of these questions, it may be a good idea to prioritize torque. This way, you can more easily accelerate at a complete stop and climb steep inclines.

What if you live or work in an area with mostly flat terrain? Do you carry mostly light cargo, such as groceries or a backpack with school supplies? In that case, prioritize speed over torque. This choice will enable you to accelerate at a slower, smoother pace and reach a higher top speed. You will also be less likely to spin out when riding off road on an ebike.

Summing Up

Choosing an electric bike with the right torque and speed balance is essential to an enjoyable ride. We hope this article has helped you better understand the differences between the two and how to choose an electric bike that fits your needs and riding preferences.

Explore the full range of electric bike options at Heybike to find your perfect match.