An electric bicycle combines the lightweight and convenience of a traditional bicycle with the ability to alleviate the burden of uphill climbs, headwinds, and carrying loads. It is essentially a bicycle equipped with a power system centered around a torque sensor, along with a motor and battery.

In the field of pedal-assist electric bicycles (Pedelecs), the most advanced technology is the "torque sensor," which serves as the core component that interprets the rider's intentions. High-end electric bikes typically use the most technologically advanced "dual-side torque sensors." This sensor technology has long been dominated by multinational companies such as Germany's BOSCH and Japan's YAMAHA. Bicycles equipped with these sensors generally cost over 2,000 euros. In mid to low-end electric bike products, "single-side torque sensors" are widely used. These sensors can only detect the force applied by one pedal and cannot fully understand the rider's power needs, leading to a significant difference in riding experience compared to high-end models. In the low-end segment of electric bikes, there is also a type of product that uses "rear axle hook sensors." Due to the lack of technical barriers, these sensors are cost-effective and widely adopted, but they perform poorly in actual use and are therefore mostly used in low-end models.

Today's article will introduce various types of sensors, analyzing their working principles, advantages, and disadvantages.

First, let's understand the power transmission method of a bicycle: the rider pedals, which then drives the chain to transfer power to the rear wheel, propelling the bicycle forward. The components involved in this process are: pedal — crank — chainring — chain — sprocket — rear hub — frame.

Rear Axle Dropout Sensor

What is Rear Axle Dropout Sensor?

The gray component shown in the image is the "rear axle hook sensor." It is installed at the junction of the rear upper fork and rear lower fork, precisely where the rear axle is mounted (the red dot in the image indicates the rear axle). This sensor acts as an additional attachment, typically made of aluminum alloy. The bicycle's forward motion transmits power through the rear axle to the "rear axle hook sensor," which then transfers it to the bicycle's frame. When the sensor receives force from the rear axle, it deforms, compressing the "pressure sensor" located in the red circle in the image. This action activates the battery motor, providing power to the electric bike. The main advantages of the "rear axle hook sensor" are its simple principle, uncomplicated structure, and low production cost. It performs well if the electric bicycle is used primarily on flat roads. However, the drawbacks of this sensor become apparent under real-world road conditions. (As indicated by the red circle in the image, the "rear axle hook sensor" is essentially a pressure sensor embedded within an aluminum alloy casing.)

Disadvantage One

The "rear axle hook sensor" attempts to measure pedaling torque through a series of transmission components such as the pedal, crank, chainring, chain, sprocket, rear hub, and frame. Each of these parts is an elastic body, leading to power loss during transmission. Additionally, due to the "elastic hysteresis" effect of metal, the sensor's force measurement is not only inaccurate but also delayed. This means it cannot interpret the rider's power needs in real-time, resulting in delayed and inaccurate power delivery, leading to a poor riding experience. It's akin to pressing the accelerator in a car only to experience a delay in power response.

Disadvantage Two

The sensor is mounted at the junction of the rear upper and lower forks. Accurate force measurement requires precise installation, and any deformation during installation due to imprecise machining can result in inaccurate sensor data. This creates high demands on the production process of the bicycle frame, inadvertently increasing the manufacturing cost of electric bikes.

Disadvantage Three

The junction where the rear upper and lower forks meet is a high-stress area of the bicycle. Bumps and impacts from various road surfaces are transmitted through this part, which is where the sensor is installed. Most of these sensors are made from aluminum alloy, known for its relatively low hardness and strength, making them susceptible to damage under real-world conditions. The accuracy and lifespan of the sensor rapidly decline with use over time. For instance, hitting a curb or having the bike blown over by the wind could deform the sensor, resulting in erratic behavior or a complete loss of power.

Although the "rear axle hook sensor" has the advantages of a simple principle and low implementation cost, its imprecision and delay in torque measurement lead to a poor riding experience. It imposes strict manufacturing requirements on bike production, shifting cost savings from the sensor to the frame processing. The aluminum alloy material is prone to deformation and has a short lifespan, which means high maintenance costs for users. Consequently, most higher-end electric bikes have abandoned this type of sensor. Giant, for instance, tried using "rear axle hook sensors" for producing budget electric bikes years ago but eventually gave up due to poor performance, frequent malfunctions, and short lifespan.

Torque Sensor

What is Torque Sensor?

With technological advancements, torque sensors have finally been applied to bicycles. As mentioned at the beginning, the biggest difference between torque sensors and rear axle hook sensors lies in their understanding of "force." The dual-side torque sensor, in particular, can measure the force exerted by both pedals. The following text will focus on this more technologically advanced sensor.

Compared to the outdated "rear axle hook sensor," the advantages of the dual-side torque sensor are remarkably clear. It is primarily installed between the crank and chainring, on the outside of the bottom bracket, and sometimes directly on the spindle. Since there is no perfect rigid body, the spindle undergoes slight torsional deformation under force. By measuring these minute deformation signals on the spindle's surface, the current pedaling torque can be accurately determined with high precision. Because it measures torque through torsion, the torque sensor is also known as a "torsion sensor."

Advantage One

Since the torque sensor is installed on the outside of the bottom bracket or directly on the spindle, there are fewer intermediate steps in measuring pedaling force. The pedal, crank, and spindle are typically made of rigid steel components with minimal elastic hysteresis effect, allowing torque signals to change rapidly with pedaling force. This results in a quick power response in electric assist bicycles equipped with dual-side torque sensors, with most reaction times being in the millisecond range, offering virtually zero delay and instant power delivery with each pedal stroke.

Advantage Two

If the torque sensor is mounted on the outside of the bottom bracket, it allows for a standardized modular design, enabling seamless installation on virtually any bicycle without the need for specially developed frames. This design also eliminates the requirement for precise frame treatment, as needed for the "rear axle hook sensor," reducing costs in both development and production.

Advantage Three

Being mounted on the outside of the bottom bracket or spindle, most of the impact forces from road bumps are absorbed by the frame's resilience and elasticity, minimizing the strain on the torque sensor. Additionally, torque sensors are typically manufactured using the bicycle’s bottom bracket technology, utilizing high-quality steel materials that offer excellent hardness, strength, and durability, resulting in a long lifespan for the sensor.

The primary distinction between a pedelec and an electric bicycle lies in how they provide assistance: the former uses the rider's pedal force to determine the level of assistance, while the latter controls power output through a throttle. In terms of riding experience, electric assist bicycles that work in harmony with the rider's feet and legs have a clear advantage. So, if the throttle of an electric bicycle could be installed on the chainring, wouldn’t that effectively upgrade it to an electric assist bicycle?

Torsion Spring Sensor

What is Torsion Spring Sensor?

The "torsion spring sensor" was developed with this concept in mind. Much like the "rear axle hook sensor" incorporates a pressure sensor, the torsion spring sensor integrates a Hall sensor. Its core principle involves mounting the throttle of an electric bicycle onto the chainring, cleverly shifting the control of power assistance from the hands to the feet. This innovative approach effectively converts an electric bicycle into an electric assist bicycle.

The appearance of the "torsion spring sensor" is quite similar to that of the "torque sensor," as both are mounted on the chainring. This similarity makes it difficult for consumers to distinguish between the two in many electric assist bicycles unless explicitly labeled by the manufacturer. As a result, many companies use the "torsion spring sensor" but promote it under the guise of a "torque sensor," earning the torsion spring sensor the industry nickname of "pseudo torque sensor." Although the torsion spring sensor can indeed be used to measure torque, its accuracy is significantly inferior to that of a true "torque sensor."

The image above labeled "TORQUE SENSOR," which is claimed to be a torque sensor, is actually a torsion spring sensor. The structure of the torsion spring sensor is not complicated; it consists of two interlocking aluminum discs. One disc is fixed to the chainring, while the other is connected to the crank, with a spring serving as the intermediary. When a rider applies force to the pedal, the crank moves the connected disc, compressing the spring and rotating the chainring. The deformation length of the spring is linearly proportional to the applied force, as described by Hooke's Law. During compression, the two discs experience relative displacement, causing the attached magnet to move. The Hall sensor detects this magnetic field change, determining the magnitude of the applied force. This is akin to how the throttle on an electric bicycle causes the magnet to move. The name "torsion spring sensor" originates from the use of the spring structure within it.

The torsion spring sensor has its advantages: it has a low technical threshold, low circuit costs, and is relatively simple to implement.

In contrast, a true torque sensor (as shown in the image below) measures the subtle deformation on the surface of a metal shaft when force is applied. This deformation is not visible to the naked eye, and there are no spring-like structures inside.

Compared to the torque sensor, the torsion spring sensor has the advantage of being more cost-effective, but it also has inherent disadvantages due to its design:

Low Measurement Accuracy: As previously mentioned, the torsion spring sensor uses a Hall sensor as its measuring element, resulting in lower accuracy. The data obtained is significantly inferior to that of a torque sensor.

High Measurement Delay: The use of a spring as an intermediary means that the connection between the crank and the chainring is not rigid. As a result, some of the force applied by the rider is lost in the spring, causing delays and further reducing the already low data accuracy.

Poor Measurement Consistency: The torsion spring sensor requires multiple springs within a single sensor, but these springs often lack uniformity. This inconsistency leads to variations in measurement accuracy even among sensors from the same production batch.

Poor User Experience: The presence of a spring causes the crank and pedal to wobble during riding, with a "kickback" effect occurring after each pedal stroke, resulting in an uncomfortable riding experience.

Delayed Power Output: This sensor structure cannot continuously measure force. For instance, when a force is initially applied, the spring compresses, but once released, the force dissipates. During transitions of force between the left and right foot, significant fluctuations occur, necessitating extensive filtering to mitigate sensor data instability, which further causes delays in measurement data.

These five disadvantages contribute to the torsion spring sensor's poor measurement accuracy and significant delay. The direct consequence is that in real-world comparisons, electric assist bicycles using torsion spring sensors deliver assistance much slower than those using torque sensors. The assistance is abrupt and lacks smoothness, failing to provide the seamless, on-demand support that riders expect. Consequently, high-end electric assist bicycle products have moved away from using torsion spring sensors.

What assist modes do Eahora electric bicycles use?

Now that you understand the three basic types of assist modes for electric bicycles, are you curious about which mode Eahora's electric bikes use? Eahora's electric bikes all feature torque sensor assist. We recommend checking out the latest models equipped with torque sensors: ROMEO PRO II, ROMEO II, and JULIET III. Click to explore these ebikes and experience the advanced technology they offer.

Summary

From a riding experience perspective, torque sensors excel by accurately measuring the force applied by the rider on the pedals, unaffected by road conditions. This precision enables the sensors to truly understand the rider's intentions, allowing the motor and battery to provide just the right amount of power for a more comfortable riding experience.

From a research and production standpoint, torque sensors offer strong versatility, eliminating the need for specialized frame designs or high-precision installation processes. Although they are more expensive per sensor, the overall cost is reduced when distributed across the whole bike production.

In terms of daily use, torque sensors boast a long lifespan and are less prone to malfunction, requiring no maintenance. This significantly lowers the barrier for riders in terms of usage and purchase decisions.

Lastly, in terms of safety, torque sensors are highly stable, unlike rear axle hook sensors that can deform over time, leading to decreased accuracy and potential failures. This instability can cause issues like sudden acceleration or loss of power, posing a safety risk during rides.

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