Experts Explain: How Do Cycling Power Meters Work?

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I’ve always been curious about how things work. And if the thing was cycling-related, I had to find the answers. This is doubly true for cycling power meters.

That’s why I contacted power2max and 4iiii to help explain the technical details. So, we joined forces and wrote this easy-to-understand explanation of how cycling power meters work.

KEY TAKEAWAY

Power meters measure the Force (torque) using strain gauges and time and distance using cadence, which can be measured with an accelerometer or, in some cases, an external magnet. In the end, no power meter actually measures power but calculates it based on the P=W/t and W=F×d formulas. Continue reading for more info.

Are you ready to learn more?

Big thanks to power2max and 4iiii for helping me with the technical explanations.

Table of Contents

What Are Cycling Power Meters?
How Do Cycling Power Meters Work?
What Is Power Averaging, How It Works, and Why It Is Important?
Summary
Cycling Power Meters FAQ

What Are Cycling Power Meters?

Cycling power meters are devices that measure (estimate is a more precise word; see the technical explanation below) a rider’s power output. They then transfer it to a cycling computer or other devices with ANT+ or BLE connectivity.

Their main advantages include objective and instantaneous data cyclists can use to improve their training effectiveness. You can read this article on Why use a power meter to learn more about its benefits or consider getting a cycling coach.

How Do Cycling Power Meters Work?

Surprisingly, no power meter measures power. To understand how power meters work, you must understand what power is. This requires some knowledge of physics:

P(ower) is W(ork) divided by t(ime).
P=W/t

And:

W(ork) is F(orce) times d(instance).
W=F×d

A power meter can measure the Force (torque) using strain gauges and time and distance by using cadence, which can be measured with an accelerometer or, in some cases, an external magnet.

What is a strain gauge?
Strain gauges measure strain on an object. This can be a crank spider, crank arm, or the axle of a pedal. A strain gauge is a very small and sensitive foil with a defined electric resistance. If the object is deformed (when force is applied during a pedal stroke), the electric resistance changes by a very, very small amount. This change is registered, and calculations are made to know how much force was applied. In the case of a rotating object like a crankset, this force is called torque.

power2max power meter being tested — power2max power meter testing | Source: power2max

The vast majority of cycling power meters are direct force power meters.

They use strain gauges to measure torque. These power meters include all power meter types except handlebar power meters.

However, there are also opposing force power meters (like handlebar power meters). Instead of strain gauges, they use other sensors (like an accelerometer, speed sensor, wind pressure sensor, and elevation sensor) to calculate power.

What Is Power Averaging, How It Works, and Why It Is Important?

Power averaging is the process of averaging power readings. It is required because the force on the crank fluctuates wildly during rotation.

The power meter will take a reading of the instantaneous power 4 times a second and average those readings.

Illustration of variable power over time

For example, on the downstroke for a 1/4 of a second, your power could be 600 watts, and the next 1/4 second on the upstroke could be 100 watts. That would be reported as 350 watts at 120rpm.

The 1/4-second measurements don’t provide valuable data, but when we average the readings, the results give the rider something meaningful to work with.

Summary

Most power meters on the market use strain gauges to measure torque and accelerometers to measure cadence. Then, they calculate the power by multiplying these two units. These are called direct force power meters and include pedal, crank arm, spider, and other commonly known power meter types.

There are also opposing force power meters. They include handlebar power meters that use different sensors for measuring and calculating your power based on your speed, acceleration, wind pressure, temperature, etc. In other words, they don’t measure the force you put into every pedal stroke but the opposing forces.

If you have any further questions regarding power meters, feel free to add a comment below.

Cycling Power Meters FAQ

How much do cycling power meters cost?

Depending on the type, brand, and other features of power meters, you can expect to pay around $300 for an entry-level crank arm power meter and up to $2000 for a high-end crankset power meter.
Learn why are power meters so expensive.
Read my how to choose a power meter article to learn more about the differences in prices based on power meter types.

Are cycling power meters worth it?

A power meter will provide you with instantaneous and objective data you can rely on. So, if you want to improve your cycling performance, you should consider investing in it. Your structured workouts will be much more effective.
Make sure to read my article on Why to Use a Power Meter, which includes many more reasons.

How do power meters in smart trainers work?

Power meters in smart trainers usually don’t use strain gauges, and the measurement mechanism may differ depending on the manufacturer.
For example, they can calculate the flywheel rotations (Wahoo trainers) or use optical torque sensors to measure the twist between two series of teeth (Elite trainers). (Source)

What are the biggest challenges when designing and manufacturing power meters?

I discussed with the power2max product developer the biggest challenges when designing and manufacturing power meters. Here is his take:
There are many challenges when you want to build a power meter. Some are related to strain gauges, some are related to the production process, some are related to the position on the bike, etc.
I want to pick some out:
You have to make sure that your strain gauges only detect the force applied to propel the bike forward. For example, if you look at a crank arm, there is not only force in the direction you pedal but also a bit left and right. Especially when you go out of the saddle, there is a lot of movement in the crankarm, i.e., torsion.
You have to know the materials you use. For example, strain gauges are glued onto a surface that deforms. You have to know everything about the surface it is glued onto and the positioning. Otherwise, you won’t be able to make the correct measurement.
Knowing the perfect design of a power meter is not everything. You also have to know how to build it. You need easy and repeatable processes that are 100% accurate to always have the same results. At the same time, you want to build a very robust product that can withstand external influences like temperature changes, moisture, or unwanted forces (like crashing or hitting a stone, or clipping a pedal in a corner). (Source: power2max)

What makes power2max power meters unique?

I also asked the power2max product developer what makes their power meters unique. Here is his answer:
After 12 years of building power meters, we still think that spider-based measurement is not only the most reliable way to “measure” power but also the most accurate and most comfortable way of measuring power for a customer.
Our power meters are basically built to be forgotten. Once they are on a bike, you only have to worry about recharging it once or twice a year or switching the battery. Our unique technology is mainly in the production process that allows us to build the highest possible quality for a very fair price.

What are the biggest misconceptions about power meters, especially spider-based power meters?

The most important value you can get with a power meter is power and, more precisely, the total power that moves your bike forward. This power is produced by BOTH legs combined and transmitted via each crank arm onto the spider, the chainrings, the chain, etc.
The first place where the power of both legs is combined is the spider. Therefore, a spider-based power meter always measures the combined torque of both legs and calculates the total power with the help of cadence.
This is important because single-sided power meters always have a bigger margin of error. In addition, the balance of the left and right leg can change during a workout due to fatigue, position on the bike, climbing gradient, intensity, etc.

Preview picture source: power2max

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