Power Meter User Guide

Power meters and the concept of training with watts have become widespread in recent years, thanks to the use of connected indoor trainers and manufacturers offering bikes already equipped with this technology. However, not all sensors are equal in terms of accuracy and reliability, and a minimum of precautions must still be taken before using one to ensure that its data is truly meaningful. Here’s what you need to know before training with a power meter.

By Guillaume Judas – Photos: depositphotos.com / ©Shimano

Power meters are no longer reserved for professionals as they were twenty years ago with the very expensive (but renowned for their reliability) SRM. Today, all cyclists and triathletes can equip their bike with a power meter, or use their connected indoor trainer to support their progress. There are different types of power meters, which can now be found starting from around €300. A connected indoor trainer that measures power can be found for around €500, while also offering other advantages, of course. This latter type of device may suffice for those who train indoors during the week for short, calibrated sessions, but who don’t necessarily need to measure their power on all their bike rides, especially on the road on weekends.

What is a power meter?

A power meter is a device that measures the power (expressed in watts) delivered by the cyclist while pedaling. It is calculated by measuring the torque applied during pedaling (force) multiplied by the crankset’s rotational speed. Strain gauges installed in the sensor deform during pedaling and form the basis of the power calculation. The data is then converted into watts using an algorithm and transmitted via Bluetooth or ANT+ to a bike computer / head unit, computer, or smartphone that the user can consult during the effort, or afterwards for analysis by transferring their file to software or an application.

Unlike heart rate, power is a raw value that is not affected by external parameters such as humidity, heat, or even fatigue or digestion. It directly shows the effort provided by the cyclist. But it also allows distinguishing one’s profile, based on the maximum sustained power according to the duration of effort. For example, a sprinter has a profile that shows very high power over a few seconds, and average power over several tens of minutes. A climber or a rouleur is not necessarily explosive, but stands out for high power maintained for several tens of minutes. Finally, a puncheur ranks among those with the highest power over four to five minutes of effort.

To compare athletes and their abilities, we do not use raw power, but the watts/kilo ratio, because the larger the cyclist’s build, the more absolute power they have, but this must be used to move their own weight. On flat terrain, the watts/Scx ratio (air penetration coefficient) is slightly more important, even if it is a less commonly used concept.

Here are some examples of power in W/kg depending on the level:

Over one hour:

• between 5.7 and 6.4 W/kg: very high-level professional
• between 4.7 and 5.3 W/kg: very good amateur level
• between 3.5 and 4.1 W/kg: average amateur
• between 2.4 and 3.1 W/kg: occasional cyclist

Over 5 minutes:

• between 7 and 7.6 W/kg: very high-level professional
• between 5.6 and 6.4 W/kg: very good amateur level
• between 4.3 and 5 W/kg: average amateur
• between 3 and 3.7 W/kg: occasional cyclist

Over 5 seconds:

• between 22 and 24 W/kg: very high-level professional
• between 18.6 and 20.8 W/kg: very good amateur level
• between 15 and 17 W/kg: average amateur
• between 12 and 14 W/kg: occasional cyclist

Beyond the natural abilities that the concept of power allows to detect, the use of a power meter primarily allows optimizing the time spent training by working precisely in the targeted zones to optimize progress.

=> SEE ALSO : Power in cycling

Different types of power meters

There are several types of power meters, each with its advantages and disadvantages:

• Crankset-based power meter: placed in the bottom bracket spindle, this type of sensor has the advantage of being less exposed and is normally very reliable. However, once installed, it is difficult to move from one bike to another. Furthermore, it only measures overall power and cannot distinguish the power of the two legs.
• Crank arm-based power meter: this is the least expensive system when the sensor is placed on only one of the two crank arms (usually the left). It has the advantage of being easily removable from one bike to another, if you have the same base crankset on both bikes. It can also be installed as an option on an existing crankset. But it is also a less precise system, as it only measures power on one leg and then simply multiplies the defined value by two. There are also power meters for both crank arms.
• Crank spider-based power meter: this type of sensor can, in some cases, be mounted as an option on an existing crankset. The sensor is only placed on the drive side, but it still measures the power difference between the two legs by deducing the deformation on each half-turn of the crankset. Here too, the system has the disadvantage of being limited to a single bike.
• Pedal-based power meter: a simple system to install, and therefore to move from one bike to another if needed, although it is sometimes necessary to respect a precise tightening torque to ensure data consistency. Some pedal pairs only offer single-sided measurement, but it is possible to upgrade the system at any time by purchasing the second pedal later. This type of sensor offers other functions, such as analyzing the pedaling angle or where force is applied on the pedal platform, in order to correct certain imbalances. However, its disadvantage is that it is exposed in case of a fall.
• Rear hub-based power meter: this type of sensor is linked to a rear wheel, which can be easily moved from one bike to another. But currently, this type of sensor is almost no longer found on the market.
• Connected indoor trainer: this type of device has been on the rise since the first lockdown and the explosion of virtual cycling. Direct drive indoor trainers are used by removing the rear wheel and installing the chain directly onto a drivetrain system placed on a stand, directly connected to an application that simulates a virtual course with changes in gradient or predefined resistance. The power supplied by the cyclist directly affects their virtual speed in the application. High-end devices offer very precise power measurement and have the advantage of being less exposed and less subject to temperature variations that could alter the calculation. Mid-range devices are less precise, and some can provide optimistic or pessimistic power, which directly affects performance in the application used. At the entry level, some indoor trainers even broadcast very fanciful information, which can explain superhuman performances on virtual cycling applications. This type of power meter is, of course, limited to indoor training use, but it can be used with several different bikes.

The accuracy of a power meter

The accuracy of a power meter is evaluated on the power delivered at a given moment, but also over time, with sometimes measurement drifts, whether during a ride, or over weeks and months of use. All manufacturers claim a certain measurement accuracy, generally from +/- 1% to +/- 2%, which means that for a power actually supplied by the cyclist of 300 watts, for example, the power delivered by the sensor will be within a range of 6 watts for the most accurate (from 297 to 303 watts) to 12 watts for the least accurate (from 294 to 306 watts). However, it should be noted that it is almost impossible for an average user to determine whether each manufacturer truly respects the accuracy it claims.

What seems most important for benefiting from a power meter and using it as a progression tool is the repeatability of measurements over time.

When you have the opportunity to test several different power meters, you can easily realize that there are sometimes significant differences between two sensors used simultaneously on two different bike computers / head units, for example, a pair of pedals with a crankset, or with an indoor trainer. From there, it is not possible to determine with certainty which sensor is accurate and which is not. One solution is to compare two sensors, one of which has a reputation for being accurate, and to cross-reference tests with another user who also has a sensor they believe to be accurate, but in any case, this is not scientific, especially considering that depending on the conditions of use (outdoor temperature in particular), a sensor can lose its accuracy. Nevertheless, what seems most important for benefiting from a power meter and using it as a progression tool is the repeatability of measurements over time. Whether a sensor is optimistic or pessimistic is not as important as long as you always use the same one and a value of 300 watts always represents the same power in January and July, regardless of external conditions, battery status, or the duration of sensor use. If you train on the road with a certain sensor and on an indoor trainer with a device that does not deliver the same values, you risk completely missing your progression goal, by being undertrained or overtrained. In short, there is no perfect solution, but the first thing to do before using a power meter – and most people don’t do it – is to calibrate the sensor before each ride.

=> SEE ALSO : Working on intensity without calibrated sessions

Why calibrate a power meter?

Calibrating a power meter is done to adjust the measuring device so that it matches a known reference and produces more accurate results. The strain gauges of a sensor are highly sensitive, and their operation can be altered by temperature variations, certain shocks, transport, or a long period of disuse. The calibration process consists of determining the zero point, i.e., the level where the sensor is not subjected to any stress. This is the starting point for any torque measurement. For pedals or a crankset, one or the other crank arm must be placed in the down position, feet off the pedals, and the procedure launched on the bike computer / head unit/GPS. This only takes a few seconds. For an indoor trainer, calibration is generally performed by a spindown procedure, which consists of bringing the indoor trainer to a target speed and then allowing it to decelerate.

Some recent power meters are equipped with an automatic calibration function, which allows the device to automatically adjust to changing conditions during the ride. But a manual calibration procedure always ensures greater accuracy and better repeatability of measurements. For a power meter on the bike, we recommend performing the operation before each ride. For an indoor trainer, a manual calibration every two or three weeks may suffice, especially if the device always remains in the same place.

Manual or automatic calibration is different from device validation, which is a more complex operation performed by the manufacturer, or by the most advanced users. This consists of a static load test. It involves applying a known force to the power meter and comparing it with the force measured by the sensor. The confusion between the two terms comes from the English word calibration, which can also mean validation or standardization.

How to use power data?