Power Meter User Guide

Power sensors and the concept of training with watts have become more widespread in recent years, whether thanks to the practice of connected home trainers or manufacturers who offer bikes already equipped with this technology for sale. But not all sensors are equal in terms of precision and reliability, and there are always a minimum of precautions to take before using one so that its data really means something. Here's what you need to know before you want to train with a power meter.

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

Power sensors are no longer reserved for professionals as there were still twenty years ago with the very expensive (but renowned for their reliability) SRM. All cyclists and triathletes can now equip themselves with a power sensor on their bike, or take advantage of their connected home trainer to be supported in their progress. There are different types of power sensors, which can be found today from around €300A connected home trainer that measures power can be found for around €500, while of course presenting other advantages.tagThis last type of device may be sufficient for those who train during the week in short, calibrated indoor sessions, but who do not necessarily need to measure their power on all their bike rides, particularly on the route the weekend.

What is a power sensor?

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 (the force) multiplied by the rotation speed of the crankset. Strain gauges installed in the sensor deform during pedaling and are the basis for calculating the power. The data is then transformed into watts using an algorithm and transmitted by Bluetooth or ANT+ to a meter, 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 to distinguish his profile, according to the maximum power sustained according to the time of effort. For example, a sprinter has a profile that shows a very high power over a few seconds, and an average power over several tens of minutes. A climber or a rouleur are not necessarily explosive, but are distinguished by a high power maintained for several tens of minutes. Finally, a puncher ranks among those who have the highest power over four to five minutes of effort.

To compare athletes with each other and their abilities, we do not use raw power, but the watts/kilogram ratio., because the larger the cyclist, the more absolute power he has, but which must be used to move his own weight. On the flat, it is the watts/Scx ratio (air penetration coefficient) which is a little more important, even if it is a less used concept.

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

Over an 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

In 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

In 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 capacities that the notion of power allows us to detect, Using a power sensor allows you to optimize the time spent training by working precisely in the targeted areas to optimize progression.

=> SEE AS ​​WELL : Power in cycling

The different types of power sensors

There are several types of power sensors, each with their own advantages.tagand their disadvantages:

• The power sensor in the crankset : placed in the axis of the pedal, this type of sensor presents the advantagetagand to be little exposed, and normally shows good reliability. On the other hand, once installed, it can hardly be moved from one bike to another. In addition, it only measures an overall power, and cannot distinguish the power of the two legs.
• The power sensor in the crank : this is the least expensive system when the sensor is only placed on one of the two cranks (usually the left). It has the advantagetagand to be easily removable from one bike to another, if you have the same basic crankset on both bikes. It can also be installed on an existing crankset, as an option. But it is also a less precise system, because it only measures the power on one of the two legs, and then simply multiplies the defined value by two. There are also power meters for both cranks.
• The power sensor in the crank star : this type of sensor can be mounted in some cases on an existing crankset, as an option. The sensor is only placed on the transmission side, but it still measures the difference in power of the two legs by deducting the deformation on each half-turn of the crankset. Here again, the system has the disadvantage of being limited to a single bike.
• The power sensor in the pedals : a system that is easy to install, and therefore easy to switch from one bike to another if necessary, even if it is sometimes necessary to respect a precise tightening torque to ensure the regularity of the data. Some pairs of pedals only offer a measurement on one side, but it is possible to upgrade the system at any time by purchasing the second pedal later. This type of sensor offers other types of functions, such as analysis of the pedaling angle or the place where the force is applied on the pedal platform, so as to be able to correct certain imbalances. But it has the disadvantagetagand to be exposed in the event of a fall.
• The power sensor in the rear hub : 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 impossible to find on the market.
• The connected home trainer : this type of device has been all the rage since the first lockdown and the explosion of virtual cycling. The so-called home trainers direct drive are used by removing the rear wheel, and installing the chain directly on a transmission system placed on a support, directly connected to an application that simulates a virtual route with changes in slope or a predefined resistance. The power supplied by the cyclist acts directly on his virtual speed of movement in the application. The most high-end devices have a very precise power measurement, and have the advantagetagand to be little exposed and little subjected to temperature variations that could alter the calculation. Mid-range devices are less precise, and some can provide optimistic or pessimistic power, which directly influences the performance in the application used. At the entry level, some home trainers even broadcast very fanciful information, which can explain superhuman performances on virtual cycling applications. This type of power sensor is of course limited to use on indoor training, but it can be used with several different bikes.

The accuracy of a power sensor

The accuracy of a power sensor is assessed on the power delivered at a given time, 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 provided by the cyclist of 300 watts for example, the power delivered by the sensor will be in 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). Note however that it is almost impossible for an average user to determine whether each manufacturer really respects the accuracy it claims.

What seems most important to benefit from a power sensor and use it as a progression tool is the repeatability of measurements over time.

When we have the opportunity to tester several different power sensors, it is quite easy to see that there are sometimes significant differences between two sensors used at the same time on two different meters, for example a pair of pedals with a crankset, or with a home 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 the reputation to be precise, to cross-reference the tests with another user who himself has a sensor that he also thinks is accurate, but in any case this is not scientific, especially if we consider that depending on the conditions of use (the outside temperature in particular), a sensor can lose its accuracy. Nevertheless, What seems most important to benefit from a power sensor and use it as a progression tool is the repeatability of measurements over time.. Whether a sensor is optimistic or pessimistic does not matter so much, as long as you always use the same one and a stated value of 300 watts always represents the same power in January and July, and regardless of external conditions, battery status or how long the sensor has been used. If you train on the route with a certain sensor and on a home trainer with a device that does not deliver the same values, you risk completely missing your progression objective, by being under-trained or over-trained. In short, there is no perfect solution, but the first thing before using a power meter – and most people don't do it – is to calibrate the sensor, before each ride.

=> SEE AS ​​WELL : Working on intensity without calibrated sessions

Why calibrate a power sensor?

Calibrating a power sensor is performed to adjust the measuring device to match a known reference and produce more accurate results.. The strain gauges of a sensor are hyper 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 0 point, that is to say the level where the sensor is not subjected to any stress. The starting point of any torque measurement. For pedals or a bottom bracket, you must place one or the other crank in the low position, the feet outside the pedals, and launch the procedure on the computer/GPS. This only takes a few seconds. For a home trainer, calibration is generally carried out by a procedure of spindown, which involves bringing the home trainer to a target speed and then letting it decelerate.

Some newer power meters have an auto-calibration feature, which allows the device to automatically adjust to changing conditions during your ride. But a manual calibration procedure always ensures greater precision and better repeatability of measurements. For a power sensor on the bike, we recommend performing the operation before each outing. For a home trainer, a manual calibration every two or three weeks may be sufficient, especially if the device always remains in the same place.

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

How to use power data?