This is how the bike power meter works

It is said that it transforms runners into robots and that it causes falls, but the GPS computer is also a little gem of technology serving performance.

It is a small black box, attached to the center of each rider’s handlebar, and connected to a power sensor installed on the bike (read below). It is called a GPS meter or on-board computer. Several manufacturers have entered the market, so there are different brands, but the operating principle is always the same: the screen reveals live, that is to say during the race, a host of very valuable data to the runner (watts, heart rate, etc.). The latter can thus better manage his effort, and therefore be more efficient over the duration of an event, as long as he completes it. Because the tool is accused of distracting runners glued to the screen, exposing them to falls. His detractors also say that he has transformed the leaders of the peloton into robots whose every pedal stroke is conditioned by the figures transmitted live.

But while many have an opinion on the subject, few really know how the device works. To find out, we made an appointment with Simon Meier, mechanic from the Swiss Cycling team, entered this week in the Tour de Romandie. This choice was not made at random: the Swiss team is invited each year to the French loop to highlight seven national hopes under contract with other teams for the rest of the season. Each rider therefore arrives for a week with his own bike… and his own computer.

“Riding with your device is essential because it is calibrated for each rider,” says Simon Meier. There may indeed be differences in results between two new sensors: when you develop 400 watts, one may display 400 and another 390. It’s a bit like when you’re on the same hike with a friend and your phone shows a difference in step counts. To make the power meter figure correspond to reality, the coach configures the device according to the runner. The latter also knows, by dint of driving, how his on-board computer will respond to this or that effort.”

There are not only intrinsic differences between the devices; there is also some in the way they collect the cyclist’s data. The “sensor” itself can in fact be located in the crankset, in the bottom bracket axle or in each pedal.

It is with a measuring device in the pedals that the athlete obtains the greatest amount of information. “This device allows you to know what power the runner is developing in each leg, the ideal being that left and right are perfectly balanced,” explains Simon. Better yet: the device detects if the shoe is tilted too far forward or backward, knowing that it is by pushing with the sole of the foot that the energy will best “pass through” the pedal.

This data is transmitted live to the cyclist, but it is especially useful for training. During the race, the runner pays attention to other parameters: watts, heart rate or even GPS coordinates. These make it possible to anticipate dangerous turns, to position yourself correctly before sharp climbs or technical descents, or to know where the refueling zones are located.

Simon Meier on the Tour de Romandie.Image: DR

Each runner can configure their home screen, i.e. select the data that appears there. It’s a bit like on a phone, when you choose your favorite apps. To obtain additional data, the cyclist must press a button directly on the device or, if their bike is equipped with the latest technology, press levers located at the ends of their handlebars (exactly the same way he would change gear). This second solution allows him to keep both hands on his bike, and therefore better control when the peloton gets nervous and it “rubs”.

Each runner turns on their counter at the actual start of the race and turns it off immediately after crossing the line, so that the recorded data reflects the efforts during the race, and is not interfered with by the seconds of inactivity which follow the race. ‘arrival. A procedure which sparked controversy this spring.

If the data is reliable, it is because most devices are connected by satellites. They work everywhere in the world, in high mountains as well as on country roads, and in all weathers. The cold tends to put a strain on the battery, but it has a battery life of 8 to 9 hours, which is more than enough to finish the race. Only rain can play tricks on the runner but again, the manufacturers have thought of everything. “Drops can “press” keys by mistake. To avoid this, the runner can deactivate the touch option,” explains Simon Meier.

The mechanic is one of the defenders of technology. He points out a little-known advantage of the power meter: “Sometimes, when you are behind an opponent known to be stronger and you see him drawing while your data tells you that you are not over-revving, you know that the other is not in a big day and you can place an attack.

However, we must be wary of a simplistic reading of the figures. To get a credible idea, the runner must take into consideration the elements that can affect his results. Developing 400 watts with a headwind and a cool temperature does not have the same value as in the middle of summer with a tailwind, “but the runners train so much that they know how to interpret the data during the race” , Simon intervenes.

In the past, it was said of a good cyclist that he knew how to become one with his machine; today, the body is still well positioned on the bike but the mind is elsewhere, entirely focused on a small black box fixed to the center of the handlebars.