Power meter

Power meter as a universal power measuring device

Anyone who rides a racing bike or mountain bike usually wants to know and ensure that they are dividing up their physical strength and performance in the best possible way. If the intensity of the effort is too high at the beginning, the rider will tire prematurely. Conversely, if the rider rides too slowly over a long period of time, the result will be a deterioration in performance.

It is therefore important to adapt your riding style so that, on the one hand, the rider does not tire prematurely and, on the other hand, you do not lose sight of your performance goal. In order to optimally combine the two, the use of a power meter as a proven means to an end is a great help.

A power meter and its physical mode of operation

A power meter works with strain gauges made of constantan with a high specific resistance. If the strain gauge is loaded, it changes its length. This change in length in the form of a stretch immediately results in a change in its ohmic resistance. A diagonal voltage measured in a bridge circuit shows a proportional change in this diagonal voltage, which serves as the output value. This changed diagonal voltage therefore also represents a measure of the stretch and thus of the current load on the crank. In this way, a continuous measurement of the force in every position of the crank over its entire circumference is possible.

Differences in the recording of measurement data

Depending on how the power meter is switched on, it can measure the current force applied either on the left pedal or on both sides, left and right. In the first case, with one-sided measurement, the power meter doubles the measured value and shows it to the rider. The second application, with two-sided measurement, leads to a more precise recording of the actual torque transfer over each full rotation of the crank. Just as a right-handed person is more skilled with his right hand than with his left, the force applied to the pair of pedals is never exactly the same on the left and right.

Physical background, data, forces, torques, power, angular velocities, rotational speeds

When the rider steps on the pedal, he transfers a force to the crank with his foot. Depending on the current angle of the pedal to the vertical, an effective lever arm is created from the crank radius to the bottom bracket axis. This effective lever arm in turn creates a torque that is transferred to the rear wheel via the chain. The torque changes depending on the angle of the crank to the vertical and is defined as follows:

M _(𝛼) = F * r * sin(𝛼)

M... (torque in Nm) F... (foot force on pedal in N) r... (distance from pedal axis to bottom bracket axis in m) 𝛼... (angle of current crank position to the vertical in degrees)

The instantaneous power that the driver transfers to the pedal is determined by the following mathematical relationship: P = M _(𝛼) * 𝜔 𝜔... (angular speed of the crank in ¹ / _sec The angular speed of the crank is made up of the following: 𝜔 = 2 * 𝜋 * n n... (speed of the crank in ¹ / _sec ) This gives the following formula for the instantaneous power that the driver transfers to the pedal: P _(𝛼) = F * r * sin(𝛼) * 2 * 𝜋 * n P _(𝛼) ... (power at the crank in watts)

Training for success

Every athlete has to train a lot and, above all, purposefully to achieve good results. This is especially true in cycling, which demands a special level of discipline, endurance and willpower from every rider. The mental strength of the rider also plays an extremely important role. A key parameter for assessing performance is the power rating per kg of body weight. A world-class rider can deliver a power output of around 6 ^W / _kg of body weight for at least half an hour. As a logical continuation of every training session, every ambitious rider monitors the level of their individual anaerobic threshold (IANS). This IANS value is a measure of the maximum power that the rider can just about deliver while maintaining a constant lactate level in the body.

More information on lactate levels

During physical exertion, the human body produces lactate, which, however, is detrimental to the muscles if the concentration is too high and leads to a drop in performance. A normal lactate level is between 1.0 ^mmol / _l and 1.8 ^mmol / _l . However, during heavy physical exertion, such as intensive cycling training, this value can rise above this normal value. In order not to negatively affect the function of the muscles, it is absolutely necessary that this increased lactate level is broken down again. It should under no circumstances exceed the mark of around 4 ^mmol / _l .

Collection of all data

A power meter has the great advantage over a sports watch that it constantly measures and displays the current performance of the rider, allowing the user to optimise their riding style. This ensures that they do not overstrain their body at times and thus achieve poorer training results. They can always adapt their riding style so that they are always just below their individual performance limit. Thanks to the measurement support of the power meter, the rider is constantly able to divide their physical performance reserves according to their needs. In addition, they ensure that their lactate level in the body remains fairly constant within the normal range. This positive side effect helps the rider with every training session, continuously increases their fitness and always brings them good training results.

Smoothing the output value of the power meter

The constant recording of the current power would lead to considerable fluctuations if the measured value were output directly and could easily be misunderstood. Therefore, smoothing electronics ensure that an average power of the last 3, 10 or 30 seconds is output while driving. Practical testing has shown that a setting of the last 3 seconds produces a good average value that does not fluctuate so much.

Consideration of zero values

In order to make the result display as realistic as possible, it is necessary to include the downhill phase that follows each uphill ride in the measurement result. If this is not done, recording only the strenuous uphill ride will lead to a power display that is too high if the rolling phase downhill is not taken into account. A power meter therefore also records the downhill ride, which is easier for the rider to manage, and provides a more realistic overall picture in the end result.

Cadence, pedaling time and pedaling index

Since calculating the mileage always requires recording the crank speed, a power meter measures the rider's cadence and uses this, together with the force on the pedal, to calculate the current mileage. A power meter records the sequence of measured values ​​over the entire journey, regardless of the terrain geometry being negotiated. This type of constant recording of each pedaling and rolling phase is reflected in the pedaling time. This pedaling time only takes into account the journey that can be completed with more or less effort, i.e. uphill. A power meter indicates this pedaling time as a percentage of the total journey.