Wind from all directions

Wind from all directions

Saturday August 10 2019 we tested our new Strydv3 with revolutionary wind port for the first time in practice. This new Strydv3 measures the air-resistance. Stryd calls this the ‘Air Power’. The measuring device includes a ‘wind port’, a small hole at the bottom of the Stryd. A high-grade sensor measures the minute differences in air-pressure, comparable to the pitot-tube used in the aircraft industry.

We were lucky that Saturday. It was a windy day. By running back and forth on 2 tracks, we were able to get proper data of the Air Power from the new Stryd. We found that the Stryd calculates realistic values for Air Power, also in variable weather conditions. However, the results for tail winds did not match theory. Meanwhile, Stryd is working on improving their algorithm so it will also be able to handle tail winds properly.

Experiences with daily use of the Stryd v3 with Wind Port

Since August we have used the Stryd v3 daily on all of our training sessions and races. Obviously, the weather conditions have been (very) different during all of these sessions.

The good news is that we found that the Air Power usually matched our theoretical calculations very well. Of course we were not able to measure the local wind speed and direction, so we had to rely on weather data. Also the wind is never constant, it changes upon the approach of rain clouds and in the wake of corn fields or trees. So we were not able to check the Air Power very accurately, but we can say that the data were always in the correct order of magnitude.

The impact of the wind direction/angle

In our book ‘The Secret of Running’ and in previous papers, we explained our approach that the total running power Pt should be equal to the sum of the running resistance Pr and the air-resistance Pa, so Pt = Pr+Pa.

Pr can be easily calculated. We know that Ron weighs 80 kg and has an ECOR of 1.04 kJ/kg/km at a pace of 5:45/km (speed 2.9 m/s). Consequently, his Pr is 1.04*2.9*80 = 241 Watts.

Obviously, Pa depends on the wind speed and direction. This is quite a complicated 6th order polynomial relationship, for which our running friend Arno Baels has prepared a spreadsheet calculator. The results are given in the figure below.

The green line gives the data for a wind speed of 38 km/h (wind force 5). These were the data of our first test on August 10. The figure shows that with a head wind Pa is as high as 90 Watts! At a cross wind (90 degrees) Pa is still 10 Watts, whereas with a tail wind Pa becomes -15 Watts. So with a tail wind, you are literally pushed by the wind.

The orange line gives the data for a wind speed of 15 km/h (wind force 3). These conditions occur regularly in the Netherlands, they might be considered average. In this case Pa is only 20 Watts facing a head wind. With a tail wind Pa is approximately 0.

Both in training and in races, you will probably experience variable wind directions as the course will never be a straight line. Part of the course you will enjoy a tail wind and part of the course you will have to battle against a head wind. Stryd PowerCenter presents the Air Power as a percentage of the total power so you are able to evaluate your performance. The figure shows that the impact of the Air Power at the lower wind speeds is around 5% of the total power, but a the higher wind speed (38 km/h), it increases to 20%!

Our book ‘The Secret of Running’ is for sale in our webshop. Also available in German as ‘Das Geheimnis des Laufens’, and in Italian as ‘Manuale completo della corsa’.