Wind tunnel testing: comparison between high and low profile carbon wheelsets

Bola takes a 50mm high 28mm wide carbon bike wheel in seven yaw angle tests, yaw angle is the angle at which the wind blows towards the bicycle and rider system. A yaw angle of 0 degrees means that the wind is blowing directly, completely against the wind. The larger the yaw angle, the stronger the crosswind. The test was conducted from -15 degrees (wind blowing from the left) to+15 degrees (wind blowing from the right), with an increase of 5 degrees each time, and a total of 7 yaw angle data were captured.

The data provided in the wind tunnel testing course is CdA, which represents the drag coefficient multiplied by the area. The drag coefficient reflects the shape of an object and the ease of air flow around it, while the area is only the size of the object. The lower the drag coefficient, the smaller the object, and the easier it is to pass through the air. Therefore, at the same power, the speed is faster. It is interesting that when the yaw angle is low, the wheel set with a higher frame height actually has a slower speed, possibly because this wheel set is much wider than the reference wheel set, increasing the frontal projection area. When the wind blows from a wider angle, the wheel sets with higher frame height will come into play, providing more "sail" effect, capturing the wind and pushing the driver forward. The following figure shows the average CdA values captured for all seven yaw angles. On the left is the average value. The middle is the reference carbon bicycle. On the right side is a reference bicycle equipped with pneumatic carbon wheels. After averaging the CdA values of the raw data, Bola found that the carbon bike wheel set with higher box height had a CdA of 0.3640, which was 0.0062 lower than the benchmark and faster. According to CdA, the required power for 40km/h can be calculated. Let's take a look at the differences in the real world.Below, it can be seen the average of 282.41W to ride at a speed of 40km/h. Using a bench bicycle will consume an additional 23.06W. It is interesting that the use of pneumatic bola carbon wheel did not significantly reduce the gap, and bola value was only 5.89W

To help connect it with the real world, if you ride at a power of 250W, it's equivalent to saving 25 seconds within 40km or increasing speed by 0.24 km/h with equal effort. If you increase the output to 350W, your speed gain will decrease to 0.27km/h. Once the testing errors of Bola are taken into account, the carbon wheelsets may actually be equal. The error of Bola testing (calculated as the difference between two tests on the same baseline at the beginning and end of the day) is 3.91W. As shown in the two bar charts below, the error bars of the two related data overlap with each other.

Comparing the best case of the reference carbon bicycle with the worst case of the high profile carbon bike wheel, the speed of the low carbon wheelset may be faster. Of course, the margin of error can work in both directions. If we consider the worst-case scenario of the reference bicycle and the best scenario of the high carbon wheel, the difference may be as high as 13.7W (40km/h). Statistically speaking, due to the existence of errors in Bola, it cannot be concluded that high profile bike wheels are faster than low profile carbon wheels. Only by relying on raw data and attempts can we believe that this is indeed the case. From the perspective of cost savings per watt, the power savings from such expensive costs are very limited. According to raw data, the price of saving less than 6W is $900, which means saving $150 per watt. Moreover, previous data from Bola shows that the difference between the best and worst pneumatic carbon bike wheel sets is only 3.87W, and it is not related to price. In other words, upgrading to a more advanced carbon wheel set will not save much power. The most interesting thing is that, given that most of the differences between high  and low profile carbon bike wheels are discovered at higher yaw angles, high yaw angles are not common in reality, so the power savings that can be achieved in real situations will be lower. Of course, purchasing new wheels does not necessarily mean pursuing aerodynamics, it may be because they are lighter, wider, and have better handling, or simply because they look better.