Bicycle.Engineering

Your aero advantage might not be what the number suggests, part 3:

Ambient wind speed plays an important role in how wind-tunnel gains translate into real-world benefits.

The stronger the ambient wind, the wider the yaw-angle distribution a rider experiences. Since the well-known “sailing effect” usually appears at higher yaw angles, more wind often means larger measured aero gains on the road.

Many yaw-weighted averages seem to be based, directly or indirectly, on publicly available wind data. That is a sensible starting point, but it comes with two important caveats:

We rarely choose to ride in stormy conditions, but those days are part of the recorded wind statistics.
Wind speeds are typically measured 10 m above ground. Close to the road surface, the wind speed is significantly lower, as shown in the distribution graph.

The result: yaw-weighted wind-tunnel averages may be biased toward higher yaw angles than what many riders actually experience on the road in normal riding weather.

: Your aero advantage might not be what the number suggests, part 2: Wind speed.
Most testing in wind tunnels is carried out at wind speeds of around 45 km/h. There are good reasons for this: for example, measurements are more repeatable at higher speeds. It also sounds like a reasonable speed, given that the average speed in many races is now at this level. However, the speed at which the peloton is riding can differ greatly from the wind speed experienced by the riders. The convoy of cars and motorbikes that accompanies the race can have a significant impact, and obviously there is drafting whithin the peloton. If we look at data from races and compare the recorded power to what it should be if we calculate it based on riding speed, most of the time there is a big difference. So even in professional racing (mass start, not time trial), we yet have to find an example where the savings over the full distance of a race matches the savings found in a wind tunnel.
Obviously 45 km/h is a speed most of us rarely achieve, but how much of an impact does it have? Quite a lot:
- at 35 km/h, you get about 47 % of those gains measured at 45 km/h
- at 30 km/h, this is down to 30%

: Last year, I wore through a set of road tires all the way to the threads. What surprised me wasn’t just that I had ridden enough to do that again, but how differently the tires aged compared to the 23–25 mm tires I used to ride at a younger age.

Narrow, high-pressure tires would quickly develop a flat spot in the center tread, giving them a noticeably squared-off profile after a few hundred kilometers.

Modern wider tires seem to wear much more evenly. Lower pressures and larger air volume distribute the load better, so the tire maintains its intended round shape for much longer.

The obvious benefit is increased tire life.

But there may be another, much less discussed advantage: aerodynamics.

Your front tire is just as exposed to airflow as your rim. And a squared-off tire with a flat leading edge is probably not a very aerodynamic shape anymore.

Which raises an interesting question:

After some real-world wear, could a well-shaped 30 mm tire end up being just as aerodynamic as a worn 25 mm tire?

: the bicycle industry has established standards that give a very distorted picture when comparing different performance gains. For example, aerodynamic gains are often communicated as measured in the wind tunnel at a speed of 45 km/h. Meanwhile, bicyclerollingresistance.com publishes rolling resistance at 29 km/h for a single wheel. Now how do the two compare? In this graph we plotted the gains at different speeds for 7 W savings in the wind tunnel (full aero bike vs. good allrounder) and 4 W published by bicyclerollingresistance.com (Continental Grand Prix TR vs. Continental Grand Prix 5000 S TR). You will be probably susprised to see that at speeds that are relevant for most riders, those 4 W in rolling resistance are worth a lot more than 7 W in the wind tunnel.
Also a fact worth highlighting is that at 15 km/h, a speed at which you are likely in climbing and putting out meaningful power, you still get 4 W of gains from rolling resistance, but just 0.3 W from aerodynamics.

: Today we show why we are often doing the same work multiple times. To achieve the best result, we try different tools and different settings. Even though they all seem to do the same, the differences can be quite significant.
The example shown here is quite simple, a profile that tapers at two different rates, with a smooth transition in between. But using native Solidworks tools, we got some waviness in the transition. Instead, we used the blend surface tool of the GW3D AddIn for the primary surfaces, and the xNurbs AddIn for the transition, achieving the smoothness we are striving for.

: In a recent podcast of by , 's Ingmar Junickel stated that the importance of high yaw angles in cycling is often overstated, probably because it makes aerodynamic gains look bigger. That is a statement I can agree with, even though I see also other ways how people ended up thinking high yaw is more likely than it really is. But how much of a difference does the weighting function make when looking at weighted averages? I looked at wind tunnel tests we did for ' wheels at 3 different standard deviations, 10°, 7.5° and 5°. The ranking of the wheels did not change, but the difference between the 35 mm Faserwerk Bergreif and the 55 mm DT Swiss ARC1100 shrank from 5 W to 3 W.