The physical phenomenon of aerodynamic drag can be explained and understood, as we did in part 1 of this series on aerodynamics in cycling. However, measuring, assessing and making practical use of it is not easy. After all, races do not happen inside a wind tunnel.
We will cover how cross-winds add to complexity in the real world, how computer simulations are always incomplete and how wind tunnel experiments are much more complicated than thought. You may embark on a journey to try to measure aerodynamic drag yourself, but is it truly necessary? Instead, you may achieve most of the gains by understanding the principles and investing in trustworthy brands.
The real-world challenge of cycling aerodynamics
Making practical use of aerodynamics is much more difficult due to its complex nature. Weight is simple, less is better and one can easily measure it at home. Rolling resistance is more difficult to assess, but again, less is better, so with some confidence, we can determine which tires are fastest for road cycling. Aerodynamic drag, however, is complex and challenging to assess. As such, the most aero setup for the individual use case outside on the road is almost impossible to determine, as many factors interplay. Still, some general principles will be discussed in the following sections.
What counts for aero in cycling is effective wind speed, not the ground speed shown on your computer
The standard for testing cycling equipment is typically based on wind speeds of around 40 to 45 km/h. However, many cyclists have criticized this, as the average cyclist rarely maintains such velocities, and if they do, it’s often only briefly. The focus might be misplaced on the wrong speed metric. It’s not about the speed at which you travel across the terrain, but the effective wind speed you encounter. This means the speed with which you hit the air in front of you will be different because of, well, wind, the meteorological kind.
Testing wheels at 45 km/h is more realistic than it may seem. Don’t forget that the natural wind is added to the ground speed shown on the bike computer. 30 km/h can increase to 40 km/h and beyond more quickly.
It is rare for the wind to be entirely still outside. Sometimes, there are just light breezes of a few kilometers per hour, while other times, you may find yourself struggling against powerful gusts of over 40 km/h and more. On average, most riders experience winds of around 10 to 20 km/h, which must be factored into their ground speed. For example, if you ride at 35 km/h with a 10 km/h headwind, your effective wind speed relevant for aerodynamics is 45 km/h. Conversely, if you have a 10 km/h tailwind, your effective speed will be decreased to 25 km/h.
Admittedly, the values the industry tests are still on the higher side and most applicable to pro racing, yet maybe not that far away from the situations you will encounter on your bike. Still, what is faster at 45 km/h will also be faster at 30 km/h, just the difference will not be as pronounced.
The pesky cross-winds: how yaw affects cycling aerodynamics
The yaw angle of the wind on the rider is influenced by four key components: the rider’s speed and direction, along with the wind’s speed and direction. While riding, we not only feel the air we push aside as we move forward but also experience varying wind intensities from different directions. Both combine to create an effective wind at the frontal edge of our bikes. Even if it may feel as if the wind is always coming straight against us, that’s rarely the case.
The yaw angle of the effective wind that cyclists usually experience ranges from 0 to 20 degrees at most cycling speeds. Generally, higher yaw angles greater than 10 degrees are much less common than those with lower angles. Additionally, as cyclists increase their speed, the yaw angle tends to decrease. Higher yaw angles can be good, as they help the so-called sailing effect by which deep-section wheels or aerodynamic frames act like a sail and effectively push the rider forward.
What is fastest depends on how you look at the data
As one can easily observe in the graph above, yaw adds another level of complexity. Which is the best model? For example, Model C (red) and Model D (green) both exhibit reduced aerodynamic drag at a yaw angle of approximately 10 degrees. In contrast, at 0 degrees and at angles greater than 12 degrees, Model D (green) shows the lowest drag. Most of us may not be aware of the yaw angles we typically encounter during our rides. If you had to choose, you might lean towards either Model C (red) or Model D (green). The decision between the two is close and may not result in a significant difference in performance. Furthermore, both manufacturers can claim that their model is “the fastest,” depending on how they present their data.
In aerodynamic testing, the yaw angle introduces an additional layer of complexity. Depending on the riding conditions and speeds, one rim may perform better than another due to differences in yaw. Again, “the fastest” is highly subjective and can vary based on these factors.
One potential solution is to use a weighted average based on typical riding conditions; however, there is currently no established standard for comparison.
Navigating cross-winds: stability may trump speed in aero wheel design
Yaw gives us another point to consider: cross-wind stability. When the effective wind comes at a yaw angle relative to the front wheel, the rider experiences side forces. You may remember the feeling when a large truck passes you, or the wind direction suddenly changes while passing a gap between some houses on the side of the road. This phenomenon is tricky and scary, mainly because it can appear suddenly without a warning. Therefore, most companies focus on crosswind stability, especially since early aero-bike wheel models were highly unstable. The goal is to reduce the steering force by adopting a different shape and making the forces more predictable so the rider can react.
Aerodynamic wheel design has evolved significantly with cross-wind stability as a primary concern. Nowadays, a depth of 40-50mm is recognized as optimal for balancing performance and safety across varying wind conditions.
Indeed, nowadays, the very deep aero wheels are gone from the pro peloton as teams value stability over a couple of watts saved. As a rule of thumb, around 40 to 50 mm depth is probably the sweet spot for general road riding.
From wind tunnel theories to winning races: it’s not just tunnel vision
In this section, we will explore the process of aerodynamic testing. Following our discussion above, it’s clear that assessing aerodynamics is a complex task. Determining the most suitable aerodynamic bike component for a specific rider and race scenario adds an extra layer of difficulty. This information will help you better understand the claims made by equipment manufacturers and enable you to make more informed choices. It’s important to note that comparing test data from different companies can be challenging because of varying experimental conditions.
The tools used to understand cycling aerodynamics are CFD, wind tunnels, and velodrome testing.
Nowadays, engineers have three primary ways to understand the aerodynamics of cycling components: computational fluid dynamics (CFD), wind tunnel testing, and real-world testing in a velodrome or outdoors.
Computational fluid dynamics (CFD) aid the initial engineering decisions
CFD, short for computational fluid dynamics, is a software-based simulation to predict how air flows around components. It requires expensive software and an accurate physical model of the part being assessed. Engineers can test multiple configurations and make informed design decisions as the entire process is executed on a computer. There are limitations, as fully modeling a moving, rotating bike wheel is very difficult, and even more so a complete bike has many moving components, not least the rider. As such, the technology is helpful, especially in the development of new bikes and parts, in steering engineering in the right direction.
The wind tunnel is the laboratory of the aerodynamicist
A wind tunnel is the aerodynamicist’s laboratory for simulating the effects of air resistance on an object moving through the air. It is a tube with powerful fans that draw a steady airflow over a fixed object under test, such as a bicycle or other components. Additionally, researchers can visualize airflow over a test subject by studying the movement of smoke or dye as it passes over the object. When the airflow changes from laminar to turbulent, it indicates that the airflow has become separated at certain points.
Various factors, such as drag, side forces, real-time air temperature, and wind speed, can be measured within the wind tunnel. Most importantly, the tunnel’s flow, direction, and wind speed can be controlled independently of the weather outside. The bike industry faces a unique challenge because the wind speeds involved are relatively low compared to those in other fields, such as Formula 1 racing. Consequently, wind tunnels designed for cycling must be equipped with high-precision, sensitive machinery, which significantly raises operational costs.
The velodrome is the first step towards real-world testing
The next step of a controlled environment is the velodrome, which is often employed, especially when optimizing the rider’s position and equipment choices. It is less controlled, but a step towards the “real world,” where testing becomes very difficult since the environment can not be controlled, and many confounders preside.
How challenging the task becomes will be more apparent once we try a little thought experiment of our own.
Thought experiment: How to assess the aerodynamic drag of a bike wheel in a wind tunnel
Imagine you aim to evaluate the aerodynamic drag of a new wheelset. How would you set up the experiment within a wind tunnel?
- Test only the rim? That omits the spokes, which is not realistic. But maybe it’s best to optimize rim shape?
- Add the spokes? Good, but should the wheel be static or rotating while you measure? Wouldn’t the fork and bike influence the measured aero data?
- Include wheel rotating, mounted on a bike? Well, is that realistic? What about the rider?
- Include a wheel rotating on a complete bike with a dummy as a rider? Well, but the dummy is not pedaling. Would that not influence the outcome?
- Test wheel with a bike and a person riding on top of the bike?
- At this point, are we still measuring the wheel’s drag, or the drag of all the other things?
Not easy.
As the scientists and engineers among you will know, every experimental setup has its own advantages and disadvantages, and the choice depends on the research question and the desired outcome. However, we need to consider how much information we get about the wheel when adding more variables to make the setup more “realistic.” At some point, we learn nothing about the wheel from all the other noise.
Aero testing is complicated and expensive. The more you strive to create a “realistic” setup, the harder it becomes to extract meaningful information from the noise. However, if the setup is too simple, it may not be relevant to real-world scenarios.
Yes, I am sorry, it’s complicated. Luckily, most of us are not designing new equipment; we are just looking to purchase some.
Aero testing requires a lot of dedication, even with the latest high-tech gadgets
If you are looking to purchase the most aero equipment within your budget, you have two options. The first is to understand the principles of aerodynamics in cycling, assume the differences at the very top are minimal, and buy from a trusted brand. Very likely, you will enjoy most of the benefits the best aero equipment has to offer. You made a good choice, and this blog is here to help you make that choice with all the articles in the Go Faster,Roll Faster, and Be Faster categories.
However, if you want to fully comprehend the influence of each equipment choice on your personal aerodynamics, there is no way around testing yourself and determining your own CDA. Indeed, some things that are commonly known to be faster may not be in your particular case. Beware, this will involve learning how to use the testing equipment, having a good experimental setup and interpreting the data. For members of The Escape Collective, I highly recommend the Performance Process podcast episode “Aero 101” for a first look at what is involved.
The Chung method enabled aero self-testing in the real world
Without a wind tunnel, there is a method to test aerodynamics on the road, commonly known as the Chung method, invented by Robert Chung. In essence, you need to ride several times at a constant power on a closed loop or out-and-back, while recording power, speed, and environmental data such as wind and air density. You then enter this data into software such as Golden Cheetah or Aerotune and use mathematical methods to determine your CDA. Aerotune briefly explains the process in the video below.
What makes this endeavor challenging is all the little interferences that affect aerodynamics: changes in wind speed, rolling resistance, the rider’s position, changes in the rider’s weight, temperature, humidity, and the accuracy of all the equipment. As such, course selection is very critical. You need to be able to hold a position and have minimal interference from passing vehicles.
Variations in rolling resistance add noise to real-world aero testing
Xavier Disley of Aerocoach highlighted the challenge of variations in rolling resistance on the Silca Marginal Gains podcast when testing for CDA. As we are currently unable to reliably measure the changes in rolling resistance on the road, this adds additional variability to the data.
The CDA from your DIY aero testing is only as exact as your least accurate measurement. What level of accuracy meets your testing target? Can you accurately control for and account for interferences? Honest opinion: With current technology, the risk of false information and misinterpretation is very high.
Every single influencing factor can change during the experiment, and if not properly accounted for, the result becomes less accurate. Ultimately, the calculated CDA value is only as precise as the least accurate measurement. The question remains: what is accurate enough for your testing goal?
Aerometers are a great idea but still have limited actionable use
Some companies have begun developing aerometers to simplify real-world aerodynamic testing and to measure drag more accurately. Most notably, Notio, one of the first and Aerosensor, one of the most popular these days. The aerometers measure dynamic pressure change and integrate with your power meter and speed sensor. Additionally, some devices detect changes in your position on the bike, such as the Forma Body Position sensor or the Aerosensor Aerobody.
For now, it seems that with these devices and methods, you may be able to understand your personal setup around more considerable differences such as position, aero vs. non-aero helmet or the effect of aero socks. But it will involve dedication and effort. Is it worth it? It is for you to judge. Maybe it’s best to book the services of dedicated aero testing and consulting firms such as Aerocoach, Swiss Side or Watt Shop.
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