What is better, more Watts or a lower CdA?
In this article, Coach Alan explores the relationship between wattage and CdA. Often understanding this relationship and its impact on speed makes for effective progression for a racer looking to improve their race speed.
Cd = Coefficient of Drag
All shapes and objects have a degree of drag when they pass through the air whilst moving or whilst air passes over them whilst blown or through the wind. How the air flows over the object causes a degree of resistance, some shapes cause more resistance than others. It is easier to visualise if you think of water and hydrodynamics rather than air. Think about a stream flowing around a rock, for example.
Below are some examples with representative numbers, lower is less resistance;
Sphere - 0.47
Half Sphere - 0.42
Cube -1.05
Long Cylinder - 0.82
Streamlined Body - 0.04
As you can see, there is a huge difference between objects being either stretched longer, squared off at the back or smoothed into a streamlined shape.
A = Frontal Area
In conjunction with shape, the frontal area impacts the overall resistance of an object to the flow of air or fluid. The larger the frontal area of an object without changes to other dimensions, the greater the drag.
As you can see in the image above, Tri Training Harder has been venturing into the wind tunnel with a few of our coached athletes. This has led to some significant improvements for athletes and knowledge and experience for me and Phillip to share with the other coaches. A core part of this learning is exploring the relationship between wattage output and drag coefficient. Regarding aerodynamics, it's easy to be completely focused only on reducing drag to the lowest number, the most efficient position. This can, however, come at the cost of being able to put out high power and ride the bike effectively and, in some circumstances, even ride the bike safely.
Increase wattage
Decrease CdA
Which of these you choose out on the roads depends on the conditions and course, but when sat still in a wind tunnel or on a trainer, the position we are looking for needs to combine both low drag and high power. This is why using a metric that looks at both aspects is critical to give a relative point for comparison. This number is the number of watts you can achieve at FTP per amount of drag the position creates; the metric to describe this is watts per metre squared or W/m2. In some cases, this may need to be watts at race pace; the individual pursuit performance on the track would be poorly expressed by looking at FTP numbers when the event is over in under 4mins. The same could be said for long-distance triathlons where sub-FTP numbers will be used.
Using the chart below, you can see examples of aero positions. Let's take a ‘good triathlete’ CdA of 0.24 and look at a range of FTP values.
By a slender margin, this demonstrates that your focus on all but a mountain time trial should be overall on reducing your drag number rather than increasing your wattage. At higher powers or speeds, this relationship will gently grow. At higher speeds the wattage required to go faster does not increase in a linear fashion; you have to push higher and higher wattages to go faster.
It is also important to understand that your focus can vary. When the course goes up or goes slow, shift the focus slightly in favour of the wattage or simply push harder. When these aspects aren’t present, or you are above your target pace, focus on aero. If you have a target time, you do not even need a power meter to apply this principle. A simple speedometer and perceived exertion will suffice!
Often after a race or time trial, athletes will assess their performance based significantly on their wattage output only. Wattage output does not directly equal performance. You may also ask, ‘How do riders know if they did a good job of being aerodynamic’. “With great difficulty” is the answer. However, products in development, such as BodyRocket, are working on this. In the meantime, this is an overlooked aspect of a wind tunnel session. The experience of looking at the live date on the heads-up display and the result of each run in addition to the coaching on a position, can make a huge difference to awareness and execution of safe aerodynamic riding.
If you want to learn more about the wind tunnel experience, you can learn more about our services here.
Alan has worked with Tri Training Harder since 2014. During this time working with a wide spectrum of athletes from beginner, to youth and junior elite athletes through to 70.3 and Ironman AG winners and Ironman Kona Qualifiers.
An active Triathlon coach since 2007 Alan has been fortunate enough to work with athletes, peers and support staff who have continutally challenged him to evolve and develop. Building on a solid foundation in swimming teaching, Alan has specifically developed swimming coaching experience having worked in High Performance Swimming environments. Alan's other passion is all things fast on a bicycle!
Since 2015 Alan has worked in conjunction with the other Tri Training Harder Coaches to significantly develop collective coaching practice both on camp and online.
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Tri Training Harder are one of the leading Triathlon coaching providers in the UK, using our wealth of experience to unite scientific and technological research with already well-established and successful best practices, to create a formula for triathlon and endurance coaching that works.
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