Aerodynamic force coefficients describe the amount of drag a body experiences at different speeds.
For example, a cyclist riding at 20mph in still air will experience less than half the drag of someone riding at just 12mph.
Specialists in the field of aerodynamic drag for cycling and other industries like to talk about force coefficients because they remain more or less constant regardless of wind speed, atmospheric conditions and scale. To be precise, with large changes in any of these changes in underlying aerodynamics can result in changes in coefficients, but for cycling purposes, we can assume they are constant.
Coefficients give us a meaningful number without worrying about test conditions. We can obtain a drag coefficient from an F1 wind tunnel model at 180kph in Brackley, and assume the same value for the race car at 280kph in Sao Paulo.
Drag is a simple function of dynamic pressure (), drag coefficient (: a constant for a given setup and body position) and frontal area (). Dynamic pressure is a function of air density () and speed () squared:
Aerodynamic drag is then calculated as:
As a cyclist we aren’t interested in separating drag coefficient from frontal area so we talk about as a single number. Drag force is just times .
Why is aerodynamic drag so important?
Lets take a cyclist on a flat road, with no wind at a constant speed as an example. Where does their power go? It all goes into friction (chainset, hub bearings, rolling resistance) and aerodynamic drag. Power is simply, force times speed. The friction force stays roughly constant with speed, but aerodynamic drag in cycling increases with the square of speed. In other words, aerodynamic power goes up with the cube of speed, whilst frictional forces just go up linearly.
Taking a typical setup we can work out how this power breaks down for different speeds.
Cycling power contributions and their variation with speed.
Aero drag power rises with speed much faster than friction: at 5kph just 10% of your power goes into aero; at 40kph over 80% of your power is required just to push you through the air.
It is, by a very large margin, your biggest problem.
The good news is that you can reduce this value for free, and you can do it right away. Increasing your power output takes weeks of hard work. Making yourself more aero takes no time at all. You just need to be able to measure it!
What does all of this mean in practice?
Let’s take a flat 40km TT. How does reducing your CdA affect your time?
Effect of CdA on 40km TT time.
Reducing your aero drag by 10% can save over 1 minute and 30 seconds.
A good set of aero wheels may cost you $2000, and typically reduce your CdA by around 0.006. The graph above tells us that would reduce your 40km TT time by about 25 seconds.
Optimising your position on the bike using aerosensor could typically save around 0.015, which could save you as much as 1 minute 15 seconds.
Adjusting Your Equipment To Reduce Aerodynamic Drag in Cycling
The most important factor in choosing the right equipment is to find the balance between weight, aerodynamics and comfort.
For example, aero wheels are often heavier than standard wheels, so you need to decide if the extra weight is worth the aerodynamic advantage.
As handlebars are in front of the rider, they have a big impact on drag.
Aero bars are of course going to be the ones that take into account aerodynamic drag in cycling. But understandably, they may not be not suitable for all riders. However, they do allow a narrower arm position and promote a horizontal torso, thereby reducing the drag and improving efficiency in elite cyclists.
Traditional vented helmets reduce aerodynamics as they suck air in – increasing drag and making it much harder for the air to glide over the top.
Aero helmets, on the other hand, have a smooth exterior which helps the air to flow over the top of the helmet more easily. This results in less drag and therefore makes the rider more aerodynamic. It’s a quick and easy way of shaving time off your ride too.
How much power can you save for the same speed?
Using the same example as above, the wheels would save you about 4W whilst the body position would save you 12W. With aerosensor, these gains in performance are immediate, and in the case of your position, free. Free speed.
Aerodynamics is the science of speed and it’s something that every cyclist should take into consideration if they’re looking to improve their performance. By understanding the purpose and benefits of aerodynamics, cyclists can make small changes that can have a big impact on their riding.