As your speed increases, the air becomes your enemy. Whilst this is hardly noticeable at lower speeds, on fast descents, during races or on long flat stretches, it becomes one of the greatest forces a cyclist has to contend with. Up to around 15 km/h, rolling resistance and other losses outweigh the impact of aerodynamics. From around 20 to 25 km/h, however, the role of air resistance becomes increasingly dominant. From around 30 km/h, it is usually already the greatest braking force, and from 40 km/h onwards, air resistance typically accounts for the lion’s share of the power required.
Yet aerodynamics is about far more than just deep rims, streamlined frame tubes or minimising the frontal area. Often, the greatest potential for improvement lies where one might not initially expect it. If you want to understand why a bike glides quickly through the air, you therefore need to take a step back and look at the fundamental principles. The key lies in the complex interplay between the rider, the bike and the surrounding airflow. We provide an overview of the key physical laws of aerodynamics.
Conclusion
Mountain bikes are getting faster and faster: uphill, downhill and on the flat. It is only natural, then, that cross-country racers are now also focusing on the importance of aerodynamics. If you want to be fast, you have to battle against the air. This isn’t just a topic for professionals with access to a wind tunnel. Even ‘ordinary’ cyclists can save a lot of energy with good aerodynamics. - Jan Timmermann, BIKE editor
1. Air resistance increases in proportion to the square of the speed
The most important basic rule of cycling aerodynamics is: the faster you ride, the more disproportionately greater the air resistance becomes. In simple terms, it can be described by the formula F = 1/2ρCAv². The key factor here is the square of the speed. If, for example, you increase your speed from 30 to 40 km/h, the aerodynamic drag does not increase by a third, but by a factor of (40/30)² ≈ 1.78. This means that at 40 km/h, you have to overcome around 78 per cent more aerodynamic drag than at 30 km/h. This explains why aerodynamic optimisation becomes particularly relevant during high-speed riding, in time trials, on flat routes and in competitive cycling.
2. The power required increases approximately in proportion to the cube of the speed
The relationship becomes even more striking when one considers not the drag, but the power required. Power is force multiplied by velocity: P = F × v. As air resistance increases with v², the power required increases, approximately, with v³. So, if you accelerate from 30 to 40 km/h, you have to generate approximately (40/30)² ≈ 2.37 times the power to overcome air resistance. This does not mean that the total pedalling power increases by 137 per cent – rolling resistance, the drivetrain and the gradient also play a role. However, it does show why those last few extra kilometres per hour on flat terrain cost so much energy.
3. The end face is a crucial factor
As well as speed and air density, it is primarily the effective frontal area A that determines how much aerodynamic drag is generated. This is not solely a matter of the actual area visible from the front. The decisive factor is the combination of body shape, orientation and airflow – which is why we often refer to the effective frontal area or the CdA value (drag coefficient × area). A compact riding position with the upper body low and as little exposed surface area as possible can significantly reduce drag. It is important, however, that a position is not only aerodynamic but also sustainable over a longer period: a theoretically perfect position is of little use if you can only maintain it for a few minutes.
4. The rider is the largest aerodynamic component
When it comes to cycling, it is not the frame that presents the greatest obstacle to airflow, but the rider. The rider and the bicycle together form a complex flow system, with the body accounting for a significant proportion of the total drag. That is why changes to the seating position, the upper body, the arms or clothing often have a greater effect than minor optimisations to the frame. For example, an aerodynamically optimised bike can lose a large part of its advantage due to an unfavourable, upright riding position. Conversely, a rider on a more conventional bike can be surprisingly fast if they adopt a good riding position.
5. A surface that is as smooth as possible is not necessarily the most aerodynamic
Intuitively, one might assume that a surface that is as smooth as possible always produces the least aerodynamic drag. The reality is more complicated. With certain shapes, controlled turbulence can ensure that the airflow stays in contact with the surface for longer before it breaks away. This can reduce drag. A classic example outside the world of cycling is the golf ball. In bicycles, this principle is utilised in, amongst other things, specialised fabrics, rim profiles and tube shapes. The specific shape and the respective flow conditions are always the decisive factors.
6. A turbulent wake forms behind the bicycle
When air strikes a cyclist and their bicycle, it cannot follow the contours everywhere. At certain points, the airflow separates and a turbulent wake with lower pressure forms behind the object. This area is a key component of aerodynamic drag. Good aerodynamics therefore aim not only to minimise the frontal area as much as possible, but also to guide the airflow in a controlled manner around the object and then recombine it. This is particularly important around the rider’s body, on frame tubes, on the wheels, and behind components such as the seat post or water bottles. The shape of an aerodynamic component is therefore always considered as a whole and not just on the basis of its front view.
7. Crosswinds significantly affect aerodynamics
In reality, a bicycle rarely rides into a headwind. Even a relatively light crosswind alters the relative wind direction that the bicycle actually ‘feels’. A component may therefore exhibit very low drag values when facing a head-on wind, but react completely differently at an oblique angle of attack. This becomes particularly evident with deep or high wheel profiles. Here, it is not just a question of minimising drag, but also of lateral forces and handling. Modern aero wheels are therefore not only analysed at a 0° approach angle, but across a whole range of approach angles. A good aerodynamic design must consequently strike a balance between low drag, stable airflow and controllable handling.
How about you: have you ever given any thought to the aerodynamics of your bike or your clothing? Let us know in the comments!
Jan Timmermann
Editor