Easy aerodynamics for cyclists and some tips

Easy aerodynamics for cyclists and some tips

Aerodynamics is the branch of fluid dynamics that attempts to describe and quantify the forces acting on an object immersed in a gaseous fluid, usually air, when the object is moving, when the fluid is moving, or when both are moving.


Any cyclist who has ever cycled in a headwind has experienced how exhausting it is to ride through the invisible mass of moving air in front of you. You will also have noticed that the faster you go, the more air resistance you experience and the more energy you must exert to overcome it. We call this force or resistance to progress aerodynamic drag.

• resistance due to air pressure
• direct friction or air friction against the surfaces of the rider and the bicycle.

The air pressure. Pressure difference

The moving cyclist disturbs the air flowing around him, forcing the air away from his surface to make way for him. This increases the pressure at the front. On the other hand, the opposite effect occurs at the rear of the cyclist, creating regions of lower pressure that result in a "pressure drag" against the cyclist.

With high pressure at the front and low pressure at the rear, the cyclist is literally pulled backwards.

Air friction

Aerodynamic designs and close-fitting clothing help air circulate more smoothly around these bodies and reduce pressure resistance, while minimising turbulence through the air by using specially designed airfoil shapes.

The frictional force is smaller in magnitude and is caused by the friction of the air in contact with the outer surface of the rider and the bicycle.

On flat courses, aerodynamic drag is by far the biggest barrier to a cyclist's progress, accounting for 70 to 90 percent of the resistance felt when pedalling.

Mathematical formula for aerodynamic drag

To better understand how each of the elements involved affect each other, we will study, without going into too much depth, the well-known formula for the frictional force or aerodynamic drag of a body in the air.

In this formula:

• ρ is the density of air in kg/m3. It varies with altitude and other parameters such as temperature (inversely proportional) or atmospheric pressure (directly proportional).
• Vw is the relative speed, in metres per second, between the air and the cyclist, for headwinds the speed of the cyclist shall be added to the wind speed.
• Cd is the drag coefficient, which is a property of the shape and surface texture of the object. It is dimensionless (unitless).
• S is the frontal cross-sectional or projected area, in m2, of the bicycle-cyclist combination. It is not the total surface area, but the area occupied by the bicycle-cyclist combination when viewed from the front.

 

 

 

The Cd coefficient and the cross-sectional area are closely related. A simple change of posture will not only reduce or increase the cross-sectional area, but will also expose different surfaces (helmet, clothing, skin...) with different surface properties to the air. Therefore the two quantities are often considered as a joint value "CdS".

It is very important to note that in the above formula the speed is squared. This means that the aerodynamic force does not double as the speed doubles, but that its value increases quadratically.

If we plot the formula as a function of speed, it can be seen that by doubling the speed from 20 km/h to 40 km/h, the aerodynamic force has gone from 7.57 N a 30.27 N

 

For simplicity we assume that there is no wind and that the atmospheric pressure is approximately equal to that at sea level.

 

Recall that the formula we are drawing is as follows:

where on the horizontal axis we represent the speed.

 

 

Reducing the CdS factor by, for example, adopting a more aerodynamic stance, which means reducing the frontal area, will result in lower aerodynamic drag force values, which will facilitate forward movement. A 20% reduction in the CdS term is depicted in the graph below.

 

In terms of power, this reduction in the CdA term has meant a saving of more than 60 watts at 40 km/h.

 

The following figure shows both curves simultaneously.

 

Conclusions and recommendations

For touring cyclists, recreational cyclists or those who are not concerned about their own personal best, it seems that the best strategy for tackling headwinds would be to slow down, exactly the same as for tackling a climb, and take it easy. As well as adopting a posture and equipment that reduces the exposed frontal area as much as possible.

In more competitive scenarios we should consider:

• Try to be more aerodynamically efficient at higher speeds, such as flat stretches and descents.
• Relax the position on low speed sections, such as ascents, where aerodynamic resistance is less important in favour of the force of gravity, which is determined by the weight of the cyclist-bicycle combination.

Just as important is that the position on the bike allows you to continue pedalling comfortably, so that the effort and fatigue are sustainable over time. The fastest position will be the one in which we minimise the impact of aerodynamic resistance, but which we can maintain for as long as possible.

 

Recommended article:  Physics of cycling: Forces. Basic notions