Cycling is like an unbalanced balance; each time practically the same forces act on the bike and its rider. However, the magnitude of these forces changes constantly. One of those forces is the aerodynamic drag force. Aerodynamic drag force is the resistive force that originates from the air that is rushing past the cyclist. The past decades a lot of research has been done towards the effects of aerodynamic drag on a cyclist’s performance. This research resulted in a lot of knowledge and performance enhancements.

The forces during cycling

As mentioned above, a mixture of forces acts on the bike and its rider. Four major forces apply on a cyclist during a training ride or race:

  • Propulsion, produced by the rider him or herself
  • Gravity
  • Rolling resistance
  • Aerodynamic drag

The contributions of the different types of forces on the resultant or net-force depend on the environmental conditions during the ride. When riding uphill or downhill, gravity may provide an extra obstacle or aid respectively. Windy environments may result in an increased or decreased aerodynamic drag, dependent on its direction compared to the rider’s movement.

On a flat surface, approximately 90% of the power output of an 80-kg cyclist will be lost due to aerodynamic drag! Needless to say, that a lot might be gained with respect to aerodynamic drag in cycling.



Aerodynamic drag

As pointed out earlier, the aerodynamic drag is the resistive force that is produced by the rushing of air past the cyclist. This overall resistive force is a resultant from two contributors: form drag and skin friction. Air will ‘bump onto the rider’ and rush past him or her. This creates a low-pressure region directly behind the back of a rider. This low-pressure region is called the wake region.  The wake region increases the aerodynamic drag. The bigger this region, the bigger the aerodynamic drag.

Skin friction refers to aerodynamic drag created by the roughness of the skin. A rougher skin will produce more friction in the boundary layer, due to which energy will be lost. However, a completely smoothened skin isn’t optimal either. When adding some texture or dimpling to the surface or skin area, the air flow will be ‘attached’ to the body for a longer time. This increases aerodynamic efficiency because it reduces the low-pressure wake behind the rider. A high-tech skinsuit, relatively smooth with dimples or texture, may improve aerodynamic efficiency by 4-6%!

The theory

The aerodynamic drag force can be calculated with the following formula:

F = ½ρ x v2 x Cd x A

F = aerodynamic drag force in Newton.
ρ = air density, which is the mass of the air (kg/m3).
v = air speed with respect to the rider. Note, when cycling 30 km/h with a 20 km/h head-wind, v is 50 km/h.
Cd = drag coefficient, which is dependent on the physical form of the rider.
A = the projected frontal area of the rider.

Cd and A together determine the total drag area or the magnitude of the low-pressure wake region.



Practical implications

There are a few methods to improve (reduce) aerodynamic drag:

  • Positioning rider on the bike
  • Equipment
  • Drafting

Positioning

The most significant improvements in aerodynamic drag can be accomplished by adjusting the position of the rider. There are a few major contributors to an aerodynamic position:

  • Torso angle. The smaller the torso angle *the angle between hips and torso*, the smaller the projected frontal area and therefore, the smaller the aerodynamic drag. Lowering of the torso can be accomplished by a combination of some simple adjustments. The saddle height can be raised, the handlebars lowered and the distance between saddle and handlebars increased.
  • Forearm angle. A small inclination of the forearm and positioning of the elbow pads forward might reduce projected frontal area.
  • Posture. Shrugging or turtling reduces the frontal projected area as well. This position can be obtained by bringing the shoulders towards the neck.

Equipment

Recently, a lot of research has been done concerning aerodynamic efficiency. This has resulted in major adjustments in equipment.

  • Helmets. Especially during a time-trial, an aerodynamic helmet might reduce aerodynamic drag. An aerodynamic helmet fills up a large part of the low-pressure wake behind the head when a cyclist is sitting in an aerodynamic position.
  • Clothing. Special skinsuits with minor texturing are developed to improve aerodynamic efficiency as described above.
  • Wheels. A wider rim reduces the exposure of the spokes to the wind. This reduces the aerodynamic drag of the wheels.
  • Frame. Over the years, several adjustments have been made on the frame of the bike to enhance aerodynamic efficiency.

Drafting

Riding in a peloton will significantly reduce aerodynamic drag. When riding in a peloton, the aerodynamic drag forces are shared among the riders. Cycling at the same speed behind another rider, will require less power output. Therefore, the speed of a peloton is faster than the speed of a solo rider. Indeed, drafting cyclists may perform 30% less work compared to the front cyclist. Riding on the back of the peloton might be very beneficial in terms of aerodynamic drag.

But not only the drafting rider will benefit from the situation. Also, the front cyclist may reduce his or her aerodynamic draft force. By riding directly behind the front cyclist, the drafting cyclist will ‘fill up’ the low-pressure wake region behind the front cyclist. This in turn reduces the aerodynamic drag force by 2-5%.

Literature

Cycling Science; The ultimate nexus of knowledge and performance. Stephen S. Cheung PhD & Mikel Zabala PhD.