# Aerodynamics

## NAQI AERO SPEED GEL INCREASES CYCLING SPEED BY LOWERING AERODYNAMIC DRAG IN CYCLISTS

###### Nikolaas Van Riet*, Greet Claes**

* Cycling aerodynamics research & development consultant

** Head of research and development, NAQI

**Abstract **- An Aero Speed Gel, consisting of vortex generators in an oil gel, applied to arms and/or legs of a cyclist, is able to significantly lower the drag coefficient of a cyclist. At time trial speeds (50 km/h) this lowering of the drag coefficient leads to gains of up to 15 watt for a typical cyclist in time trial position. This translates to a potential gain of almost 1 second per km. For a triathlete tested at 39 km/h the potential gain was 307 seconds or a reduction in drag of 6%

###### Index Terms - cycling aerodynamics, time trial, vortex generators

The importance of aerodynamics in cycling cannot be underestimated. In professional cycling speeds above 40 km/h are the norm. In time trial (TT) average speeds above 50 km/h are common. At 40 km/h 85% of the power of the cyclist used to combat the aerodynamic resistance, also known as drag. Rolling and mechanical resistance only accounts for 15%. At 50 km/h aerodynamic drag accounts for 90% of total power.

It must be clear that most potential in increasing a cyclists speed must be found in optimizing aerodynamic drag. The suits, shoes, bikes and helmets of a cyclist are already optimized for aerodynamic drag. The exposed skin s there for a potentially interesting area for aerodynamic optimization.

### II. THE SCIENCE BEHIND AERODYNAMICS IN CYCLING

Aerodynamic drag: As you cycle through the air, your bike and body need to push the air around you, this creates higher pressure in front of you and lower pressure behind you. Because of this, the air exerts a net force against you as you ride. There are a few things that dictate how much force the air exerts against you. The faster you ride, velocity V (m/s), the more force the air pushes and pulls you back. You and your bike present a certain frontal area A (m2) to the air. The larger this frontal area, the more air you have to displace, and the larger the force the air pushes against you.

This is why cyclists and bike manufacturers try hard to minimize frontal area in an aerodynamic position. The air density Rho (kg/m3) is also important; the more dense the air, the more force it exerts on you. Finally, there are other effects, like the smoothness of your clothing and the degree to which air flows laminarly rather than turbulently around you and your bike. Optimizing your aerodynamic positions also helps with this. These other effects are captured in a dimensionless parameter called the drag coefficient, or Cd.

###### Figure 1 Power in function of velocity. Rolling resistance in purple, aerodynamic drag in orange and drivetrain losses in red.

Picture by: https://www.gribble.org/cycling/power_v_speed.html

The formula for the aerodynamic drag acting on a cyclist in metric units, is:

Fdrag(N) = 0.5 . Cd . A . Rho . V2

With A the frontal surface area of the cyclist in m2, Rho the density of the air in kg/m3, V the speed of the cyclist in m/s and Cd the drag coefficient. The power Pcyclist (Watt) that must be provided to your bicycles wheels to overcome the total resistive force Fresist (N) while moving forward at velocity V is:

Pcyclist = Fresist . V

With

Fresist (N) = Fgravity + Frolling + Fdrag

Fdrag is by far the dominant force in this equation. To give an example: For a typical rider of 70kg, with a bike weighing 7kg, a frontal area A of 0,0509m2, a drag coefficient Cd of 0,63, on a 0% slope, with normal air density, and a cycling speed op 50km/h, we get Fdrag=38N (527 Watt), Frolling= 3,8N (53 W). Or stated differently at 50km/h, for a cyclist in normal position and conditions, almost 90% of the power he/she is producing goes to overcoming the air resistance.