Aerodynamics 101

Why Aerodynamics?

Think about riding your bike down the street. As you cruise down the road at 16 km/h (10mph), the speed seems effortless. Then, you accelerate to 25, 35, 40, and finally to 48km/h (30mph). All of a sudden you have progressed from an easy cruising pace of 16km/h to a full-out effort [sprint], in order to achieve 48km/h.

So, what changed? You did not put on any additional weight during the duration of your ride; the shape of your body and equipment did not drastically alter; the wheels contacting the ground did not suddenly become exponentially more "sticky."

What changed was the composition of the air. The air, which you are traveling through, became denser. As a result of your increased velocity, you are now interacting with more particles per second, thus creating a more dense fluid to travel through.

Let's think about air for a second... What is air, in a scientific sense? Simply put, air is various molecules [micro billiard balls] combined together, primarily composed of oxygen, nitrogen, and carbon dioxide. What we are really concerned about is why 16km/h air so much easier to travel through vs. 48 km/h air.

What do we see has happened? On the left: when you look at the composition of air at 16km/h, you notice that (1) the particles are spaced farther apart, thus it takes more time for those particles to collide (2) there are fewer particles. Therefore, the air is easier to travel through, as a result of a lower density.

But, what has happened to the air at 48km/h? More particles collide in a shorter timeframe; therefore, the air is effectively "denser" and, as a result, more energy is required to travel through. As you travel more rapidly, more particles are colliding with you per second.

"Air" friction affects everything moving through it, although you may associate aerodynamics with airplanes and Formula 1. It impacts everything from a soccer player running down the pitch to a cyclist traveling 14km/h uphill. We, as athletes, are always combatting the air pressure around us.

How much of our propulsion energy actually goes to overcoming our aerodynamic signature? Upwards of 80% of all the energy expended [by the athlete] is "used" to move him or her through the air. More astonishing is the amount of power it takes to increase you speed from 16km/h to 48km/h.

Bear with us, as we dive into the mathematical inner workings of power and aerodynamics.

Applying the laws of classical physics to aerodynamic drag requires some math. Our first step is to outline the benefit of a reduction in aero drag to real world time and distance calculations.

The fundamental formula to start with is the equation for power

P = F*V = F*(d/t)

The basic assumption is that the athlete is not going to output more power, so P [power] is held constant.

Therefore, as F [force] drops, V [Velocity] will go up by the inverse amount to keep P constant.

Similarly, over a fixed distance [d], if F goes down, time [t] will also go down by the same percent.

To put it another way, if we decrease the aero drag force by 10%, to keep P constant your equation looks like this:

P = F*V*(1.1/1.1) = (F/1.1)*(1.1*V)

Another way to look at it is with two events, 1 and 2. For each, power [P] is constant. Therefore:

P1 = P2

F1*V1 = F2*V2

F1*(d1/t1) = F2*(d2/t2) F1/F2 = (t1/t2)*(d2/d1)

If we want to know the time saved over a particular course, d1 = d2, therefore d2/d1 = 1 and we get:

F1/F2 = t1/t2

F1/t1 = F2/t2

Since we are comparing a fixed set of conditions [event 1], we can set F1/t1 to be a constant:

C1 = F2/t2

C1*t2 = F2

Any percent change in force will have the same percent change in the time (e.g. if F2 is 10% lower than F1 then t2 will be 10% shorter than t1). To get the 100 grams of drag reduction = ~1 sec/kilometer, we just need to look at the first formula, P = F*V and approximate gravity as 10 m/s²rather than 9.8 m/s². The rule is not dependent on the length travelled or speed since F is being measured directly.

What does all that mean for you?

Rule by computational physics for aero drag: 100 grams of aero force reduction ~ 1 second per kilometer time reduction for equal power output by athlete

We now understand that air friction plays a crucial role in an athlete's performance and that neglecting aerodynamics will yield suboptimal results. Realizing that aerodynamics is important is not enough, though. In order to obtain useful results, we need to have the proper method, the right tools and to ask, "how can we be better?"

Speed Read

  1. Air Density: As your velocity increases, so does the velocity of the particles colliding with you per second.
  2. Over 80 percent of all energy expended goes to overcome air pressure
  3. 100 grams of aero force reduction is approximately 1 second per kilometer in time reduction