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by Rob Raulings

Perhaps this article should be called Aero drag and what it will do to you. Aerodynamics are a much vaunted, but little understood topic in cycling - this primer will give you the inside story and help you realise why you should understand it.

The total power (or force) required to move a bicycle forward must overcome 5 sets of resistive forces, which are:

1. Aerodynamic power to push you and your bike through air (1/2rC_dAV_a²V_g) (~85%).

2. Rolling resistance power (C_RRW_TV_g) (5-15%)

3. Power to rotate wheels (F_wV_g³) (~1%)

4. Power to overcome gravity on a hill (W_TV_gSin(Arctan(Road Grade)) (varies greatly)

5. Friction losses in the drive and bearings (small except for chain line cross over) (1-2%)

Note that on the flat most power (~85%) is used to overcome aero drag forces, with about 5-15% of total drag coming from rolling resistance of the wheels. The other forces involved, power to rotate the wheels, and friction losses in the drive train are relatively minor.

All this changes when you get to a hill, however. As you ride up a steeper and steeper hill, overcoming gravity becomes the number one force you need to deal with. This is the reason most good climbers look like underfed chickens - their lower total mass allows them to ride up with far lower power requirements than you or I.

C_RR is the coefficient of rolling resistance (a dimensionless number like 0.004)
W_T is total weight of bike and rider (Newtons ie Weight in Kg * 9.8)
V_g is ground velocity (m/sec)
V_a is air speed (m/sec)

F_W is factor related to the power to rotate the wheels
C_dA is the co-efficient of aero drag and frontal area of the cyclist (a number like 0.235)
r is the air density (lower altitude and temperature = increased air density, sea level 24C = 1.232)

By putting all the factors together we can derive the formula for calculating power:

Power = 1/2rC_dAV_a²V_g + C_RRW_TV_g + F_wV_g³ + W_TV_gSin(Arctan(Road Grade)

If the power being produced by the cyclist is larger than the sum of all the forces, then the cyclist accelerates, conversely if the power being produced is less than the sum of the forces, the cyclist decelerates.

Does this equation actually stack up? The answer is yes! It is possible using web sites like analyticcycling.com to actually calculate speed with a given power input or power given a speed input.

Now we come to one of the most often overlooked factors in aerodynamics - the aero drag of the bluff body on top of the bike (ie you) which accounts for about 80% of all aero drag, while the bike itself only accounts for about 20%. So it makes significant sense to get the aerodynamics of your basic body position correct. TCR can assist in getting your base position right - you need to consider the following important aspects:

Horizontal Torso: By getting your back parallel to the ground, you markedly decrease frontal area and the co-efficient of aero drag. In general, the flatter your back, the further forward, and the higher your seat must go to achieve the same torso-leg angle (roughly 90 degrees).

Narrowly spaced elbow pads. A narrower aerobar pad spacing is more aero, but can constrict breathing. You need to find your happy medium.

Knee tracking: It is essential for good aerodynamics to keep your knees in and close to the top tube.

Table 1: Predicted 40k time, flat course, calm conditions, 3 body/drag positions, for various types of cyclists. Also, time saved by good and excellent drag positions compared to typical drag position.