The forces acting upon a bicycle and its rider are complex and subject to many external and internal variables. The following diagram provides a simple overview of these forces.
Newton’s first law states that: “Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.” Therefore, a pedalling rider would endlessly accelerate unless there are forces to oppose this. The four types of resistance opposing the progress of a cyclist are:
- Air resistance: Energy lost due to pushing air out of the way.
- Rolling resistance: Energy lost due to tyre deformation as they roll.
- Gravitational resistance: Kinetic energy turned to potential energy as one goes uphill. Potential energy is then transferred back to kinetic energy going downhill, however, much of this is lost due to additional air resistance and braking.
- Mechanical resistance: Energy lost due to friction in the drive train and wheel hubs.
Not all these resistances are of equal value and are dynamic based on speed, terrain, maintenance of bike etc. The good people over at ridefar.info did some clever calculations and estimated the proportions of total resistance overcome by cyclists in the ultra-endurance cycling race, the Transcontinental Race. For their method see: https://ridefar.info/bike/cycling-speed/summary/#Basic_Method.
From this, we can see that even on a hilly course, air resistance is the largest force in inhibiting a cyclist’s progress. As rider speed increases, the proportion of air resistance increases even more. To understand this, we will look at the air resistance (drag) formula:
As we can see, the drag force is proportional to velocity2. Therefore, a doubling of velocity leads to air resistance increasing four-fold.
Steve Gribble, The Computational Cyclist, has a fantastic interactive program that calculates the power required by the cyclist to maintain different speeds in different parameters (https://www.gribble.org/cycling/power_v_speed.html). With no wind, a flat road and all other variables being equal, a cyclist must produce 150W to maintain 30km/hr, 225W to maintain 35km/hr, 321W to maintain 40km/hr, 444W to maintain 45km/hr and 594W to maintain 50km/hr. This is all due to the air resistance drag force.
This leaves us with the obvious goal of reducing drag force to improve bike race performance. A reduction in drag force will allow us to output a lower power for the same speed or go faster for an equal power output. The variables that we can control to do this are CD and A. We will go into CD and A in further detail later.
The goal of Pure Agilis’ data analytics and research team is to enable the dedicated triathlete or cyclist to better understand CDA and provide low cost and accessible options of measuring their own CDA. This will assist in making informed decisions about bike fit and equipment choices to optimise race performance.