The Physics of Cycling Aerodynamics: A Technical Guide

At speeds above 25 km/h, aerodynamic drag becomes the dominant force a cyclist must overcome, accounting for up to 90% of total resistance at 45 km/h. Understanding the physics behind cycling aerodynamics is crucial for anyone looking to improve their performance, whether through position optimisation, equipment selection, or clothing choice.

This guide breaks down the complex physics of cycling aerodynamics into its core components. We'll explore how air flows around a cyclist, why certain positions are more aerodynamic than others, and the fundamental principles that govern speed on a bicycle. While some concepts may seem abstract, each has practical implications for how we can ride faster with the same power output.

Core Equations and Concepts Of Cycling Aerodynamics

The Reynolds Number (Re)

Where:

• ρ (rho) = Air density (kg/m³)
• V = Velocity (m/s)
• L = Length (e.g your body or bike) (m)
• μ (mu) = Dynamic viscosity of air (kg/m·s)

Alternative form: Re = (V × L) ÷ ν Where:

• ν (nu) = Kinematic viscosity (m²/s) = μ/ρ

What Is The Reynolds Number?

General definition: The Reynolds number is a dimensionless quantity that predicts flow patterns in different fluid flow situations. It represents the ratio of inertial forces to viscous forces within a fluid. Re can be seen as a way of understanding the properties of the air and how it behaves for aerodynamic purposes. Air with the same Reynolds Number will behave the same way, even when the input variable may be different.

Application to cycling: In cycling, Re typically ranges from 200,000 to 500,000, indicating turbulent flow. This affects:

• How air separates from the rider
• Where transition points occur
• Equipment design decisions
• The effectiveness of surface treatments

Bluff Body

General definition: A bluff body is an object that, due to its shape, causes significant flow separation when moving through a fluid. The separated flow creates a wide wake behind the object. Basically a bluff body is any 'unaerodynamic' shape. An aerofoil or a bullet would not be a bluff body, a cyclist or a bus would be.

Application to cycling: A cyclist is a classic bluff body, with:

• Approximately 80% of drag from pressure differences
• Large wake formation
• Complex flow separation patterns
• Significant dependence on Reynolds number
• Multiple separation points

CdA (Coefficient of Drag × Frontal Area)

General definition: CdA combines two crucial aerodynamic properties:

1. Cd: How effectively an object moves through air.
2. A: The projected frontal area

Application to cycling:

• Units: m²
• Typical values:
  • Pro TT: 0.17-0.22 m²
  • Road race: 0.24-0.28 m²
  • Casual: 0.30-0.35 m²
• Power required increases linearly with CdA
• Used in the drag equation: F = 0.5 × ρ × v² × CdA

Pressure Drag

General definition: The force resulting from pressure differences between the front and rear of an object moving through fluid. Created by flow separation and wake formation. High pressure is created infront of an opbect as it moves through the air. Low pressure is created behind an object in turbulent flow. This combines to both push and pull an object to resist its forward movement. 

Application to cycling:

• Primary source of aero drag (~80%)
• Affected by:
  • Body position
  • Equipment shapes
  • Clothing fit
  • Wake size

Viscous Shear

General definition: The resistance force created by fluid layers sliding past each other at different velocities, creating friction.

Application to cycling:

• Secondary source of aero drag (~20%)
• Affected by:
  • Surface roughness
  • Material properties
  • Boundary layer state
  • Local flow velocity
• Important for clothing design

Boundary Layer

General definition: The thin layer of fluid immediately adjacent to a surface where velocity changes from zero at the surface to free stream velocity.

Application to cycling: Two states:

1. Laminar:
   • Smooth, parallel flow
   • Lower drag
   • Unstable and sensitive
   • Usually exists at the front of objects
2. Turbulent:
   • Mixing between layers
   • Higher surface drag
   • More resistant to separation
   • Dominates at cycling speeds

Wake

General definition: The region of disturbed flow behind a moving object, characterised by reduced pressure and irregular flow patterns.

Application to cycling:

• Size indicates drag magnitude
• Affected by:
  • Rider position
  • Equipment choice
  • Flow separation points
  • Reynolds number
• Critical for drafting effects

Understanding these aerodynamic concepts helps explain why certain positions, equipment choices, and design features work effectively in cycling. The interaction between these phenomena will determine your overall aerodynamic performance.