Aerodynamics and wheel design

Aerodynamics refers to the interaction between airflow and a moving object.  At Williams Cycling, our goal is to develop wheels that are designed to minimize drag and create wheel stability regardless of riding conditions.   We use tools such as CFD and real world wind tunnel testing to achieve our design goals.  

A2 Wind Tunnel

The information below highlights terms regarding wheel aerodynamics and rim design.

Yaw Angle
The angle at which airflow (wind) interacts with a wheel.  For instance, a direct headwind is considered a 0 degree yaw angle.  Wind blowing at a 20 degree angle would be considered a crosswind. 

Types of Wind
Meteorological wind = wind that blows due to weather.

·    Resistance wind (thrust) = induced by the cyclist by moving through air.  For instance, assume 0 meteorological wind and a cyclist riding at 20 mph, the cyclist creates their own 20 mph headwind by moving through a still body of air. 

·    Effective wind = the combination of meteorological and resistance air.  These two winds combine to create one wind on the rider. 

Resistance wind dominates meteorological wind.  On average, athletes ride significantly faster than the meteorological wind is blowing.  Research modeling suggests that approximately 66% of wind yaw angles experienced by riders are lower than 10 degrees. 30% of wind yaw angles are between 10 to 20 degrees.  The vast majority of your riding will take place between 0 to 20 degrees yaw angle. 

Sample effective yaw angle calculation:  Bike speed = 25 mph, wind speed = 6 mph, wind yaw angle = 15 degrees.  Effective yaw angle = 3 degrees.

NOTE.  Even though wind speed is 6 mph and wind angle (yaw) is 15 degrees, bike speed at a 25 mph headwind dominates effective wind yaw angle calculation.   The cyclist will feel an effective wind yaw angle of 3 degrees.

The relative opposing force imparted on an object as it moves against still air.  It is the force exerted by airflow that resists the forward motion of an object.  Example, the faster you ride, the more opposing force you feel while moving through air.  The below pictures shows how air flows around a wheel at different yaw angles.

Laminar Flow
Laminar flow is the air that moves across a wheel with no disruption or turbulence. 

Stalled Air Flow
Air that separates from the wheel and leaves pockets of spiraling air in its wake. This leads to air flowing in reverse direction which creates high drag and impedes movement.

Leading Edge
The leading edge of a wheel is the first object that meets airflow.  Assuming a direct headwind, or 0 degree yaw angle, the tire would be the leading edge.   Airflow hits the tire and evenly flows around the rim.  The rear fairing at the back of the wheel is hidden from the wind.  When wind yaw angle increases above 0 degree, the back fairing is exposed to airflow and is considered the second leading edge. 


Side Force
Wind flowing past the surface of a rim exerts a force on it (total force). Lift (X force) is the component of this force that is perpendicular to the oncoming wind flow direction. It contrasts with the drag force (Y force), which is the component of the surface force parallel to the flow direction. If the fluid is air, the force is called an aerodynamic force.  In other words, X force opposes Y force at a perpendicular angle. 

Total wind drag is largely dominated by side force. 

Rim Depth

Rim depth plays a major role regarding aerodynamic performance.  A deep section rim profile increases rim surface area.  
As wind yaw angle increases from 0 to 15 degrees, airflow attaches to the wheel and wind drag lowers to optimal performance.  As the yaw angle increases past 15 degrees, airflow separates from the wheel, wind drag increases and creates  stall.  (Graphs below)

Graph below:   Note that optimal drag reduction occurs at 15 degree yaw angle. 
In other words, The Williams System 58 wheel creates the least amount of wind drag at 15 degree yaw angle. 

Graph below:   Note that optimal drag reduction occurs at 15 degree yaw angle.  In other words, The Williams System 85 wheel creates the least amount of wind drag at 15 degree yaw angle. Note that an 85mm deep section rim creates less drag at 15 degree yaw angle than a 58mm deep section rim.

Rim Shape (Airfoil)
A deep section rim profile increases rim surface area.  The larger the rim surface area, the more potential wind side force. 
As the depth of the rim increases, the extent of the total drag minimum is also seen to increase. The goal of any rim designer is to create an airfoil rim that maximizes side force and minimizes drag.   

Wheel Stability
Aerodynamic performance is only one part of rim design.   Wheel stability is critical for steering control.   To create a deep section aerodynamic wheel that is stable in cross winds, we must equalize wind force on the front (wind side) of the rim with the back (non-wind side) of the rim. 


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