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.
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 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.
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.
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. In theory, when wind yaw angle increases, lift force becomes greater than drag force. The more wind angle, the more lift force occurs until critical angle of attack is reached (maximum lift). When wind angle goes past critical angle of attack, the wheel stalls and no longer benefits from side force lift. This is when you feel the bicycle push you around on the road. Typically, this happens when there is swirling wind and a wind gust blows into the cyclist at a sharp angle (sideways). This can be a very uncomfortable feeling for the cyclist.
Rim Shape (Airfoil)
A deep section rim profile increases rim surface area. The larger the rim surface area, the more wind side force. The goal of any rim designer is to create an airfoil rim that maximizes side force and minimizes drag.
Wheel Stability and Aerodynamic Drag
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.
Torus vs. V Shape Rim
A torus is a donut shape rim. A V-Shaped rim is a V-shaped tori.
The torus shaped rim is a low drag shape with regard to a bicycle wheel. This is because the leading edge of a rim must also function as the trailing edge and vice versa. Low drag in both directions parallel to motion equals good crosswind performance.
Ellipse (Torus) shape rims work very well in cross winds because they create similar low drag in both directions parallel to motion. V-shaped rims created low drag in one direction and high drag in the other direction parallel to motion.
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.