A golf ball and a bicycle have a lot more in common than you think. The same dimples that cover the surface of that little white ball now provide a new and exciting level of aerodynamics for wheels.

Since the 1984 Olympics when track cycling showcased aerodynamic bicycle components, subsequent designs have pushed the envelope. Cyclists constantly test the boundaries of maximum speed while trying to minimize the impact of airflow and wind.

It's no secret that the knowledge to produce fast wheels comes from countless hours spent in the confines of wind tunnels. This is where bicycle teams and manufacturers test and study the effects of wind resistance on riders and their equipment. One of the most fascinating developments in recent times has been a patented design from Zipp, a well-known bicycle component manufacturer. This company has successfully produced a doughnut-like, bulged and dimpled wheelset that radically reduces drag.

To grasp the concept of this unique design, it’s important to understand the factors that affect wheel aerodynamics:

Wind Speed: This is a combination of the current weather conditions and the speed of the cyclist. The stronger the wind speed, the more energy it takes to pedal.

Wind Angle (sometimes referred to as Yaw or Apparent Wind): The relationship between the bicycle speed and direction, and meteorological wind speed and direction. Generally, the faster the cyclist is moving, the lower the wind angle. Most aerodynamic wheels are tested and optimized with a zero to 20-degree wind angle in mind.

Air Pressure Drag: Relates to the force of air separating from the surface of an object. Smooth air flow (referred to as a laminar boundary) moves in a streamlined fashion while chaotic air flow (referred to as a turbulent boundary) is more energetic, often described to look like tiny eddies. The crossover between laminar and turbulent occurs with changes or imperfections in the surface. The edge between the tire and the rim is good example.

Surface Friction (also known as skin friction or direct friction): When wind contacts the outer surface of the bicycle and rider. Compared to air pressure drag, surface friction is much less of an issue.

At first glance, Zipp’s leading-edge technology is somewhat counter-intuitive to standard aerodynamic designs. The mainstream of ‘fast’ wheels are built with rims and spokes that have distinct characteristics. A deep rim (measured from the outer to inner edge of the rim surface) with a tapered profile (no hard edges) undoubtedly improves efficiency. In addition, bladed or ovalized spokes have also been proven to reduce drag and increase speed.

Building on the current trends, the folks at Zipp have taken things to the next level by discovering a way to make air ‘stick’ to the wheel surface. Using their knowledge of wind conditions and angles in addition to drag and surface friction, their wheel designs have most certainly impacted the future of cycling aerodynamics.

One key aspect of their innovative design is the toroidal rim. With an elliptical profile, the shape of the surface is bulged rather than tapered. Through their research, Zipp discovered that traditional styles experience a fair amount of drag as airflow transitions between the tire and the rim. Consequently, they found that a toroidal form provides a more flawless transition.

The other critical element in the equation is the dimpled surface of the rim. The pattern is very similar to that of a golf ball. Although this type of textured exterior inherently creates more surface friction, its impact is minor compared to the overall reduction of drag. The dimples act as a catalyst in energizing airflow around the wheel. Specifically, air hits the small indents on the rim surface that cause an early transition from a laminar boundary to a turbulent boundary. Even though this forces the airflow to become chaotic, it is also causes the air to stay attached to the rim surface.

In essence, Zipp has found a way to create ‘sticky’ air. The combination of a toroidal rim with a dimpled surface prevents airflow from becoming detached from the wheel. For cyclists, this means that they will experience a significant reduction in drag over a wider range of wind angles. Beyond wheels, Zipp is now testing this new technology on other components like hubs.

As the wheels of a bicycle continue to turn, so too will the life-cycles of aerodynamic technology. Who knows what the future holds …

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