The Dimple Effect

Zipp's rim dimpling

The tiny detailing that can be seen on a couple of bikes and accessories has so much more science in it than you could possibly imagine, I'm getting excited at the prospect of enlightening you.

Lets start of with why they are there. These dimples are put specifically there to make the surface less aerodynamic. Confused already? Well by making it less aerodynamic, they make it more aerodynamic. I'm making this worse aren't I? Righto, there are two types of drag, pressure drag, and skin friction. Pressure drag accounts for most of the resistance while you are cycling, whereas the skin friction, which is just the friction generated as air passes over the surface, is only a minimal force, so aerodynamicists don't mind sacrificing a little of skin friction to improve the overall performance of the product in the pressure drag sector.

Now to look to a sport that is far less interesting than our own for a few more cues on why the dimples are there. Golf seems like a good place to start. The dimples on golf balls are using a similar principle. And when you have ever tried to explain why they are there, you give a vague description saying that they are there to make the ball fly straight. But it is so much more than that. The dimples are designed to generate a small amount of turbulence which in turn creates a 'boundary layer' which allows the next layer of air to pass smoother around the body, and ultimately stick to the surface for longer. This 'sticking' allows much cleaner airflow and reduces pressure drag considerably and reduces the pressure tail behind the trailing edge. This attribute also makes wheels with this surface treatment to handle better in cross winds because this boundary layer is smoothing the airflow, reducing the pressure drag and from greater yaw angles minimizes the effect of crosswinds on the wheel. All in all, extremely clever.

As well as seeing this in Zipp's line of wheels, Ridley has started employing this on their new road bike the 'FAST' by using a paint job that does this. This utilises stratigically placed strips of 'rough surfaces' that generate this turbulent airflow and creates a boundary layer which makes the air stick to the airfoil for longer and makes the air rejoin clearly which is a huge cause of turbulent airflow and hence pressure drag. These strips are placed where the air starts to detach from the frame, making it the ideal  airflow over the foil.
Also we are starting to see skinsuits with a certain dimpling on their surface. And considering that the body on a bike is two thirds of the drag produced, this dimpling is set to make a huge difference as it is making a difference where it counts.

So now you see the monumental effect of the humble dimple.