Take a look at a World Tour race. Look around at any mass participation cycling event. Even take a look at the bikes in the weekend bunch. You will notice a recent trend that has seen more and more riders using deep section rims. These technologically advanced components are taking the cycling world by storm. Riders cite their aerodynamic benefits as a key reason for choosing deep section rims, and in the case of carbon the stiffness that they can add. Of course there is also the fact that they can make any bike look like a speed machine. What I have wondered is why are deep section rims better? How do they actually affect the aerodynamics of a bike when really only the tire is exposed to the frontal wind resistance?
Wind resistance is one of the key barriers to speed in cycling. Professionals and amateurs alike go to huge lengths to overcome wind resistance. This resistance has a number of factors involved including frontal area and turbulence. Most of the efforts are focussed on reducing or smoothing frontal area. Wheels however present a small frontal area, their limitations to aerodynamics center around the tendency of wheels to create turbulence.
Before considering why deep section rims it pays to think about the mechanics of a bike wheel while it is in motion. The wheels contact the ground and maintain circular motion. They translate circular motion into forward motion. Looking at the cyclist-bike-road system a wheel has motion in a number of ways depending on your point of reference. For instance, relative to the ground the portion of the tire contacting the road is stationary, the wheel skewer has a velocity equal to that of the rider and the top of the wheel has a velocity twice that of the rider as it rotates forward. This is essentially the way the wind ”sees” the bike wheel.
As I mentioned earlier turbulence also affects wind resistance. A cyclist with low drag will have a laminar, smooth flow of air over their surfaces. Uneven objects and movement can upset this laminar flow and generate turbulent air. This air can influence laminar flow and essentially increase the resistance of the rider. This is where wheels come in. As the wheel rotates and the upper portion of the wheel is moving at twice the speed of the rider. This forward movement includes not only the rim and tire but also the spokes. As these move through the air they create turbulence. The rules of wind resistance and flow dynamics determine that as speed increases resistance increases also, but at an exponential rate.
Consider also that speed = distance/time, that is as distance increases so does speed. As the distance of the spoke from the center of the hub increases so too does its speed. As the speed the spoke moves through the air increase so too does the amount of wind resistance it creates. The physical manifestation of this is that it requires far less power or effort to go from 20km/h to 30km/h than it does to increase from 40km/h to 50km/h. This is thanks to the exponential relationship of resistance to speed.
So, put simply, long spokes equal more turbulence, equals more resistance, equals more power required to attain a given speed. An elegant solution to this is to simply decrease the length of the spokes by increasing the depth of the rim. Rims have a smooth surface and allow laminar flow to occur over them. The exponential relationship here is advantage as only a small increase in rim depth can provide a substantial reduction in turbulence.
Limitations? Increasing rim depth also increases rim mass. From physics we can recall that force = mass x acceleration. Acceleration is a product of change in distance over change in time. More force is required to rotate something with a greater mass. After all, light wheels are seen as super fast too. Advanced composites and alloys have allowed rims to become deeper with little weight penalty. Take the Fulcrum Racing Speed for example;
Thanks to advances in carbon fibre deep wheels are now available as clinchers, increasing their versatility.
In cycling wind rarely just affects a cyclist from a direct frontal angle, we have to deal with crosswinds. This can be where deeper section rims cause issues. They increase cross sectional area from a side on point of view and can effectively act like sails, increasing drag and allowing winds to push a rider around. This is one of the reasons why low profile rims have not been superseded. It is also one of the reasons that we do not all simply ride around with full disc wheels.
So it is shorter spokes, rather than the deeper rims that give the benefits. The deeper section of the rims allows the laminar flow that spokes cannot generate. The changes over the last decade has been huge, with continuing advances in composites who knows where things may go. For now, I’m pretty happy with my 35mm deep Fulcrum Racing Quattro’s. They are a good compromise for everyday cyclists, especially when you add a 3rd and 4th factor to aerodynamics and stiffness; those of reliability and price! Still one day I will have to try it for myself and see if I can notice the difference between 35mm alloy rims and 80 carbon rims (Fulcrum Racing Speed XLR).