Herriott Sports Performance

Before you set out on the quest for the best bicycle wheels on the planet, get a bike fit. Your bicycle accounts for approximately 25 percent of your vehicle’s (rider/bike) aerodynamic equation. That means 75 percent of the equation is rider position. Aerodynamic tweaks in your position can gain you much more than a different frame or wheel set can provide. Wheels account for 7 to 11 percent of the total aero package, the fork: 6-9 percent, the frame: 4-9 percent, and 2-4 percent falls into the “ other” category. Wheels, then, are still a very important component when it comes to the full aero package.

What’s the actual difference between wheel sets? I’m not going to compare a low-profile 32-hole wheel to a deep-dish carbon wheel. I don’t see anyone racing on 32-spoked wheels anymore. So I’ll compare a Zipp 404 to a Zipp 808: 3.2 watts 50kph, 1.25 percent 250 watts. What that really means is 16.5 seconds faster every 22 minutes. This is aerodynamics only. Weight is a whole other beast to take into account, I’m sticking with aerodynamics. Why is it 16.5 seconds faster every 22 minutes?

Aerodynamic drag consists of primarily three aspects. The first is surface character. Surface character is what the surface consists of…like smooth, or dimples on a golf ball, or hair on a tennis ball. The surface character effects the flow of air. The second aspect is frontal surface area. That one is pretty self-explanatory, smaller frontal areas have less drag. The third aspect is shape. A great example of shape is how a football can be thrown much further and with more control than a volleyball.

Okay, strap down, here we go into the world of aerodynamics. Total drag is the combination of skin friction (“good drag”) and pressure drag (“bad drag.”) Pressure drag is what causes turbulence. Pressure drag is air that spins off an object when air flow hits it. Oval or elliptical objects have less pressure drag than round or blocky objects. Skin friction is a layer of deflected air that hovers right at the surface of an object. This layer creates an isolation layer around an object that can keep pressure drag (“bad drag”) from forming. Zipp has added skin friction to it’s rims in the form of dimples — just like a golf ball — since dimples are more marketable than hair, as in tennis ball hair.

Laminar flow is undisturbed air. Air is laminar before it hits an object and eventually returns to laminar flow after an object passes through it. The quicker that air becomes laminar after going around an object, the less drag it will have. Skin friction drag returns to laminar flow far before pressure drag does. This is why Zipp has added skin friction to the rims. Skin friction is smooth and consistent drag. Pressure drag is rough and chaotic. In the total drag equation, the more skin friction you have means the less pressure drag you have and the smoother air will return to laminar flow quicker…less drag.

So, that explains the surface aerodynamics. But what about shape? Is there a magic shape? Actually, yes. NACA (the aerodynamics research predecessor to NASA) studies showed an aerodynamic aspect ratio of around 3:1 minimized drag. Aspect ratio is length/width comparison. A 14-inch taper leading back from the widest point creates an ideal aspect ratio. Using this data, most race tires are 23mm wide (the width.) Multiply 23mm by 3 (3:1 ideal aspect ratio) and we get 69mm. Subtract 21mm (tire hight) and we get 48mm. Roughly 48mm. is the rim depth (with 14-inch taper) we’re after to achieve the ideal aspect ratio. That probably explains why there are so many wheels with about a 50mm depth. Some manufacturers also figured out that aerodynamics are better with a wider than normal (same width as the tire) braking surface (tire/rim junction.) But is that 50mm depth really the best for a bicycle wheel? No. Wheels have spokes and these are key mitigating factors. The 81mm rim depth of a Zipp 808 does not make it more aero than the 58mm depth of a 404, but the shorter spokes used in the 808 make it more aero than the longer spokes of the 404. The drag is reduced when the length of the spoke is reduced. The number of spokes is important, too: more spokes, more drag. In general, a deep rim can be built stiffer, therefore allowing for fewer/shorter spokes without sacrificing strength/rigidity.

Parts on your bike and your body are related to each other and affect each other. Air flow around one object will effect the air flow around the other. These different layers create what is known as boundary layers. The bike and rider are dynamic objects and there are many exposed and moving parts between the bike and the rider that create turbulence and lead to inconsistent and uncontrolled boundary layers between them. So, what this means is that you may have a very aero wheel set. But maybe your narrow 19mm tire is a mitigating factor that creates a boundary layer of air between the rim and the tire that creates pressure drag (turbulence.) And now your aero wheel set is not as aero as you thought. There are boundary layers of air between the fork leg and wheel, your legs and the seat tube, etc. Mizzuno, and more recent Oval and Ridley, have put slits in the fork legs and seat stays to create a vacuum and direct the boundary layer of air between the spokes and the frame/fork. Boundary layers complicate everything discussed because they take all that nice flowing air, even objects with optimal aspect ratios, and can drive it into each other, causing turbulence. This is why the wind tunnel is so important. It measures your drag. Small things like tire selection and subtle body position changes can make a big difference on total drag. One thing leads to another. So, to get aero, get a bike fit, use you aero knowledge to pick an aero wheel set, then get into the wind tunnel and optimize your position.