A2 Wind Tunnel Report
Clearly the primary consideration when investing in "aero" wheels is aerodynamic quality. Serious wheel manufactures will provide wind tunnel test data for review by customers. This data is routinely provided as "grams of drag at 30mph" through a range of yaw angles spanning +/-20 degrees.
To help in our discussion of yaw, let's define two kinds of wind. There is the wind that blows due to weather, perhaps a south westerly breeze blowing at 10 kph, from now on we'll call it the meteorological wind. And there is the "wind" that we perceive due to cycling at speed, even on a flat-calm day with zero meteorological wind you experience some of this "wind" in your face. Of course this isn't wind in the true sense, rather a cyclist moving through a (still) body of air experiences a sensation the same as if the rider was static and the wind was blowing at the speed of the rider. Many people would refer to this wind as wind resistance and so we'll call it resistance wind.
Given that most of us ride significantly faster than the meteorological wind is blowing, most of the time, the resistance wind tends to dominate. For example, if we ride at 40kph with a 10kph full-on sidewind (meteorological wind approaching at 90 degrees to our ride direction) the effective wind has a yaw angle of just 14 degrees. In fact modelling suggests that somewhere between 50 and 70 percent (let's say 2/3rds) of effective wind yaw angles experienced by a rider are lower than 10 degrees, the faster your ride, the higher this percentage. The same research suggests that a further 30 percent (let's say 1/3rd) of effective wind yaw angles are between 10 and 20 degrees.
The Relevance of Yaw
So why does yaw matter? Wheels present different drag profiles depending on the yaw angle of the wind. Wheel manufacturers are now particularly keen to demonstrate the advantages of using their equipment in terms of drag reductions at specific, meaningful yaw angles, especially the 0-20 degree kinds that most of us experience, most of the time.
On 6/11/2014 Williams submitted new tech System 58 and 85 carbon clinchers for wind tunnel testing at A2 Wind Tunnel. Along with the Williams wheels, brand new ZIPP 404 and 808 firecrest wheels were submitted for testing. The idea was to determine how Williams new tech wheels would perform vs their intended competion.
Wind tunnel protocal was strictly monitored. The same exact tube and new 700x23 Continental GP 400S tire was used on each wheel for each test. All testing conditions were identical. Below are two charts. Chart one is WIlliams 58 vs. Zipp 404 firecrest wheel. Chart two is Williams 85 vs. Zipp 808 firecrest wheel. Note that both the Williams 58 and 85 wheels were faster from 0 - 12.5 degree yaw angle in the tunnel. This graph is a significant win for Williams as our wheelset price point is $1,199 vs. $2,700 for Zipp Firecrest. We believe this proves that extremely fast wheels can be manufactured and sold at a price point many athletes can afford.