A set of deep-section aero wheels will give you a significant advantage over alloy clinchers most are specced with.
These aero wheels are fast in two ways. First, their smooth airfoil shapes pass through the air a lot more cleanly than a box-section rim. The tyre is the leading edge and splits the air. Behind it, a deep rim controls the airflow, helping it to come together again smoothly. A shallow box-section rim can’t do this, so air behind it is turbulent, causing more drag.
Secondly, deep wheels aim to ‘sail’ on crosswinds by making the airflow follow the shape of the wheel as it passes over it at an angle, creating forward thrust. In this way the wheels’ own drag decreases as the wind angle increases, until they reach a ‘stall’ point at which the drag sharply increases again because the airflow detaches causing turbulence and an area of low pressure behind the rim. Wide rims enable much higher stall angles, often beyond what you’re likely to encounter on the road.
The power figures are higher for the greater 12.5º wind angle because the drag of the total bike and rider system is higher, even though the contribution of the wheels is smaller. Overall drag will always be lower at 0º (i.e. head-on or no wind) because the much smaller frontal area of the bike and rider outweighs the drag reduction of airfoil shape wheels and the frame at higher yaw angles.
It is possible for most aero wheels, especially discs, to have negative drag at high yaw angles, actually propelling you forward, but it’s slight so the net result is a reduction in total system drag. Track bikes operate in windless velodromes, so their wheels and frames tend to be skinnier airfoils with minimal frontal area.
Wide wheels knowingly give up some speed at 0º in order to gain lots at higher yaw for the simple reason that you very rarely ride in calm conditions.
As well as the superb 'swooshing' noise a set of deep-section wheels makes, you'll also notice how much easier they make maintaining your momentum. When you're riding on the flat, a quality set of deep-section wheels will make holding a given pace that little bit easier, giving you the option to kick on, or keep something in the tank for the run without compromising bike speed.
Race wheel jargon buster
Clinchers
Wheels that grip the tyre between lips that run along the outside edges of the rim. Will only work with ‘clincher’ tyres and require an inner tube.
Tubulars (AKA ‘TUBS’)
Wheels that don’t have the raised lips to grip the tyre, and rely on the tyre, which is stitched together to enclose an inner tube, being taped or glued to the rim.
Yaw angle
Any angle off 0º of wind direction. For example, a wheel test at 10º of yaw has the wheel turned 10º off full-frontal (0º).