DEEP RIMS DISSECTED

Drag, turbulence, yaw angles… we highlight how and why deep-section wheels are faster than shallow rims...

DEEP-SECTION 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

in our results graph on p83 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 SUCH

Wide wheels such as those in this test 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.