WHAT IS STALL ANGLE?
Deep section wheels are fast because they streamline the airflow as it passes over the wheel surface. The smoother or more laminar the airstream as the flow leaves the wheel, the faster it goes. If the wind angle, or yaw, is too much (the wind is coming more from the side instead of head-on) the wheel stalls. As a wheel stalls airflow no longer comes off the wheel smoothly, instead it leaves the surface and becomes turbulent. As the yaw gets steeper, airflow is turbulent at the wheel surface as too, and even more turbulent as it leaves the wheel.
When a wheel stalls out, drag increases, because the wind no longer “sees” an aero shape that it can flow over smoothly; instead the wheel looks more like a flat surface. Instead of flowing over, the wind pushes on the rim. Turbulence as airflow leaves the wheel creates constantly changing pressure gradient on the downwind side and starts to buffet the wheel. A wheel that is hard to handle is a wheel that has stalled. The deeper the rim, the higher the drag when it stalls. The greater the stall angle, the more crosswind a wheel can tolerate before it stalls out, slows down, and gets hard to steer.
