The aerodynamic advantage comes mainly from the rim height, the profile and the width of the rims and the number, length and shape of the spokes. Since the spokes cause a large air turbulence, it is advantageous if as few or short spokes rotate on the impeller. The solution is high profile rims.
A high-profile rim solves two problems:
Firstly, the high rim reduces air resistance where it is greatest - at the edge of the impeller.
And second, thanks to the high profile, you can significantly reduce the number of spokes without softening the wheel. The higher the rim, the fewer spokes are needed. With us the front wheels with 80mm and 90mm rim height optionally have on request 16 instead of 20 spokes.
Unfortunately, so does the crosswind or gust vulnerability. From 60mm rim height with side winds of 30km / h and / or gusts it is difficult to keep the wheel with V-profile rim straight.
The reason is not only the pressure of the wind on the entire bike with the driver but the pressure difference at the front wheel between the front part and the rear part . Front is the tire that divides the air. If the wind comes from the side, the air molecules on the leeward side (lee) have to flow faster than those on the wind side (windward). It creates a (horizontal) buoyancy or suction on the leeward side.
On the rear half of the wheel it is similar, only that the air molecules can not follow the shape of the pointed front edge of a V-profile rim and 'tear off'. It creates a vortex. This does not create a pull to the side. The front wheel is thereby rotated or steered in the windward side.
Thus, the wind pushes the entire driver with the wheel to the side and in addition steers the front wheel in the same direction . This makes the handling more prone to bumps and the heavier the higher the front wheel rim is. The wheel feels wind-prone and the longer ride is uncomfortable or annoying .
The solution is to prevent the stall on the rear half of the front wheel. This is achieved with a possibly round front edge of the rim and a strömungsangepassem profile. This is called U-shape or U-profile rim .
Due to the new wide rims (distance between the braking surfaces) in connection with similarly wide tires very aerodynamic profiles are offered. The production of a carbon U-profile rim is much more complex than that of a carbon V-profile rim. The manufacturing processes have improved enormously in recent years, so that the production costs could be significantly reduced.
The U-profile only makes sense from 45mm upwards, since below the crosswind has little attack surface and the profile already has a U-shape.