Originally Posted by mihlbach

Thats a load of bull. The 40mm threshold is a myth perpetuated by people who just repeat what they hear. Of course rims over 40 mm are more aero than those less than 40mm, but the aerodynamics gains of rim depth decrease with increasing depth. In other words, you derive more aerodynamic benefit going from 20-30mm than you would from 30-40mm. Keep in mind that deeper rims are also heavier. A 30 mm Kinlin rim is barely heavier than an OP, but 40+ mm deep rims are much heavier. Everything is a compromise, and 27-30 mm is the sweet spot for a general purpose aluminum clincher road rim.