A rotating frequency analysis showed that two samples of C and S-type asteroids belonging to the Main Belt, but not to any families, present two different values for the transition diameter to a Maxwellian distribution of the rotation frequency, respectively 48 and 33 km. We found a better estimate for the transition diameter, respectively DC = 44 ? 2 km and DS = 30 ? 1 km. The ratio between these estimated transition diameters, DC/DS = 1.5 ? 0.1, can be supported with the help of the YORP (Yarkovsky?O?Keefe?Radzievskii?Paddack) effect, although other physical causes cannot be completely ruled out.