As stated earlier, the number of teeth (N) on a spur gear is directly related to its radius, or diameter; therefore, the ratio of the radius (rD) to (r A) in Eq. (10.43) must be the same as the ratio of the number of teeth (ND) on the fixed ring gear (D) to the number of teeth (NA) on the planet gear ( A). Therefore, Eq. (10.43) can be rewritten as