Fixing the sun gear, and using the planet carrier and ring as input and output shafts gives another overall gear ratio: {(N_S / N_R) + 1}.

A final ratio may be obtained by fixing the ring and allowing the sun gear and the planet carrier to rotate. For this configuration the overall gear ratio is: {(N_R / N_S) + 1}.

The number of teeth on the gears is constrained by the meshing requirement so that:

N_R = (N_S + 2N_P ). Also: {(N_R + N_S) / # of Planets} = an Integer

In an actual epicyclic box, dog clutches select the gears by fixing various gearbox components. This system is compact, easily provides three or four speeds, and is enclosed so that dirt is not an issue in its performance. A diagram on the next page shows a Sturmey Archer three-speed hub with the middle gear engaged (i.e. the ring gear is locked to the input shaft.).