i hav a confusion prevailing in the design of flywheels...

(for understanding ref, divide the wheel into 4 equal quarters...)

i hav seen some flywheels in forging machines tht hav some holes on it radially n equally placed in all the four quarters of the wheel...

in some other flywheels i hav seen tht the mass is not equally distributed radially... tht is holes hav been drilled in three of the quarters and the remaining one quarter is left undrilled.. or else, the flywheel is setr hav a greater mass on one or two of its adjacent quarters... why is this being done?...