Captain Hat, on 31 January 2012 - 06:24 AM, said:
You now have well over half of a 100-ton 'mech rotating about a much narrower joint, meaning much higher pressure on the rotating ring, much higher torsion on it when the torso turns and a much shorter moment arm for the actuators to get leverage on. Even with astonishing advances in technology, you've gone from spinning just the turret to spinning the entire weight of the tank around a much smaller ring with much higher lateral stresses imposed as the vehicle moves.
Keep in mind that a lot of mechs don't have a "turret ring" equivalent. They are built more along human lines with joints and myomer to "twist" the torso rather than just having it rotate on a track or ring. Not that that changes things drastically, it still takes time to turn better than half a mechs weight.