Flywheel housing question

[marc silva]

I have often thought about why does the flywheel housing crack. After removing my engine and transmission to install a new engine I noticed my flywheel housing was almost cracked in half at the starter. Then it hit me. How can the engine be bolted solid at the rear, and granted it is in rubber but in my opinion it is basically a solid mount compared to springs in the front. I know the frame flexes but think about it. The motor hangs off the rear mounts causing some stress on the flywheel housing as the front bounces on the springs. Am I incorrect in my thoughts?