Second-order forces acting laterally
During the power stroke the piston is pressed against the cylinder wall due to the angle of the connecting rod relative to the cylinder. At higher engine speeds, however, the inertial force is much greater. It can then be said that the crankshaft pulls the piston down and due to the angle of the connecting rod relative to the cylinder the piston is pressed against the cylinder wall, but this time on the opposite side. The combined gas and inertial forces acting in a sideways direction vary periodically twice per crankshaft revolution and cause the engine to vibrate in a lateral direction.
