The force exerted by the thrust of the connecting rod on the crank and transmitted to the rail with each revolution of the driving wheel. Rotating masses must be balanced but since this is only the wheels and the connecting rods, this is reasonably easily done by balance weights. Reciprocating masses such as pistons, piston rods, etc are much more difficult to balance. They are balanced at the wheel centres and on the crank axle itself. In fact the design of the crank axle may be inherently self-balancing to some extent. It is not desirable to balance 100% of reciprocating mass because this would result in the load on wheels dynamically changing during rotation (and this is exactly what hammer blow is). It was common practice to balance 60% of reciprocating mass but this was found to cause hammer blow, so was reduced to 30% on two cylinder engines. The balance is spread unequally over all coupled wheels. Four-cylinder engines, because of their cycle, are self-balancing, so balance weights are not used. Once again, adding weights to them actually causes hammer blow. Engines must not be operated under power without connecting rods as the unbalanced forces can actually destroy the track. See Wheel Balancing.