Inline-fours larger than 2.0 L usually have balance shafts whilst engines over 3.0 L are seldom used in passenger cars. In contrast, the flat-four has much less secondary imbalance at the expense of larger rocking vibrations, that cause it to rotate back and forth around a vertical axis. This is because the cylinders cannot be directly opposed, but must be offset so the connecting rods can be on separate crank pins, which results in the forces being slightly off-centre.