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Secondary Forces

The balance shafts counteract secondary forces. Secondary forces are caused by reciprocating mass (the parts that go up and down) — they have nothing to do with the rotating mass (the parts that go round and round). The parts that cause the secondary forces are the pistons and the top part of the connecting rod.

The pistons have a faster average speed in the top half of the stroke and slower speed in the bottom half of the stroke. This is due to the connecting rod and how it rolls around with the crank. As the crank moves, the connecting rods move laterally. This outward movement adds to the vertical movement, causing the speed of the piston to accelerate by pulling it down the bore more than the crank movement alone. As the crank moves towards the bottom of the stroke, the connecting rod returns to the middle horizontally. This motion subtracts from the downward motion of the piston.

In the standard configuration of a four cylinder internal combustion engine, while the two pistons in the middle are travelling up, the outer two pistons are moving down, and the opposite is true when they complete the other half of their rotation. This means two pistons travelling in the top half of the stroke are moving faster than the two pistons in the bottom half of their stroke. This imbalance is what causes the secondary forces.

The frequency of the vibration is twice the engine RPM, while the strength of the vibration is determined by the rod to stroke ratio and the mass of the reciprocating parts. The heavier the pistons and connecting rods, the greater the amplitude. The larger the stroke is compared to the bore, the larger the connecting rod angle becomes. The greater the angle, the more piston speed is affected.

The reason that motors such as the Honda H22 and the Mitsubishi 4G63 have balance shafts is because of their displacement size and long stroke compared to small piston bore. Smaller motors (such as the B16A and B18C5 don’t have these balance shafts because of their smaller size and lower weight. V-shaped motors reduce the effect of this force in a tradeoff for other side-effects while flat boxer engines such as the type found in Subaru’s or Porsche’s are naturally balanced and do not suffer from this force or the complications in trying to negate them.

The Balance Shafts

There is a balance shaft on each side of the crank. They are weighted on one side, rotate in the opposite direction, and twice the speed of the crank. When both weights are pointing downwards, the upward force of the pistons and rods are counteracted. Balance shafts do not stop the secondary forces from acting on the crank and the bearings, and only act to brace against it so that passengers in a car do not feel the effects of this force. The secondary forces are a low frequency shaking force. The force put on the bearings during each combustion event is many times greater than the second order forces.