The purpose of a crankshaft in a car is to convert linear motion into rotational motion. A crankshaft comprises of several parts which, working together, cause a car's wheels to move in circular motion. The piston and flywheel are two main parts that cause the crankshaft to rotate in circular motion. The major role of a crankshaft is to transform the linear motion of the pistons into rotational motion that is transmitted to the load through the connecting rods. The crankshaft also uses counterweights during its functioning which balance the weight of the connecting rods and allow for the movement of the crankshaft. Crankshaft Only Will be inspected before shipping Pistons Rocker arms Crankshaft, technically speaking the crankshaft is the part of your engine that changes the up-and-down motion of your pistons into a circular motion that can be used to turn the wheels. It's centrally located inside the block of the engine.