Why is the middle of the pulley hollow?

Reduce the weight of the pulley. Traditional pulleys have heavy weight and low air permeability, and hollowed-out pulleys have portability to increase comfort, so hollowing out the middle of pulleys is to reduce the weight of pulleys. The pulley belongs to hub parts, and its relative size is relatively large. General manufacturing techniques are mainly casting and forging. Generally, the smaller one can be designed as forging and made of steel.