Compared to constant force springs, variable force springs have a pulling force that changes with the spring's extension. Typically, the initial force is lower and increases as the extension becomes greater. These springs are commonly used in store display racks. At the beginning, the spring is fully extended (maximum tension and rack fully stocked) and fixed to the rack. When the front row of products is taken, the second row is pushed forward into the original front position as the spring retracts. When only a few items remain on the rack, the spring no longer needs to exert a strong pulling force to push the items forward. Therefore, the spring is designed with a variable force characteristic.