Compression springs do a couple of things. First they supply the pressure to engage the belt. They also determine the rate at which the CVT transmission changes from low to high gear. A 1000rpm spring will go into a higher gear than a 2000rpm spring. This NCY 1500 rpm compression spring is specifically for the GY6. Measurements: Wire Diameter: 4.2mm Inner DIameter: 48.5mm Length: 154.0mm Looking for more performance upgrades? This spring is also available as part of our Stage 1 GY6 150 Performance Kit !