The bigger the spring, the stiffer it is and the later in the RPM range the clutch will engage. The smaller the spring, the softer it is and the earlier in the RPM range the clutch will engage. Optional springs for the Kyosho clutch. Harder springs will cause the clutch shoes to engage at a higher RPM. If you find you need a bit more punch on the bottom RPM of your car this will help. You can tune the Kyosho clutch using soft springs (0.9) medium springs (0.95) or these hard springs (1.1). You can also use combination's of the springs to give you a different engagement feel or a more progressive engagement. Add on top of this the ability to mix aluminum and carbon shes and you have a very flexible clutch system.