Etan Premium Quality Trampoline spring 180 mm set of 10. This Etan Premium spring set of 10 pieces is suitable for trampolines with a spring length of 18 cm. The Etan Premium springs are made of the best spring steel. So that your trampoline jumps smoothly for years to come. When determining the correct spring length for your trampoline, make sure that you measure a trampoline spring that is not stretched. Remove it from the frame and then measure from end hook to end hook.