Designed to make coil shock setup leaps and bounds easier, we use the Trail Rear Shock Spring for our Sprindex coil shocks because they let us adjust our spring rate across a 50-70lb range. Competing coil shocks only allow you to adjust spring rate by swapping individual coil springs, so these Sprindex springs both make initial setup much easier, and fine-tuning down the line since you can adjust spring rates by increments of 5lb.