Older motorbikes use a pair of coilovers, one on each side of the rear swingarm, but most modern bikes use a monoshock with the single main coil spring wrapped around the outside of the damper. The spring is often sat on hardware like a seating ring or washer to keep it concentric around the shock absorber and unable to scrape under compression, plus spreading force evenly on the preload nuts, making tuning easier.
We offer multiple brands of shock springs in many different sizes and in many different spring rates - including linear (straight) rates as well as progressive rates which start softer and get firmer during the suspension travel. Shock springs are measured using their inner diameter (in millimetres), as they need to be big enough for space around the damper, and their total height with no pre-load applied (also in millimetres) - learn more about springs & preload.