Manufactured by a high-tech company involved in rubber components on Formula 1 cars. They have a special shape and material, and include a formulation which helps stabilize their intended performance throughout a race. Depending on number of races completed, they will require annual inspection and evaluation. The rubber formulation will break down over time, even if not in use. Sold each.
This rubber spring is slightly taller than the original one and will require the car to be fitted with an aftermarket adjustable strut set. On original Hi-Lo brand adjustable struts, the threaded center bolt must be shortened by 3/4" to prevent bottoming on cars with 10" wheels. On extremely lowered cars, it is possible the Hi-Lo would require machining to obtain sufficient clearance.
See C-STR0687 for a softer road-going compound rubber springs.