This may be a progressive spring... but not the only type that is. I do not subscribe to the theory that the wire diameter has to change to be progressive. By the admission of the two Mech. engineers on this thread alone (and my personal experience and knowledge) if you add to the coils in a section of similar wire diameter spring the rate in that section goes down... so it is safe to say in my estimation that any spring wound with one or both ends wound with more coils will be softer in those sections. It can make them either progressive or staged depending on how the coils are arranged. I agree that 99% of all of the springs I have seen for RC cars are straight rate FWIW. I have seen progressive springs... but I don't know if they would help or hurt suspension tuning (usually I have only seen them on Traxxas type of vehicles).