The length springs needed depends on several things, including the frame dimensions, type of solo seat, and style of seat spring mounts being used. Ideally the seat is mocked up and a measurement can be taken to determine what length seat springs are needed. You do not want to slide back while riding, so the solo seat should be mounted so that it is parallel to the ground. Most solo seats have a bit of a kick up in the rear to help cradle the rider and keep them from sliding back. All of the solo seat springs we carry are heavy duty and up for the task at hand!