Swift Springs Standard Coil Over 2.5" OD 8" Barrel Type. Unlike others, Swift designed barrel springs with ease and consistency of vehicle setup in mind, our barrel spring design has a maximum of 4″ outer diameter (except B5 and B4 series) (Usually located on the center coil of the spring). Because most other manufacturers do not design their barrel springs with this rule, most of their barrel springs outside diameter will differ from one rate to another. This creates a hard time for racers especially with tight clearances to test and see which rates will fit and which rate does not. A heavier rate spring can interfere with the suspension A-arm or other suspension components.