For the 2004 model year, the F-150 was redesigned on an all-new platform. The new platform starts off with a fully boxed in frame and introduced rear shocks to mount outside of the frame for decreased wheel hop and improved ride quality. This new body style kept the fully independent front suspension introduced in the last generation but added vacuum driven front wheel hubs for the 4 wheel drive versions. The previous generation had full time connected front axles. The improvement saves fuel and by default goes into a wheel locked position. Should a failure occur in the vacuum solenoid, system, or hoses the wheel hub defaults to a 4wd position to keep from leaving a driver stranded. Internally, a 3-valve version of the 5.4 Liter V8 was introduced and replaced the previous 2-valve version. Externally, the eleventh-generation model was different from its predecessor, with sharper-edged styling; a major change was the adoption of the stepped driver's window from the Super Duty trucks. Regardless of cab type, all F-150s were given four doors, with the rear doors on the regular cab providing access to behind-the-seat storage. Ford also introduced additional variants of the F-150. The FX4 Off-Road package available since the 2002 model year became its own trim level. A sportier version of the F-150 became available as STX, replaced by FX2 Sport in 2007.