The front suspension used a double wishbone while the rear is a solid rear axle type. The 4runner continued to use a body on frame construction design and a solid rear axle for strength and durability compromising interior room and on-road handling. Toyota's other mid-size SUV, the Highlander is a crossover which is not designed for off-roading. The optional 4WD systems were full-time on V8 models while "Multi-Mode" or part-time on V6 models, both systems used a lockable Torsen center differential. A new suspension system, X-Relative Absorber System (X-REAS), became standard on the Sport Edition and optional for SR5 and Limited models, a rear auto-leveling height adjustable air suspension is sometimes included with this option on Limited models. The X-REAS system links the dampers diagonally by means of hydraulic hoses and fluid using a mechanical center valve which reduces body roll during hard cornering. All 4runners were equipped with skid plates for the engine, transfer case, and fuel tank to prevent damage during off-roading. The Hill-Start Assist Control (HAC) system prevents the 4runner from rolling backwards on inclines and a Downhill Assist Control (DAC, 4WD only) modulates the brakes and throttle automatically without driver inputs for smooth hill descents at very low speeds, both electronic aids are standard on 4WD models.