The Difference Between Closed & Open Knuckle 4x4 Front Axles IDN-124

You will often see a 4x4 beam-type front axle referred to as a "Closed Knuckle" or "Open Knuckle" axle. What does this mean?

In simple terms, the steering knuckles are the parts that pivot at the ends of the axle beams when you turn your steering wheel. With a closed knuckle axle, the back side of the steering knuckles are sealed to keep lubricant in and water out of the axle u-joints and knuckle bearings/bushings. The axle u-joint totally enclosed by the steering knuckle and knuckle ball welded to the end of the axle tubes. The axle u-joint is not visible unless the axle is disassembled.

With an open knuckle axle, the axle u-joint is exposed and it is readily visible with a mere glance. Open Knuckle Axles can use either ball joints or king pins which have their own seals.

Up until the mid-1960's, almost all production 4x4s through the 1 ton class had closed knuckle front axles. Ford started the trend to transition to open knuckle axles with the 1966 F100 and Bronco. At the start of the 1976 model year, all domestic 4x4s rated for 1 ton or less was built with open knuckle axles. The biggest drawback to closed knuckle axles is maneuverability. Closed knuckle axles can usually only turn 28 to 30 degrees. Open knuckle axles can often handle a 40-42 degree turn.  A vehicle with an open knuckle front axle has a dramatically smaller turning radius compared to a similar vehicle with a closed knuckle axle. Theoretically, closed knuckle axle parts bathed in lubricant should outlast open knuckle parts, but due to leakage and lack of maintenance, closed knuckle axles often have premature part failures. 

Years popular domestic 4x4s transitioned from closed to open knuckle axles:
1966: Ford F100, Bronco
1969: Chevy Blazer
1970: All other Chevy and GMC 4x4 models
1971: IHC Scout
1972: Dodge W100, W200
1972: IHC 1/2 ton and 3/4 ton Truck (most)
1972: Jeep CJ and Commando
1974: Jeep Wagoneer and Truck
1975: Dodge W300
1976: Ford F250