Locking hubs

Locking hubs, also known as free wheeling hubs are an accessory fitted to many four-wheel drive vehicles, allowing the front wheels to be manually disconnected from the front half shafts.

Because many 4WD vehicles, especially heavy duty 4×4 trucks, do not have a center differential or equivalent (e.g., a viscous coupling), they should be used in 4WD mode only when traction is limited: small differences in rotational speed between the front and rear wheels that occur when driving in 4WD on a dry, hard surface (such as pavement) cause a torque to be applied across the transmission, a phenomenon known as transmission wind-up can occur, and can cause considerable damage to the transmission or drive train; on loose surfaces (dirt, snow, mud, gravel), these rotational speed differences are absorbed by tire slippage.

Consequently, most 4-wheel-drive vehicles are fitted with “part-time” 4WD systems: they can be switched from 2-wheel-drive mode (“2WD”), where only the rear wheels are driven by the drivetrain, to 4-wheel-drive mode (”4WD”), where all 4 wheels are driven by the drivetrain. Many of these vehicles will spend most of their time in 2WD: locking hubs allow the parts of the drivetrain that are used for 4-wheel-drive to be disconnected. With the hubs disengaged, and the transfer case in 2WD, the entire front axle and differential are inactive.