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.
Suggested benefits of locking hubs include better fuel efficiency, quieter operation, less vibration, and lower wear; however, there are no reliable scientific studies to prove these claims. Exactly how great these benefits are is open to debate, with many feeling that they are outweighed by the disadvantages below.
In older vehicles, manual locking hubs are used to disengage the front wheels. This requires getting off/out of the vehicle to engage or disengage the front wheels. If road conditions are irregular (snow, then dry pavement, then snow again, repeatedly), these vehicles can be used in 2WD mode with the locks engaged (by disengaging 4WD with the lever or switch INSIDE the vehicle) and 4WD needs only to be engaged from inside the vehicle when road conditions require it.
In more modern 4WD vehicles, automatic locking hubs are often used which, as the name implies, engage automatically when 4WD is activated from inside the vehicle. The main advantage is that the driver does not need to leave the vehicle to activate 4WD, or drive the vehicle in 2WD with the front axle engaged. The disadvantage with this system is that most designs require the vehicle to move some distance (usually a whole wheel turn, often going backwards) after engaging 4WD, in order for the hubs to engage or disengage (in many cases, 4WD can be engaged with the vehicle moving). This might not be possible if the vehicle gets completely stuck before 4WD has been engaged, meaning automatic hubs require more caution on the driver's part.
Exposed hub locks can be broken or damaged by off-road conditions, rendering 4×4 useless and leaving the vehicle stranded. Also, in some axle designs (such as those used on older Land Rovers), the top swivel bearing can become starved for lubrication (which is normally supplied by oil slung up by the rotating axle), unless the hubs are locked every few hundred miles. Also, since locking hubs generally do not require a key to operate, they can be maliciously locked or unlocked by persons other than the vehicle owner.