When it comes to locking differentials (AKA lockers), there are really only two types: manually selectable and automatic. An automatic locker doesn't require input from an outside source to function. In other words, the driver doesn't have to make the decision for the locker to be locked or open when in use.
With an automatic locker, the unit is usually locked, both axles connected, while driving in a straight line. This provides equal torque to both drive wheels and, all things being equal, equal traction at the tire-to-road surface interface, too. Automatic locking differentials are designed to keep both axles, and the attached wheels, in a continuous drive mode. This drive mode is automatically circumvented when turning the vehicle to provide differentiation in the speed of the wheels/tires on the inside and outside of the turn. The differentiation is necessary to provide a smoother and tighter turn than would be possible with a solid axle. It also reduces noise and tire wear and greatly improves handling.
However, when compared to an open differential an automatic locker can be noisy, producing bangs and clunks when disengaging in a turn. This is especially true when the locker is under load when entering a turn and then the load is removed (lifting off the gas) while in the turn. Handling in a turn, or on an icy or slippery high-crowned road, can be affected to. With four-wheel-drive vehicles running an automatic locking differential in the front, when in 4WD, the steering can be stiff on hard surfaces. This is especially true at low speeds, but is not present in 2WD with the front hubs unlocked. On the up side, these quirks are easily learned and experienced users of automatic lockers can make them seem almost as smooth as an open differential on the street. Best of all, with an automatic locking differential, you have maximum traction at all times without ever thinking about it.
Selectable Locking Differentials (AKA on-demand lockers) require input from an external source, usually a human, to engage or disengage the unit. Typically selectable lockers are turned on and off with the simple flip of a switch. The user can drive around town and on the trail unlocked and enjoy the smooth manners of an open differential until added traction is anticipated. When the driver foresees a need for improved traction the selectable locker is engaged to maximize traction.
With all but a few exceptions, selectable lockers are open when off and fully locked when on. As a result, a selectable locker provides all the smooth driving traits of a standard open differential and the extreme traction of a fully locked axle in one package. When locked, these units provide equal torque to each driven wheel regardless of what surface the wheels are on, or even not on if one tire is in the air. The down sides are the driver/operator must anticipate the need for each mode and turn the unit on and off. On the downside, selectable lockers require support components, such as air compressors, cables, switches, etc. These are susceptible to failure and can take a perfectly good on-demand locker off line.
There is more to be said regarding the various types of automatic lockers and selectable lockers, but we'll include those details in future installments of Gear Talk.
Gear Talk is produced in association with Yukon Gear & Axle.