A type of differential. Some sources call it a manual locker because it must engaged and disengaged by the operator, usually with a switch on the dashboard. The switch activates a valve, which allows air pressure generated by an on board compressor and stored in a reservoir to engage the locking mechanism and solidly lock the axle shafts together. When engaged, an air locker allows for no wheel-speed differentiation. When disengaged, an air locker functions as a conventional open differential. Air lockers should not be engaged on hard, dry, paved surfaces.

An all-wheel-drive (AWD) vehicle has a driveline with the capability to send power to all four wheels, usually on a full-time basis. Most AWD vehicles are designed for all-season, instead of all-terrain, operation: ground clearance is usually closer to that of a car than a 4x4 truck, and the single-speed transfer case lacks a low-range.
Also see Four-Wheel Drive.

The angle at which a line drawn from the leading part of the vehicle's bodywork and touching the front tire meets the ground. It gives an indication of the steepness of a step-up that a 4x4 can approach without its nose digging in. The bigger the angle, the closer it is to 90 degrees, the better for offroad work.
Also see departure angle.

The minimum internal angle of an inverted-V ramp that a vehicle can drive over without dragging its belly, i.e., a measure of the sharpest bank it can drive over. A typical figure is about 150 degrees; the smaller the better.

In full-time four-wheel-drive or all-wheel-drive applications, the center differential is part of the transfer case. A transfer case with a center differential (or a viscous coupling) is necessary for full-time four-wheel-drive operation because differentiation in speeds occurs between the front and rear axle, just as between the two wheels on either side of a single axle.
Also see differential.

The lowest gear reduction of which a particular vehicle is capable; calculated by multiplying transmission First gear by transfer case low-range by axle ratio.

The angle at which a line drawn from a vehicle's rearmost bodywork and touching a rear tire meets the ground. It gives an indication of the maximum steepness of a step-down that a vehicle can negotiate without dragging the rear bodywork. Long body overhangs reduce the departure angle. The closer the angle is to 90 degrees the better for offroad work.
Also see approach angle.

A mechanical device consisting of pinion gears, a pinion shaft, side gears, and the differential case. The axle shafts are splined to the side gears. The pinion shaft is also called a "spider," especially when the differential assembly uses four, instead of two, pinion (spider) gears. The ring gear is bolted to the differential case. In an axle, it receives engine power via the driveshaft and drive pinion. As the ring gear turns, it turns the differential case. The differential case transmits power to the differential pinion gears via the pinion shaft (spider). In normal, straight-ahead operation, the differential pinion gears transmit power to the axle shafts. In certain situations, the differential pinion gears move inside the differential case. For example, when the vehicle is turning to the left, the "outer" wheel turns faster than the "inner" wheel, causing the side gears to rotate at different speeds. When this occurs, the differential pinion gears rotate on the differential pinion shaft. This prevents the driveline from binding. However, an "open" differential sends only as much power to both wheels as is necessary to turn the wheel that requires the least power to turn. Thus, if one wheel is on ice, and requires almost no power to spin freely, an open differential will send only that much power to the wheel on the other side of the axle, too, although the other wheel may have plenty of traction. Often, this is not enough power to move the vehicle.
Also see limited-slip and locker.

Different four-wheel drive systems have different drive modes. The usual drive modes available with a part-time system are 2-Hi, 4-Hi, and 4-Lo. More sophisticated full-time four-wheel drive systems may offer additional drive modes such as 4-Hi-Lock, and 4-Lo-Lock, which lock the center differential for maximum tractive capability.

In four-wheel-drive applications, an electric shift system may be used to select drive modes instead of a transfer-case lever. Typically, a lever-operated system will offer a transfer case neutral position, whereas an electric shift system will not.

A vehicle with four-wheel drive (4WD) has a driveline with the capability to send power to all four wheels. Typically, the vehicle is designed for off-road, rough-terrain operation. The transfer case will include a low-range mode, and the vehicle will have more ground clearance than a typical passenger vehicle.

To provide shift-on-the-fly capabilities, many part-time four-wheel drive systems use some type of front axle disconnect. The front axle disconnect is normally part of the front differential assembly. As part of a shift-on-the-fly four-wheel drive system, the front axle disconnect serves two basic purposes. First, in two-wheel-drive mode, it disengages the front axle from the front driveline so the front wheels do not turn the front driveline at road speed, saving wear and tear. Second, when shifting from two- to four-wheel drive "on the fly" (while moving), the front axle disconnect couples the front axle to the front differential side gear only when the transfer case's synchronizing mechanism has spun the front driveshaft up to the same speed as the rear driveshaft. Four-wheel drive systems that have a front axle disconnect typically do not have either manual- or automatic-locking hubs. To isolate the front wheels from the rest of the front driveline, front axle disconnects use a sliding sleeve to connect or disconnect an axle shaft from the front differential side gear. Different manufacturers use either vacuum or heat to move the engagement sleeve. Toyota's Automatic Disconnecting Differential (ADD), for example, uses a vacuum actuator; GM's Insta-Trac uses an electrical element that heats up, causing the air in a sealed chamber to expand, moving the sleeve to engage the front axle.

A full time 4WD system provides engine power to both the front and rear axles on all surfaces at all times. This usually requires a transfer case with a center differential, a viscous coupling, or both. Full-time 4WD improves traction and handling on paved surfaces in inclement weather, but may reduce fuel economy.

A transfer case drive mode, like neutral or low-range. Most conventional part-time transfer cases offer a 2-Hi as well as a 4-Hi mode. High-range is for normal-speed operation. In most instances, high-range is a straight 1.00:1 ratio, offering neither a gear reduction (underdrive) nor an overdrive. Some full-time systems are not a straight 1.00:1.

Automatic-locking hubs automatically engage, or "lock," the hub and tire and wheel assembly to the front drive axle's axle shaft when the operator engages a four-wheel drive mode. When released, or "unlocked," the axle shaft is disengaged from the hub body assembly and the wheel can rotate freely on the spindle. To unlock most automatic-locking hubs, the operator must select a two-wheel drive mode and drive the vehicle straight backwards at least 10 feet. The hubs of any part-time four-wheel drive system should always be unlocked before driving on dry, hard-surfaced roads.

Manual-locking hubs perform the same function as automatic-locking hubs. However, the hubs must be manually locked or unlocked, usually by twisting a part of the hub from a "free" position to a "locked" position. Advantages of manual hubs include greater flexibility of operation, durability, and they provide the option of flat towing of the vehicle without a trailer.

A suspension design whereby each wheel can operate independently from the other.

A type of traction adding differential. Most limited-slip differentials are either cone-type or clutch-type. Both types allow a limited amount of inter-axle slippage during cornering, and both use some type of springs to provide a pre-load, or static load. When one wheel is slipping, both types use the separating force generated by the specially-designed differential pinion gears, combined with the static load, to bypass the pinion gears and transfer a greater proportion of power directly from the ring gear to both axle shafts.

Clutch-type limited-slip differentials have splined side gears that accept clutch packs. The clutch packs are composed of eight to 10 friction discs. The friction discs are either splined to the side gear or eared to fit into the differential case. When one axle shaft is slipping, the separating force tends to move the side gears out, compressing the clutch packs. Because some of the clutch discs are eared and fit into the differential case, the power flow bypasses the pinion gears; thus the differential transfers a greater percentage of the power available at the ring gear to the slipping wheel than would an open differential. Clutch-type limited-slip units need a differential lubricant with a special friction modifier, or "anti-chatter" additive.

Cone-type limited-slip differentials also use static load as well as side gear separating force to increase the outward force on the side gears and essentially bypass the pinions. Instead of clutch plates, a cone-type limited-slip uses the side gear force to push cones splined to the side gears into mated machined surfaces inside the differential case.

A third type of limited slip differential is the True-Trac from Tractech. The True-Trac has no springs to provide a static load and uses friction between the differential case and its parallel gears to transfer torque. The True-Trac is a compound planetary gear set that functions as a limited-slip.

A suspension design where the entire drive axle is a solid unit. Also known as a solid axle.

A locker, or locking differential, is a type of traction adding differential. A locking differential essentially locks the axle shafts together, transferring all of the power from the ring gear to both axle shafts, regardless of either wheel's lack of traction. Where limited-slip differentials are normally "unlocked" and only begin to function when one tire slips, a locking differential, except for an air locker, is always locked, and only "unlocks" to allow for wheel-speed differentiation when cornering. A locking differential is typically used in a rear-axle application; a locker installed in the front axle can impair steering.

The transfer case drive mode that provides maximum gear reduction to the wheels and multiplies engine power. Low-range is for slow-speed operation. Where the case's high-range ratio may be 1.00:1, low range ratios are typically between 2.20:1 and 3.50:1.

Currently the least expensive and most common 4WD system. A part-time 4WD system is designed to be operated only on reduced-traction surfaces. The transfer case lacks any mechanism to allow front-to-rear axle speed differentiation. Thus, any accumulated driveline bind must be released via tire scrub. Extended use of part-time 4WD on a high-traction surface, such as dry pavement, can adversely affect handling and damage the driveline. Some, but not all, part-time 4WD systems allow the operator to shift from two- to four-wheel drive "on-the fly."
Also see Shift-on-the-Fly.

A vehicle with a four-wheel drive system designed to shift between two- and four-wheel drive while moving is said to have a shift-on-the-fly system.

A suspension design where the entire drive axle is a solid unit. Also known as a live axle.

In shift-on-the-fly four-wheel-drive applications, a synchro is a mechanism that synchronizes the speed of the to-be-engaged front output shaft to the speed of the rear output shaft, spinning the front driveshaft up to road speed before the front drive axle is engaged. Every part-time shift-on-the-fly four-wheel drive system needs some type of synchro. Many use a synchro in the transfer case identical to the synchro found in manual transmissions. Others use a magnetic clutch similar to that used on an air conditioning compressor.

A mechanical device installed inside a differential, either a locker or a limited-slip. A traction-adding differential is designed to combat an open differential's tendency to send only as much power to both axle shafts as is needed to turn the shaft that requires the least power to turn.
Also see differential.

A mechanical device with the capability to send power to a front driving axle as well as a rear driving axle. Transfer cases drive the front output shaft with either a geartrain or a flat Hyvo-type chain. Transfer cases designed for four-wheel-drive vehicles offer a low-range drive mode, where all-wheel-drive vehicles typically have a single-speed transfer case.

An ingenious device that can act as a limited-slip differential. A viscous coupling is a small sealed drum filled with a measured quantity of silicon fluid. Inside the drum are two sets of interleaved plates. One set of plates is splined to the outer drum and one set is splined to the inner shaft. In a typical application, a viscous coupling is installed in a transfer-case center differential to act as a limited-slip. The outer drum is attached to the front output shaft, for example, and the inner shaft is attached to the rear output shaft. As long as both front and rear output shafts turn at the same speed, both sets of plates in the viscous coupling turn at the same speed and there is no relative motion between the plates. If the front shaft were to begin turning faster than the rear output shaft, indicating front wheel slip, the two sets of plates would develop relative motion and begin slicing through the silicon fluid. The silicon fluid would heat up and solidify, momentarily locking the sets of plates together and counteracting an open differential's tendency to transfer only limited amounts of power. With the plates locked together, relative motion stops; the silicon fluid cools down and returns to its liquid state, freeing the plates.