Seamless Pressure
Other options for separating front and rear drive are clutch packs like those found in automatic transmissions that are hydraulically powered (often externally) and add pressure as slip is determined, and gerotor-pump systems like that used in the Grand Cherokee's optional Quadra-Drive system. AWD systems in cars rarely use a center differential, preferring some sort of clutch pack, hydraulic, electromagnetic, or electromechanical device for operation.

Regardless of how power is applied to the front driveshaft in the above systems, the front axle itself must also be engaged. In the early Jeep days, the front axle was always connected, even if not being driven from the transfer case, as in 2WD. Then manual locking hubs were put on the end of the axles to connect the wheels.

Manual-locking hubs require you to physically rotate the hub to lock it and engage front drive. This is inconvenient, but it's inexpensive and means you can still drive if anything in the front drive breaks and allows use of any aftermarket front differential. Some enthusiasts have converted their vehicles to have locking hubs on the rear wheels as well. Whatever breaks in the driveline, as long as the wheel bearings are still good, can be pushed or towed home.

Automatic hubs replaced manual hubs as a question of convenience (you don't manually turn anything). Auto hubs are more complex than manual hubs, but less so than a disconnect system. The '03 Expedition uses a vacuum-powered hub disconnect that gives the same effect as auto hubs, but doesn't require slow speed or reversing direction to unlock front drive.

Feeling Disconnected
The next advance is disconnect systems, like Toyota's ADD and GM's Insta-Trac. With this type of system there are no locking hubs and one front wheel is always linked to the front differential. The other wheel has a disconnect system on its axle shaft, which engages by a sliding sleeve or dog clutch--vacuum, heat, electric motor, or hydraulically powered--to link both front wheels to the differential. These systems are convenient, but don't always engage instantly, and aftermarket front differential options are limited because one side of the differential is always driven.

Some manufacturers, like Jeep, have come full circle and don't use any front-drive disengagement system on various models. These systems offer ultimate simplicity, but if anything breaks in the front drive, the vehicle may be undriveable, and aftermarket front-differential options are limited as with a disconnect system.

For those vehicles derived from front-drive cars or minivans, the AWD tends to use either a VCU for the front/rear connection (Vibe/Matrix, RAV4), or run primarily as front drivers adding rear drive only as required (MDX, CR-V, Rendezvous). In many of the latter cases, the driveshaft going from front to rear is always engaged, with the rear-drive engagement handled at the rear axle.

The VCU systems work the same way as 4WD transfer cases, with a limited amount of front/rear slip and drive to both ends all the time. The Aztek/Rendezvous and CR-V use a gerotor pump just ahead of the rear differential, which adds rear drive automatically and only when front slip is detected. The Acura MDX uses just one differential, between the front wheels, and is always driving its rear driveshaft. Instead of a conventional differential in back, it has a ring-and-pinion gearset with an electromagnetic clutch pack attached to each axle shaft. Wheel-speed sensors determine what is slipping where and engage the clutch packs individually as required, and since it's electronically controlled it can think ahead. If the vehicle is in first gear and the throttle is hit hard, the system knows the front tires will spin and add rear drive before tire spin actually happens.

Axle Differentials and Traction Control
The axle differential is what enables the outside tire to travel further than the inside tire on a corner. In most standard applications, the differential is open, meaning it has no internal devices to minimize speed difference from side to side. With an open differential, a tire that loses traction will spin, while the other does little or nothing to propel the vehicle. That's why most four-wheel drives don't drive all the wheels equally. Open differentials are the least expensive, require no special maintenance (just routine gear-oil changes), have no affect on steering, and work best with most traction control and anti-lock systems. They allow independent control of each wheel.

A limited-slip differential does exactly that: limit the amount of slip side to side so the vehicle will corner comfortably but won't spin a tire excessively in loose traction situations. In new vehicles with fresh limited-slips, you often hear a scuffing noise from the inside rear tire on a tight turn, as it tries to travel a similar distance to the outside rear tire. Factory limited-slips, which generally add $200-$300 to the sticker, are often clutch-pack units, and since clutches are friction devices, they will, over time and miles, wear out and become unlimited-slips. Clutch material also ends up in the lubricating oil and may damage bearings if not changed. Names used for these differentials include Trac-Lok and PosiTraction, and clutch-pack limited-slips have only the slightest effect on steering, but do require a lubricant modifier when the fluid is changed.

Other vehicles use a different approach to limited-slip, with the Hummer and Ranger FX4 packages choosing a Torsen (for torque-sensing) unit. In the Jeep Quadra-Drive system, a gerotor limited-slip is used in both front and rear axles. The final option in axle differentials is a locking differential, and factory applications are limited to the rear axle on some Toyota products, Hummer H2, and front and rear axles on the Mercedes-Benz G500 (which is why some consider it the only true 4WD factory-built vehicle in the U.S.). Some vehicles list a "locking differential" on the option sheet, though the majority are limited-slip devices and not true lockers.

A locking differential, when activated--by compressed air, electrically, or hydraulically--mechanically links the left and right axle shafts 100 percent, allowing no difference in left and right wheel speeds. On the trucks mentioned, the lockers are off in normal driving, so the differentials behave like open units and have no effect on handling or steering.

When the differential is locked, power is distributed equally left and right, so if one tire has good traction and the other is on ice or dangling in free air on a rutted trail, the vehicle will be propelled forward by the tire that has traction. Therein lies the principle disadvantage of a locking differential--with equal power to both sides, it tends to make the truck go straight ahead. This is further amplified with a locker in the front, because the steering wants one tire to go further than the other and the locker wants them to go the same distance. For that reason, a locking front differential should be used only when traction is minimal. Any aftermarket locking differential except those that can be unlocked, like the ARB Air Locker, must be equipped with locking hubs to isolate it from the steering on pavement.

The popularity of anti-lock brakes has made traction control an alternative to traction-aiding differentials such as limited-slips and lockers. ABS offers a means of both reading relative wheel speed and applying a brake at only one wheel, making electronic traction control a comparatively inexpensive addition.

If the sensors detect one wheel turning faster than the other side of the axle (or all three other tires), the ABS pump can apply brake pressure to the spinning wheel. This not only stops the spinning, but since power is not freely transmitted through that wheel, the open differential delivers some drive to the wheel that wasn't spinning. In some vehicles, if the applied braking action doesn't prove effective enough, throttle intervention cuts back engine power as well. This traction control is usually enough to keep going on road in snow or mud. However, in more serious conditions, electronic traction control may not be enough. It tends to take away power at times you need it most, and if the hill is long enough, it's possible to overheat some components.

Which system and options you choose will depend on your accurate assessment of requirements, availability, and cost. Along with the understanding that next year will always bring something new, as the '03 Range Rover, Hummer H2, and Jeep Wrangler Rubicon offer advances over today's versions. TT

What's In a Name?
The companion chart lists most of the SUVs and pickups sold in North America available as four- or all-wheel drive. The column headings are defined as follows (information in parentheses indicates optional systems or engines).
Name:The oft-trademarked name given to the drive system by the manufacturer.
Type: All- or four-wheel drive.
Default: Which wheels are driven at vehicle startup prior to any driver intervention.
4 HWY: Can the vehicle be operated in 4WD on pavement?
Low-range: The low-range gear ratio in the transfer box expressed as "x.xx:1," with high-range noted only if it's not the standard 1:1.
Center diff: The mechanism used to split power among the front and rear axles, even if not a differential. Note that "lock" means the unit can be locked for off-highway use, and (rr) indicates the unit is located at or in the rear axle CP=hydraulic clutch pack; EM=electromagnetic clutch pack; GR=gerotor; VCU=viscous coupling unit.
TQ split: The front/rear torque split designed into the system at startup. This value may or may not be variable.
Trac Aid Diff: Lists the limited-slip (ls) or locking (lk) differentials available by front or rear axle and whether standard or optional.
ETC: Electronic traction control on front, rear, or all four wheels; standard unless listed as "opt." Does not apply to 2WD versions.
2WD?: Shows if a two-wheel-drive version is offered. Is it front or rear drive?
Engine: Standard and (optional) engines for the base 4WD or AWD model.
Hp/Tq: Horsepower/torque for the standard and (optional) engines.

GLOSSARY
axle tubes: Hollow tubes that enclose the axle shafts and lubricating oil and extend from the differential housing to the brakes.
clutch pack: A series of friction plates that transfer more power as they're pushed together tighter.
driveshaft: A metal tube or tubes used to transmit power from the transfer case or transmission to the front and/or rear axles.
Gerotor: A self-contained hydraulic pump, progressive and tunable in operation, that reads speed differential between two sides (left/right or front/rear) and automatically pressurizes a clutch pack to limit slippage. When speed differential occurs, the pumps turn at a different speed to generate various pressures.
ring and pinion: A gear set consisting of two gears, the "driving" pinion and the "driven" ring, used to provide the 90* angle to transfer power from a longitudinal driveshaft to a lateral axle shaft.
thixotropic silicone fluid: The silicone fluid used in some viscous-coupling units that becomes less viscous (ie. thicker or stickier) as heat is generated.
torque: The moment of a system of forces tending to cause rotation on an axis. Engine torque can be calculated by (hpx5252)/rpm.
torque converter slip: Transfers engine output to the transmission hydraulically, with the engine turning one set of blades and the counter set turned by the action of the hydraulic fluid between them. Unless the converter is "locked," there is some slip between the two sides of the converter. This is why your truck doesn't stall when stopped and still idling in gear.
torque split: The percentage split in torque applied to front and rear driveshafts, often variable, and usually controlled by the transfer case.
transfer case: A housing (often separate but occasionally part of the transmission assembly) that contains the gears or chainset required to drive both driveshafts. It may also contain reduction gears for low-range and a differential.--GRW