Many years ago I was exploring a trail in the mountains east of Tucson, Arizona, in my FJ40, and came upon a couple in a shiny new 4x4 Toyota pickup. They had managed to high-center the transmission skid plate on a rock ledge, so that the right front and left rear tires of the truck hung just an inch or two in the air and spun uselessly when the fellow applied throttle. The couple, new to backcountry driving, was bewildered that their “four-wheel-drive” vehicle had been rendered completely immobile by such a minor obstacle, and were convinced something was wrong with the truck. I picked up a fist-sized rock from the side of the trail and kicked it solidly under the hanging rear tire. “Try it now,” I said—and the Toyota bumped free. They were astonished and even more bewildered—until I explained how a differential works.

When any four-wheeled vehicle makes a turn, each wheel needs to rotate at a different speed because it travels a different line than the others, and thus a different distance. If, for example, the two rear wheels were locked together with a solid axle, the tires would scrub horribly every time the vehicle turned, and handling would be affected dangerously. Thus we divide that axle in two and connect them with a differential—a system of gears that transfers power from the driveshaft to the wheels while allowing them to rotate at different speeds in a turn. In a four-wheel-drive vehicle we add another driveshaft and differential at the front, so those wheels can be driven as well.

But an open differential, as this is known, has a major inherent flaw, especially for those of us who own 4x4s: If one tire loses traction, the differential gears, in effect, route all power to that side of the axle. Thus if you stop with one wheel on a solid surface and the other in mud—or hanging in the air—the tire with grip will remain stubbornly motionless while the other just spins. (Technically both wheels are receiving the same amount of torque, but the amount required to spin the low-traction tire is not enough to move the vehicle with the other tire.) So my friends in the Toyota pickup found themselves in a situation in which the power being delivered to the front axle was expended on the tire that was off the ground, while the same thing happened in the rear. Once I stuffed that insignificant rock under the hanging rear tire, the full torque of the engine was available and the truck pulled itself free.

Ironically, the drawbacks of open differentials became more apparent with the advent of so-called full-time four-wheel-drive vehicles such as Land Rover’s Defender. In a part-time 4x4, the front and rear driveshafts are locked together at the transfer case when four wheel drive is engaged, so power is always equally distributed to both axles and therefore to at least one wheel in front and one in back—but you cannot drive on pavement with four-wheel drive engaged or the tires will scrub and the gears will bind. The Defender (along with other vehicles such as the Mercedes Benz G-Wagen) allows engine power to be directed to both ends of the vehicle, regardless of the surface, by employing a third differential in the transfer case to allow the front and rear driveshafts to turn at different speeds on pavement and prevent binding. To ensure equal power to both ends of the vehicle on trails, the center differential can be locked manually. But if that lock fails—as happened to me in a Defender 110 on a remote route up the west wall of Kenya’s Great Rift Valley—you are left with a one-wheel-drive vehicle. When my front axle unloaded on the steep grade, all power went to that axle through the open transfer-case diff, and as soon as one front wheel unloaded I was completely immobilized with a single tire spinning fruitlessly—on a scary 40-percent grade with a steep dropoff. (I was towed the rest of the way up the escarpment when, improbably, a battered Land Cruiser appeared driven by none other than Philip Leakey . . .)

Over the decades, various attempts have been made to allow the differential to function properly in turns while maintaining full traction when needed. Way back in 1932 the engineering firm ZF, at the request of Ferdinand Porsche, invented a “limited-slip” differential to prevent the high-powered Auto Union Grand Prix cars Porsche had designed from spinning their inside tires when exiting turns. In the succeeding decades, many American manufacturers installed limited-slip differentials in their trucks to help enhance traction on slippery surfaces, and several types were developed. Some used clutch packs to transfer power, others (Torsen, Quaife) used gears, and some—especially those designed to be installed in transfer cases—used hydraulic fluid. 

But limited-slip diffs are just what their name implies—they cannot completely prevent loss of traction in challenging conditions. The best way to ensure full traction when it’s needed is to lock both halves of the axle together, thus ensuring ideal power delivery even if one wheel is off the ground. But once you do that you’re right back to our initial problem with tire scrub and gear windup. The trick, then is to have the differential lock only in conditions where it would be beneficial—or to have it locked all the time except when turning. This can be accomplished either automatically or manually.

In 1941 a fellow named Ray Thornton patented an automatic locking differential he called the  Thornton NoSPIN Differential. It was manufactured by the Detroit Automotive Product Corporation and installed in many WWII military vehicles. The NoSPIN employed a series of clutch packs and a spring-loaded cam gear, which kept the axles locked together unless the vehicle was turning a corner, when the cam disengaged the clutch packs from the spider gears and allowed the wheels to rotate at different speeds. In the 1960s American truck manufacturers began installing the NoSPIN in light-duty trucks as an option—and it gained its nickname, the Detroit Locker. While extremely effective, the Detroit Locker suffered from noisy operation on pavement as the gears engaged and disengaged, and handling that could sometimes be jerky as power transferred between one and two wheels. (Later models have attenuated these characteristics somewhat, but the transition is still noticeable to the driver.) A similar product made by the Eaton Corporation was introduced on 1973 General Motors light trucks, and Eaton subsequently bought the parent company of the Detroit Locker. 

The mechanism of the Detroit Locker and its relatives replaces a large part of the differential, requires precision resetting of pinion backlash, and is thus expensive and time-consuming to install as an aftermarket option. Not so with so-called “lunchbox” lockers such as the clever Lock-Right, which replace only the spider gears and can be installed in an afternoon by a competent home mechanic. The Lock-Right and its kin keep both axles locked together until the vehicle turns, at which point the internal drive plates ratchet past each other and allow the outside wheel to turn faster than the inside wheel. On the trail, full power is available to both wheels, even if one is airborne.

Opinions differ on the “lunchbox” nickname—some claim it’s because the unit will fit in a lunchbox, others because you can install one in the time it takes to eat a sandwich and chips (somewhat optimistic). Regardless, this type of locker is the most affordable and easy way to gain true diff-locking capability for your 4x4. However (there’s always a however), the Lock-Right-type lockers are restricted in strength depending on what the carrier is designed for, and are generally not recommended for tires over 33 inches in diameter. They can also be noisy on the street, as the ratcheting becomes noticeable around corners. And I would strongly dis-recommend them for installation in the front axle, as steering will be significantly affected (and, as with the Detroit Locker, they should never be installed in a front axle that does not have free-wheeling hubs).

Automatic lockers have their fans, but arguably the best locking differential is one the driver can control. The Australian company ARB popularized the air-activated locking differential after buying the rights to the Roberts Diff-Lock in 1987—and it transformed the capability of the Land Cruisers and Land Rovers in which it was first deployed. For most driving, a differential with an ARB unit installed acts as a normal open differential—no noise, no steering effects or increased tire wear. But when traction is lost—or, significantly, when the driver observes a spot ahead of the vehicle where traction might be lost—the locker can be engaged and the obstacle traversed smoothly and easily. Once back on a solid substrate the diff can be unlocked and returned to normal function. A small compressor (which can double for filling tires) activates the locker by pushing a sliding pin in the differential. The ARB locker is now available for a huge range of vehicles, and while its installation is as complex as that of the Detroit Locker (with the wince-inducing addition of needing to drill and tap a hole in the differential housing for the air line), its reliability and astounding capability has been proven over millions of miles.

It took a few years, but vehicle manufacturers caught on to the benefits and capabilities of selectable diff locks. Toyota introduced an electrically operated rear differential lock in its TRD package for the Tacoma, and optional front and rear diff locks in the 70 and 80-series Land Cruisers. The Mercedes G-Wagen has front and rear locks, as does the Rubicon version of Jeep’s Wrangler, and the Ram Power Wagon, among other vehicles. Until you’ve climbed a 45-degree slope in a vehicle with both diffs locked—and, thus, true four-wheel drive—it’s difficult to imagine the gravity-defying traction available. Even experienced passengers gasp and scrabble for handholds as high-noon sun floods through the windshield. (Showoffs are advised to keep in mind that with the front diff locked steering is very difficult; it should only be employed when absolutely necessary and for as short a distance as possible.)

To explore the next step in making four-wheel-drive vehicles truly four wheel drive we need to do a 180-degree turn and look at . . . brakes: specifically anti-lock braking systems. 

While ABS has been around in one form or another since 1929, when a primitive mechanical system was developed for aircraft, it was Mercedes Benz that introduced the first fully electronic, multi-channel four-wheel anti-lock braking system as an option in 1978. ABS relies on a deceptively simple system of sensors at each wheel, individual hydraulic pumps for the calipers, and a computer control. The sensors do nothing more than count the number of rotations per unit of time for each wheel. When one or more of the sensors detects a wheel turning slower than the others during braking—as when a tires locks and stops rotating altogether, increasing braking distance and hampering steering control—the computer reduces braking force to that wheel, pulsing the pressure many times a second to maintain static friction between tire and surface. Soon this system was exploited to provide electronic stability control (ESC), to help road cars maintain traction in slippery conditions.

And then—lucky for us—a light went on in an engineer’s head that this system could also be used to enhance traction in four-wheel-drive vehicles. It’s accomplished by exploiting the characteristics of the open differential.

Recall the offside wheels on that poor Toyota pickup spinning helplessly in the air. With an electronic traction-control (ETC) system, those versatile ABS sensors send that information to the computer, which applies braking force to the spinning tire or tires. The open differential is “tricked” into increasing torque to the tires on the ground, and the vehicle pulls itself free. Land Rover debuted ETC on its 1993 Range Rover, and off-road driving has never been the same. Advances in programming and technology have since brought us to the point that some vehicles can maintain forward progress with traction to only one wheel.

That would be miraculous on its own, but engineers weren’t finished yet. One of the most challenging conditions facing a driver on trails is a steep descent. In an older vehicle such as my FJ40, if you stomped on the brakes in a panic on a steep downhill section, the unloaded rear brakes would lock and the vehicle would instantly try to swap ends. Descending such slopes meant using first-gear-low-range engine braking and careful cadence foot-braking to make it down safely. In a vehicle with automatic transmission (and thus little engine braking) the situation was even dicier. Enter hill-descent control: Punch a button, point the vehicle downhill, and steer. The ABS and computer can selectively brake individual tires if necessary to maintain a steady walking pace and prevent lockup on truly hair-raising slopes. No driver, no matter how skilled, can equal that.

I remember my initial experience in a vehicle equipped with ETC and hill-descent control. At first the chattering, juddering progress up and down steep ridges was alarming—it sounded like something was seriously wrong. But I soon got used to it and realized how effortlessly I was conquering obstacles that had required all my attention in the FJ40. 

Is electronic traction control, then, superior to manually lockable differentials? The definitive answer is: It depends. Remember that a skilled driver using manual diff locks can anticipate the need for extra traction and respond in advance, thus frequently avoiding drama of any kind. By comparison, a traction-control system must detect a difference in wheel speed before it reacts, and the computer must decide if action is required or if the driver is simply turning. In some vehicles I’ve driven a considerable amount of throttle—and trail-damaging wheelspin—is necessary before the system kicks in. Increasingly, however, manufacturers are incorporating driver-selectable, terrain-specific algorithms that quicken response when the vehicle is in low range, for example. These algorithms can also modify throttle response and shifting to suit conditions. Land Rover was a pioneer in this technology with their Terrain Response dial. Some vehicles, such as Jeep’s Wrangler Rubicon, incorporate both ETC and manual diff locks—the very best of both worlds.

One could argue that these computer-controlled tricks reduce the skill formerly required of the driver. Indubitably true to an extent—surely I feel I paid my dues with my leaf-sprung, open-diffed 1973 Land Cruiser over the years. Yet in the sybaritic, climate-controlled cockpit of a Land Rover LR4 or Jeep Wrangler Rubicon I can traverse terrain that would have the FJ40 struggling. If technology makes it easier for new enthusiasts to get out and explore the backcountry, I’m all for it—even if I don’t get to show off as often getting them unstuck with a fist-sized rock.