Here it is appropriate to consider what many manufacturers-most, actually-ignored or hoped would go away for decades: Single wheelspin on a front or rear axle. At best, limited-slip diffs were offered as an option. At worst, there was nothing, and you prayed, off-pavement, that you had enough suspension articulation to keep the wheels on the ground so they could continue giving traction and didn't spin in the first place.
Rally cars or four-wheelers? The Porsche Cayenne has exceptional off-pavement driveline sp
With the wider spread of ABS, the manufacturers found they had ready-made speed sensors available at each wheel and, with the inventory equivalent of a piece of fuse-wire and some sealing wax, wheel-braking traction control was there for the taking. So if a wheel left the ground and began spinning, the speed sensor checked what was going on and had the brakes nail it before it got too serious. It was an elegant concept, affordable to manufacture and, with tweaking, effective for moderate use. So, no need for limited-slips any more? Not so fast.
Jack up the back end of your 4x2 car, turn the rear wheels slowly, then brake one wheel. The opposite wheel turns twice as fast. So simply braking isn't enough. It has to be modulated-er, fudged a little. And if it is happening a lot, the brakes get hot and worn. So while modern designs are very smart and the systems effective for most types of off-pavement use-Land Rover's new LR3, which relies on traction control exclusively for the front axle, is a good example-there are limits. Which brings us back to limited-slips and locking diffs for the really serious stuff.
...like the Range Rover Sport, its larger wheels limit a buyer's choice of all-terrain tir
And that is what Jeep has done for Quadra-Drive II. Electronic limited-slip diffs (ELSDs) go in the center, rear, and front diff positions. And, as hinted above, in the front axle, the ELSD backs off the locking of the front axle when the (stability control) steering sensor tells it there is a tight turn going on. (Put an old Type 461 Mercedes G-Wagen into a tight turn with its 100-percent mechanical front diff-lock engaged, and you'll wish you hadn't!)
Interestingly, Jeep retains the traction-control system on Quadra-Drive II: "As backup," they say-to the thud of fainting accountants. Jeep's ELSDs are electronically controlled versions of their earlier Gerotor automatic-locking diffs, in which the different shaft speeds generated hydraulic pressure to engage the locking clutch. Now this is triggered-and elegantly backed off-by the electrics. (There was the tiniest hint that the ELSDs needed some help at extreme high temperatures, so braked-wheel traction control was kept on the list.)
For absolute, no-nonsense, 100-percent diff locking, a metal-to-metal dog clutch is the business, and that is what the Mercedes G-Wagen has, front and rear. Rather the "thinking man's 4x4," as well as being an exceptional performer, Mercedes relies on the driver to disengage the diff-locks when not required-just as we do on selectable (Type 1) 4WD. Lawyers and warranty-folk blanch at the idea of "must disengage," and the spectre of "customer misuse" has tended to favor the limited-slip.
Suspension articulation is a double-edged weapon, as anyone with a first-edition Discovery will attest. Lots of articulation typically means excellent terrain compliance, but also more on-road body roll. Driveline engineers, armed with something to control single-wheelspin, came to the rescue, fitting beefier sway bars to rein in nautical handling characteristics at the expense of keeping wheels on the ground in the rough.
Then Nissan came up with the idea of a disconnecting rear antisway bar on the Patrol a few years ago, and the message has gotten around. The Porsche Cayenne has them front and rear, and so, in due course, will the Grand Cherokee. Here, they will likely yield an extra couple of inches of droop on a hung wheel-worth having in order to get traction from four rather than three wheels.
Adjustable air suspension is also increasingly used to bridge the gap between the four-wheeler and the road-hugger, but there are limits to its effectiveness if, on full extension, you are left with little relative wheel movement, one to the other. The interdependence of drivelines and suspension systems amounts to "articulation versus diff-locks," and the increasing use of wheelspin-controlling devices is the result.
Inside the cockpit, increasing complexity and the importance of safe-OK, idiot-proof-operation makes demands on ergonomic design. Most of us prefer levers to pushbuttons, but they take up cockpit space. Nissan, for one, can always be relied on for simple, pragmatic designs, and the Xterra's and Pathfinder's rotating-switch sequential selection for their auto-selecting four-wheel drive ensures clear, error-free operation. Similarly, the status of the VW Touareg's advanced driveline is crystal-clear just by looking at the controls (the rear diff-lock, when fitted, is next in the 5 o'clock dial position).
On paper at least, it is hard to see how Jeep's Quadra-Drive II could be bettered when the disconnecting antisway bars come on line. Only Jeep has been brave enough to offer a center, rear, and front diff lock-and also a means of keeping it tame in turns. "Limited-slip" is an elastic term in comparison with the G-Wagen's metal-to-metal dog clutches, but the electronic control of the Gerotor clutch clamps should be an improvement in sensitivity and top-of-the-graph oomph.