Jeep Grand Cherokee 4WD System Breakthrough - 4x...For?Posted in How To: Transmission Drivetrain on September 1, 2005
Anybody for Insta-trac? Grip-lok? All-drive? Sure-tork? How about Claw-grip? Sorry, forgot to add the little (tm). You've seen this stuff before, felt you should be impressed but didn't like to admit you hadn't a clue what it really meant-and all too often, the salesman didn't know either, not in any detail. Does it have a center differential? Is it a form of traction control? Is it even four-wheel drive? And that's just the start!
To get a feel for what Jeep has been up to with the latest Grand Cherokee, you first have to discard all the crazy trade names-for which nearly all the OEMs are guilty-get down to what the basics are, and take it from there. Take a look at the accompanying chart of the three generic 4x4 systems. Despite all the names, there are really only three basic designs. They aren't UN-sanctioned titles, but calling them Types 1, 2, and 3 is as good a way of labeling them as any. So we have:
Type 1: Good old selectable four-wheel drive, where you're in two-wheel drive most of the time and manually engage 4WD when needed off-pavement; there's no center diff, so to avoid transmission windup, you don't use it on hard grippy surfaces. World War II veterans will be nodding at this.
Type 2: Auto-engaged four-wheel drive, found on what have come to be known as "soft-roaders" (i.e., pavement-biased luxury SUVs), usually with no low-range gears. Like Type 1, there's no locking center differential, and again you're in two-wheel drive most of the time with 4WD automatically engaging when a significant difference in the rotational speed of front and rear propeller shafts is detected. Where a viscous coupling might have done this job by itself, locking up (nearly) as relative speed differences rose, these days this detection is usually electronic and is used to trigger faster engagement of a clutch pack to link the two shafts together. Unlike Type 1-where there is a "hard," metal-to-metal connection between front and rear propeller shafts-with auto-engaged 4WD, it's invariably a "soft" connection with a degree of designed-in slip in the clutch or viscous coupling. A good example of this is the BMW X5, which uses speed, acceleration, steering input, lateral g-force, and color-of-your-grandma's-eyes sensors to produce a very smart algorithm for dialing in 4WD when needed.
It is fundamentally important to differentiate between this kind of driveline, where the drive to the second prop shaft is only through the clutch (or viscous coupling), as opposed to some type of permanent 4WD, where a similar clutch (sensing shaft speed differences) is used to prevent excessive shaft speed differences across a "hard drive" differential: In other words, a 4WD system that uses a limited-slip.
That leaves what may arguably be the best of all:
Type 3: Permanent four-wheel drive. This is what it sounds like-a "hard," metal-to-metal, shafts-and-gears connection between the two propeller shafts, with an essential center differential between them to accommodate the different distances travelled by front and rear wheels in corners and the like.
An obvious question: If it's best, why don't all 4x4s have permanent four-wheel drive? As a rule, it boils down to a number of factors, including driveline wear, fuel mileage, manufacturing cost-and to be fair, user need.
Customer needs do vary, and this is where Jeep has been first to grasp the nettle and offer sensible, graded driveline options in one model. These cater to the owner just wanting a robust, capacious carry-all-where two-wheel drive will do just fine-through the man who simply wants to cope with an icy driveway or side road without having the bother of any levers or buttons to operate (rather like the X5), and all the way up to the lantern-jawed, dam-construction site-boss who really does need the best off-pavement capability there is.
The Jeep design engineers surely got it right, but when the marketeers were let in, the idea of having to say to a customer, "Yessir, I think our second-best driveline will do well enough for you!" was likely too much, and out came the fancy labels again. What nevertheless resulted were Options 1 through 4, as the diagram shows, and it is notable that in the step-up to where four-wheel drive is offered, it is a permanent, full-time system-shafts and gears with a center differential. In the upper two options, that differential is a limited-slip unit that prevents excessive front-to-rear prop-shaft speed differences in difficult conditions.
Delving deep into Jeep's tech info eventually yields the diagram here, and the detail is worth studying over a long cool beverage. The bean counters must have been kept out of that meeting, too, for they would surely have had an attack of the vapors at footing the bill for two different transfer cases-the NV140 on Option 2, and the two-speed, limited-slip-equipped NV245 for Options 3 and 4 (called Quadra-Trac II and Quadra-Drive II, respectively).
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.
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.
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.