Subscribe to a magazine

Jeep Grand Cherokee 4WD System Breakthrough - 4x...For?

Passenger Side View
Tom Sheppard | Writer
Posted September 1, 2005

Has Jeep come up with a pragmatic 4WD recipe for the upscale SUV?

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.

By contrast, a nameless sinner's unguarded range-change toggle switch could easily be engaged incorrectly.

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 VW Touareg's well-laid-out pop-up controls effect simple display of the 4WD system while ensuring that the center diff (4 o'clock position) cannot be locked unless you are in low-range.

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).

Load More Read Full Article

Comments

Advertisement