https://ift.tt/35bC3xL Jeep Grand Cherokee L Chassis Deep-Dive: Don’t Call Me Giorgio

Executives on hand at the unveiling of the 2021 Jeep Grand Cherokee L were uncharacteristically tight-lipped about their new baby’s architectural pedigree, as if loose lips risked sinking their just-christened ship. Now that trucks are rolling off the newly refitted Detroit Assembly Complex Mack Plant, Jeep veep Jim Morrison finally divulged some family tree details about the Grand Cherokee’s all-new WL platform.

An All New Platform

Okay, there’s nothing entirely new under the sun, and surely a few bolts, bits, and bobs carry over, but Jim swears this architecture essentially started from a clean sheet. The team that designed it was, of course, intimately familiar with the Giorgio platform that underpins the Alfa Romeo Stelvio (and Giulia sedan) and presumably will spawn Maserati’s Grecale SUV. But the engineering that lends itself to a great road-burner SUV (and sport sedan) doesn’t necessarily facilitate deftly tiptoeing across the Rubicon Trail. This WL is optimized for sure-footed off-roading, for handling payload and towing burdens that rank near the top of its class, and for easier access to the cabin.

Sophisticated Suspension

Up front, the lower control arms are replaced by pairs of ball-jointed links, so that now when you turn the wheel, instead of simply pivoting about a fixed “king-pin axis” defined by drawing a line through the two ball joints, the knuckle sweeps the shape of a cone. This “virtual” steering axis has the effect of reducing or eliminating the scrub radius (determined by the distance from the center of the tire contact patch to point where the steering axis contacts the ground). The upshot is that the new arrangement promises an improved feeling of stability. In back there is a revised five-link setup.

Adaptive Damping with Air Suspension

The Quadra-Lift air suspension now comes paired with adaptive dampers (active damping was strictly reserved for SRT and Trackhawk models in the WK2 Grand Cherokee). This pairing provides a more settled ride, especially during abrupt transitions, either left and right or between varying surfaces. And to prevent ingesting dust when out on the trail, the air suspension system is completely closed, and a pair of new air tanks back beneath the third-row seat allow it to more quickly drop to the access height, falling 1.8 inches in seven seconds without venting any air to the environment.

Higher Strength, Lower Weight, Stiffer Structure

The WL platform is making its debut in its extended form—15.1 inches longer overall than the WK2 model on a 7.0-inch longer wheelbase—to accommodate a whole extra row of seating. And yet comparing like trim levels, the curb weight difference between the outgoing WK2 two-row Grand Cherokee and the new L is 68 pounds or less—within 1 percent. This was enabled by expanding the use of high-strength steels (now only 29 percent of the unibody structure is constructed of mild steel), and by expanded use of aluminum. The front shock towers are cast aluminum, while several other pieces are assembled from aluminum extrusions and castings: the shock-tower brace, front suspension and powertrain cradle. The hood is also aluminum (as before). The body is 13 percent more resistant to twisting, which Jeep owners may notice when placing one tire up on a big rock, and it’s 18 percent stiffer in bending (useful when bounding over whoops). Improving these figures gets harder as the “box” gets longer.

Identical Fuel Economy

Buy a 2021 Grand Cherokee and you’ll get an EPA rated 19/26/21 mpg with a V-6 and rear drive, 18/25/21 if you choose 4WD, and 14/22/17 mpg if you spring for the V-8 4×4. And those numbers apply equally, whether you grab a deal on the outgoing WK2-platform two-row version, or choose the spanking new, long-wheelbase WL version. (Expect next year’s WL two-row GC to do even better.)

Holding the line on mass helped, but a few other technologies made a big difference too: All 4×4 models get an active transfer case that disconnects drive to the front when it is not needed, reducing parasitic losses. Active grille shutters improve aerodynamics, as does extensive underbody aero shielding on models without the off-road package’s skid-plates. The net result is an improvement of 13 counts of aero drag, dropping the Cd from 0.370 to 0.357—that’s more than enough improvement to cover the slight increase in frontal area (from 31.62 sq ft to 31.93 sq ft) that mainly comes with the 2.2-inch increase in width. Overall “drag area” (CdA) improves 2.6 percent.

Lower Without Losing Ground Clearance

Mounting the front differential directly to the engine helped lower the powertrain 1.6 inches, allowing for the hood line, line of sight, and seating hip-points to be lowered slightly. The door sills were also lowered by 0.3 inch on standard suspension; even more with air suspension. But the redesigned underpinnings essentially preserve minimum running ground clearance, which drops 0.1 inch with the standard suspension and gains the same amount with air suspension. But clearances to the fuel tank, front and rear axles improve by more—0.7, 0.4, and 0.8 inches respectively for the standard suspension.

Active Engine Mounts

New engine mounts capable of changing their stiffness are positioned to align with natural frequency node points for the entire powertrain. This reduces the noise, vibration, and harshness that gets transmitted from the powertrain to the unitized body and chassis.

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