Off-road terrain suits the Grand Cherokee well. It seems a good combination of luxury and capability. RAMEY WOMER, BEST RIDE JUDGE
A cut-away view of the Dana Hydra-Lok differential illustrates elements of the hydraulic pump and clutch plates that transfer torque between wheels.
A Catia model of the Grand Cherokee front suspension reveals the rubber bushings that isolate the spring tops from the unibody, a track-bar damper (tuned for NVH), and a specially developed steering damper (partly obscured by the knuckle-toknuckle tie rod).
This year's Grand Cherokee features a three-link rear suspension consisting of a central A-arm and two control arms. The new design eliminates problems caused by asymmetries of the five-link suspension employed in the previous model.
We had a difficult decision to make when we launched the new Grand Cherokee," admits Daimler-
Chrysler supervisor of Jeep steering andsuspension Wilbert Williams. "We had todecide whether or not to keep the solid axlesuspension of the older model. Cherokeecustomers said they wanted a good turningcircle, nimbleness, and a positive road feel.These all contributed to the ‘Jeepness' theyliked."
Apparently the design team made all theright decisions. The redesigned GrandCherokee garnered Top Finalist honors,though it retains a solid front axle ratherthan the more "carlike" independent frontsuspensions found on other contestants.Best Ride judges eagerly accorded it highmarks for its off-road handling, tracking,and performance through mud.
"A solid axle can give better articulationthan an independent suspension under certain conditions," explains Williams. "Tocompensate for the unsprung mass thatgoes with a solid axle design, we did a lot ofsuspension tuning that effectively made themass transparent to the driver."
Agile handling comes in part from the Grand Cherokee's use of light-but-robust hydroformed chassis components. DaimlerChrysler designers also put rubber bushings on either end of the coil springs and between the axle and control arms to keep wheel vibrations out of the chassis and help quiet the ride. The new design boasts 30% better torsional stiffness than its predecessor which, among other things, minimizes any body roll and gives the vehicle a sense of solidness.
Solid-axle designs can have well-recognized problems with shimmy and tramp — vibrations caused by gyroscopic forces between the steered wheels — as the suspension travels. Grand Cherokee engineers had these in mind when they laid out their new chassis.
"We designed and tuned a steering damper to reduce lateral inputs caused by suspension design geometries," says Williams. "The damper connects to the tie rod rather than the drag link, as in the older Jeep design."
A newly designed steering gear with a rack-and-pinion valve is another big contributor to the Grand Cherokee's nimbleness, says Williams. "We went through nearly 50 iterations of valve designs before coming up with the final configuration."
The rear suspension of the newGrand Cherokee connects the differential to the unibody via a single centralA-arm and two lower control arms.
Gone is a track bar used in the old five-link design. Eliminated as well is an inclination for the vehicle to move laterally because of it.
"In the old design, the axle rotated about the track-bar arc during articulation. This is an asymmetric rotation that shifted the vehicle laterally, causing a phenomenon we call head toss," explains Williams. "Using an A-arm in the rear gives us a symmetrical suspension articulation that eliminates that tendency."
Mounting angles for suspension components are of course critical for ride qualities. DaimlerChrysler engineers used Adams CAD modeling software, along with other specialized suspension modelers, to check ride, roll, and compliance factors during design. "We modeled the vehicle at every juncture of the program and optimized subsystems iteratively," says Williams. "This was one of the ways we blended all the suspension elements to give the driver a sense of confidence and a feeling of being connected to the road."
HOW DID IT DO THAT?
One of the most-talked about features on any of the vehicles competing at the Best Ride event this year could be found on the Jeep Grand Cherokee. Called QuadraDrive by DaimlerChrysler, it is based on an innovative limited-slip differential developed by Dana Corp.'s Spicer Light Axle Group.
The differential is billed as the first limited-slip device able to work as both a front and a rear axle. At the test track, it let a Grand Cherokee do things no other sport ute could: climb over an 18-in. step, and get through "Anaconda Alley," a serpentine ditch designed to force diagonally opposed wheels off the ground. The Quadra-Drive equipped Grand Cherokee was also one of the few vehicles able to surmount no-traction hill. Conveyor rollers installed in the path of left-hand wheels on this 17% rise let only the wheels on the right-hand side generate traction.
Key to getting through such obstacles is a hydraulic pump built into the differential. As one wheel loses traction and spins, a gerotor pump creates hydraulic pressure in proportion to wheel speed. Hydraulic pressure, in turn, compresses a clutch pack, transferring torque to the opposite wheel.
In contrast, most ordinary limited-slip differentials employ two clutch packs, one for each wheel. There must be at least some traction on the slipping wheel to engage clutch plates and transfer torque to the opposite wheel.
The use of clutch packs also explains why conventional limited-slip differentials may cause pulling if used on a steered axle. Ordinary limited-slip configurations typically employ a preload on their clutch plates. If the preload exceeds the available traction on one of the wheels, the two wheels tend to lock in synchronism, making steering problematical.