Engineering the LX; Chrysler Group builds a quintessentially American car: a large, rear-wheel-drive car with just a touch of Mercedes.
DaimlerChrysler's decision to scrap the front-drive LH line and replace it with the rear-drive LX large car platform is very astute. The 120-in. wheelbase gives plenty of interior room, the rear-drive chassis and fully independent suspension provide the base for a pair of very credible performance vehicles, optional all-wheel-drive takes care of Snow Belt worries, Chrysler can borrow parts--like the five-speed automatic transmission that is mated to the Hemi V8--from Mercedes, and the layout is different than anything the domestic competition has to offer. Obviously, someone in Auburn Hills has been thinking.[ILLUSTRATION OMITTED]
UNDER THE SKIN
"Thirty-seven percent of the LX body-in-white is high-strength steel," says Tom Seel, supervisor, Body-in-White Engineering, Premium Vehicle Platform Team at DaimlerChrysler, "which is 17% greater than the amount we used in the 1998 LH." According to Seel, dual-phase steel is used for the front rails; load management and durability parts are stamped from 280 Mpa steel; the passenger safety cage utilizes 340 Mpa sheet; the front rails are stamped from DP590 and the rear rails from 550 Mpa steel. A geometric assessment of the body sections resulted in the use of two-piece welded octagonal front rails, a double C-section rear rail (inner and outer), a load-bearing cross-car roof bow, and a three-layer body side. "The LX chassis has a one-piece outer ring supplemented by a two-piece inner ring and reinforcements," says Jack Broomall, director of DaimlerChrysler's Vehicle Development, Activity and Premium Product Team. "And the spare tire is mounted on an angle (down at the front, up at the rear) so it stands up in a crash to help absorb the energy."
The bare body shell weighs 914.8 lb, has a torsional stiffness of 13,200 lb-ft/degree, a claimed first torsional moment of 40 Hz and a first bending moment of 48 Hz. "Not only does this give a solid base on which to mount the fully independent suspension," says Seel, "it helps the LX's crash performance as well. A 35-mph head-on barrier crash brings almost no intrusion into the passenger compartment, while a 40-mph offset has minimal intrusion." By using a "tire catcher" section along the A-post, engineers are able to redirect the front wheels away from the passenger compartment, feed this energy into the reinforced lower rails, and use the tire sidewall to absorb some of the energy. In addition, the LX was designed to meet future rear crash standards, including a 50-mph offset rear crash. "We used multiple impact zones, including the spare tire and well, to protect the fuel tank," says Seel. To reinforce his point, Seel adds that the LX body-in-white has 4,277 spot welds and 22,366 cm of adhesive holding it together.
But anyone can do a stiff structure if money is no object. The trick is to build a strong, light structure that doesn't break the bank. "Compared to the LH platform," says Seel, "we've reduced equalized variable costs by 15% and capital tooling costs by 40%. Plus, we've designed the structure around strategic subassemblies that will allow us to build new models and variants quickly, easily, and inexpensively." The latter confirms an open secret: the LX platform will not only add high-performance versions of the 300C and Magnum RT, but distinct new models as well.
[ILLUSTRATION OMITTED]
UNDER THE ALUMINUM HOOD
The base engine for the LX is the 190 hp 2.7-liter V6 found in the mid-size Chrysler Sebring and Dodge Stratus. Next up, and standard in the 300 Touring 300 Limited and Magnum SXT, is a 3.5-liter V6 with a triple-plenum intake manifold that produces 250 hp and 250 lb-ft of torque. It is mated to the four-speed automatic, and is the volume engine/transmission combination for the LX line. At the top of the heap sits the 5.7-liter Hemi V8 with its 340 hp and 390 lb-ft of torque, more than enough to push about 4,000 lb down the road quickly. And it is the first high-volume American production engine to offer an integrated cylinder deactivation system.
The MDS (Multi-Displacement System) unit replaces a conventional roller cam follower with a hydraulically pressurized design consisting of an outer sleeve and piston unit to which a roller follower is attached. When the pressure in the solenoid is released, the outer housing slides over the piston, which prevents the transfer of energy to the pushrod. "By trapping air in the cylinders," says Broomall, "we're able to reduce the pumping losses and increase the sealing force on the deactivated valves without introducing any temperature spikes into the system." Buyers will see fuel economy improvements of 10% to 20% depending on conditions, but MDS has little to no effect on EPA fuel economy numbers due to the dearth of situations in which the system can engage. The transformation from V8 to V4 operation takes just 40 milliseconds.
[ILLUSTRATION OMITTED]
Compared to the Hemi found in the Ram pickup--where the horsepower rating is 345 hp, but torque comes up 25 lb-ft short of the LX motor--the car engine has new intake and exhaust manifolds, and a torque curve tuned to passenger car requirements. The engine exhales through a 21-liter muffler and twin 15-liter resonators, and uses a cast-aluminum oil pan and liquid-filled engine mounts to reduce NVH levels.
The Hemi is mated to a Mercedes-derived five-speed automatic built in Kokomo, IN. DaimlerChrysler says this transmission uses bearings where other automatics make do with bushings, has lubrication passages in the clutch pack to promote fluid flow through the discs, and uses slots in the case to scavenge oil flung from rotating parts. The latter means parasitic losses are cut by not having these parts continuously rotating in oil. Unlike the four-speed unit, this transmission features Chrysler's "AutoStick" which allows the driver to manually shift the transmission one gear at a time.
Six months after launch, an all-wheel-drive system will be offered on the 300 and Magnum. It will add a front differential and a transfer case with a planetary gear center differential. The torque split is fixed at 38% front, 62% rear. Available only with the 3.5-liter and Hemi engines, the AWD system necessitated a change to the front cradle and minor modifications to the suspension. It includes all-speed traction control and a vehicle stability system, as do rear-drive LXs equipped with these engines.
AT THE CORNERS
The front suspension is a short and long arm design with a high upper A-arms (which places the upper ball joint above the tires), and the lateral lower links and tension struts attach to the steering knuckle via separate ball joints. Gas-charged coil-over damper units, and an anti-roll bar attached to the lower A-arms complete the package. AWD models use forged upper control arms and single-piece cast-iron lower control arms that are packaged around the front-drive system. In back, there is a five-link aluminum independent suspension with coil springs, gas-charged dampers, and--on Hemi-equipped models--an anti-roll bar that attaches to the rear knuckles. Everything but the coil springs and dampers are mounted directly to a steel subframe that attaches to the body via four large rubber mounts. Urethane jounce bumpers are designed to progressively engage on sharp bumps to minimize harshness and enhance body control.
Four-wheel discs are used on all LX models, as are low-drag calipers. The Hemi-powered Dodge Magnum RT and Chrysler 300C utilize twin-piston aluminum calipers and 13.6-in. vented rotors up front, and single-piston aluminum calipers and 12.6-in. vented rotors in the rear. The aluminum units are gray anodized for corrosion resistance. Lesser models use 12.6-in. discs all-around (the fronts are vented) and single-piston calipers.
It should be emphasized that the LX was designed, engineered, and developed in America, with minimal reliance on the Mercedes parts bin. It is not a cost-constrained E-Class Mercedes with Chrysler or Dodge bodywork. Rather, it is a modern, capable, sport/luxury platform with world-class NVH and performance levels. In short, it's a pleasant surprise.
By Christopher A. Sawyer, Executive Editor
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| Title Annotation: | On Cars |
|---|---|
| Author: | Sawyer, Christopher A. |
| Publication: | Automotive Design & Production |
| Geographic Code: | 1USA |
| Date: | Apr 1, 2004 |
| Words: | 1327 |
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