Prowler advances aluminum technology

DN Staff

October 6, 1997

13 Min Read
Prowler advances aluminum technology

Born from a "retro car" concept that made the auto show rounds, Plymouth's Prowler pounced off the brainstorming board and into the hearts of hot-rod enthusiasts. Picking it up, Chrysler engineers realized their toughest task would be remaining true to the original hot-rod theme. With an $80 million budget, they did, thanks to innovations in materials and design.

A few examples include a die-cast magnesium instrument panel, composite brake rotors, rubber couplings in place of u-joints, and Kevlar body straps. Plymouth brand's first icon since the Barracuda, the "Purple Cat" as it has been called, packages many clever ideas to bring this exotic car to market for about $40,000. While some innovations such as the stainless-steel exhaust manifold have made it to high-volume production for 1998, the impact of others remains to be seen.

Because of the car's "blue-collar appeal," and in the true spirit of hot-rod building, Prowler uses many components from other vehicles. To meet the relatively low budget, 45% of Prowler's dollar value consists of carryover parts, according to Executive Engineer Craig Love. For instance, the 3.5-l 24-valve SOHC V-6 engine was borrowed from Chrysler's 1997 full-sized sedans. Inverted interior door handles were borrowed from the Viper. A modified steering wheel from the Grand Cherokee takes the Prowler through the turns, and the heating and air conditioning system combines Neon and Cirrus parts.

While the rack-and-pinion power steering uses lines unique to Prowler, a modified Chrysler minivan steering gear is mounted under the frame for easy access. The power steering pump is carried over from the current full-size sedans, while the steering reservoir is from the right-hand-drive Neon.

Moreover, the Prowler's electrical system includes components that range from the Dodge Ram's window-lift switches to the instrument-cluster board from a Cirrus or Breeze. According to Team Engineer James Finck, the company's minivan body controller makes it all work together. "Using modified carryover components allows us to offer the features of a luxury car without incurring all of the normal investment it would take to tool it up from scratch," he adds.

While 45% of Prowler's dollar value is in carryover parts, the remainder was invested in advanced engineering and cutting-edge technology. "The core idea of the work on Prowler is that we walk before we run," says Love. "The point is to try out new things in low-volume applications before applying new engineering principles to high-volume production cars."

Purple-cat passion runs deep among metallurgists as well, because the biggest design innovation may be Prowler's future impact on aluminum production. Nearly 900 lbs of the 2,862-lb roadster consist of aluminum--including body, frame, and suspension parts. It may be the most significant production effort with aluminum to date, according to Prowler Materials Executive Saad Abouzahr. Prowler uses virtually every known alloy and form of aluminum, from extrusions and castings in the frame, to suspension components made with a new semi-solid forging (SSF) technology.

As a result, the aluminum intensive Prowler weighs in 21%--roughly 600 lbs--lighter than a comparable vehicle with steel components. Will the switch pay off? Conventional wisdom says aluminum cars should be noisier, less able to hold up in a crash, and rust more easily. Abouzahr says that's old thinking. "These are high-strength aluminum alloys, the kind used in airplanes, boats, and sporting equipment."

Responsive powerplant. Prowler's powerplant produces 214 hp at 5,850 rpm, and 221 lb-ft of torque at 3,100 rpm. The engine has a cast iron block, forged steel crankshaft, aluminum heads, and aluminum pistons. The normally silver intake manifold is painted black, and the valve covers are silver, like old hemi valve covers.

For a quick throttle response, Prowler uses a Blinter cam similar to the one introduced in 1993 when Chrysler launched its first cars with this engine. Back then, consumers complained about the sharp throttle response, according to Prowler's Program Manager Paul Doolan. So engineers added curvature to the cams to slow down the response. "We took Prowler back the other way, giving it a crisp throttle response."

To transmit power to the wheels, Prowler has a rear-mounted, 4-speed, electronically controlled (4EATX) fully adaptive transaxle with the AutoStick system. The driver can choose between automatic or dropping it into manual mode with the flick of a switch. In contrast to other Chrysler cars with AutoStick, Prowler's transmission has no automatic upshift.

On other cars with AutoStick, putting the selector in first gear and pressing the accelerator to the floor results in the transmission taking only so many rpms before upshifting. On Prowler, that doesn't happen. It stays in first right up until the fuel is shut off at 6,400 rpms. Estimated cruising range, depending on the driver's habits, is about 250 miles.

Prowler's drivetrain layout has the engine up front, driving 20-inch rear wheels through a rear-mounted transaxle. A 52-inch long aluminum prop shaft runs down the middle of the car, connecting the engine-mounted bell housing to the transaxle bell housing, that also contains the torque converter.

While engineers intended to carry over the concept car's five-link independent rear suspension, an evolution of sorts took place. To simplify design and make the suspension more controllable, engineers went to a four-link independent suspension consisting of two adjustable upper lateral links that are actually modified steel Cirrus/Breeze/Stratus components: one upper trailing link, and an aluminum lower control arm. The rear springs and shock assembly are modified from the Dodge Viper and tuned specifically for Prowler.

The front suspension is a double "A" arm system. The spring/shock assembly mounts inboard, and is actuated by a pushrod-rocker mechanism. Prowler is the first production car to use this assembly, although it has been featured on Indy and Formula One cars. As on the rear, modified Viper springs and shock assemblies are used. The steering knuckle, rocker, and both upper and lower control arms are aluminum. The control arms, which are visible outside the body, are clear-coated for long-term finish protection.

All of the suspension engineering has paid off according to Chassis Engineer Joe Crowley. "The tires are a big part of that--they have a big footprint. The suspension geometry is set up to achieve a high level of handling. During evaluation at our proving grounds, we pushed limits with Prowler that we can't with a normal car. We're close to our Dodge Viper in a lot of things."

The wheelbase stretches 113 inches with an overall car length of 165 inches. Width is 76 inches, while Prowler stands 51 inches high with the top raised. Standard interior features include power door locks, power windows and dual air bags, and a 320W Infinity(R) sound system. Leather seats, unique to Prowler, are Chrysler's first that use Intergram's mold-in-place process. Other features include a remote keyless entry system, electronic speed control and air conditioning.

In a special meeting at Chrysler Corp.'s World Headquarters, engineers dissected the Prowler for Design News. Here are the major technical details and innovations:

A. COMPOSITE BRAKE ROTORS. 20% silicon carbide ceramic-particle reinforced aluminum brake rotors provide a 50% unsprung weight savings--6 to 7 lbs per wheel, at a cost of about $3/lb. Because aluminum melts at 1,200F, and mechanical properties start deteriorating at about 900F, engineers had to design the cooling vents so that the rotor's maximum temperature would not exceed 600F. Because inorganic brake linings transfer material and score aluminum rotors, special organic linings are used.

B. SMC BODY PANELS. The front and rear quarter panels, rear valance panel, and "motorcycle" fenders consist of sheet molding compound.

C. KEVLAR STRAPS. Two DuPont Kevlar straps on each side tie the body tub to the frame to distribute side-impact loads.

D. RUBBER-COUPLED DRIVELINE. Engine output shaft is hard coupled to the drive plate on the crankshaft flange, while flexible rubber couplings replace u-joints within the propshaft assembly. The rubber couplings partially damp out torque peaks in the rotational direction produced from the engine, preventing the propeller-shaft tube from conducting noise.

E.MAGNESIUM INSTRUMENT PANEL.The instrument panel (IP), the first of its kind in the industry, bears the load of all gauge parts to reduce noise, vibration, and harshness (NVH). It's a die-cast magnesium cross-car structure as opposed to a more typical panel with a welded structure made out of multiple components. With standard IPs, dimensional control proves difficult and scrap rates are high. This technology may have future applications for high-volume production because of the consistency of the die-cast parts. Prowler's IP combines more than 20 stamping and plastic components in a single thin-walled casting, making it 8 lbs lighter than conventional IPs.

F.ALL-ALUMINUM SEATS.An all-aluminum frame and seat pan saves another 7 lbs. Intergram supplies Chrysler's first leather-covered seat produced with the foam-in-place process.

G. ALUMINUM WHEELS.Because aluminum's molecular structure becomes highly oriented during spinning, welding a spun rim to a cast spider proves difficult. The material's structure seeks a more random state when heated. CA-based American Racing, the supplier, worked to develop the narrow-electron-beam welding technology that Chrysler required to precisely control heat application and ensure part uniformity.

H. RUN-FLAT TIRES.Front wheels sport P225/45HR17 tires. The 20-inch P295/40HR20 rear wheels may be the largest in the industry to make it on a production passenger car. All tires are Goodyear Eagle GS-D Extended Mobility (EMT) radials with run-flat capability (50 miles at 55 mph). A low-pressure sensing system registers on the instrument panel when tires need air.

I.HEADLAMPS.Prowler's unique headlamps are an integral part of the front painted body panel structure. The show car's flush, saber-shaped headlamps had a strong angle that made it difficult to obtain a legal beam pattern, so engineers used ellipsoid projector-beam technology from Bosch. The technology doesn't require large reflector surfaces, so all engineers had to do was bulge the light housings out from the sides, then make sure that the interior surfaces were properly curved to limit refraction.

J.BODY TUB. Body tub components consist mainly of marine-grade aluminum alloy, 5454-O temper. Welding aluminum demands a lot of energy. One alternative, used extensively on Prowler's body, employs rivet bonding instead. A shank of metal is driven through two sheets of aluminum and epoxy adhesive helps optimize joint stiffness.

K. FRONT FASCIA.The front fascia, front bumper covers are made of reaction injection molding (RIM) compound. Prowler's frame absorbs energy in stages during a crash. System met 30 mph impact standards by the third design iteration.

L. GRILLE.The grille consists of acrylonitryle-butadiene-styrene. Grounding, a very important issue in today's electronically controlled vehicles, is accomplished with special rivet nuts crimped into the aluminum frame. Corrosion between dissimilar metals requires special coatings.

M. STAINLESS-STEEL EXHAUST MANIFOLD. To meet new emissions requirements, engineers reduced upstream thermal mass. Moving the converters from under the car to under the hood, replacing heavy cast-iron manifolds with thin-walled stainless-steel manifolds, and using light-weight V-clamps to make the connection reduced time to steady-state converter operation.

N. 320-W SOUND SYSTEM. Prowler features a 320W Infinity(R) sound system with a six-CD changer behind the passenger seat and a cassette player in the instrument panel. There are two speakers in the instrument panel, two drivers in the doors, and a sub woofer in the tuned speaker enclosure that also serves as the back panel behind the seats.

Flat-hemming aluminum

Aluminum's lower elongation means steel can handle greater stress without work hardening. That's why flat-hemming aluminum requires two things:

  1. Increase hem radius to prevent splitting.

  2. Constantly move the stress point along the fold to avoid work hardening.

Advanced manufacturing

Just as Prowler put new materials and designs to the test, its manufacture helps Chrysler to evaluate other ideas:

Engineers enhanced scrolls with engineering data to make them suitable for use as operation description sheets (ODS). Updated daily at plant level, the new ODS system increases manufacturing flexibility and reduces costs.

  • Farmington Hills, MI-based Bamel Fastener Co. provides a single point of contact for hundreds of different fastener suppliers.

  • Fasteners, delivered on a daily or weekly basis, come in small-lot containers. Flow-through racks put the fasteners within the operator's reach, help maintain just-in-time inventory, and improve visual management on the factory floor.

  • Assembling painted body panels and powder-coated control arms without gouging or scratching, requires in-system damage control. Watch and belt-buckle wraps, special 3M tape, plastic sleeves, and protective racks and tooling all help minimize damage to Class A surfaces during assembly.

How to make aluminum suspension components.

In the early 1970s, MIT technologists discovered that metal alloys, when agitated sufficiently during solidification, form a rounded globular microstructure instead of the conventional dendritic microstructure. These semi-solid metal alloys exhibit non-Newtonian, thixotropically viscous behavior. When subjected to a shear force, the apparent viscosity of a semi-solid alloy at a relatively high volume fraction of solid decreases. As a result, the alloy begins to demonstrate flow characteristics similar to that of a high-viscosity fluid, such as heavy machine oil. Today, Alumax Engineered Metal Processes Inc. (Bentonville, AK) uses this technology to produce suspension and wheel components that weigh up to 9 kg. The plant can produce more than 10,000 tons annually. Raw material production for semi-solid forging (SSF) feedstock takes place at the Ferndale, WA-based Intalco Aluminum reduction facility.

Here's how the four-step process works:

Magneto-hydrodynamic billets or slugs are made from aluminum alloy 357 in a patented casting process that employs an electromagnetic stirring device placed around a specialized, direct chill solidification front.

  • Slugs, charged to a precisely controlled, continuous induction heating furnace, are returned to the semi-solid state. The material, somewhere between 40% to 60% solid, has the consistency of toothpaste.

  • The semi-solid is automatically transferred to a forming process that is similar to plastic injection molding. Material, pressurized into a permanent die, is then water quenched at the press.

  • Post forming secondary operations include gate, runner and overflow removal, T-5 or T-6 heat treatment, various quality assurance tests, tumbling, and packaging.

According to Manager Chassis and Powertrain Jon Rasbach, the SSF parts have lived up to expectations, meeting the minimum 9% elongation, 45 ksi UTS, and 33 ksi YS. Proper die design requires close work, up front between the design engineer and process engineer to eliminate cold shots.

Prowlers control arms, rocker arms, and front and rear knuckles are semi-solid forged. Although there is room for yield improvements, SSF advantages include:

Near-net-shaped parts, formed with a greater degree of predictability, reduce machining costs and time.

  • Mechanical properties are uniformly improved over typical casting processes.

  • Parts are less inclined to shrink and have fewer microscopic holes, both of which could weaken the part over time.

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