To the average driver, it's invisible: a big, bulky, non-descript lump of metal that dutifully bears their vehicle's largest loads in virtually every situation. Most drivers never see it; few ever give it a second thought. They know only that it's down there, somewhere beneath the powertrain. And it's probably made of steel.
Probably. On one of the world's most popular vehicles, however, this is no longer the case. The front suspension cradle--the beefy structural component that carries huge static and dynamic loads on Chrysler's minivans--is aluminum.
To many, that may sound almost like a contradiction in terms. Structural aluminum? Aluminum, after all, has built a reputation as a lightweight material. But experts say there's no reason it can't be used in automotive's toughest structural applications.
"Aluminum may have some of its best applications in structural components," notes Dr. David Cole, director of the Office for the Study of Automotive Transportation at the University of Michigan. "It serves very effectively there."
Indeed, looking at the advantages of Chrysler's aluminum cradle makes one wonder why the industry hadn't done it earlier. Check, for example, the list of items that can now be mounted on the suspension cradle: ABS control unit, brake junction block assembly, leak detection pump, power steering pump oil cooler, and a host of items that weren't mounted on the steel version.
What's more, aluminum provides better dimensional integrity: about a half-millimeter across the length of the part, compared to about three times that for steel.
Best of all, however, is the weight reduction. In an era when engineers work frantically to cut a pound of weight from a vehicle, Chrysler engineers reduced the weight of the minivan's suspension cradle by at least 14 lbs. While conservative estimates held that a stamped steel cradle would have weighed more than 40 lbs--the aluminum version weighs a scant 26 lbs.
Chrysler managers are ecstatic over the thought of having removed so much weight. "Weight reduction is one of the few universal goods," notes Bob Gasparovich, who served as manager of suspension engineering when the cradle was developed. "It makes fuel economy better. It makes the vehicle perform better, handle better, and stop easier. It does just about everything that's good."
No easy choice. In retrospect, the advantages of aluminum now seem obvious. But it wasn't always so. Steel had long served as the material of choice for structural components. Its strength and low cost made it an easy selection. Plus, engineers understood it. It presented no unknowns.
But when Chrysler engineers began redesigning the minivan, nothing was taken for granted. For the front suspension cradle, their first priority was functionality.
What's more, Chrysler designers were adopting the cab-forward configuration that later made the LH vehicles so successful. That meant that there would be less room under the hood and, as a result, a tighter packaging environment for the cradle. To meet those packaging constraints, they needed to maintain unusally high levels of alignment integrity.
Chrysler engineers first discussed the idea of alternative materials in 1991. "We knew the part would have a complicated architecture," Gasparovich recalls. "And we worried about whether we could make a steel part with the dimensional integrity we needed."
When engineers investigated aluminum as an alternate material, they found that it met those needs. An aluminum cradle could be cast in a single piece, with more material placed wherever they needed it. The benefits of the casting process were twofold: providing greater strength in key areas; vastly improving dimensional integrity by eliminating the need to weld as many as five steel pieces together.
A cast aluminum cradle also offered another advantage: Adding an attachment to it was simple. To do so, assemblers would merely need to drill and tap a hole. Attaching a component to a steel cradle meant welding a bracket into place, then adding nuts and bolts.
For Chrysler, the changeover to aluminum was a cultural one. "We've been a steel-based company for a lot of years," notes Bernie Swanson, who served as chassis executive engineer for the minivan platform during the development of the aluminum cradle. "We've made aluminum transmission cases and engine parts, but we haven't made load-bearing members."
If the decision were being made for a low-volume vehicle, it wouldn't have been as difficult. But this was the minivan--the Chrysler minivan. For more than a decade, it had dominated the industry, serving as a cornerstone of Chrysler's comeback. Yearly production volume ranges between 700,000 and 800,000 vehicles. And aluminum was more costly than steel.
In the spirit of Chrysler's new team approach, the company's engineers met with other platform members who had expertise in materials, finance, procurement, design, and a range of other disciplines. Together, the team hashed it out, finally reaching a decision that would have once been unthinkable.
"We wouldn't have made this decision 10 years ago," notes Gordon Rinschler, general manager of the minivan platform. "It never would have gone past the stage where it was a twinkle in the engineer's eye. Someone would have said that it cost too much, and it would have been all over."
Chrysler executives say that the decision-making process on the cradle is typical of the company's new platform-team approach. Co-located team members met frequently, often informally, before finally settling on aluminum. That approach helped to break down walls that might have once existed between departments. "Everyone who needed to be part of the cradle decision was probably within 100 feet of each other," Gasparovich says.
Packaging challenge. Once the decision was made, however, the hard part started. Swanson and a team of engineers and designers began the task of designing the new cradle.
One of the most difficult aspects was the packaging. Chrysler product designer Ken Dostert configured the new cradle, employing the advantages of the casting process to place more material where needed. Dostert shaped the cradle to accommodate the multitude of powertrain and steering configurations. He determined proper thicknesses for heavily loaded areas and ensured that clearances were sufficient in the tightly packaged environment. He also made allowances for various attachments and "windows" in the cradle.
The all-wheel-drive version, for example, required a special, semicircular cut-out for the steering's power transfer shaft. The cut-out would later enable the steering rack to be replaced without having to remove the cradle, thus reducing repair and warranty costs.
Designing the cradle in this way would have been impossible with stamped steel, engineers say. But with a casting, it was possible for them to adjust the design as needed. The advantages of the process even extended to the stress analysis procedure. In the past, the most difficult part of analyzing a steel cradle was modeling the stress concentrations on the welds. But because the casting consisted of a single element, instead of a welded assembly, engineers were spared the task of guessing at stress concentration factors.
"This was probably the most reproducible finite element model we've ever had," Swanson says. "We didn't have to worry about going back and forth between the software model and the bench tests to determine the correct stress concentration factors."
10.4 X 106 psi
30 X 106 psi
Supplier partnerships a key. For Chrysler's engineers, the biggest challenge in developing an aluminum cradle was on the process side, not engineering. "The question is not whether we can make one, but whether we can make 800,000 of them," Swanson explains
Indeed, aluminum was by no means a new material for Chrysler. The automaker had used it in engine blocks and heads, as well as in transmission cases and wheels. But no one had ever made a structural aluminum component in such high volume (Chevrolet's Corvette uses aluminum suspension components, but in lower volumes).
To turn the cradle into a reality, Chrysler found a supplier, CMI-Precision Mold, Inc., Bristol, IN, and began working with its engineers. CMI assigned two engineers, Chad Bullock and Tim Glilliland, to interface with Chrysler on issues of manufacturability. Together, the two teams re-shaped the product for the casting process. They developed special heat treating steps for the aluminum parts, then constructed high-tech molds with built-in distortion, which compensated for the heat treating.
CMI's involvement stretched all the way to the software level. Within Chrysler, Dostert generated a wire frame model on CATIA software, then called upon CMI engineers to "surface" it. CMI engineers also performed mold flow analyses and thermal feasibility studies.
"This process has broken some of the old paradigms," notes John Romain, a Chrysler product engineer assigned to the cradle project. "The old way was to have the vehicle engineer throw it over the wall and walk away. We didn't do that here."
Throughout the process, Chrysler counted on its suppliers for major commitments. CMI, suddenly facing the task of building 800,000 large aluminum parts per year, nearly tripled its manufacturing space, from 110,000 sq ft to 300,000.
Chrysler, meanwhile, worked with test equipment suppliers to ensure that the aluminum parts met specifications. For example, they collaborated with suppliers of X-ray equipment on the development of special systems to check the aluminum parts for porosity. Because porosity is a key measure of the aluminum's strength, they devised a system that would take as many as 30 different views of the aluminum cradles to look for acceptable porosity levels. A knowledge-based computer program checks each "picture" to see if the part meets specifications. "When you take a big step like this on a high volume piece, you end up dragging the entire industry with you--from the firms that make the part, to the companies that build the X-ray and bending equipment," says Swanson.
Aluminum test-bed. The new suspension cradle, which quietly debuted with the introduction of the 1997 minivans, has yielded a multitude of benefits for Chrysler. Its design enabled Chrysler engineers to employ a fully isolated front suspension, thus improving the vehicle's noise, vibration, and harshness characteristics. It also allowed for the front suspension to be reliably built and repeatably positioned in an automated assembly fixture for installation into the vehicle.
Despite the recent growth in the industry's use of aluminum, experts say that its popularity could drop off as quickly as it grew. "The future of this material in vehicles is very dependent on fuel economy standards and volatile material costs," says Cole of the University of Michigan. "If those things change, everything changes."
For that reason, Chrysler engineers view their use of aluminum as a moving target, with ever-changing goals. "From a vehicle standpoint, we can say that we've been successful," Swanson concludes. "But we can't be completely satisfied. The way to remain competitive is never to be satisfied."
How aluminum cradles are made
To develop a manufacturing process for their aluminum cradle, Chrysler engineers collaborated with designers at CMI-Precision Mold, Inc., Bristol, IN. Together, the two firms developed a high-tech, 10-step process to cast, heat treat, straighten, and inspect the aluminum parts:
Step 1. Reverbratory furnace melts A-356 alloy and introduces molten material into 187' heated launder.
Step 2. Gravity die cast machines produce 80-lb cradle.
Step 3. Multiple-stage trimming machines saw and mill. Casting reduced to about 30 lbs.
Step 4. Continuous Roller-Hearth processes product through solution furnaces, polymer quench, and double-age oven.
Step 5. Automated straightening cells use laser-targeted anvils to gently bring castings within strict tolerances.
Step 6. Mechanical and microstructural properties are evaluated at materials lab.
Step 7. Six-station transfer line qualifies castings for machining
Step 8. Castings undergo final machining.
Step 9. Automated system X-rays each casting. Computerized recognition system identifies defects.
Step 10. Castings undergo partial assembly of lower isolators.
Steel-aluminum cradle comparison
Items mounted on Chrysler's aluminum suspension cradle,versus a list from the minivan's old steel cradle:
Aluminum Cradle Mounts Steel Cradle Mounts
Lower control arm
Steel Cradle Mounts
Lower control arm