Cadillac will make its debut in the compact luxury market segment this year, thanks to a massive engineering effort that has resulted in a lightweight, high-strength body structure.
The structure makes liberal use of aluminum, magnesium, high-strength steel, and ultra-high-strength steel. It played a key role in bringing the new ATS sedan in at a curb weight 500 pounds lighter than that of its closest predecessor, the Cadillac CTS.
"We counted mass on all of our structural members in terms of grams," David Masch, vehicle chief engineer of Cadillac, told us. "We even ended up debating the size of fasteners."
The ATS body structure employs a combination of aluminum, magnesium,
high-strength steel, and ultra-high-strength steel.
(Source: Design News)
Cadillac's design effort called on teams of structural engineers, chassis engineers, materials specialists, and mathematicians to create a body structure that would manage loads intelligently. By positioning structural members to be in shear, rather than in bending, the designers made cross-sectional areas of virtually all members smaller. "Putting members in shear is the most efficient way to manage loads," Masch said. "The more you can get those structural members in line with the loads, the more efficient you can make them."
Cadillac's approach to cutting weight out of its vehicles reminds me of the diet plan, Weight Watchers, which advocates a slow and methodical approach to weight loss unlike some other plans that look for bigger, more immediate results. I think Cadillac's commitment to cutting mass in the ATS by reexamining every facet of the design, down to the fastener level, can't help but be the more effective way to ensure a lighter, yet still highly stable and high performance vehicle. Chuck, obviously new material choices and close attention to customer requirements played key roles in its redesign effort, but what about use of CAE software? I'm assuming that FEA played a key role in the weight reduction redesign operation.
This story is a significant look at incremental engineering to achieve a goal. The "big" weight-reduction wins aren't there anymore, but there's still gas mileage savings to be achieved with every -- as you say -- gram you can shave off the weight of a car. Cadillac focused its engineering team on this task and achieved it. There's an important lesson here about the team engineering process which extends beyond the car. It applies to the entire auto industry, as well as to smaller-scale product design such as in medical miniaturization.
Chuck, thanks for a great article on an important topic. This is a good lesson about how to do lightweighting the right way. Although the headline of the article says that they cut mass one gram at a time, it's important to note that Cadillac has a well thought-out overall approach, rather than just blindling chunking mass out of components.
I've worked on projects where design groups working on each subsystem were each mandated to reduce weight by 10%. There was no "big picture" view. Designers frantically scrambled to take weight out of components wherever possible, reducing wall thicknesses, adding holes and pockets, etc. In many cases, this resulted in poor decisions, many of which had to be reversed later at significant cost.
In contrast, Cadillac started out with load path optimization. Then they built the vehicle around the optimized load path. This is a much smarter approach, which focuses around making sure that mass is utilized efficiently.
It's also noteworthy that Cadillac's lightweighting strategy made extensive use of steel. The idea that lightweighting always consists of replacing steel with aluminum is simply wrong. In some cases, steel is the best way to get the strength and stiffness you need with a minimum of material.
Another thing worth noting is that plastics and composites apparently were not a major part of this lightweighting effort. I'm sure that Cadillac probably looked at the possibility of using plastics or composites for some of these parts. It would be interesting to hear their reasons for staying away from these materials.
Beth: I didn't ask David Masch about CAE and FEA at the auto show, but I'm sure that played a big role. These days, it's almost unheard-of to do a structural design like this one without the aid of FEA. The beauty of FEA is that it enables engineers to manage stresses and strains in a way that allows them to optimize the cross-sectional area of members, and therefore optimize the weight.
Beth, here's an addendum to my earlier response: Cadillac's official press info says, "Advanced computational development helped determine the most efficient design." The press info also makes special note of the fact that a team of mathematicians was involved in the design.
Great article, Chuck. With all of the strong metals, I can't help but think one of the objectives is to protect the driver and passengers in the case of a collision. I know one of the fears involved in small-car purchases is the problem that comes when a small vehicle hits a giant SUV. Perhaps these metals answer that fear.
Great point about computational analysis, which ties in with the recent crop of stories Beth Stackpole has done about CFD and FEA (computational fluid dynamics and finite element analysis) seeing broader usage. Of course, the automakers have long performed such analyses.
Your comments all seem to applaud Cadillac's design efforts. And, for the redesign points noted, they all may be justified. However, at 3400 pounds for a 'compact' SUV, that seems a little heavy for the mission of fuel efficiency for the next 5 years US standards. For all of the money spent for this redesign, it appears that the computational dynamics forgot to include the aerodynamics of the vehicle. And, when cars are so expensive, is there really that much usable space in this vehicle? In my 25+ years in commercial aviation, we always always had to consider the aerodynamic loads as the key to fuel efficiency. Apparently, Cadillac did not based on the shape of this vehicle, and it was not mentioned in the article or any of the comments I read. Engineering wisdom is based on past successes and failures. The Dodge Intrepid of 2000 has more space, one of the lowest coefficient of drag at 0.42, and a weight of only 3200 pounds with the highest level of torsional ridigity of any passenger car since then, and at a much lower price while easily achieving 28 to 32 MPG at 70 MPH.
Honda sells more cars than Cadillac. Maybe the reason for that success is that Honda started and still employs design engineers from the aviation industry. And, 'powdered metal connecting rods' - that is a recall just waiting to happen.
@windy9: As a rule of thumb, I've found that it's not a good idea to use powder metal in any application which sees significant tensile loads (like a connecting rod). That being said, powder forged connecting rods are widely used in the automotive industry. According to this article, more than 500 million powder forged connecting rods were produced between 1986 and 2005. So it's not exactly like this is something new which Cadillac came up with for the ATS.
The powder forging process produces a significantly denser part than conventional powder metallurgy (nearly 100% dense), so the mechanical properties are much closer to a wrought material.
As far as aerodynamics, you have a valid point, but I'm not sure that people who buy luxury cars necessarily want something that looks like a 2000 Dodge Intrepid.
The question of whether engineers could have foreseen the shortcut maintenance procedures that led to the crash of American Airlines Flight 191 in 1979 will probably linger for as long as there is an engineering profession.
More than 35 years later, the post-mortem on one of the country’s worst engineering disasters appears to be simple. A contractor asked for a change in an original design. The change was approved by engineers, later resulting in a mammoth structural collapse that killed 114 people and injured 216 more.
If you’re an embedded systems engineer whose analog capabilities are getting a little bit rusty, then you’ll want to take note of an upcoming Design News Continuing Education Center class, “Analog Design for the Digital World,” running Monday, Nov. 17 through Friday, Nov. 21.
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