2014 Cadillac CTS: Seriously Light
Here’s a look behind what went into engineering the 2014 Cadillac CTS, which is the lightest car in its class. Why is that important? Says Dave Leone, GM Chief Engineer, Performance Luxury Cars, “We did that to enable the driving character we wanted to achieve.”
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As Dave Leone has it, in developing the 2014 Cadillac CTS, he and his cadre of engineers—a group of people who had worked on the Cadillac-redefining ATS—went to school on the competition. As the Chief Engineer, Performance Luxury Cars for GM, Leone knows that if they’re going to attract people to Cadillac, those people are probably going to be inclined to shop BMW or Mercedes, so Cadillac needs not to be competitive, but flat-out better.
One of the reasons that the ATS has proven to be so popular is because of its performance. This is predicated on a variety of things, notably the fact that the car is comparatively light. The mass for that car ranges from 3,315 lb. for a vehicle with a 2.5-liter engine, automatic transmission and rear-wheel-drive to 3,629 lb. for an ATS with a 3.6-liter engine, automatic transmission and all-wheel drive.
So because the CTS is based on the architecture of the ATS, and because, Leone says, they realized the importance of weight vis-à-vis not only ride and handling, but the mass that the engine must move, they had a mantra when developing the CTS—“Lower, Longer, Leaner”—and while the design team could address the lower and longer parts (with the executional assistance of their colleagues in engineering and manufacturing, of course), Leone’s people went hard at the “leaner” part. “Within the engineering team we said we’re going to count every gram, every engineer, every day,” Leone says. “Every gram had to earn its way on the car or it didn’t get there.” And they executed changes to things that already seemed light on the previous-generation CTS, changes that are based on materials, changes based on design.
Leone estimates that when all was said and done, about 70% of the improvements in weight can be accounted for by clever design engineering and 30% by material replacement.
What did they achieve? Compared to the second-generation CTS, “Our body is 8.5% lighter and 40% stiffer. The overall car is 7% lighter.” So, speaking to the torsional stiffness, Leone says base car to base car, the torsional stiffness of the 2013 model is 19.2 kN•m/°and the 2014 is 26.8 kN•m/°. Lighter. Stiffer.
So that’s CTS-to-CTS.
What about the aforementioned competition?
Leone makes a comparison between the 2014 CTS with a 272-hp turbocharged 2.0-liter I4 and the 2013 BMW 528i with a 240-hp turbocharged I4. He says they are comparably equipped. The CTS has a curb weight of 3,616 lb. The 528i comes in at 3,814 lb, a 198 lb. difference. Leone points out that this means the CTS needs to move 13.3 lb per horsepower and the BMW 15.9 lb.
A similar situation holds when six-cylinder models are compared. The CTS has a curb weight of 3,745 lb. and the BMW 535i weighs 4,090 lb.
Getting to a reduced mass was essentially a component-by-component exercise. And here are some examples.
B-pillar. In the 2013 model it is made with high-strength steel. It is a combination of eight pieces with a constant gauge that are welded together. The 2014 B-pillar is a two-construction, with the main pillar being a tailored rolled blank made of ultra high-strength steel. It varies in thickness from 1.4 to 1.9 mm, with the greater thickness located where there is greater strength needed. Additionally, there are scalloped edges on the 2014 B-pillar, with material removed where it isn’t needed for structure. Not only is the 2013 B-pillar more complicated to produce (i.e., the blanking, fixturing and welding), but it weighs 14.02 lb. The 2014 B-pillar weighs 10.19 lb.
Doors. A material change. In 2013, both doors are steel. For 2014, aluminum. The front door is 33 lb. lighter and the rear 22 lb.
Instrument panel structure. It is a cast magnesium component in the 2013 CTS that weighs 21.6 lb. The 2014 replacement is made of aluminum, using a central tube to which extrusions and brackets are welded. It weighs 14.4 lb. Not only is it some 36% lighter, but Leone points out that to produce the magnesium casting, they had to deploy a 500-ton press. The 2014 alternative mainly requires a few welding fixtures. “The aluminum structure is lighter, stiffer, and cheaper. Appreciably,” Leone says.
Engine cradle and support. In the 2013 CTS, a 61.4-lb steel component. In the 2014 CTS, it is an all-aluminum assembly (extrusions and castings) that weighs 24.4 lb.
Engine mounting bracket. For 2013, they’re made with cast aluminum and weigh 3.6 lb. For 2014, they’re cast magnesium and weigh 1.5 lb.
Front lower control arms. In both 2013 and 2014 they’re aluminum. But doing a redesign of the structure, the weight was taken from 12.9 lb. to 6.1 lb.
Rear suspension lower control arms. Again, for both models, steel. But the redesign brings the weight to 4.8 lb. from 8.3 lb.
Rear cradle for the differential, springs, and suspension attachments. Steel for 2013 and ’14. But 69.4 lb for the former and 54 lb for the latter.
But a question arises.
Leone worked on the first-generation CTS and headed up the engineering efforts on the second and third generations. They did smart engineering back then. But when you look at the differences made in engineering for this third-generation vehicle, it is astonishing how different things are on a component-by-component basis. So what happened?
“We had a change in design philosophy,” Leone answers, and then goes on to explain the difference in the genesis of the first-generation CTS, which appeared in 2002 (the second appeared in 2008) compared with the third. “When we designed the original CTS, it was also to have the SRX and the STS as part of the family,” he says. That is, both a crossover and a larger sedan. “The platform”—code-named Sigma—“was scalable, but the CTS was the little guy and it carried what we call ‘scar mass’ so that it could handle the components that went across the larger, heavier vehicles.” What they did was considered the biggest possible powertrain, the largest tires and wheels, and the worst load cases possible and engineered accordingly.
“What we did for the ATS and the CTS”—both are the Alpha platform—“is took the highest penetrating options and packages, the most popular configuration—in this case the 2.0-liter turbo with 17-inch tires and wheels—and took the most heavily penetrated market, the USA, and designed it to withstand the loads.”
That is, rather than designing the vehicles to not only accommodate the largest, the heaviest, and the worst-case scenarios, they engineered the vehicle for the most-likely scenarios. Leone says they took “intelligent risks.” Reinforcements are deployed as needed, not everywhere. The fundamental platform, Leone says, is capable and durable: “You can take it anywhere, and it feels rock-solid.” It is engineered for what it needs to do.
But there is the materials cost question, as well. “Is it more expensive? Yes it is, because aluminum doors are more expensive than steel doors,” Leone admits. He continues, “But it is not as expensive as you might think, because the designs are more efficient. The premium materials aren’t free, but the efficiency is.”
Which is to say that intelligent engineering makes a measurable difference vis-à-vis everything from mass to cost.
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