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The all-new C1 platform's mix of steels ranges from mild to wild-strong, but the application of each was modeled to the ounce.

Cadillac XT5’s new platform cuts weight—at less cost

It now practically goes without saying that a newly introduced vehicle will tout a weight savings compared with not just the predecessor model, but also with competitor vehicles that may (or may not) have been developed at an earlier point in the industry’s pell-mell lightweighting initiative.

So few were surprised when GM engineers announced during the recent media introduction for the 2017 Cadillac XT5 that their all-new midsize crossover is a ringing 292 lb (132 kg) lighter than the SRX it replaces. No, the revelation came when they noted that XT5’s new global C1 platform essentially eschews any “mixed-materials” strategy increasingly employed across the industry to achieve mass reduction on a vehicle architecture-level scale.

This seems to be a distinct materials-philosophy shift for GM's luxury unit. Compared with the aggressive lightweighting efforts for the brand's two newest cars, the 2017 CT6 full-size sedan and the midsized CTS, there is virtually no aluminum or other lightweight metal or composite found in the C1 structure—or anywhere on the XT5 body. Instead, GM engineers first approached this new modular crossover platform with the notion of “driving out waste” by carefully analyzing and then optimizing how the C1’s various steel alloys were used—and joined—said Paul Spadafora, the CT5’s Chief Engineer.

Spadafora told Automotive Engineering that the C1 structure employs a range of high-strength steels and that by intricately modeling how each contributes to the overall chassis assembly, then paying particular attention to optimizing the joints not only between those steels but throughout the structure, “every ounce was looked at” and eliminated if not required.

“As different high-strength steels come together,” he said, “the joints have room for optimization.”

Despite gaining 2 in (51 mm) in wheelbase, an inch (25 mm) in track, and serving up 3.2 in (81 mm) more rear-seat legroom compared to the outgoing SRX, the new XT5 has nominally smaller exterior dimensions. Meanwhile, the vehicle is at minimum one full test weight class below SRX, revealed Larry Mihalko, the XT5 Vehicle Performance Manager. And, it's two classes below SRX in some trim levels.

Perhaps even more a signal of the difference a couple of years makes in today’s weight-scrutinizing product-development cycles, the XT5 is a staggering 650 lb (295 kg) lighter than the like-sized Mercedes-Benz GLE-Class crossover and 100 lb (45 kg) lighter than Audi’s Q5, which itself is 7 in (178 mm) shorter than the new Cadillac.

“We did it with steel,” asserted Mihalko. “We were able to do it without [extensive reliance on] expensive materials.” The strategy is an unmistakable departure from Cadillac’s CT6 flagship, where GM’s new Omega platform is a combination of aluminum and high-strength steels and the body-in-white also is a complex multi-metal mix. In all, 64% of the 2016 CT6 is said to be aluminum. The CT6, reported Automotive Engineering in April 2015, is “arguably the industry’s most aggressive combination of lightweight materials, forming technologies and new joining methods in a non-exotic sedan.”

Prior to the CT6’s launch, Cadillac took a similar path with the all-new 2014 CTS midsize sedan, where a weight-prioritizing development program cut poundage 7% compared with the previous CTS and made the car some 200 lb (95 kg) lighter than an equivalent BMW 5-Series.

Despite the CTS’s acknowledged dynamic excellence, however, its hardline lightweighting translated to a cost that put the CTS at a disadvantage in the hyper-competitive midsize luxury sedan market. Johann DeNysschen, GM Executive Vice President and President of Cadillac, flatly admitted to journalists at the XT5 launch that the aluminum-intensive CTS “costs a lot of money to make.”

Whether the experience from its newly lightened sedans had any influence on the C1 platform’s “steel only” development direction is difficult for outsiders to know. But what’s certain is that the C1 achieved a high degree of mass reduction with a 10% upgrade in torsional stiffness (and hit its first-order beaming-mode targets, said Mihalko) without resorting to high-cost light materials.

However, the platform is not the only source of the crossover’s significant diet. Also contributing were the design of the vehicle’s all-new 5-link rear suspension, which accounted for 70 lb (32 kg) of the XT5’s total weight loss. Advances to the dissipative materials used for acoustic attenuation chopped another 30 lb (14 kg).

But for the C1 architecture, which will underpin a spate of different-sized future Cadillac and other GM-brand crossovers including the upcoming GMC Acadia, design optimization was, in effect, more important than the materials themselves.

“It all goes back to analytical tools,” Mihalko claimed. “They just keep getting better.”

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