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www.ADandP.media the same in the orthogonal direction." McLaren's F1, for example, used then-current Formula One technology, and was designed around pre-preg materials and the use of an autoclave. Using this technique, he says, "You can make something as complicated as your mind can imagine the tool being because it is manually broken out of the tool afterward. If it needs 27 pieces that you take out one-by-one, that's what you do." However, the cycle time for each tool and autoclave is eight hours, which limits production to a few parts per day. Says Thomson, "The game changer for us in utilizing a material and its properties is in the use of sheet molding compound." On the new 720S sports car, the door shut area and visible portions of the structure inside the cockpit are formed using carbon SMC. Its cure time, which takes place in the tool, is just a few minutes. Resin transfer molding (RTM), which has a somewhat longer cure time than carbon SMC, also cures in the tool and is much quicker than the pre-preg/autoclave setup. And though Thomson remains tight-lipped about the actual process, it is thought that McLaren's Monocell is made up of multiple pieces bonded together into a single unit via RTM. "With every car we've introduced since the 12C," says Thomson, "we have iteratively improved on the Monocell concept." When the 650S replaced the 12C, it carried over the previous car's 75-kg. (165-lb.) Monocell intact. Which meant that, except for the Spider models with their retractable hardtop design, the upper structure consisted of a stamped aluminum windshield surround mated to roof structural elements stamped from boron steel. The less expensive 570S has a revised Monocell tub with lower side sills for improved ingress and egress. And, since cost and cycle time are more critical, steel replaces aluminum for the windshield surround. However, with the introduction of the P1 hybrid supercar in 2013, the Monocell begat the Monocage; the distinction arising from the use of woven carbon fiber for the complete roof structure, including the integrated snorkel air intake. With the Monocage II, McLaren mates the third iteration of the Monocell to a pre-bonded upper structure with woven sections, mimicking the construction used in the P1. Cost reductions and design targets drove the change in material, giving the 720S sports car thinner A-pillars, larger door openings and more glass area. 35 AD&P; ∕ OCTOBER 2017 CHASSIS