The 2020 is cool in many ways. It has a , and a . But by far, the coolest thing about it is its carbon fiber monocoque chassis. And you can’t even see from the outside, which is a shame.

In 2017, the coupe came out swinging with a new 4.0-liter V8, twin-turbo engine nested in a carbon fiber chassis called the . Unlike the that underpinned its predecessors, the MP4-12C and 650S, the Monocage II is different because, as its name suggests, it is a “cage.”

It has additional structures like the A-pillars/windscreen surround, above-engine supports and a roof beam to which the 720S’s dihedral (two-axis) doors are partially hinged.

This is what it looks like:

And here’s how the doors hinge to it:

Obviously, you can’t build a convertible with that roof beam there. And so the new, beamless structure that underpins the 720S Spider is called the Monocage II-S. Because there’s no longer a roof beam to attach those doors to, McLaren had to do a little re-tooling.

“We designed the lower A-pillar to accommodate the unique, single hinge needed for the Spider, versus two hinges for the coupe,” Ian Digman, McLaren’s global head of product management, told me via email.

“That hinge was specifically designed for the unique door, but maintains the dihedral operation.”

Honestly, McLaren could have just made the door the regular swing-out kind for the Spider and probably saved everybody a lot of trouble. But it went through the extra effort to move the hinge placement so that they still swung dramatically upwards. I can’t imagine there was much space in that area to work with, either.

You’d think that removing a roof structure such as the beam would mean that additional fortifications would be necessary elsewhere. Kind of like when you take the plastic lid off your paper coffee cup, it isn’t as rigid unless you add another cup to it.

Curiously, though, the Monocage II-S doesn’t need additional strengthening over the coupe to maintain torsional rigidity, according to a .

Andrew Kay, chief engineer to the 720S Spider, confirmed this during the media drive. “There really aren’t any changes,” he said, waving his hand over a Monocage II-S chassis on display. “We really didn’t need to do much to the core structure of the car.”

Since most of the chassis’s strength lies in the thick piece of carbon fiber that makes up its lower half, that part didn’t need to change. The “tub” part of the Monocage II-S is identical to the Monocage II, as is the windscreen surround.

That’s why the Spider can keep the same thin A-pillars and windscreen surround as the coupe (which, by the way, can supposedly withstand a load of 3.3 times the weight of the vehicle on each corner).

What is different, though is the fact that the top of the windscreen surround has been tapered slightly for the convertible top, as well as the all-carbon fiber rollover protection structure—which you can see jutting upwards behind the seats.

“This carbon element is structural,” Kay continued, pointing it out. “It’s all molded and bonded into the tub. It enables us to mount seat belts and the roof. And it can take 1.8 times the weight of the vehicle.”

Hanging out of the back of the chassis is the new over-engine rear structure that’s also been redesigned so that the roof stows into it. Those are really the only bits of the structure that differentiate the Spider from the coupe.

It’s remarkable that the Monocage II-S didn’t need much in the way of bolstering over the regular Monocage II in order to maintain the same stiffness between the two cars. Even Ferrari needed to beef up its tub for the 488 Spider variant, according to :

“The multi-metal tub has been bolstered at the front and rear to compensate for removing the roof panel. In doing so, Ferrari claims the Spider has the same torsional rigidity as the coupe.”

Really, after pulling the roof beam off and moving some door parts around, all McLaren needed was just to add some rollover protection on the top for the Spider and a place to store the roof.

“What’s that roof beam for, then?” I asked. “It seems pretty non-essential.”

“Just to keep the rain off,” Kay shrugged. “Pure and simple.”