Image by: Naperville Cadillac (Art by Jason Torchinsky)

A Florida-based coachbuilder that specializes in turning hard-top cars into convertibles got its hands on a 464-horsepower Cadillac ATS-V Coupe, and morphed it into something amazing—a convertible. Here’s how the company did it, from reinforcing the unibody to custom-forming the top to yield the perfect shape.

The Cadillac ATS-V is a hell of a car, with great steering, a sharp but comfy suspension, a ferocious engine, and even an impressive cooling system. But as solid as it is, the ATS-V is missing one thing—or should I say, it isn’t missing one thing: a roof.

Cadillac of Naperville in Illinois has got that problem solved, as it’s selling a 2017 Cadillac ATS-V Coupe that has been converted into a convertible for over $100,000. The car actually looks damn good.

To learn more, I called up the dealer, and—after failing to find anyone there who knew how the Cadillac ended up a convertible—I did some Googling, eventually finding the custom body shop that built the car: Convertible Builders, LLC. I talked with the owner, Jeff, and his dad Larry, the sales manager. The duo broke down how their company—which Jeff says specializes in “cars that should have been made by the factory as convertibles, but they didn’t”—pulled off this amazing build.

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Image by: Convertible Builders

“We reinforce underneath substantially prior to cutting the car, because in a unibody vehicle, the top is part of the strength and structure of the car,” Larry told me.

Those reinforcements (some of which are shown above under a red ATS) run longitudinally inboard of the rocker panels, and also laterally as “cross pieces” that he says don’t affect servicability but do provide “a significant twisting benefit.” Jeff also mentioned diagonal beams connecting the longitudinal reinforcements, saying “Basically, what we do is build triangles underneath [the car], and that’s what’ gives us our strength.”

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The reinforcements still allow for the removal of the exhaust, driveshafts, engine and transmission. “Everything’s welded in,” he said. “Years back, people used to try to bolt them in for [serviceability], but we just build [reinforcements] around everything that’s there so it’s completely serviceable. You can even remove the fuel tanks.”

In addition, there’s a heavy-gauge sheetmetal plate called a torque-box connecting the underbody reinforcements to the front engine cradle. “That’s what eliminates the cowl shake,” Jeff said. “The last thing you ever want to do is be driving with your top down and look at your rearview mirror and see the thing shaking all over the place.”

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After the top gets the axe, the team installs interior reinforcements, especially to the B-pillar to allow it to handle door slam loads and also side impacts. “That is most definitely for side impact protection,” Jeff told me. “We bring [the reinforcement] down at an angle from pretty much the top of the B-post inside that panel towards the center of the car.”

On top of that, Convertible Builders ties the two shock towers in the rear together as well as the strut towers up front for even more rigidity (in the case of the Cadillac, Jeff recalls, a front strut tower brace already existed).

Another area that gets extra reinforcement is above the windshield between the A-pillars, just in front of the soft top—this area is called “the header,” and it’s the only area that the shop has to paint.

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“There’s three layers [of steel] on that header,” he said. “We weld in reinforcement steel across that header.” These reinforcements, and especially the act of welding the three-layers of the header together, act to tie the A-pillars together, and provide more stiffness to the area above the windshield, where the roof fastens.

“It adds rigidity to the car. Also, those are the attachment points where our roof comes forward with the hooks, and we hold the roof there,” Jeff told me. “As well as, you need a nice finish. So actually, when you sit in that car with the top down, you look up there, and it’s a nice smooth, painted piece. And it looks like it came from the factory that way.”

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In the end, the car has been substantially stiffened. When I asked how much weight the finished car has gained over the original, Larry told me that the company’s builds usually average somewhere close to 100 pounds heavier. He pointed out that most of the added weight is down low, so the final product has a lower center of gravity than the hard-top car on which it’s based.

Image by: Convertible Builders

But the reinforcements are just part of it, since building the top itself is also a fairly complex process. For each new design, Convertible Builders installs a “rigid jig and fixture” into the hard-top car, moving steel rods into position so that they touch the roof in a number of places.

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Technicians “lock” those rods in place via welds so that, after the original roof has been cut off, they can shape the soft top’s bow structure to match the original roofline.

“For every model, we design a top specifically for that car,” Jeff said, proudly. “We identify where that roof is before we cut it...We build [the new top] to those points, so we maintain the factory roof-line of the car.”

Apparently the company manages to get pretty darn close. “The biggest compliment we can get,” he continued, “is people think it’s a simcon,” Jeff went on, referring to a “simulated convertible” (a vinyl top).

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Image by: Convertible Builders

That soft top consists of three layers: the outer black top layer, padding beneath it and the headliner. It folds into the upper front part of the trunk. Getting that top to fold perfectly into place requires some more custom work, with Larry saying the factory hinge would get in the way. Hence why Convertible builders put together their own special design shown above.

And that foldable top doesn’t require modification to the rear seat, either. “These cars obviously aren’t designed to be convertibles,” Jeff said. “The big thing that we pride ourselves on is we don’t alter the rear seat at all. We don’t narrow it, we don’t lean it forward...These seats are completely intact.”

Larry says his company has been doing this kind of work for over 40 years, and specializes in a number of cars including new Dodge Challengers, of which he says there are 500 customized convertibles already on the road today (including Hellcats, which Jeff says he’s been told actually handle better than the hard-tops on which they’re based).

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An 840 horsepower Dodge Demon is arriving at the shop soon. That’s going to be a convertible that will remove hair from your head and move it down to your chest. As you might imagine, like with the Hellcats, Convertible Builders will reinforce the crap out of that thing.

H/t Stanford via Tips@jalopnik.com