While some people might think that if you have a car, say a coupe, and turn it into a convertible, chances are the convertible will be lighter than the original version, given that you’re chopping off the top.
Which is far from being the case.
That’s for a few reasons. One of the main ones goes to the fact that by removing the top, the structural rigidity of the vehicle is reduced. (Here’s one not-very-sophisticated way of thinking about this: If you have an egg, it is more difficult to crush in your hand if the shell is complete; remove part of the shell and then try the crush test, with a wad of paper towels nearby.)
This means that many convertible producers have to add structure to the vehicle, which adds weight. Weight is also added in order to have the mechanisms in place that allow the top to retract, especially if this is a convertible that has a retractable hardtop.
All of which is to get us to a place where the benefits of carbon fiber can be realized. The McLaren 12C Spider, which is based on the 12C Coupe, which means that there is a carbon-fiber chassis structure, weighs just 88 lb. more than the Coupe.
According to McLaren, thanks to that carbon fiber “MonoCell,” which weighs a mere 165 lb., the Spider “requires no additional strengthening.”
In case you’re wondering about the consequence of having a convertible version of the 12C: in a run from 0 to 124 mph, the Spider accomplishes it in 9.0 seconds, or 0.2 seconds slower than the 12C Coupe. If you’re running from 0 to 62 mph, the two are a wash, at 3.1 seconds.