Why are soft-top cabriolets heavier than hard-top coupés?
Answers:
The weight isn't in the roof, it's all the extra mechanisms and motors required to lift it up and down.
There is also a bunch of other hidden stuff that has to be added to the car, to make it more rigid. Otherwise the car would twist due to not having the rigidity of a hard roof.
The normal car body gets strength from the entire body. That is why it is called UNIBODY. When you cut off the top half, there is no benefit from UNIBODY construction where loads are distributed.
So, to gain the required stiffness, they reinforce it with heavy bar/channels of steel. It adds significant weight to the car.
When you take the lid of of a car you lose its monocoque strength and has to be strengthened with extra bracing. This is also the reason why they are more expensive.
Typically 75 - 100 kilos of meaty metal gets added, then think of motors and braces in the fabric frame. It's usual for a soft-top to feel twisty and sloppy, compared with the coupe.
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