Turning a car into convertible.

[DI48LO]

My friend wants to play with his Opel Calibra, because he has nothing to do with it, and have other cars and bikes. I keep telling him, that he must weld some tubes under the car, so it won't split in half at the first bump. Oh yes, and it has a very stiff suspension. He says, it won't happen anything.
Does anybody have knowledge about this? I'm sure that if the top is cut off, the car will be very unstable, but many times he was right, eaven if I could prove it with laws of physics.
I'm waiting for your opinions.

[Mattk]

They tried to turn a van into a convertible on Top Gear, didn't quite work. He can do what it wants, but it won't be street legal.

[DI48LO]

We don't care if it will be street legal or not, it's just for fun. By the way, it's registration is expired. I just want to know if it will be safe, so I can go with him to a ride or not

[pitfield]

It'll flex a bit so the handling will be very different, but it shouldn't snap on bumps.