for those who don't know:
as gibby said, the heavier floorpan will make up for the weight of the missing roof.
however, i think its worse than that.
if you can imagine a car being a shoe box with the lid on, the box is quite rigid, it wont flex much. however, if you remove the lid, you'll find the box will now flex quite badly.
a car is much like the principle you see in that shoe box.
if you put the lid onto the bottom of the shoe box, you'll find that it flexes just as much as without the lid anywhere near it, yet with the lid underneath the weight of the box is still the same as when the lid was on top.
the key to Cabr designers is to put in enough metal underneath in such a way to limit that flexing (which i think is called Scuttle - not sure). the increase in cold hard metal then takes the vehicle way beyond the original weight.
the problem with the new cabs by renault and someone else with the automatic roof is not the weight of the roof or mechanism so much as the increase in weight from reinforcing the damn thing - its a big car, with a huge open deck - and you still have a metal roof and the power mechanism.
so when merc make a cab that doesn't weigh as much as a Sherman, you begin to understand how well designed they are in the first place and why they tend not to go for the "grinder" option to create a cab.
it could also be a reason why a mk2 cab was never made.
