I've never heard of anyone using bracing under the body or dropping the tank to fit a luggage rack. Do you have any more info on this? From what i've been told by the US forum guys the only way they were ever installed was with Wellnuts.Ensure that it the rack has been fitted correctly, as specified when new and dealer-fitted. That means having suitable bracing underneath the body and requires the tank to be removed for correct fitting. Just putting bolts through the fibreglass will see the rack and tops flying off at anything more than low speed.
I stand corrected - I knew they were a "dealer fit" item (you lose marks at NCRS level because or them!) but I'd read in the past about those just "bolted" flying off. I seem to recall seeing photos of bracing, but maybe that was just an add on. But, I see that even the later C3 (80 on I think) that had a roof carrier option had only rather hefty looking nuts and washers or well nuts underneath!Dazaa is correct. They were fitted by the dealer not the factory and the dealer isn't going to start dropping the tank on a brand new car. Same fitting for the passenger side mirror base plate.