Yes that's my take on it as well. They don't know and can't be ars** to figure it out.
It is feasible that the convertible would act differently to the normal 500, some sort of resonance due to the structure? but they're not really a full convertible so I wouldn't think there would be much if any difference in structural rigidity.
My money would still be on drive shaft(s).
I'm sure it's a tricky one to pin down though. My other car (Mercedes G Wagen) is notorious for drive train vibration to the extent owners have spent many thousands and hundreds of hours to finally give up and sell it.
Someone with the right knowledge and technical equipment would figure it out I'm sure but who that would be I wouldn't know.
I appreciate your frustration and potential financial loss but life's too short sometimes!
Thanks for comments too. I have wondered about drive shafts but as it doesn't seem to vary that much with speed, I think it's more likely to be a combination of an over-rigid structure compared to hatch (a solicitor petrolhead friend was shocked at how hard it felt when he tried it) and possibly premature wear. I'm thinking bushes as it could be some kind of ripple or resonance effect from bumps. Don't know...
Yes, someone would be able to sort it out (as I'm sure they have with your G Wagen), but at 9 months old and 7k miles or so, it wouldn't be reasonable to expect the owner to carry out design engineering work or live with a complete annoyance.
I do also agree that life's too short and would normally cut my losses or live with it. The only reason I'm going to continue pursuing this is because (from my experience of now trying 5 different 500Cs), it's definitely
not present (or perceivable) when its new, it develops over time and gets worse as the miles go on. And now, according to Fiat it's a design feature that isn't listed in any brochure I can find!
So anyone test driving a nice smooth one today at their nearest Fiat dealer would be potentially disappointed when they reach 7-8k miles, especially if they are any kind of car enthusiast. On the basis of Fiat's letter, every 500C will get the vibration at some stage!
Without sounding funny about this too, I suspect that many, if not the majority, of 500C owners are much younger than me and may not have my driving experience and mechanical sympathy -
I don't mean Fiat Forum members!, but the general buying public. I'm not being arrogant at all or class myself as a great driver, btw, but I've been driving for 30+ years and have owned, driven, fixed (and even help prepare for racing) all sorts and do know a little of what's what.
My other principle is why on earth should any car buyer (talking all makers here) be penalised in some way for a failing or design issue that the manufacturer is not prepared to fix?
It wouldn't be acceptable if a customer bought a new TV that always lost sound after 37 minutes of being powered up and after engineers had tested it, it turned out that it isn't actually suffering a manufacturing fault, but a wrong component had been specified. There would be uproar if the buyer was told to live with it or buy another one when it had been provided with a warranty. So why should car manufacturers selling £16k products treat their customers any differently to a TV maker selling £500 items?!
I don't think I can change the world, but in today's supposedly consumer-led society, maybe something does have to change.
I'm also not going to bankrupt myself over this, but equally I'm not going to be steamrollered into submission.
Anyway, rant over. Thanks again for the comments RussH, Cheers,