I use Thule 591s, but I've seen a qute a few high-end road bikes with fork-mounted carriers. What is the reason, what are the advantages?

Because clamping carbon downtubes is bad.

It holds the bike more securely? Watch a bike strapped to the 591 and see how much wobble the front wheel has as you drive around…also watch how much wobble the bike has attached to the carrier.

I'm guessing as the front wheel is removed there is less bike sticking up so less drag (somethign roadies know more about than me), but also traditionally, road bikes have always been carried with the forks clamped to the rack (look at older pictures of the Tour cars).

I guess (and I've no idea if this is correct), the perception is that they are a better design for carrying road bikes – not ideal for front suspended forks in some cases (check your warranty) and might be best if the sus fork is locked out, but as road bieks don't have this issue, it is probably an easier way for them to move bikes around (as it has always been that way on the road)…

Lighter frame materials? Is the frame so 'fragile' that it can't withstand a bike frame clamp?

Otherwise I've no idea…

I feel from an engineering perspective having the bike held by the forks, like it is in use make more sense than clamping the down tube. On carbon frames especially, whilst the tubes might have great strength when loaded in a riding situation they are not designed to be clamped and can be very weak loaded like that. Some high end frames guarantee T&Cs state it void it carried in a frame clamping racks. Windsurf carbon masts are clamped but the clamp area is strengthened to compensate. Extreme "aero" frames can also be a fiddle in down tube mounted clamps. Also, with the seat post removed too the bike is very much lower than with the other setup with less frontal area with makes it whistle less, less of a drain on fuel economy, can get into some car parks I would otherwise struggle and often in a lower price band on some ferries.

Mine has clearance for discs on the mtb but you have to get an adapter to carry maxle equipped bikes.

Ah pap, I own a pair of Atera GiroAF carriers suppose that means I now have to buy a third carrier for my bling new carbon road bike!

I have one of each by Thule, fork clamp and tube clamp. I got the fork mounted one cause it looked gucci and profesional. It does hold the bike more securely so preferable for a long drive…..however it is a pain in the arse to use so I bought a 591 which I use 99% of the time.

How is it a pain to use – are you using it with a bike with a bolt through fork? With that one I grant you it takes another 20secs or so, but with the others the front wheel is off and the bike on the roof in less than 10 secs.

Conversely whenever I'm in friend's cars, adjusting the arm to the right length and size seems a chore and as I ride 6 different bikes doing that all the time would drive me nuts. I guess you get used to what you know.

I think fork mounted ones are far better, although Pace forks didn't used to be warrantied for them. You certainly don't need to worry about locking forks out, the only time I'd say they're a faff is if you have a bolt-thru fork.

Saying that, I only ever stick my bike in the back, safer, more secure, doesn't affect my fuel consumption. Much easier!

Problem I have is that I need to take out a baby seat if I carry my bikes inside the car then faff with seats etc. By the time I've done that and taken the bike(s) out of my lockup it adds about an hour to my biking day!