Transporting a Carbon MTB I took a punt and purchased a Mountain bike second hand, unseen (off our local equivalent to eBay).  It turns out it is a much higher spec bike than expected - including a carbon frame. 

Until now I have used Thule 591 roof racks to carry my bike, which clamps the down tube. Can I carry a carbon bike using these racks, if not, what is the recommended way to carry it?
I don't know the official recommendation, but the clamp could be problematic with a carbon DT. 

Here's the thing, though: I don't think the clamp is strictly necessary. The purpose of the arm is to keep the bike from swaying laterally - you'll accomplish that as long as the arm is somehow attached to the frame. What I would do is put the DT in the clamp, but don't tighten the clamp: instead, use a rope or strap to tie the arm to the DT.

Another option: don't use the arm at all. Instead, use ratchet straps on either side to keep the bike upright. A lot of people do that when transporting motorbikes. This method seems more secure, but it might be difficult without a second set of hands.