Why do bike manufacturer's put the gear cables on the down tube, when they should know that, that is the place a cross racer is most likely to grab hold of the frame to shoulder the bike. Also there the cables are most prone to getting caked in mud!!!
Ah, Top tube cable for rear mech so that cable isn't on the side of the down tube where you would grab hold of, but doesn't the cable for the front mech rub on your back and get covered in mud? Just an observation but I prefer the clean lines of no cables on the down tube.
In 99% of 'cross races you may carry the bike for 2% of the time. The rubbing on the back thing is really a red herring. The clean lines gained by top routing cables come at a price of more bends in the outer of the cable and make shifting more likely to go awry. The dirt thing is theoretically interesting but I've never had a shifting problem with downtube routed cables in 'cross - principally because dirt and cables only become a bad combo when outers are involved.
What is it like to grab hold of the downtube with cables on when racing? I'd experiment with cables on the downtube but it would be an expensive way to find out. As regards the top routing of cables, I've had totally enclosed cables for quite some time and not had a problem with shifting and I agree that the more the cable is enclosed the more likely you may be to have resistance on the cable. But personally I don't like the idea of having metal cables interfering with me grabbing hold of the downtube. I think at the end of the day it's down to choice, but that choice is becoming more limited, I would like a Scott Addict frame but I can't get past the cables on the downtube thing!
I am seriously considering a single speed cyclocross bike, weird aren't I.
I did edit the above post to make more sense, but it didn't upload for some reason!
Grabbing hold of the downtube with cables on it is pretty much how you'd imagine. Very easy. No discernible difference to top mounted. It's 1.5 mm or whatever it is of cable, that's all.