big post ahem. I guess that I'll start by saying that I'm basically referring to aluminum clinchers, only because there aren't really any 'disc only' aluminum cross
tubulars available yet. Sure you can use something meant for rim brakes but that's not what this thread is about. I know that tubulars are the standard but tubeless is realistically becoming quite a good option for lots of people, especially when using rims better suited to the task like NoTubes. Now if only there were a few more UCI legal tyre choices. There's already plenty of carbon road and MTB tubulars that are suitable as well as a few clinchers so that's not a big issue if you're financially able to make that choice
A quick search found this: http://forums.mtbr.com/showthread.php?p=5351110
MTBR:Real disk brake weight - post #5 wrote:
Avid BB7 (type N)/Avid Ultimate levers/PowerCordz cable/iLink housing/liner: 267g
Avid Juicy 7: 266g
Avid BB7 (type N)/Paul Love levers/PowerCordz cable/iLink housing/liner: 251g
Avid Elixir CR: 250g
Shimano XTR PM: 248g
Avid Elixir R: 246g
Hope X2: 237.5g
Marta SL: 237g
Avid BB7 (type N)/Extralite Ultra levers/PowerCordz cable/iLink housing/liner: 225g
Avid Juicy Ultimate: 226g
Avid Juicy Ultimate: 219g (with trimmed housing length)
Avid BB7 (type N)/KCNC levers/PowerCordz cable/iLink housing/liner: 214g
Formula R1: 183g
So I've got no idea how accurate things are after a few years (and that refers to MTB brake levers) but it shows that the BB7 with some tricks isn't such a weight penalty compared with hydros (admittedly a few years out of date). That is without rotors and other hardware which is common between all set ups. Not only that but when you use really light small rotors which is acceptable on a cross bike, the weight penalty over rim brakes really isn't too bad compared with existing aluminum clinchers with machined sidewalls and cantilevers.
Doesn't this assume there is a good light disc(k) fork availible? AFAIK, and I could be wrong, the 3T fork has not be released, and the only other options are ridged MTB ridged forks... which have some geometery issues.
The same argument could be said of the frames though. Very few 'race' frames have been made with disc mounts previously so if you were set on discs with a real 'race' frame, going custom is probably your best option and then using the Trigon disc fork would be perfect because adjusting things to suit the slightly taller A2C would be very easy. Those forks have a respectable weight and the durability has been proven on many XC bikes. Not to mention the fork issue but also very few rear 130mm disc hubs exist so you'd be best off building something with 135mm rear spacing and being able to choose from many light and durable MTB hubs.
You're right, 29er rims are the obvious choice for a disc specific CX rim, but imagine if ENVE or Stan's narrowed down their 29er rims slightly and shaved a few grams off! That'd be the ultimate...
I don't know if making the clinchers narrower would be worth it? The extra width can be fantastic for low pressure performance and basically the only common clincher that weighs less than the Crest is the 340 which is 4mm narrower for only small weight savings (albeit with a brake track). With that said, a disc only aluminum tubular would be pretty sweet. The only one's I'm aware of are MTB rims and they're wider than necessary for the base tape on a cross tubular and IIRC weigh more than something like a Major Tom or HED Belgium tubular.