So this bike is supposed to be the balls. Tough as all hell, light, compliant.

Why not stick a rigid MTB fork on there and fly?

What do the roadies here think....would it survive single track XC racing? I'm not talking about hucking cliffs. I was thinking more along the lines of endurance racing.

I know the frame is used competitively by a lot of X-ross riders. Not sure about an actual MTN bike race. I am not sure if the warranty on the frame would be covered on a MTN bike course.

Cross yes, MTB, don't know... All they did for the cross version (which isn't available yet for consumers as far as I'm aware) is put a different fork in to take cantilever brakes and put pegs on the seat stays for canti's as well.. other than that, it's a stock R3 frame.

Why don't you get a high end MTB? Since they are made to support cantilevers, have mountain specific geometry and designed to be stronger?

Tire clearance?

I don't think of an r3 as compliant in mtb terms. Big tires will help but what can you really fit? How are you going to feel after 22 hours of bouncing over roots rocks and logs on 32s with a super stiff frame.

Since you seemed more worried about the frames survival then your own.... Yeah it will probably survive if you don't crash it. Like any lightweight bike mtb/road/cross whatever though how far you can ride until that one crash that ruins it is just a crap shoot.