Not sure about 1x9, but I use 1x10 for XC races. Yes, like 'sometime' said, you need a chainguide! I use C.Sixx - south african and really good, but quite expensive! I also run a 38T up front and 36-11 cassette (29er), but also have a 36 and 34T, for when the course has some proper climbs. But go and google 'sram xx1'
