i'm looking to put a cassette and rear mech on my inbred 29er swap out. ideally a single chainring with a 9 speed cassette. currently running 32/14 singlespeed. i read something about new shimano rear mechs with some kind of clutch mechanism - if i put one of these on, could i get away without a chain device to stop the chain dropping off the front? or would i still need something like the bionicon chain guide?