Chucky - what shifters are you running on the roadrat?
If it's road STI shifters then bear in mind most are double only these days so a triple might not be worth it if you have to buy triple compatible shifters too. If it's flat bar shifters then you should be okay.
Plus not sure if mtb front mech it going to handle a 50t front road chainring, probably will but again something to consider especially as road front mechs have different pull than mtb ones (though I'm sure people do use them successfully).
If you thinking of touring then got for a triple I'd say.


