I'm building up a 140mm trail bike with a 13.5" BB I'm tempted to run 180mm cranks. I've had them before on a hardtail years ago, and thought the felt great. Love the torque you get. I'm planning on riding this bike hard. Am I asking for constant pedal strikes? I run 170mm on my DH bike w/o issue. It also makes the math with the gearing work out really well. A single 30t front ring and 180mm arms should yeild a pretty decent low gear.