Bike Forums - When to Overhaul BB

Take the chain off the chainrings up front so you can get a better idea of how the bottom bracket is spinning.

With your front shifter, shift the chain to the inside of the bike, the smallest chainring. Now create yourself some chain slack -- push forward on the tension pulley of your rear derailer, towards the front of the bike. This pulley wants to spring towards the back of the bike whenever you let go - that's its job, to keep tension in the chain as it moves to different sprocket sizes. Again, gently push it forward so it pivots towards the front, and the chain will droop along the bottom. Now take this chain slack and pull it towards the chainrings up front, drooping the chain on the inside of your smallest front chainring. The chain can rest on the shell of your bike's frame, around the outer part of where the bottom bracket lives.

Let me know if that was confusing and I'll take/find you a photo.

Spin the crank, which should no longer have the chain attached to it. It can spin freely in place. The smoother and longer it spins, the better condition your bottom bracket is it. If you give it a strong spin and it stops after a couple rotations, it's suffering from a lot of friction. Listen for crunchy noises; there shouldn't be any (or many). Try to push your crank arms laterally, towards the center of the bike's midline, or laterally away from the bike. There should be no side-to-side play of the cranks.

Mountain bikes are hard to estimate wear on bearings, because conditions vary so much. You have a high-end bike, so you wouldn't disassemble and regrease the bottom bracket; it's made to be disposed and replaced as a single unit. But you don't need to cross that bridge until you're sure your bottom bracket is toast.