Trek 26" mtb disc, how to change up crank set /gear size? What should I consider with regard to the front gear (derailleur) selector unit when changing up from a 42 to a 48 tooth crankset? I have changed a few similar cranksets but not done a gear ratio lift. A previous attempt (grip shifts) resulted in only one of the 3 crankwheels usable. Thanks.
There are several things to consider. As @Batman has suggested you may have a clearance issue between the 48 tooth gear and the chain stay. The chainstay is the part of the frame that extends from the bottom bracket to the rear drop out. Because it tends to widen as it gets closer to the rear axle it may hit the largest ring gear. You may also have an issue with the bottom bracket axle length. Different offsets may move the new crank in or out. Offset is the distance from the inner face of the smallest ring gear to the point the axle enter the crank arm. This may be mitigated by staying with the same brand but not guaranteed. You may also be beyond the tooth capacity of the rear derailleur. This is calculated by adding the tooth difference of the cassette and the tooth difference of the crank. A 12-32 cassette would have a difference of 20, a 22-32-42 crank has a difference of 20. The derailleur must have a 40 tooth capacity. The front derailleur must be compatable with a 48 tooth gear. If all this seems like a lot your chain may be to short when you are done.