what do you mean what happens? The bike doesn't explode or anything if that's what your wondering. Smaller cogs in the back just means more speed on the flats and downhills and more effort uphill. Also, more tightly spaced gearing means you may find a gear that more closely fits a given situation. I changed to a 12-26 road cassette on my MTB that I commute on and absolutely love it. I can still climb just fine and now my gears are spaced more closely together so I can spin a cadence in my comfort zone more often. I've gone from averaging 11mph on my 15mi commute to 13-14mph on my commute. I kept the same 22-32-44 crankset on it, too.