The bigger tire in front helps with control. A good 1.9 rear tire is fine, but I find 1.9 tires in the front too narrow and not sticky enough in turns.
I run 2.1 front and rear because I like the looks better, but I did put a 1.9 on the rear to try it out, and I found my bike accelerated better than with a 2.1. I guess the extra traction and wieght of the 2.1 slows me down a little.
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Booyah!!