What about the cogs and the derailleurs?
And if it rains 3 days in a row?
Tah's why you ride fixed gear in the rain
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Seriously, I started commuting about 6 months ago. I've ridden in a couple of rain events (somewhat unexpected) so far this year. I fully intend to continue commuting through the winter (mainly so I don't have to get on the fluid trainer like I did last year). I agree with the people who responded that as long as you have the right gear you'll be OK. I just ordered some rain pants to take away one more excuse to avoid riding in the rain.