I have not yet had a chance to try it in the rain, but it has very little turning resistance and pretty decent pressure on the rim so I really doubt there will be a rain problem. If you are riding in deep mud there would be a sticky mess to deal with (at least if the mud came up to the rim, but then a bottle dyno would be sunk too. If you ride mud you need a hub unit. I ride almost entirely on road or gravel so no problems like that for me.