Wait, what an I saying? So there's skiing already in Bozeman (as long as you're willing to use or make rock boards) and about 4 inches of snow on the ground in town. Riding isn't done for those who would rather be astride wheels than atop planks. It might be tough, you might (will) struggle, and you might have to drive a bit to get to the riding- but you can ride. Trust me.
For the purposes of this post I'm going to try to stick to real trails, with real singletrack. In the winter you might not always have this luxury- but you'll usually be able to get that fat-tire fix. I'm also going to stick to regular mountain bikes- I don't have a Pugsley (hell yeah I want one though).
Pipestone
Most Bozeman folks know about Pipestone. It's a little climatic anomaly just over an hour west from town. Nestled in the rain shadow of the Continental Divide the whole area tends to stay relatively free of snow. And by relatively, I mean it will have three to four inches of the white stuff compared to the three to four feet we'll have in Bozeman. The best times of the year to ride at Pipestone are in the spring (late March through early late May) and autumn (October to late November). It is often possible to ride in midwinter- I've been there in January (in shorts!) during a warm snap and in six inches of the lightest powder at 8ยบ in February after a few hours of skiing fresh powder (there's a story here, but I'll save that for another time).
South Cottonwood
A nice side benefit to riding a packed, icy trail is that the trail is insulated from your tires, so you can't do any damage. Yes, you can slide around corners to your heart's content. Braaap!
Sourdough Canyon (Bozeman Creek)
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Please respect the work the BSF does, especially regarding the classic ski track set. Sourdough is a multi-use trail but screwing up the skiers' fun won't win the mountain bike community any favors.
East Gallatin Recreation Area
Well, there's a few ideas for you. Remember a few key details to make any attempts at riding more successful- fatter tires, a period of cold snow-free weather, and platform pedals. Disc brakes and singlespeeds make things a bit easier but are not necessary. This winter I might experiment with building a set of 24" wheels with fat tires- I'll certainly document that if it happens.
Remember, skiing is tons of fun but there's only one way to satisfy a craving for mountain biking. With a bit of preparation you can sucessfully ride in the winter- even in Montana!