if your riding deepish snow then your best bet is a mud specific tyre, one with long widly spaced knobbles. If your riding ice then you want a studded tyre, nokian make these but you can also make them yourself by screwing screws through your tyre. not really worth doing if we only really get a few days of snow a year, but if you ride wooden objects a ot, then I recken studded tyres would be ace, but really crap on rocks and metal.