When is a mountain bike not a mountain bike? When it’s a KtraK and can ride over snow, ice and sand. KtraK is a universal attachment for bikes of all sizes and shapes that turns the front wheel into a ski and the back wheel into a tank-treaded all-terrain propulsion unit. It’s extreme to look at, but familiar in execution: If someone can ride a bike, they can ride a KtraK. Requiring no tools to install if the bike has a quick release (or a multiwrench if not), it takes all of two minutes to set up, and you’re ready to rock the beach or boogie down a ski slope. Avid dirt bike racer Cal Douglas is looking at a KtraK: “It could be the next big thing, or just fun for us crazies.” An adult size ($400) is also available.