I just got a used mountain bike and on my first trip my rear wheel stopped turning with the cassette. When I pedal it turns the cassette and the gears still change smoothly but it doesnt turn the rear wheel anymore. What can it be? How can I fix this?
Really hard to say without knowing what hub you're running. Most likely the pawls failed inside the cassette body. The pawls and springs are the mechanism which allow cassette to spin freely independent of the wheel (like when you coast or spin the cranks backward). When forward pressure is applied to the cassette the pawls and springs engage and thus move the rear wheel. Fixing them can be a very simple process involving nothing more than a couple of allen wrenches or a rather more involved process requiring replacement of the cassette body. Really depends on the specific hub involved.