Can you tell us the component group? Your bike is probably '86-'89. Mid-High end Schwinn MTB. Later models came with triple butted frames at some point they were TIG welded and higher in the hierarchy. For that era (based on your crank) they came with the 500M group (Shimano Exage 500LX), which is a great group. (The shifters do not look 500M to me, though, more like 300M). Any frame stickers?