i have a 1972 schwinn deluxe twinn. i bought it all in boxes, not sure which way the chains go on. its got a small single sprocket on the front and 2 sprockets on the rear with a five speed cassette. i think the front chain goes on the the small inside sprocket on the rear and the rear chain goes on the outside large sprocket then goes on the cassette. but when i have it that way and shift to the largest cog on the cassette the chain crosses so much that it pulls it off the sprocket. what am i doing wrong? thanks