Schwinn bicycle serial numbers are located on the underside of the bike, on the opposite side of the location of the service sticker. This is the metal space that connects the pedals. Schwinn serial numbers begin with "IBD" or "SN," followed by 11 alphanumeric characters.
As of June 2015, the SchwinnCruisers.com offers a serial number lookup service that allows owners to see when their cruisers were made. This service works for all cruisers made from 1948 to 1982. Records from before 1948 are unavailable; these records were lost in a Schwinn factory fire in 1948. The website is not affiliated with Schwinn.