Dating schwinn bicycles
For example, the back of cranks are usually marked with crank arm length in mm, typically in the range of 165 to 185.
Seatposts are marked with outside diameter, also in mm.
The Appendix gives dates of initial manufacture for the vast majority of derailleurs made from 1920 to 1999.
Other manufacturers of bicycle components have date marks on their pieces.
This makes dating the components an interesting archeological investigation, but one not necessarily related to the date of the bike. Trek owner Larry Osborn made this observation, and suggested this as a supplementary way of dating a Trek (and other bikes as well).
Fueled by this first realization, and with the help of other bike folks, Larry and I have sorted out other codes (a project still ongoing).
This clearly is when the component was made and not when the bike was made, but unless the component or bike manufacturer had lots of stock lying around in inventory, the date should be a fairly good indication of the year of the bike.
The rear derailleur can often be dated to a year or two by referring to the book "The Dancing Chain - History and Development of the Derailleur Bicycle", by Frank Berto, 3rd edition 2009.Dia-Compe cantilever brakes don't appear to have a date code, but supposedly the 981, 983, 986 brakes were first introduced in 1981, 1983, 1986, etc.Following Ben's lead - I checked three sets of Dia-Compe G calipers and all have the four-digit date code on the back of one of the arms.Of course, all this assumes the bike has the original component.
The most likely components to be original are the stem, handlebars, seatpost, and brakes.The rear derailleur freewheel/cassette and chainwheels are probably the first to be changed on a bike.