Rather than replicate what can be found on the internet in a variety of places I’m just going to offer some prewar information along with some other observations to help locate and approximately date a bike. You will read in many places that no serial number lists exist prior to the fire at the Schwinn factory in August of 1948. This is not exactly true. Enough bikes have positively been dated by records such as invoices, build sheets, dealer ledgers, and dated photographs that a decent idea of serial number ranges can at least get us in the ball park. Other things such as how the bike was equipped and who sold it can also provide clues to when the bike was built.
Before using this information make sure you have a Schwinn. I can’t tell you how many times I’ve been asked “what year is my Schwinn” when, in fact, the bike was another make. Also keep in mind a serial number will not tell you what model, options, colors or any other specific information like an automobile vehicle identification number (VIN) will. There are also no breakouts on exact production figures of models. Schwinn only released total annual production numbers as did most other manufacturers.
A few words regarding the online lists. The bikehistory.org site has a place where you enter your serial and supposedly it gives you the year. About half the time this is wrong. Schwinn duplicated numbers through the years so you really need to know the differences between models/equipment to accurately date a bike. Personally if I do an online lookup I use the Angelfire site. A few of the books published also contain the Aug 1948 and up serial number lists. There are also sites that have many of the yearly Schwinn catalogs which are helpful to identify the model and year of a bike-Google “Schwinn Catalog” and you will get several sites listing these catalogs. Not all catalogs are posted online so you would do well to buy some of the books available if you plan on collecting Schwinns. My book reviews here https://vintageamericanbicycles.com/index.php/books/
For bikes built before 1952 the serial number is located underneath the crank hanger/bottom bracket (turn the bike upside down) (Fig. 1)
For bikes built from 1952-1969 the serial number is located on the rear drop out (where the axle comes through) (Fig. 2)
For bikes built from 1970-1982 the serial number is located on the head tube on the lower left hand side just above the bearing cup (Fig. 3).
Starting in 1976 another four digit number is on the head badge itself which is a Julian date with the first three numbers being the number of the day and the last number the year. In Fig 4 this would be the 296th day of the year in either 1978 or 1988. By identifying the model of the bike you could determine which decade this is.
For bikes built before 1952 you need to narrow it down further by determining if the bike is a prewar (WWII) or a postwar bike? The normal ‘tells’ for a prewar bike are rear facing dropouts and no provision for an integral kickstand mount *. Postwar bikes will have front facing dropouts, an integral kickstand mount, and an electro-forged frame which will have a ‘seam’ on the bottom bracket shell (see Fig. 11).
*These generalities do not hold true for Paramounts and some other odd models such as the low priced, postwar Meteor which did not have an integral kickstand mount.
A couple of caveats on the serial numbers; some early tandems, some Whizzer equipped bikes, and many of the early lightweights e.g. Paramount have specific serial numbers. Later Murray, Japanese, or Hungarian built Schwinns have a numbering system all their own.
As discussed above it is possible to at least get an idea if not the exact year on many Schwinns built before August of 1948. Dating or even accurately identifying Schwinns prior to the early 1930s can be challenging. The bikes built before the advent of the balloon tire bicycle are very similar to almost every other manufacturer and were usually of a simple diamond frame or motorbike configuration. Of course if it has a Schwinn head badge that can make it easier but Schwinn, like nearly all other manufacturers distributed bikes through tire stores, department stores, hardware stores, and hundreds of other retailers before they installed their own dealer network in the early 1950s.
A couple of other ways to help identify the year on prewar bikes is pulling the crank to see if it is dated (Fig. 5) or to see if you can date the hub https://vintageamericanbicycles.com/index.php/hub-dating/
For some prewar bikes, especially those sold by Mead and other large volume distributors, the numbers may indicate a year or two earlier than when the bike was actually sold. It is important to remember that even though there is a date associated with the serial number this was stamped well before the bike was actually built and may, or may not indicate the actual year of the bike. Lastly you will find pre-Aug 48 lists with exact letter/number ranges that vary from what I’ve provided here. Because no official list exists collectors continue to debate this. Adding to the confusion is the fact that anomalies exist for which there is no explanation and some periods did not follow sequential numbering for whatever reason.
The earliest number I have seen (I own the bike) is 1898 on a Model 22 Gents Roadster. This serial number runs parallel to the bearings (Fig. 6). I am unsure how long this continued but I suspect not too much past the turn of the century. Certainly by the teens Schwinn was stamping their numbers in line with the crank.
The following is a brief synopsis of the prewar numbering system drawn from multiple sources but based largely on work done by Tim Brandt. For the sake of simplicity in some cases I have generalized the cutoffs. For this reason you may find a bike that bleeds into another year. This information should be used to get you in the ballpark from there you need to really evaluate the bike to date it accurately and in some cases this may not be exactly possible especially if you only have a partial bike or frame.
The early numbers were hand stamped and consisted of numbers only. Beginning in 1935 a letter preceded the serial number followed by either four or five digits.
Before 1935 Four, five, or six numbers (generally large, handstamped)
1935-A letter (A-K) followed by four numbers (large, handstamped) (Fig. 7)
1936-same as 1935 but may have five numbers (K-Z/A) (Fig. 8)
1937-small, machine stamped letter (R-W) followed by five numbers
1938-same as 1937 letter (W-B) followed by five numbers (Fig. 9)
1939-42 the style changes to a larger, heavier font (Fig.10)
1939-letter (B-D) five numbers
1940-letter (D-F) five numbers
1941-letter (G-H) five numbers
1942-letter (I) five numbers
The post war bikes will have the built in kickstand mount and have the ridge indicative of the electro-forged frame (Fig. 11)
1946–letter (K-B) five numbers
1947– letter (C-D) five numbers
1948– letter (D-E) five numbers
Hopefully this helps making the identification of your Schwinn easier. Enjoy the ride!