Even if you find the serial number, its absolutely no use in identifying the bike without a brand/manufacturer. There is no global index like auto VINs.
Serial numbers are used for several different purposes.
- Identifying a frame through production, for logistical and tracking reasons
- Identifying a completed bicycle through a sales process
- Uniquely identifying a bike for warranty purposes
- Identifying a bike for insurance purposes.
At no point is a bicycle frame serial number used to identify the bike's model or age or manufacturer. A frame's serial number is not related to a car's Vehicle Identification Number (or VIN.)
For a frame to lack a serial number could be any or
- The frame was hand made by a small-run frame builder, or home made. Most frame builders would use a serial number of some format though, even though their monthly output could be single digits.
- The frame has been repaired and the serial number was lost by replacement.
- The frame has been stolen and resold, with the serial number ground off or filled-and-painted-over.
- The frame was legitimately repainted and the serial number is obscured or hidden (my bike's like this - the S/N is visible but illegible.
- The serial number is covered by an accessory (very unlikely)
But most likely, you simply haven't found the serial number. Common places for serial numbers:
- Under the bottom bracket
- On top of the bottom bracket housing
- Up the side/front/rear of the seat tube
- Up the side/front/rear of the down tube
- Up the side/front/rear of the head tube
- Inside/outside the chainstays
- Inside/outside the seatstays
- Inside/outside the rear dropouts.
Less common areas to find a serial number
- INSIDE the bottom bracket housing
- Down inside the seat tube.
Places you won't find the serial numbner
- On any mechanical part - they may have their own serial number, but it won't be a frame serial number
- On the fork - again it may have a fork S/N or replicate the frame's SN or it may not have a serial at all
- Inside any closed tube - the headtube, BB and seat tube are the only tubes in the frame with some level of visibility, so it won't be anywhere else.