Finding and recording your bike's serial number is probably the most important thing you can do to keep your bike from being stolen. It means you can prove to the police that the bike is yours; and yet most people don't do it, probably because it's rather hard to find. Here's how you do it. Flip your bike upside down and look at the bottom bracket where the two pedals meet. It should be there. If its not, checkout our diagram which shows the most common serial number locations.