Because this information doesn't exist anywhere. I will try to explain any that aren't immediately clear.
Girl Bike: -Get 12 stars -Do a back flip and get 3 stars
Retro Bike: -Win 5 games on multiplayer -Like Bike Race Page -Like Top Free Games Page [note: you can actually get away with not liking them. just clicking on the buttons to go to the page is enough]
Acrobatic Bike: -Race 100% wheelie and get 3 stars -Do 2 front flips and get 3 stars -Do 4 back flips and get 3 stars
Bronze Bike: -Get 24 stars in Desert -Get 24 stars in Arctic -Get 24 stars in Dunes -Win 3 consecutive races. [This means you have to go on multiplayer and win 3 games in a row. Whether this is with the same person doesn't matter. If you win 2 games against one person, then play another and lose, then go back to the first person and win again, it won't count. It should show your current and highest win streak on the left side of the bike list in the shop. This applies to the other bikes that have a "consecutive races" requirement as well.]
Ninja Bike: -Win 250 games on multiplayer.
High Tech Bike: -Create a game with a FB friend -Create a game with a random user -Create a game sending a SMS -Create a game sending an email [Hit the "Start" button on multiplayer then hit "Find" to get the SMS and email options.]
Police Bike: -Play with 5 FB friends
Silver Bike: -Get 24 stars in Hills -Get 24 stars in Beach -Get 24 stars in Savana [Note: Top Free Games apparently can't spell (unless they decided to switch to speaking Italian for just that one word then switch back).] -Win 7 consecutive races. -Beat a Bronze Bike. [For this you have to play multiplayer, race against someone else who is using the Bronze Bike, and win. What bike you are using does not matter. This applies to all the other "Beat X Bike" requirements as well.]
Hog Bike: -Beat a Ninja Bike -Beat a Ghost Bike -Beat a Police Bike -Beat a Super Bike
Ghost Bike: -Play with 20 FB friends
Gold Bike: -Get 24 stars in Desert 2 -Get 24 stars in Arctic 2 -Win 12 consecutive races -Win 5000 games on multiplayer -Beat a Silver Bike