You can create your own tracks by going to www.bikerace.com and designing it anyway you want!
Race it at least once and click on Save.
Then, open the Bike Race app, tap on Single Player and scroll until you see User Created Levels. Fill in the Search by Code field with the code generated for the level you just created and enjoy playing your own level!
You can also share the code with your friends to race against them!
Please be mindful that you must access the website from a desktop device or laptop, and not a mobile device.