There are two challenges that you will unlock as you progress through the game, both of which take place on types of bikes. Both require you to pass through a number of checkpoints, with each one adding more time to your overall timer. The BMX Challenge is unlocked at the beginning of the game, and can be found by climbing onto the bike sitting at the skate park just south of Glen Park. The NRG Challenge is found by getting on the bike at Easter Basin in San Fierro. For both, you will want to get their respective skills up higher before attempting them. If you’re going for 100%, then you will need to do both, as they are required.