The Chiliad Challenge is a set of three races down certain paths with the Mountain Bike that is found on the top of the mountain. You can start this challenge as soon as you get to The Badlands, although you need a Cycling Skill of at least 40%, but 100% is definitely recommended. Also, the challenge is only available from 7:00 to 18:00 on the in-game timer.
These are some of the more difficult races in the game, simply because you'll be racing downhill on a mountain bike on top of a mountain. You cannot kill any of your competitors by means of drivebys or you'll fail. You have 25 seconds to get on your bike again should you fall off. The races are done in the order named below and you must finish in first place to unlock the next one. After you finish a race, you'll have to backtrack back up the mountain in order to start the next one/redo it. The big point is to race INTELLIGENTLY and you also cannot kill any other competitors.
SCOTCH BONNET YELLOW ROUTE
This is the first race in a set of three. You're up against five other competitors and things won't be easy. Use R1 for sharp turns and make sure to not go too fast. Use this race to get a feel of racing on Mt. Chiliad, since you'll probably be doing these races for a good while. Make sure you pass through each checkpoint as you head down the mountain. There is a large straightaway after the 9th checkpoint and the homestretch is right after the 15th checkpoint or so. Make sure to use these two areas to get a heads up on the competition.
For some reason, this race seemed a bit easier than the first one. At the first fork you encounter not too far down the path, you'll be going right instead of left like in the first one. Be careful of the very sharp turns on this new path and use the abundance of straightaways to get ahead of the competition. There is a huge straightaway between checkpoints 21-23, so this is your chance to gain a big lead when you get there. Remember to race intelligently as you head towards the finish line.
This race is pure hell. It starts at the top of the mountain, but the real starting line is somewhere in the woods closer to the base. You'll be up against only two competitors this time, but they're the least of your problems at first. Not too long after you start, you'll come upon a wooden plank walkway that you must traverse for a short while. If you fall off, then you must head back to the beginning of it and get on again.
The competitors seem to hog the walkway like they were driving on the freeway. Take extra careful precautions on the walkway. Make sure to go as slow as you are able to to make it through this section. After the walkway, there are numerous opportunities to blow by the competitors, so make sure you don't screw up on the walkway. After a sharp hairpin turn, it's smooth sailing to the finish line. Make sure to not fall off of the mountain on the final path to the finish line.