Grand Theft Auto: San Andreas/Chiliad Challenge
Get on the mountain bike at the top of the mountain to start the challenge. You can only do this between 7:00 and 18:00.
The challenge is a set of races conducted like any others, with checkpoints and competitors, just on a bike. There are three races, and you have to finish first in each to compete in the next, or to ultimately finish the mission.
 Scotch Bonnet Yellow Route
This course winds down the southwest side of the mountain, and ends near the trailer across the street from Angel Pine. If you rode up the mountain this is probably the reverse of the route you took to get there.
After a few times down you will get used to the motions of braking into the hairpin turns and accelerating on the straightaways.
 Birdseye Winder
This course starts and ends in the same place as the first one, but winds down the north face and then all the way around counterclockwise. This one's pretty tough because there are many places to fall off course.
The first two turns are the same as in the first race. Then comes a long, nearly straight stretch. Watch for the slight rights; if you are going too fast you will run into the fence which will result in an annoying delay. After that is a big hairpin turn with a cliff on the outside. Use the back brakes to slow down without losing control. After the big turn, watch out for the slight lefts or you will ride right off the mountain.
The next turn is around a scrub of bushes and is protected by a cliff wall. So it's a pretty safe turn; just don't get lost in the bushes and run off the side. The next turn is very tight and very downhill. Keep to the left to be safe, and if you spin out while braking you can recover in the right direction. After this turn don't go crazy with acceleration--there are some "slight" lefts with cliffs on the right and you can fall off.
The next big turn to the right is also dangerous, but you won't be going so fast at this point anyway. Then it's kind of bumpy to the next turn. You can just go straight towards the checkpoint, run off the cliff and land near the checkpoint after the course turns.
Now is a long, somewhat straight stretch where you can let it out and accelerate all you want. But when you emerge from the trees there will be some left turns with guard rails on the right. Hanging up on these guard rails can bring you to a very frustrating abrupt stop, so slow down and/or cut the corner by staying on the hill on the left.
There are two of these left turns with guard rails. The next one has a wooden fence and is a little less treacherous. After this you're nearly home free. There's one more downhill hairpin turn. As you accelerate towards the finish line, take care to stay to the right because there are a few more slight turns that can result in you falling off the cliff.
 Cobra Run
This race is shorter and easier than Birdseye Winder but has one very tricky section--a boardwalk. You start this race like the other two (get on the bike), but the actual starting point is a little further down the mountain, on the east face.
The beginning of the race is downhill down a steep ravine. Watch for the posts in the middle of the path. There's a jump next--watch your speed because you will need to brake right after you land. Enter the boardwalk and make your way down. Use the back brake (and front brake if you need it) to keep yourself from falling off.
If you do fall off the boardwalk, you can bunny hop back on. You can also bunny hop up into some of the checkpoints and thus continue down the hill next to the boardwalk. But the trees are big obstacles there.
The boardwalk ends in a unique stunt jump. After that, you're still in a woodsy section so watch the trees. Next is another big jump; if you miss it you will hit a ridge and stop abruptly. But it's not fatal.
After this is a sharp right turn and you are on the path going down and around the side of the mountain. Go through the archway, speed up, but keep to the right and don't fall off the cliff. Make sure you bear left at the fork in the road to go down towards the finish line.
The hardest part of the Chiliad Challenge is not speed; it's staying on the track. At the beginning of the race focus on steering and you will glide right into a high position. Use pedal power to push ahead, but only in the straight sections. Remember holding the pedal button accelerates, and tapping it accelerates faster but with less controllability. Remember gravity is on your side! Use the rear brakes around the hairpin turns.
Avoid the fences and side walls or you can get stopped or turned around. You can use bunny hops and turn in the air to get awy from one of these hangups. Also try to keep away from the competitors because they (by malice or incompetence) can run you off the course.
A Sanchez and Mountain Bike spawn near the trailer at the finish line. That should help you with getting up the mountain. Alternatively, go the San Fierro Airport and take a helicopter.
You can make the trip from the save house in Angel Pine to the top of the mountain in about three minutes. So save your game at the house around 22:00. You restart at 04:00. Leave the house, get in the first car you see, drive to the finish line, and pick up the Sanchez. You should get to the top right around 07:00, then hop on the Mountain Bike and get started. You can probably get two attempts at the race per day.
In first two challenges you can get an advantage in the beginning by blocking the road near the starting point with one or two trucks. Park them on same flat area near the BMX, so your opponents will just smash into them losing a lot of time, while you can just jump over.
If all else fails, instead of destroying your keyboard/controller in sheer frustration, use the sniper rifle to remove the AI competitors (the safest way to do this is when all the AI competitors have finished and stationary, if done mid-race they may get spooked and may leave the finish area). Unlike any other weapon, use of the sniper rifle will not forfeit the challenge and it will keep going, allowing you to win should you be unable to do so the regular way.
Progress towards 100% Completion.
After you complete all three races you can race them again, in the same order.