Lonely Mountains: Downhill

Unlocking bikes require bikes parts. To acquire bike parts, you need to complete challenges. The quickest way to get this is to collect 24 bike parts and unlock the Trailblazer for 4 bike parts, the Pacebreaker for 6 bike parts, the Boar for 6 bike parts, and the Javelin or Geronimo for 8 bike parts.

Bikes - Parts required
Grasshopper - 0 (you start with this bike)
Trailblazer - 4
Pacebreaker - 6
Boar - 6
Javelin - 8
Geronimo - 8

Each of the four non-DLC mountains has 8 bike parts, all during "Day" Time of Day mixed between the beginner and expert challenges. Completing challenges unlocks mountains, trails, outfits, paint jobs, and bike parts. To unlock Beginner challenges, you must complete the "Explorer" challenge which is finish the trail with checkpoints. To unlock Expert challenges, you must complete all of the Beginner challenges. Here is a list of the challenges that have bike parts as the reward:

Naming convention ==> "M1T1" = Mountain 1, Trail 1

M1T1 - Expert - 2:30 or less
M1T1 - Expert - 14 Crashes or Fewer
M1T2 - Expert - 2:05 or less
M1T3 - Beginner - 2:15 or less
M1T3 - Expert - 2:05 or less
M1T3 - Expert - 1:45 or less
M1T4 - Beginner - 2:00 or less
M1T4 - Expert - 1:50 or less
M2T1 - Beginner - 3:30 or less
M2T1 - Expert - 2:45 or less
M2T2 - Beginner - 3:20 or less
M2T2 - Expert - 3:00 or less
M2T3 - Beginner - 3:00 or less
M2T3 - Expert - 2:10 or less
M2T4 - Beginner - 2:10 or less
M2T4 - Expert - 1:50 or less
M3T1 - Beginner - 2:30 or less
M3T1 - Expert - 2:00 or less
M3T2 - Beginner - 2:45 or less
M3T2 - Expert - 2:30 or less
M3T3 - Beginner - 2:20 or less
M3T3 - Expert - 1:50 or less
M3T4 - Beginner - 2:50 or less
M3T4 - Expert - 2:25 or less
M4T1 - Beginner - 2:40 or less
M4T1 - Expert - 2:00 or less
M4T2 - Beginner - 3:10 or less
M4T2 - Expert - 2:30 or less
M4T3 - Beginner - 2:45 or less
M4T3 - Expert - 11 Crashes or Fewer
M4T4 - Beginner - 2:40 or less
M4T4 - Expert - 8 Crashes or Fewer