Lonely Mountains: Downhill

Bikes are unlocked by collecting parts from various challenges spread out over multiple tracks.

EDIT -- the original solution is outdated as of patch 1.0.4 -- Bike parts are now universal instead of tied to a specific bike. Completing challenges will award you with bike parts that you can use to unlock a new bike and pop this achievement.

(I already have all the bikes unlocked through the old system, so I cannot see how much each bike costs. If someone would let me know which bike is the cheapest I'll update the solution with that info)

The first one you are able to unlock is the Pacebreaker, which you get for completing the following challenges:
- M1T1 - Expert - 2:30 or less
- M1T3 - Beginner - 2:15 or less
- M1T3 - Expert - 2:05 or less
- M2T1 - Beginner - 3:20 or less
- M2T2 - Beginner - 3:20 or less
- M2T3 - Beginner - 3:00 or less
(Where M1T1 refers to Mountain 1 Track 1 etc.)

Unlocking bikes require bikes parts. To acquire bike parts, you need to complete challenges. The quickest way to unlock this achievement is to collect 4 bike parts and purchase the Trailblazer.

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