I have all 70 achievements so i know what ill be talking about.
First off, the two best bikes are cycle king and farty pants(gay name but still). The reason these are the best is because the king is the best at wheelies and farty pants is a fliper. (Hint: farty pants makes front/back flips too easy, my personal best is a multiflip frontflip, witch is basically 6 frontflips at once)
now for the hardest achievements,
-Sneaky start, absolute hardest one, what you do is the following. Start the level you get the achievement from and dont touch any keys, right before the word go apears(i mean when get set disapears and go almost apears) press and hold the right and top keys(or d and w), then instead of go itll say sneaky start, if done correctly just complete the level so it counts.
-another one people are finding hard is 'moon bonus level-wheelie', i did this pretty easily. choose the king as your biker and the second the race starts do a backward wheelie, dont go fast just try too stay near the start(dont worry if the rest of the racers get a head start, ive given them a 30 sec head start and still come in first on the moon) now if you cant get the 10 secs before the first ramp restart the level and try again, that simple, when u get it win the level in atleast 19th place.
last thing, use farty pants for everything that dosent need a wheelie, with him you can do a frontflip just jumping, a triple frontflip from a normal ramp a quadruple frontflip from a really good ramp, a quintuple frontflip on a really good ramp with some boost and a multiflip frontflip with the right conditions(6 flips) usually with this guy i keep a boostx5 throughout the levels.
farty pants and sir colin are the best on the moon i had 7 backflips with farty pants on 1 ramp. sir colin is good because he has a really good bunny hop
Sir colin is probably the best overall. As others have said, letter f for front wheelies and cycle king for back. One other tip is I find the brothers work great for collecting horns. Theyre extremely stable, with a large bike, and it just seems that its very easy to do it with them. For trying to finish all the upgrades, use sir colin and go to the first map. When you get used to it, you can get up to 2x boost (higher actually slows you bc of the time it takes to get it there)and complete the lvl in under 40 seconds every time. It still takes some time to finish, but its not too bad.
