Courtesy for this video goes to Achievement Hunters. In this video they show you four achievements, Big Wheelie, Big Stoppie, Big Air and Mascot Jr. For Big Air you have to jump 50m with a bike in one spot, they used a different spot than me though. I used the Construction Pit where you go off the top (when you do you can see the white cones infront of you) using the Zipper 150cc. All you have to really do is use your boost good and you should get it.