3 days i was fighting with C2 and few hours ago i succeeded. There are 2 similar tutorials in the forum and arcade "http://www.scirra.com/tutorials/240/physical-bike" and "http://www.scirra.com/arcade/addicting-example-games/34/html5-physics-vehicle".
The 3rd is:
dl.dropbox.com/u/67937942/Car%20game%2C%20Suspension.capx
You can make front/backflips but you need to use both left and right buttons, you can play with gravity and speed i suggest you not to touch the "shaft" physics, if you want to make suspension harder or softer just change the length or the angle of the shafts.