This is probably an easy one: how can I get the rotation around one axis (the Y axis) ?
Well, this is not a Bullet specific question, but a math question.
Yo need to extract Euler angles from a quaternion. There are a lot of methods for doing it (and a lot of different conventions/combinations).
Some of these methods can be found in the btMatrix3x3 class (getEulerXXX(...) or something like that); other are missing.
I remember an old post about it, in which I posted some methods that work for my needs:
http://www.bulletphysics.org/Bullet/php ... 40&start=0.
Hope you may find it useful.