Hello Max,
>>>> The frame.zaxis.x value is always in [-1...1]
Yes, thats correct, but if frame.zaxis turns over ry>90deg (z-axis downwards) you will get the same result as between 0-90deg. Another problem:
>>> rx = Math.atan2(frame.zaxis.y, frame.zaxis.z); If frame.z-axis lying on or parallel x-axis -> frame.zaxis.y=0 && frame.zaxis.z=0 -> atan2 will get a bitch ... (NaN) If you catch the situation - you have to check also the orientation:-(

I hate Euler angles now, but I'm to stupid to understand Quaternions in that way, that I can solve the problem with it:-(

Have a nice day
Karsten