Solving the Yo-Yo's Limit Angle for Acceleration

[dany-vai]

Homework Statement

We have a yo-yo, formed by a little cylinder tied to a little rope, and two big disks as wheels, one for each side of the central cylinder. We do a force T on the rope; its slope is an angle θ over the horizontal x-axis.
We are given the mass m of the cylinder, the mass M of each wheel, the radius of the cylinder R1 and the radius of each wheel R2. We want to know the limit angle θ , for the acceleration to be positive or negative.
I try to explain it better: under a certain angle, the yo-yo have a pure roll in the positive direction, over this angle it goes "back" and rolls towards the negative direction.

The Attempt at a Solution

I put in all the forces, the weight $mg$, the reaction $N$, and the friction force (static, since it's roling) towards the left side.
My equations are:
T*sinθ + N = m*g
T*cosθ - f_{s} = m*a_{cm}
f_{s}*R2 - T*R1 = I_{G}*α = I_{G} * a_{cm} / R2

But I'm always obtaining a non-sense answer solving the system, I guess I'm doing wrong something about the signes. Can someone help me?
Thanks a lot.

 

[lightgrav]

when it rolls, its rotation is around the point where it contacts the ground.
If the torque around this point is zero, it does not roll (alpha = 0).

 

[dany-vai]

Thank you very much| Now I got it :D