[ r_2 \cos\theta_2 + r_3 \cos\theta_3 = r_1 + r_4 \cos\theta_4 ] [ r_2 \sin\theta_2 + r_3 \sin\theta_3 = r_4 \sin\theta_4 ]
The angle between the coupler and follower—critical for force transmission. Values near (90^\circ) are ideal; below (40^\circ) or above (140^\circ) cause poor mechanical advantage. 4 bar link calculator
[ \mathbf{r}_1 + \mathbf{r}_2 = \mathbf{r}_3 + \mathbf{r}_4 ] [ r_2 \cos\theta_2 + r_3 \cos\theta_3 = r_1