How, oh how, do I apply this newfound instant center knowledge, you ask?
Well, now the instant center of the coupler link (yellow), can be determined. Four bar linkages, like the one shown at left, have two centers of rotation at ground (green dots) and one instant center. The velocity of the link joints (small blue points) must be instantaneously moving in a direction tangent to their circles of travel. By sketching lines extending from the blue and black links (green lines), the instant center is found, and the ratio of the two velocites can be determined by their distance from the instant center. (Click Here!)
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