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To all of you with the same question: I found the original code of company Sunfounder and a manual with the original chassis dimensions. So i can tell now that length_a is femur, length_b is tibia and length_c is coxa length. All measured from pivot point to pivot point - what can be a bit tricky for the coxa construction. And length_side is the body lentgh - also measured from coxa pivot to coxa pivot. The original chassis has a square outline - therefore there is only one length.