Motorized Wheel Chair Source

Unless we know exactly where the center of gravity of gravity of the object that you want to lift, there is no way we can precisely determine the length of the shorter legs of the sling by calculations. What is normally done in cases like this is to use a double groove equalizing thimble. Without any clips on the legs of the sling, lift the load slightly above the ground to allow the legs of the sling to adjust so that the thimble align directly above the center of gravity of the load. Once this is attained, provide rope clips on the rope at the throat of the thimble to prevent any unnecessary movement of the thimble relative to the lengths of the legs of the sling.

If an equalizing thimble is not available, construct the sling using two lifting ropes of the same length. Attach the ends of each rope to the hitching points or lifting padeyes of the load. Run the two wires through a heavy duty shackle. Lift the load slightly above the ground by means of the shackle to allow the lengths of the sling legs to adjust until the shackle is directly above the center of gravity of the load. Mark the points of the lifting ropes where they bend over the shackle. This will give you exactly what the lengths of your sling legs should be.