Sqaure wheel by Hart's A-frame Linkage.[br][br][b]Literally, this is "Invention of Square Wheel"[/b] [br]cf. related word : [url=https://en.wikipedia.org/wiki/Reinventing_the_wheel]"Reinventing the wheel"[/url] (wikipedia, Reinventing the square wheel)

Hart's A-frame is an exact straight line drawing apparatus.[br][br][b]■ Characteristics:[/b][br]① Shape is an exact square wheel. (size is changing: big ⇔ small. □OIO'I' is square. CO ⊥ CI )[br]② Foot trace is exact straight line. (+/- 45° line or horizontal.) ---- [b]not cycloid[/b] curve.[br]③ Long stride.[br][br]Tip: This method cannot be applied to Chebyshev linkage. (∵ Chebyshev shape is a left/ right symmetry, so, #4 foot goes under ground.)[br][br][b]■ max span this A-frame[/b][br]Suppose, B(0,2+y), End(x, y(B)) [End is bent bar angle 180°, stretched.][br]where,[br]x[sup]2[/sup]+y[sup]2[/sup]=(4+3)[sup]2[/sup]=7[sup]2[/sup]=49 ①[br]x[sup]2[/sup]+(y+2)[sup]2[/sup]=(6+2)[sup]2[/sup]=8[sup]2[/sup]=64　　②[br]②-①[br]4y+4=64-49=15 then y=11/4=2.75[br]so, 2+y=4.75[br]x=√(49-121/16)=√(784-121)/16=(1/4）√663=6.437196595[br]