Hart's A-frame Wheel

Sqaure wheel by Hart's A-frame Linkage. Literally, this is "Invention of Square Wheel" cf. related word : "Reinventing the wheel" (wikipedia, Reinventing the square wheel)
Hart's A-frame is an exact straight line drawing apparatus. ■ Characteristics: ① Shape is an exact square wheel. (size is changing: big ⇔ small. □OIO'I' is square. CO ⊥ CI ) ② Foot trace is exact straight line. (+/- 45° line or horizontal.) ---- not cycloid curve. ③ Long stride. Tip: This method cannot be applied to Chebyshev linkage. (∵ Chebyshev shape is a left/ right symmetry, so, #4 foot goes under ground.) ■ max span this A-frame Suppose, B(0,2+y), End(x, y(B)) [End is bent bar angle 180°, stretched.] where, x2+y2=(4+3)2=72=49 ① x2+(y+2)2=(6+2)2=82=64  ② ②-① 4y+4=64-49=15 then y=11/4=2.75 so, 2+y=4.75 x=√(49-121/16)=√(784-121)/16=(1/4)√663=6.437196595