Spur / Pinion = PDR
Sample PDR: 84T spur / 22 T pinion = 3.818
PDR x DTR = FDR (DTR = Drive train ratio, or ratio of pulleys)
For an Xray, the DTR is 1.7 (34/20)
Sample FDR: 3.818 x 1.7 = 6.49
Tire Diameter x 3.14 (value of PI) = Tire Circumference
Tire Circumference / FDR = Rollout
Sample tire circumference: 63 mm x 3.14 = 197.82mm
(or 7.788 inches)
Sample standard rollout: 197.82mm / 6.49 = 30.48mm
7.788” / 6.49 = 1.2”