Doc Sprocket
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Keep in mind that the decorative (non-structural) tube they use in that instructable is DEAD thin. Way thinner than you are going to use.
For tire sizing, the LAST number is rim diameter- IE 13x5-6. 13 is the overall height in inches, 5 is the width at its widest point (sidewall bulge, not tread) and it fits a 6" diameter rim.
For tire sizing, the LAST number is rim diameter- IE 13x5-6. 13 is the overall height in inches, 5 is the width at its widest point (sidewall bulge, not tread) and it fits a 6" diameter rim.