Insert a decimal point in the product so it has the same number of decimal places equal to the total from step 1.

The three-digit number after the dash is the more important. This is the bead diameter at which the tyre fits onto the rim. If the rim is not marked with its bead diameter you can read this from the tyre – it’s the one dimension they have in common. When you've multiplied the ones digit by every digit in the top number, move to the tens digit in the bottom number. Proper fraction button and Improper fraction button work as pair. When you choose the one the other is switched off.Multiply as above, but this time write your answers in a new row, shifted one digit place to the left. Two tyres with the same ISO markings are completely interchangeable, even if one seems to be metric and the other is also marked in inches. For example, 650B and 26×1½ are both 38-584 and should be marked accordingly. Look for the ISO numbers and you always know exactly where you are. Proceed right to left. Multiply the ones digit of the bottom number to the next digit to the left in the top number. If you carried a digit, add it to the result and write the answer below the equals line. If you need to carry again, do so.

That’s how the diameter and circumference (circ – equals distance rolled in one turn of the wheel) are calculated in the Tyre size table. These figures are useful for setting your cycle-computer and precisely calculating gears. Nowadays almost all rims have hooked flanges (required to retain folding tyres) that will hang onto tyres up to three times their width. This increases the cushioning effect: you get more comfort out of the same tyre at the same pressure when you fit it to a narrow rim – yet it still rolls just as easily.As far as possible, then, use these columns to convert to ISO dimensions and then have nothing more to do with those old markings! Custom sizes refer to frames that are made in sizes that are not part of the standard size range. Custom frames can be made to fit a specific size or aspect ratio of artwork, or they can be made to fit a specific space on your wall.