Scooter Steering assembly

Although it is sometimes bewildering, the numerical size-coding may be designated all in inch-pattern, or in a combination of millimeters and inches. For example, a size listed as 3.50-10 signifies the inch measurement of the scooter's rim; in this case, 3-1/2 inches by 10 inches, or the inside diameter of the tire. The hybrid metric/inch-pattern coding is where it becomes complicated. This will read as three sets of numerals separated by a slash and a hyphen. An example is the common size 120/70-10. The code can be broken down into the following parts:

In 120/XX-XX; the numbers before the slash mark tell you the width of the tire at its widest part when mounted on a wheel and inflated to the recommended pressure, but without any weight on the wheel, measured in millimeters. This sequence will almost certainly be in two or three digits and will likely range from about 90 to 150.

Following the slash will be a two digit number which is a percentage, usually between 50 and 90 percent. This percentage is called the Aspect Ratio. If you multiply the width (the first number) by the aspect ration number, the result will tell you the height of the tire section between the bead and the tread. In our example, we multiply 120 x .70 and can calculate that this tire has a height of 84 mm. The final set of numbers, those which follow the hyphen, tell the height of the rim in inches.