The first number in the tyre's description indicates width, expressed in millimetres. This is obtained by measuring the distance between the two most distant points on either flank, with the tyre mounted upon the rim. An example of tyre size description is 215/55 R16, where 215 represents width. However, actual width often will not be exactly 215, but rather just slightly below or above this value. Such variations are tolerable, since standard width categories are defined by successive 10 mm intervals - therefore, 205 and 225, respectively, are the standard width sizes nearest to 215.
The value for width is followed by the aspect ratio indicator. This represents the height of the tyre flank, or the distance between the rim and the edge of the rolling surface, in relation to width. For a 215/55 R16 tyre, the second number, 55, is a percentual value that describes flank height as a percentage of width, a parameter known as aspect ratio. To determine the absolute value represented by this, in millimetres, the following calculation is necessary: multiplying width, 215 mm, by the percentual value, here 0.55, equals 118.25 mm. The flank height of this tyre is therefore 118.25 mm. 55 is a fairly average value for aspect ratio. An aspect ratio of 80 is generally considered high - and such values are often omitted in the description.
In typical size descriptions, such as 215/55 R16, numerical indicators are also accompanied by the letter R. This letter indicates a tyre of radial construction, which is a common type for modern models. In the past, diagonal construction predominated, but has since become largely obsolete. Radial construction employs reinforcing cord plies within the rubber matrix that are disposed at right angles from the sense of rotation, rather than diagonally. This enhances grip values during hard cornering and on wet road surfaces. The letter R is followed by yet another numerical indicator. The number 16 in the 215/55 R16 description represents the size of rim, in inches, that should by equipped with such a tyre - this helps you correctly chose the rims that match your tyres, should you prefer not to buy a complete wheel and tyre set from the beginning.
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