Conversion Charts and Other Good Info
Cross Reference Info|
There are 5 different parts of a tire size:
The first mumber or second letter in a tire size represents the nominal width. Width is measurted in a straight line from the furthest point on one sidewall, across the tread, to the furthest point on the opposite sidewall. The different size numbering systems specify widths in different measurements.
Aspect Ratio indicates a tires cross-sectional profile. The smaller the number, the lower the profile. It expresses the height to width ratio as a percent. A 90 aspect ratio means the tire's height is 90% of it's width. The aspect ratio appears immediately after the width in the Metric, Alpha and Low Profile Inch numbering systems.
Speed Ratings are internationally recognized maximum speeds at which the tire may be used with maximum load when the maximum listed inflation pressure is used. Maximum loads and inflation pressures are found on the sidewalls of the tires. Speed ratings are coded by a letter, which appears directly after the width, aspect ratio, or as part of a three digit Load/Speed Index, found on the tire directly after the complete size designation. The letters and their corresponding speeds are listed below.
The rating system shown above indicates the top speed for which a tire is certified. It does not indicate the total performance capability of a tire.
The Tire Construction, when included in the size numbering, is listed after the speed rating. The two options for tire construction are Bleted (B) or Radial (R). A belted tire has fiberglass, Kevlar, or aramid fiber belts for added strength and load capacity, however not all belted tires will have the B designation. If a tire does not have the Radial (R) designation, it is a bias-ply tire.
Rim Diameter is the diameter of the rim/wheel on which the tire will be mounted, in inches.
Some tires may have additional information in their size, for example WW, meaning it's a white wall tire. If the size is followed by TT, it means it is a tube type tire, which requires an inner air tube. TL means it's a tubless tire. Others may have M/C at the end of the size, which simply means it is a motorcycle tire.
There are four different motorcycle tire size systems currently in use. The systems can have up to five parts listed above. The different systems are:
The metric tire size system is the most common and also the most descriptive. Metric sized tires are used on just about every type of motorcycle, from the latest sport bikes, to cruisers, touring bikes, and everything in between. An 'M' sometime precedes a metric size when there is no speed rating used. It simply means that it is a motorcycle tire. Another letter is sometimes used after the speed rating to indicate belted (B) or radial (R) tire construction.
In the first example above, 180 is the width in millimeters, 55 is the aspect ratio (cross-section height is 55% of the width). Z is the speed rating (149+mph), R specifies it's a radial construction tire, and 17 is the wheel diameter in inches.
Alpha System(i.e. MT90S-16 - MT90HB-16)
The Alpha size designation is most often used for touring motorcycle tires. They can be belted even without the B designation, which can appear after the speed rating.
In the example above, M states that it is a motorcycle tire. T is the width as listed below. 90 is the aspect ratio (cross section height is 90% of the width). S is the speed rating (112mph) and 16 is the wheel diameter in inches. Since there is no designation of construction after the speed rating, it is a bias ply tire.
The Standard Inch systems are found on older motorcycles. Tires with these inch markings do not include as aspect ratio designation, which is 100%. It means the cross section tire height is 100% of the width. Inch size tires are all bias ply construction and not belted. They are no longer being fitted on new motorcycles and are gradually being phased out of use.
In the above size, 3.25 is the width, H is the speed rating, and 19 is the rim diameter in inches. The aspect ration is 100% of the tire width.
Low Profile Inch System (i.e. 4.25/85H-18 - 4.60S-16)
This is also called the 82% Series Size System because the aspect ratio, when unspecified, is 82%. The tire's cross-section height is 82% of it's width, except in cases where it's listed as 85%, as in the first exapmle above. This system is limited to very few sizes and the motorcycle manufacturers are no longer using tires with this size system.
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