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CUSTOMIZING
PERFORMANCE TIRE AND WHEEL RELATIONSHIP
Tire and Rim Association (TRA) Yearbook
To maintain consistency in specifying a tire's measurements,
representatives of all major tire companies and most minor brands
formed The Tire and Rim Association (TRA) in the United States.
The TRA has established technical specifications that are based on
engineering principles and approved practices. Every year they
publish the TRA Yearbook, which contains all TRA standards and
related information approved by the Association. These standards
include:
- Tire Designations
- Tire Load Ratings
- Dimensions
- Approved
- Rim Widths
- The Design Rim or Measuring Rim
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Standards for Metric tires are established through a similar
association called the European Tire and Rim Technical
Organization (ETRTO). The approved rim width range for each tire
size has also been carefully selected by the TRA and ETRTO
organizations. For example, the P255/50VR16 tire is approved to
be mounted on wheels ranging from 6-1/2" to 10" (refer to pg. 44).
Past experience and engineering practice has shown that wheel
widths outside these ranges will stress the tire in a manner that
may result in poor service and potential tire failure.
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It is important to note that there are some differences
between P-Metric and Metric approved rim width ranges
(in general, Metric tires have a slightly narrower range).
The TRA also has developed a measuring rim for each tire size
that enables all manufacturers to measure their tires on the same
size wheel. For 50-series tires and higher, the measuring rim width
is 70% of the tire's section width rounded off to the nearest
0.5." For example, a P255/50VR16 tire, which has a design section
width of 10.04", is measured on a 7" wide wheel.
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Mid-Range Rim Widths
Provide a balance between handling capabilities and ride quality.
The wheel's width influences handling and ride quality. Always
choose a rim width within the range of the tire's acceptable rim
width specification.
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For tires that have an aspect ratio lower than a 50-series,
the measuring rim is 85% of the tire's section design width.
Therefore, a 255/40ZR17 tire with the same 10.04" section width
would be measured on an 8.5" wide wheel. The measuring rims for
Yokohama tires are listed with their dimensional data in this
guide.
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As mentioned previously, the actual dimensions of a tire are
dependent on the rim on which it is mounted. The dimension that
changes the most is the tire's section width—a change of about
0.2" for every 0.5" change in rim width.
For an A032R in the P255/50VR16 size, Yokohama's dimensional
data indicates a section width of 10.2", an overall diameter of
26.0", and a tread width of 9.1" when mounted on a 7.0" wide
wheel.
While the tire size designation indicates that this is a
50-series tire, let's see how the tire's aspect ratio is
calculated. A tire's aspect ratio is the relationship between its
section height and section width. Since the section height is not
provided, we must calculate it.
Determine the section height.
Subtract the rim diameter from the overall tire diameter and
divide by 2 as shown in the following formula:
(Overall Diameter - Rim Diameter) Ö 2 = Section Height
To determine the section height of a P255/50VR16 tire:
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Note:
More than one dimension may vary with a change in rim width;
however, the Yokohama tire dimensions listed in this guide are
accurate for the measuring rim widths specified.
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