MadSci Network: Science History
Query:

Re: why are tyres measured in metric/imperial??

Date: Wed Mar 28 22:56:27 2001
Posted By: Adrian Popa, Director Emeritus, Hughes Research Laboratories
Area of science: Science History
ID: 979596670.Sh
Message:

Greetings:

Reference: http://www.dunloptire.com/lib rary.html

Having been part of the General Motors Corporation for the past 12 years I am
always coming across odd standards and nomenclature that no one seems to
know how or why they were first used. I have not been able to come up with a definitive
answer to your question; however, I believe the historical facts which are
provided on the referenced Dunlop web site along with my comments do provide an answer.
Let me start by presenting some background information about the USA (US for short).

First, the US has been the major market for tires for most of the past century.

Second, although scientists and technologists in the US have embraced the metric
system, the common man in the street (including members of Congress) know
little or nothing about the metric system and actively resist any changes from the
English (as we call them here) system of weights and measurements.

Third, government bureaucracies have cast English units into law making it very
difficult to change to metrics given the feeling of the voters. This includes the
powerful US Department of Transportation (D.O.T.).

Before the introduction of European made radial tires for passenger cars into the
US in the 1960s, American-made tires used English units to denote the size of a
tire. Some size examples are: 6.00X13, 8.25X14, 5.60X15, 7.00X14, 7.50X15,
6.00X15 The first 3 numbers of the size measured the width of the tire casing at
its widest point. For example, the 6.00 of the size 6.00X13 is 6 inches wide. This
called the section width. The 13 of a 6.00X13 is the diameter of the wheel, in
inches, that the tire is to be mounted on. For each diameter of tire there was only
one standard width!

While domestic tire manufacturers and the D.O.T. were changing the tire sizing
systems from Numeric to Alpha-Numeric during the 1960's, the European tire
manufacturers went to a Metric system of sizing predominantly because of
radialization. Some metric size examples would be: 165SR13, 175/70SR13,
185SR14, 195/70SR14, 155SR15, 215/60HR15, 165SR15.

You will notice there is little similarity here in some of the numbers to numeric or
alpha-numeric sizing except for the last 2 digits; 13,14 or 15 which is still the
diameter of the wheel, in inches, that the tire is to be mounted on. However,
where the numeric system used a 6.00 or 7.00 to denote the cross- section of a tire
in inches, the Metric system uses the numbers 165 from the 165SR13 to denote
the measurement of the cross-section in millimeters.

Thus because historically Americans did not need to know about tire dimensions,
except for the wheel diameters, European tire manufacturers were able to use
metric dimensions for tires except for the diameter which was written into
previous US law to be measured in inches. Thus, all European manufacturers
needed to do to get into the vast US market, which did not produce radial tires at
that time, was to change one number on the tire into inches to meet US law.

Also, the 1960 radialization and technology increased the tire manufacturer's
ability to improve the performance of the tire dramatically by increasing the
section width and reducing the sidewall height of the tire. This change of the
aspect ratio needed to be reflected when stating the size. An example of section
width increase in size on the same wheel diameter would be 155"82"SR13 or
155SR13 to 175/70SR13 or 205/60R13. All 3 sizes essentially have the same
sidewall height but notice the section widths increased from 155 mm to 175 mm
and 205 mm as the aspect ratio respectively changed from "82" to 70 to 60. The
overall tire diameters of these 3 sizes should be within 3% of each other.
However, this 3% difference in diameter did cause some problems for the general
public in the US because standard automobile odometers indicated less mileage
using the reduced diameter of radial tires. The general public thought that the
radial tires were reducing their carís fuel economy.

Today, the global market, and concerns about tire safety have resulted in an
ongoing series of international meetings trying to standardize tire parameters,
including loading. If you are interested in how complex it is to change standards
that have been in use for 40 years check out the following web site:

http://www.mailgate.org/gov/gov.us.topic.transport.road/msg00856.html

Best regards, Your Mad Scientist
Adrian Popa


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