MadSci Network: Science History Query:

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 car�s 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

MadSci Network, webadmin@www.madsci.org

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