Interpretation ID: nht74-4.43

DATE: 01/15/74

FROM: AUTHOR UNAVAILABLE; E. T. Driver; NHTSA

TO: Farrar & Farrar

TITLE: FMVSS INTERPRETATION

TEXT: This is in reply to your letter of December 17, 1973, regarding rules and regulations pertaining to tire ply bonding and adhesion strength, and to tire ratings as related to number of plies.

There are no Federal regulations that contain specific requirements for ply bending and adhesion strength, per se, in tires. However, a measure of tire structural strength is provided by the plunger energy test included in Federal Motor Vehicle Safety Standard No. 109, copy enclosed. The high speed and endurance tests also form a part of the tire evaluation process. Poor bonding and adhesion of plies could cause premature separation and failure in these tests.

There is no categorization in the Federal standard that relates tire mating to number of plies, except that labeling of tire sidewalls is required to express values for load rating, maximum inflation, and number of plies. The enclosed booklet entitled "Consumer Tire Guide" shows in its tables of tire load limits how ply ratings have been related to load ratings in tire industry practice. Recent trend in the industry has been to phase out the ply rating system and to substitute in its place the "load range" system denoted by markings with letters.

With regard to test methods and procedures used in tire analysis, we recommend your examination of the following standards of the American Society for Testing Materials (ASTM) and the American National Standards Institute (AMSI).

ASTM 885-72 "Tire Cords, Tire Cord Fabrics and Industrial Filament Yarns made from Man-made Organic Base Fibers" 1973 Edition or ANSI-L 14.231-1973 (2nd Edition) approved August 10, 1973.

ASTM D 2969-71T "Tire Cords, Tire Cord Fabrics, Filaments, and Strands made from Wire"

ASTM D 2970-71T "Tire Cords, Tire Cord Fabrics, and Industrial Yarns made from Class"

ENCLS.