Interpretation ID: nht94-5.41

DATE: May 12, 1994

FROM: John Womack -- Acting Chief Counsel, NHTSA

TO: Martin M. Sackoff -- Executive Director Of Laboratories, International Testing Laboratories

TITLE: NONE

ATTACHMT: ATTACHED TO LETTER DATED 2/7/94 FROM MARTIN M. SACKOFF TO NHTSA OFFICE OF CHIEF COUNCIL (OCC - 9646)

TEXT: Dear Dr. Sackoff:

This responds to your letter to this agency with reference to Federal Motor Vehicle Safety Standard No. 109, New Pneumatic Tires.

Your specific question addressed S 4.2.2.4, Tire strength, which states: "Each tire shall meet the requirements for minimum breaking energy specified in Table I when tested in accordance with S 5.3." You asked for an interpretation of the term "breaking," whether it means a blowout of the tire or the breaking of the tire caused by the plunger used in the test specified in the standard.

The breaking energy test is a measure of the resistance of the tire to bruise or damage due to impact of the tire with road hazards. This agency tests such resistance in accordance with the procedures of S5.3, Tire strength, of the standard. In that test, a cylindrical steel plunger is forced perpendicularly into the tire rib at the rate of 2 inches per minute at five test points equally spaced around the circumference of the tire. The inch-pounds of force required to push the plunger into the tire is continuously monitored. As the plunger pushes into the tire, the resistance to the plunger force increases. That resistance requires ever-increasing force applied to the plunger to continue pushing it into the tire. Ultimately, one of two things will happen:

1. The plunger will push all the way to the rim; or

2. The tire cords, plies, innerliner, or other components of the tire will stretch, separate, crack or break so that the resistance pressure of the tire diminishes. The "breaking" of the tire at that point does not require an actual blow-out although, obviously, a blow-out would constitute a "breaking."

The plunger force is measured just prior to contact with the rim as in 1 above or just prior to the force reduction

2

described in 2 above. The measured force is then combined with the penetration of the plunger into the tire as specified in S5.3.2.3 and S5.3.2.4 of the standard. The breaking energy value of the tire is then determined by computing the average of the values obtained at the five test locations on the tire. Table I, Appendix A of the standard specifies the minimum breaking energy of tires based on tire type, size, composition, and inflation pressure.

I hope this information is helpful to you. Should you have any further questions or need any additional information, please feel free to contact Walter Myers of my staff at this address or at (202) 366-2992.