AN ANALYSIS OF BEAD COMPRESSION GROOVES By Dennis Carlson and John Warren Taylor

WHAT IS A COMPRESSION GROOVE?

Compression Grooves Deep Compression Grooves Have Been Associated with Over-deflection(OD) and Tire Bead Design for Decades Early Bead Designs Did Not Control Compression Groove Growth and Failures Occurred Design Changes “Cured” this Problem-Chafers, Protectors, Turn-up Designs and Stiff Flippers

“Severe Bead Chafing” from a Late 1970’s RMA Book

Patents to Control Compression Grooves

Patents, Cont.

Patents, cont.

WHY DO CG’S FORM ? The Flange Area is an Area of High Compression Stresses (Hinge Point) The CG’s Form Due To Compression Set of the Rubber Over Time There Can Also Be a Small Amount of Chaffing (movement) The Sidewall “Bends” Outward in the Footprint-This Increases The Stress

WHAT DOES A COMPRESSION GROOVE LOOK LIKE?

Another example

And Another

Analysis of Compression Grooves- Procedure 75 tires were run by the DOT as part of the UTQG (Unified Quality Grading System) Wear Test (see CFR ) These tires were run for ≈7200 miles. Tire pressures were checked 3 times a day. In short, these tires were run under ideal conditions of usage. After the test, the compression grooves were measured.

The Compression Grooves Were Measured with a Digital Caliper- Width and Depth

Tests of Measurement Technique A Plaster Cast Was Made of the CG Region- Measurements Agreed Profilometer -Measurements Agreed Repeatability-18 Measurements Taken in the Same Area by a Semi-skilled Person- Coefficient of Variation ≈ 9% for Depth and 4% for the Width Measurement

Profilometer

Results-All Tires Had CG’s WIDTHDEPTH MAXIMUM0.316"0.113" MINIMUM0.001"0.001" AVERAGE0.156"0.023"

ALL TIRES HAD COMPRESSION GROOVES

Biggest Differences Were Between Tire Manufacturers Michelins had the smallest- Average Width of.085” and Depth of.016” Goodyears had Average Width of.210” and Depth of.031” (Kelly-Springfields were slightly larger but the sample size was smaller and KS is a part of GY)

Importance of Compression Grooves? In the old days, bead durability was an important issue. Tires could fail prematurely in the bead. In modern tires, this is not an important issue. Compression Grooves are used by some as an indicator of over-deflection.

Mis-Use of Compression Grooves-1 “Any Compression Groove Indicates Over- Deflection” Fact-Compression Grooves are developed under normal operating conditions. Sources –This Paper, the Cottles Paper and the Standard Testing Laboratory (STL) paper.

Mis-Use of Compression Grooves-2 “CG’s are a good indicator of over-deflection” Fact-Because tires develop CG’s under normal conditions and the wide variation between manufacturers, CG’s are a poor indicator of over-deflection.

Mis-Use of Compression Grooves-3 “GC’s equal Over-deflection(OD) Equals Tread- Belt Separations” Most of the Tests That Have Been Run to Show the Link Between CG’s and OD Do Not Separate the Tires. The Standard Testing Laboratory (STL) Test Did Fail Tires But After ≈9000 miles of extreme OD. Other Tires went miles Without Failure. The Failure Mode was Not Given.

Mis-Use of Compression Grooves-3 cont. The Amount of OD in the Most Severe STL Test was Equivalent to Loading a Car to GVWR and Then Putting an Additional 23 People in the Car. Some Tires Lasted miles What Good is This Indicator?

STL DATA - RAW STL Bead Groove Study Subjective Ratings Tire Size=P205/70R14 Load(lbs)Pressure(psi)T&RA %Subjective RatingWidthDepthMileage New Rim Modified Rim

STL DATA- Sorted Same Data Sorted by Severity of Condition Load(lbs)Pressure(psi)T&RA %Subjective RatingWidthDepthMileage New Rim Modified Rim

Future Investigations Deep Wheel Weight Impressions- So far appears to be from bad WW installation Wheel Flange Paint Loss- Seems to be universal with all steel wheels.

Deep Wheel Weight Impressions