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 49 575.104)

• 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

•

• WIDTH DEPTH• MAXIMUM 0.316" 0.113"• MINIMUM 0.001" 0.001"• AVERAGE 0.156" 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 20000 miles Without Failure. The Failure Mode was Not Given.

Mis-Use of Compression Grooves-3cont.

• 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 20000 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 Rating Width Depth Mileage• New Rim• 1433 35 100 1.5 20000• 1628 35 114 1 20000• 1055 26 85 1 20000• 1199 26 97 1.5 20000• 1403 26 113 0.75 20000• 1628 26 131 3 0.24 0.05 20000• 2017 26 163 5 0.35 0.11 20000• 1199 20 110 1.5 20000• 1628 20 149 3.5 0.26 0.06 20000•

• Modified Rim• 1628 35 114 1 20000• 1199 26 97 1.5 20000• 1403 26 113 1 20000• 1628 26 131 2 0.17 0.04 20000• 2017 26 163 2.5 0.2 0.06 10150• 1199 20 110 1.5 20000• 1628 20 149 3 0.26 0.06 15600•

STL DATA- Sorted•

• Same Data Sorted by Severity of Condition• Load(lbs) Pressure(psi) T&RA % Subjective Rating Width Depth Mileage• New Rim• 1055 28 85 1 20000• 1199 26 97 1.5 20000• 1199 20 110 1.5 20000• 1403 28 113 0.75 20000• 1433 35 100 1.5 20000• 1628 35 114 1 20000• 1628 26 131 3 0.24 0.05 20000• 1628 20 149 3.5 0.26 0.06 20000• 2017 26 163 5 0.35 0.11 20000•

• Modified Rim• 1199 26 97 1.5 20000• 1199 20 110 1.5 20000• 1403 26 113 1 20000• 1628 35 114 1 20000• 1628 26 131 2 0.17 0.04 20000• 1628 20 149 3 0.26 0.06 15600• 2017 26 163 2.5 0.2 0.06 10150•

•

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

Deep Wheel Weight Impressions