16 Rubber & Plastics News ● November 26, 2012 www.rubbernews.com

Effect of underinflation on tire operating temperatureExecutive summary

Thermal imaging is used to capture operating temperatures of tires. The operating temperatures are recorded and compared for tires at varying

levels of inflation pressure and speed, under a constant load. The deflection of the tires at each set of conditions, the static footprint pres-

sures and the operating profiles are also measured and compared.

By Jenny PaigeCooper Tire & Rubber Co.

A tire uses its inflation pressure formany functions.

It allows the tire to carry load, trans-mit forces, absorb shock, provide gripand resist wear.

For a tire to provide optimum per-formance characteristics, it has to con-tain the right amount of pressure.

This pressure is what gives the tirethe majority of its stiffness to carry theweight of the vehicle as it travels downthe road.

It has been stated in previous work,that a tire’s structure only accounts for10-15 percent of a tire’s load carrying ca-

pacity.1

Tires are designed to handle a certainlevel of deflection against the road.

When a tire is underinflated or over-loaded, it will deflect beyond its de-signed deflection.

A tire operating in this state is re-ferred to as overdeflected.

The relationship between load and in-flation pressure for which a tire is de-signed is not manufacturer specific. It isguided by the tire and wheel industries.

Tables exist which can be referencedto determine the maximum load a spe-cific tire size can carry and the inflationpressure necessary to carry that load.2

Also, the Federal Code of Regulationsrequires that tire manufacturers in-

clude the maximum load capacity andinflation pressure on the sidewall ofevery tire legal for highway use in theU.S.3

Common knowledge recognizes andnumerous technical articles discuss un-derinflation as a leading cause of tirefailure.1,4,5

Much less has been published thatdocuments or depicts how a tire is af-fected by underinflation or overloading.

This study documents how tires re-spond to different inflation conditions aswell as speeds.

Three different methods are used tocapture and measure the effect of apply-ing different stress and strain cycles inthe form of inflation conditions and

Each tire was inflated to a specifiedinflation pressure (see Table II) andallowed to condition at ambient tem-perature for at least 24 hours beforetesting.

The tires were loaded to 1,448 pounds.Table III shows this load as a percent ofthe calculated load the tire can carry ateach test inflation pressure.

The calculated load at each test infla-tion pressure was determined per theTire and Rim Association Yearbook andEngineering Design Information.2,7

The following tests were performed:One tire of each group was used to es-

tablish static contact (footprint) pres-sure on a flat surface for each conditionlisted in Table III. This was done usinga pressure mapping system.

One tire of each group was used torecord static vertical deflection againsta flat surface for each condition listed inTable III.

Thermography testing was performedto record operating temperatures foreach condition listed in Table III, ex-cept for test 4, which will be discussed inthe results section.

Thermography testing was carried outusing a single position of a two-positionmachine.

The test machine was located in aroom that was climate controlled to75°F.

The machine contained a smooth sur-face steel test wheel, 67.23” in diame-ter.

The test machine used a mechanicalloading system with a feedback controlloop to load the tires to 1,448 pounds.

Each tire was tested at 65 mph for 45minutes. Thermographic images werecaptured every five minutes.

After 45 minutes, the tire was stoppedand a final thermographic image wastaken.

The final inflation pressure and ambi-ent temperature were recorded.

The tire was allowed to cool back toambient temperature.

The tire was then retested at 90 mphfor 45 minutes using the same test pro-cedure, including the same load and in-flation pressure, as the 65 mph test.

A schematic of the test setup is shownin Fig. 3.

One tire of each group was used to ac-quire dynamic tire profiles, operating at65 mph and 90 mph for each inflationpressure listed in Table II.

This is done by operating the tire onthe test wheel with a load of 100 pounds,and using a laser profilometer to meas-ure the dimensions of the tire as it oper-ates.

The profile is taken at the top of thetire, about 90° from the contact patch.

ResultsStatic deflectionResults for the tires are shown in

Fig. 4.Thermographic imagesThe thermographic images are shown

in Figs. 7 and 8. The maximum temperature is marked

on the image. The images in Figs. 7 and 8 are those

taken of the stopped tire, after 45 min-utes of operation.

Figs. 7 and 8 omit the condition ofhighest overdeflection, as specified inTable III (test 4).

No thermographic data was taken forthe highest overdeflection condition.

This condition specified 140 percentload (15 psi).

speeds to tires: (1) static footprint or con-tact patch measurements; (2) dynamicprofilometry; and (3) thermographic sur-face temperature measurements.

Experimental designTwo groups of four new passenger

tires were involved in this study.A representative photo of each tire is

shown in Figs. 1 and 2.All tires were initially inspected and

found to be free of any design issue ormanufacturing anomaly.

Each tire was then mounted on its de-sign rim width per the 2012 Tire andRim Association Yearbook.2

Each tire was mounted using theRMA standard mounting procedure.6

Fig. 1. Tire A.

Fig. 2. Tire B.

TECHNICAL NOTEBOOKEdited by Harold Herzlichh

Technical

Fig. 3. Thermographic test setup.

Fig. 4. Results.

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TechnicalIt raised concern as to whether the

tire would fail and possibly harm thethermographic camera.

Out of caution, tire A was run at 15psi and 90 mph without the thermo-graphic camera in the room.

The tire failed after running 25miles. A forensic inspection revealed aseparation below the No. 1 belt and a1.0-inch radial split on the serial sideshoulder.

There was also a separation betweenbelts at the same location, and intermit-tent visible trapped air at the belt edgearound the tire.

It was decided to remove this operat-ing condition from thermographic test-ing.

Figs. 9, 10 and 11 show the failedtire.

Dynamic cross sectional profilesLaser profiles were taken at 65 mph

and 90 mph for tires at each inflationpressure listed in Table II.

Figs. 12 and 13 show the data for thehighest and lowest inflation pressuresonly.

The figures overlay the profiles for 45psi and 15 psi, for both 65 mph and 90mph.

There are insets on the figures of boththe shoulder and sidewall area of thetires, for ease of comparison.

Tables V and VI then list the meas-urements taken from all profiles, for the

dynamic tire’s maximum section width,maximum section height, and shouldersection height.

Height measurements are taken fromthe top of the rim flange, shown as thezero location on the figures.

The shoulder section height wasmeasured on the profile at -3 inchesfrom the tire centerline. This places themeasurement on the serial side of thetire.

DiscussionAs the inflation pressure decreased,

the temperature of the tire increased.The maximum temperature occurred inthe shoulder area for all tests.

This increase in temperature is attrib-utable to an increase in deflection of thetire. Consider the tire to be like a simplespring.

The principles of Hooke’s Law apply:

F=kx Where F is equal to the load placed on

the tirek is the stiffness of the tire, largely be-

cause of the inflation pressurex is the deflection of the tireAs the inflation pressure is decreased,

the stiffness of the tire (k) decreases. In order for the tire to carry the load

(F) when k is decreased, the deflection(x) must increase.

Fig. 4, a graph of the measured static

deflection, shows this trend. As deflection increases, there are as-

sociated changes in the footprint. The length of the footprint in the cir-

cumferential direction increases, as seenin Figs. 5 and 6.

Tire B shows an increase of 83 percentin length from 45 psi to 15 psi.

Tire A increases 85 percent. There is also movement of the contact

pressure toward the shoulders as the in-

Fig. 5. Tire A static footprint pressure profiles (45 psi, 35 psi, 25 psi, 15 psi, respectively).

Fig. 6. Tire B static footprint pressure profiles (45 psi, 35 psi, 25 psi, 15 psi, respectively).

Table IV. Tire footprint dimensions.

Fig. 9. Tire A tread separation.

Fig. 11. Tire A radial split in shoulder.

Fig. 10. Tire A separations in shoulder.

See Tire, page 18

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18 Rubber & Plastics News ● November 26, 2012 www.rubbernews.com

Technical

flation pressure decreases.The inflation pressure in a tire causes

tension in the carcass cords which, in

turn, pull the belt edges radially down-ward.

As the tire rolls, and any given sectionof the tire enters the contact patch (be-coming deflected), the tension on thecarcass cords is lessened, allowing thebelts to flatten and causing some lateraland circumferential movement of the

TireContinued from page 17

Fig. 7. Tire A thermographic images (16x6.5 inch wheel).

Fig. 8. Tire A thermographic images (16x6.5 inch wheel).

belt wires. This phenomenon is explained in de-

tail in Chapters 6 and 7 of The Pneu-matic Tire1.

When this same section of tire rollsout of the contact patch, the cord ten-sion is restored until the section makesanother revolution into the contactpatch.

This cyclic tension/relaxation of thecarcass and belt cords causes stress andstrain reactions in the rubber matrixsurrounding the cords.

Rubber compounds generate heat as aresult. The heat generation is directlyproportional to both the amplitude andfrequency of these tension/relaxation cy-cles.

The amplitude of the cycles is affectedby inflation pressure.

Greater deflection causes larger cordand wire movement in the footprint re-gion, causing more stress and strainwithin the rubber matrix and more heatgeneration.

The frequency of the cycles relates tothe tire’s rotating speed.

The faster the tire is traveling, themore times per minute the tire will cyclethrough tension and relaxation, thus themore often the rubber matrix is cyclingthrough stress and strain, and generat-ing heat.

Rubber is also not a good conductor ofheat, so the heat tends to build up insideof the tire structure.

The shoulder area of the tire is tradi-tionally the thickest section of the tire,which makes it prone to greater heatbuildup.

Also, as the inflation pressure de-creases, the contact pressure in the foot-print transfers toward the shoulders ofthe tire.

This, combined with the shoulder’sability to insulate heat, is the reason forthe maximum surface temperature atthe shoulder during overdeflected tireoperation.

Again referencing the thermographicimages, another trend in the data showsthat at the higher operating speed, 90mph, the tire temperature is greaterthan at 65 mph.

The trend in the data shows that for atire with lower inflation pressure, an in-crease in speed causes a greater in-crease in temperature than for a tirewith higher inflation pressure.

Stated another way, the overdeflectedtires are more sensitive to speed andrun hotter at an increased speed thanthose properly inflated.

Figs. 12 and 13 show the impact ofspeed on the tire profiles.

The data reveals that at a constant in-flation pressure, an increase in speed re-sults in growth in the shoulder diameterof the tire, and a reduction in the sectionwidth.

It makes sense because the centrifu-gal force on the tire is a result of the ve-locity squared.

Furthermore, the section width de-crease is more pronounced in tires oflower inflation pressures.

The tire is less stiff and more respon-sive to the centrifugal force acting on it.

As was stated before, the pressure in-side the tire creates a tension on the fab-ric cords that extend around the lengthof the tire profile.

The tension in the cords is used to bal-ance the centrifugal force.

So as the shoulder diameter grows asa result of speed, the tire width narrowsand the sidewall becomes flatter, direct-ing a greater amount of the cord tensionin the radial direction to balance the in-creased centrifugal force.

This leaves less lateral cord tensionto resist sidewall bending under load.Thus, the amplitude of the stress/straincycles in the rubber matrix is in-creased, causing greater heat genera-tion.

Conclusions● The footprint or contact patch of a

tire increases or lengthens and movesthe contact pressure outward to theshoulders as the overdeflection in a tireworsens.

● The operating profile of a tire altersresulting in greater stress and strain cy-cles in the shoulders as the overdeflec-tion in a tire worsens.

● Thermographic imaging revealsheat generation increases in the tireshoulders consistent with the increasedstress and strain from the longer foot-print and altered profiles.

● At all pressures and speeds meas-ured, the maximum temperatures gen-erated were in the shoulder or belt edgearea.

● Forensic examination of the testedtires finds observable failures to be in-cipient and actual belt edge separationsand failures in the shoulder area consis-tent with the increased contact pres-sure, altered operating profile and moresevere heat generation in the shoulderarea.

Table V. Tire A dynamic profile meas-urements.

Table VI. Tire B dynamic profile meas-urements.

Table VII. Percent increase in shouldertemperature from 65 mph to 90 mph.

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Fig. 13. Tire B profile comparison.

Fig. 12. Tire A profile comparison.

TechnicalReferences1. The Pneumatic Tire (2005). Washington, D.C.:National Highway Traffic Safety Administration,U.S. Department of Transportation, DOT ContractDTNH22-02-P-07212012.

2. Yearbook of the Tire & Rim Association Inc.(2012), Copley, Ohio: The Tire & Rim Association.3. Title 49 CFR Ch V, Parts 571.139, Section 5.5,Federal Motor Vehicle Safety Standards and Regu-lations, Washington D.C.: Office of Vehicle Safety

Compliance.4. Thiriez, Kristen K, Ferguson, Eric, Rajesh, Sub-ramanian (May 2005). 12 & 15 Passenger Vans TirePressure Study: Preliminary Results, NHTSA’s Na-tional Center for Statistics and Analysis, Washing-

ton DC., DOT HS8098465. Grant, Joseph (2004). Rim Line Grooves as an In-dicator of Underinflated or Overloaded Tire Opera-tion in Radial Tires, Presented at the InternationalTire Exhibition and Conference paper 45D.6. Demounting and Mounting Procedures for Pas-senger and Light Truck (LT) Tires, Wallchart, Rub-ber Manufacturers’ Association.7. The Tire and Rim Association Engineering De-sign Information. Copley, Ohio: The Tire & Rim As-sociation.8. Tire Safety Everything Rides On It, NationalHighway Traffic Safety Administration, DOT HS809 361.9. Beach, D., and Schroeder, J. (2000). “AnOverview of Tire Technology,” Rubber World, 222(6), 44-53.

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