Effects of aspect ratio on tire performance.

Soon after auto racing began, engineers realized that getting more rubber on the road was one way of increasing cornering speeds. One of the more interesting early attempts to get more rubber on the road was by running dual rear tires. However, the more typical solution was the use of tires with wider tread and section width. This was the first of numerous steps toward lower tire aspect ratios. Since then, due to ever-increasing vehicle handling requirements and the existence of trends to high performance tires, passenger car tire aspect ratios have been decreased. Today, racing tire aspect ratios have gone as low as 0.25.

Tire aspect ratio is defined as tire section height divided by section width (figure 1). Aspect ratios are also referred to by series. For example, a 0.50 aspect ratio tire (such as a P265/ 50VR15) is said to be 50-series.

[FIGURE 1 OMITTED]

Passenger car tire aspect ratios historically have decreased and continue to decrease further. The aspect ratio of the first pneumatic tires, specifically designed for passenger cars (1905-1915), was about 1.10. Until the 1930s, aspect ratios remained at approximately 1.0. In 1984, the aspect ratio of 60% of produced tires was 80%. In 1996, this amount was reduced to 10%, and most of produced tires aspect ratios were between 50% and 65% (refs. 1-4).

Effects of aspect ratio on stresses and deformations of tires Distribution of stresses and deformation in tires is one of the primary properties that could determine many other tire properties and performance, and is greatly affected by aspect ratio.

Studies show that when the aspect ratio is decreased, the stresses in belt cords increase, while stresses along the carcass cords and circumferential stresses in the bead wires decrease. Also, the distribution of stresses seems to be more even in lower aspect ratio tires. According to the new tire design theories (such as TCOT, D SOC), the stresses in the belt cord increase, and the tension of the carcass cords decreases simultaneously, which leads to a marked improvement in tire properties. Decreasing aspect ratio also leads to decrease of normal displacement in sidewalls and radial displacement at the crown center (ref. 5).

Handling performance

In lower aspect ratio tires, reduction of flexible sidewall height improves the structural integrity of the tire and increases radial, lateral and circumferential stiffness. Increased lateral stiffness improves cornering performance. Increased circumferential stiffness improves handling during acceleration and deceleration, particularly in combination with cornering. Shorter sidewalls, in low aspect ratio tires, make it possible to increase rim diameter without increasing tire outer diameter. Higher rim diameter is desirable for anti-lock braking systems (ABS). However, it must be considered that there is a greater possibility of damaging the wheel when driving over deep chuckholes or other objects. Also, forces associated with irregularities in road surface will be transmitted more directly from the tire to the suspension system of the vehicle due to the reduced radial flexibility (refs. 4 and 8-10).

Hydroplaning resistance

Accumulation of water as a film under the footprint, which causes a tire to lift from the road surface and lose traction, is called hydroplaning. As the aspect ratio of a tire is lowered, or the width of the tire is increased, the tire footprint area increases. The larger footprint area reduces the average pressure of the contact patch. Since footprint pressure is closely related to hydroplaning resistance, lower aspect ratio tire hydroplaning resistance is not as high as that of high aspect ratio tires. Tread void can be increased to improve hydroplaning resistance, but this reduces dry cornering capability (ref. 4).

Rolling resistance 

Intuitively, one will accept that lowering aspect ratio would increase a tire's radial stiffness and dimensional stability. This reduces the deflection of a tire and decreases rolling resistance, and thus improves fuel economy and reduces polluting exhaust emissions.

Lowering the aspect ratio also increases circumferential stiffness; hence energy loss at high speeds is reduced. At low speed, lowering the radial tire section height is less effective on rolling loss, in respect to bias tires. But, at high speeds the section height has considerable effect on rolling resistance (refs. 7-10).

Tread wear As mentioned before, lowering the aspect ratio reduces the average pressure of the contact area. It also reduces deflection of the tire due to more dimensional stability. These could result in improving the tread wear.

Figure 2 shows the effect of aspect ratio on tread wear. The effect of aspect ratio on tread wear is more obvious in bias tires than radial ones. Aspect ratio is more important when the tread pattern has higher void volume or when the tire is being used in higher severity conditions (ref. 6).

[FIGURE 2 OMITTED]

High speed performance

Experiments show that a tire's capability to sustain high speeds is primarily related to the following two factors:

* A relatively low level of heat generation; and

* a tread compound with good hot tear strength.

Naturally, the reduced tear strength yields lower high speed test results; however, the drop off in high speed performance may be academic. Tread compound tear strength is related to the tread compound's hardness. As the compound becomes harder, the tear strength increases. However, its traction is reduced. Figure 3 shows the relationship between traction and indoor high speed test performance.

[FIGURE 3 OMITTED]

From the high speed capability point of view at least, lower aspect ratio tires can successfully use softer tread compounds. It seems this is due to the more uniform stress distribution of these tires as compared to high aspect ratio tires. The use of a softer compound increases the traction of the tire on the track. At high speeds, this is very desirable for vehicle handling (ref. 4).

Run flat performance

When normal inflation air pressure is lost, such as when a tire is punctured, the relatively thin and flexible sidewall of a tire can collapse and buckle in such a manner that the sidewall fails to provide its normal functions. These include radial flexibility, distribution of the weight of the vehicle, and transmission of the forces of acceleration, deceleration or cornering from the wheel to the road.

In lower aspect ratio tires, because of improved tire structural integrity and shorter drop height (the distance a wheel drops during air loss), the chance of the tire beads un-seating from the wheel is reduced. And it enables better control to be maintained in bringing the vehicle to a safe stop (refs. 4 and 9).

Production

Production of low aspect ratio tires is more complicated and difficult than high aspect ratio tires.

Low aspect ratio tires are prone to a problem called 'reverse curvature,' due to the increased width of the tire. The term reverse curvature refers to the tendency of carcass cords in the crown region to dip radially inward at the equatorial plane. This deformation creates points of inflection that may lead to premature failure. So, the proper design and assembling of belt and carcass plies of low aspect ratio tires is very important (ref. 11).

The condition of the sidewall is strongly interrelated to the value of the aspect ratio. In a low aspect ratio tire, because of shortening the sidewall height, these parts play a more critical role. This is shown in figure 4 (ref. 8).

[FIGURE 4 OMITTED]

The tendency of considerably increasing the peak value of tension strain is an unavoidable fact remarkably appearing as the value of aspect ratio becomes small. This causes the occurrence of cracking and its growth to be considerably higher when aspect ratio is lowered. So, lower aspect ratio tires need to use sidewall compounds with optimized properties (refs. 8 and 9).

The role of the bead section in low aspect ratio tires is more prominent too. Due to the shorter distance between the rim and the road surface, it is difficult to produce high rigidity and stiffness, required to provide greater support for carcass and sidewall of these tires, and to build up the tension smoothly. To achieve these goals, the construction of the bead section is much different than high aspect ratio tires. For example, the overall height of the apex is increased approximately 100% and more stiff compounds are used for various parts of the bead zone. Also, a thick rubber part that is called a pad or rim cushion is used in low aspect ratio tires. The pad is a relatively hard and high modulus rubber strip located below the sidewall, and gives more stiffness and rigidity to the bead section of the tire. Therefore, it helps the tire to be fitted on the wheel rim. It also serves to protect the sidewall from being damaged by the rim flange when the tire is subjected to a large flexing deformation (refs. 7 and 9).

Softer compounds for the tread and more stiff compounds in the bead area of low aspect ratio tires need more precautions to be taken into account during mixing and extrusion processes.

Also, assembling of the tire's components and building the green tire is more difficult and needs better precision. This could reduce the productivity of low aspect ratio tires compared to high aspect ratio tires (refs. 8-12).

Other considerations

In addition to the above mentioned subjects, lower aspect ratio tires have a more cosmetic appearance and sleek shape to match the aerodynamic design of new modern automobiles. For these reasons, the trends to lowering the aspect ratio of tires, to meet high performance tire requirements, is continued. Today, even a tire without sidewall, with an aspect ratio of 0.2, has been invented (refs. 9 and 10).

This trend is not limited to passenger car tires (refs. 13-15). In a new and patented work, Bridgestone has developed an ultra-low aspect ratio tire for bus and truck use which has the technology for traditional dual-mounted drive tires to be replaced with a single tire. A number of benefits is reported, such as greater durability, low level of noise, improved ride and comfort, reduction in rolling resistance (up to 10%), weight savings of between 80-110 kg on the drive axle, and less width (175 mm) than equivalent dual tires (ref. 16).

References

(1.) D. Beach, J. Schroeder; "An overview of tire technology, " Rubber World, vol. 222, no. 6, pp. 44-53 (Sept. 2000).

(2.) "The tire industry: A new perspective to 2005, " The Tire Industry, Dec. 1997.

(3.) R.A. Ridha and W.W. Curtiss, "Development in tire technology, " in Rubber Products' Manufacturing Technology, ed. by M.M. Hall, A.K. Bhowmick and H.A. Benewey, Marcel Dekker Inc.

(4.) J.T. Warchol and R.C. Schroeder, "Ultra low aspect ratio, high performance tire development, " SAE Technical Paper Series, No. 841290.

(5.) G.Z. Wu and X.M. He, "Effects' of aspect ratio on stress and deformation of radial passenger tires," Tire Science and Technology, TSTCA, vol. 20, no. 2, April-June 1992, pp. 74-82.

(6.) A.G. Veith; "Tire treadwear--a comprehensive evaluation of factors: Generic type, aspect ratio, tread pattern, tread composition. Part II, results of primary treadwear test series," Tire Science and Technology, TSTCA, vol. 14, no. 4, Oct.-Dec. 1986, pp. 219-234.

(7.) M.H.R. Ghoreishy, M. Karrabi and M. Razavi, "Development of optimized compounds for the components of the bead section of a low aspect ratio steel-belted radial tire," Iranian Polymer Journal, vol. 10, no. 2, pp. 115-123 (2001).

(8.) United States Patent, No. 5, 746,860.

(9.) United States Patent, No. 6,499,521 B2.

(10.) United States Patent, No. 4,811,771.

(11.) United States Patent, No. 4, 967, 817.

(12.) United States Patent, No. 5,693,160.

(13.) United States Patent, No. 4,082,132.

(14.) United States Patent, No. 5,634,995.

(15.) United States Patent, No. 4,112, 994.

(16.) "Ultra-wide bus and truck tire by Bridgestone," Tires and Accessories; no. 7, Aug. 2001, p. 56.

Vahdat Vahedy and Mir Hamid Reza Ghoreishi, Iran Polymer Institute