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Goodyear’s EMT tires can run without air, for at least 80 km (50 miles). This performance is derived from a thick, reinforced sidewall with specially engineered rubber compounds capable of withstanding the heat and stress of low-pressure operation while maintaining handling and comfort. 

Reinventing the Tire:

ALGOR FEA Chosen to Verify Wheel Design for Goodyear “Run-flat” Tires

Well-inflated tires improve gas mileage, handling and passenger comfort. More importantly, you really can’t leave home without them. Or can you? The only way to get on the road again if a normal tire becomes flat is to change it. However, new “run-flat” tires have been developed to enable automobiles to be driven at normal speeds for limited distances even with no air pressure.

Goodyear, the world's largest tire company with annualized sales of more than $14 billion, developed a run-flat solution called an extended mobility technology (EMT) tire. During product development, the company realized that running the tire at low inflation pressures might adversely affect the wheel. A parametric study of inflation pressure and stresses in a common wheel was conducted for Goodyear at the University of Akron using ALGOR FEA software to determine whether low inflation pressures would damage the wheel.

EMT Tires Developed to Run Even When Flat

The EMT tires can run without air, fully loaded, for a minimum of 80 km (50 miles) and a maximum of 150 km (93 miles) at 80 km/h (50 mph), depending on the vehicle on which they are fitted. The run-flat performance is derived from a thick, reinforced sidewall with specially engineered rubber compounds capable of withstanding the heat and stress of low-pressure operation. If inflation pressure is lost, the design prevents the tire from coming off the rim, without significant effects on handling and comfort. 

Wheel Stresses Investigated

Because tires deflect so much more than wheels, the wheel is usually assumed to be rigid during tire design. “Most wheels are over-designed in order to meet durability and fatigue standards,” said John Stearns, who conducted the parametric FEA study of the wheel for Goodyear as his doctoral project at the University of Akron. “Tire manufacturers don’t often have to study stresses in the wheel for a particular tire design. However, because run-flat tires have more weight, thicker sidewalls and a different range of possible operating pressures, the standard tire wheel needed to be re-evaluated in order to ensure safety.” 

Stearns modeled a typical aluminum wheel with conventional styling in Pro/ENGINEER based on an actual, common wheel and 2-D drawings provided by Goodyear. The geometry was captured from Pro/ENGINEER with ALGOR’s InCAD technology. “Using InCAD technology is a lot cleaner than using IGES file translation, offering more control and less hassle with file export settings,” said Stearns. 

The wheel geometry was modeled in Pro/ENGINEER and captured using ALGOR’s InCAD technology. 

Stearns generated a hybrid finite element mesh consisting of a combination of brick and tetrahedral elements, with mesh refinement near the area where the tire is mounted to the vehicle. He experimented with the number of elements through the thickness of thin parts such as the rim to arrive at a model consisting of about 35,000 elements. “One of the reasons I chose ALGOR for this project was its flexible meshing capabilities, which were needed for the complex geometry of the wheel,” said Stearns. 

The wheel was fully constrained at the five lug nut holes and partially constrained at the center hole such that the wheel was only free to rotate and slide on the axle. The loading on the model needed to consider both the weight of the car and the inflation pressure of the tire.

The weight of the car on the tire was considered to be 1,000 pounds, which is the maximum load for which the tire is rated. Given that the average car is about 2,000 pounds, which is distributed over 4 tires (500 pounds per tire), the 1,000 pound maximum is about twice what a tire would actually have to support. This weight was applied as a pressure load onto the bottom section of the wheel in a parabolic distribution. Specifically, this load is carried by the part of the wheel called the bead seat, which is the flat area just inside the outer rim. This distribution of weight was calculated from a standard formula for this type of application. In addition to pressures that simulate the weight of the car, Stearns added the inflation pressure of the tire. In a series of three cases, he considered inflation pressures of 0 psi, 17 psi and 35 psi. Inflation pressures are normally derived from the car manufacturer on a vehicle-by-vehicle basis. However, 35 psi is common inflation pressure setting. 

This drawing illustrates how the weight of the car was applied to the wheel. This distribution of weight was calculated from a standard formula for this type of application.

For each of the inflation pressures, a linear static stress analysis was performed. Stearns looked at the deflection, maximum principal stress and maximum principal strain results. “The results revealed that the inflation pressure does affect stress in the wheel,” said Stearns. “Higher stresses were found at lower pressures. That may seem counter-intuitive unless you consider that the pressure helps to maintain the wheel’s round shape. At lower pressures, the weight of the car deflects the wheel into an oval shape which results in higher stresses.” 

Evaluation of maximum principal stresses showed that the wheel experienced greater stress at lower inflation pressures. Above is the maximum principal stress contour in ALGOR for the wheel at 0 psi inflation pressure. 

In the laboratory, Stearns and Goodyear personnel studied the three tire inflations with strain gauges and found good correlation between the experimental and FEA results. “Depending on the location of the readings on the wheel, correlation was as low as 5%,” said Stearns.

 

In the laboratory, the EMT tire was tested at various inflation pressures (left). John Stearns of Goodyear (right) shows the equipment that measured the wheel’s performance. The laboratory results correlated well with ALGOR linear static stress analysis results. 

Although stresses were higher at lower inflation pressures than at the recommended pressure, the magnitude of the stresses was low enough that a standard wheel could be safely recommended for EMT tires as long as stringent wheel tolerances are met. 

By the end of 2002, Goodyear will have sold more than one million EMT tires. Goodyear EMT tires are original equipment on the Chevrolet Corvette, Daimler Chrysler Prowler and the Mini from BMW Group. It is also an option on the BMW 3 Series cars sold in North America. Stearns has also used ALGOR for other tire, wheel and manufacturing component projects. 

Goodyear, located in Akron, Ohio, makes tires, rubber products and polymers for automotive and industrial applications. John Stearns earned his Ph.D. from the University of Akron and is currently working for Goodyear as a Senior Development Engineer. He has also worked for the Rubbermaid Corporation.



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