State College, PA —Drive on under-inflated tires and you're flirting with disaster, as every Ford Explorer driver in America now knows from Firestone's Wilderness tire recall.
But what most don't know about tire failure is that it's caused by "standing waves."
"Every tire has a maximum speed limit at which a standing wave occurs along the tire circumference, causing deformation, a temperature rise, and eventual failure," says Moustafa El-Gindy, director of the Crash Safety and Vehicle Simulation Research Center at Penn State University's Pennsylvania Transportation Institute (PTI).
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Tire pressure vs. threshold speed
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| Tire pressure |
Standing waves start at |
57 psi |
155 mph |
38 psi |
124 mph |
19 psi |
87 mph |
Testers usually determine this threshold speed by rotating prototype tires in contact with a test drum. But El-Gindy and his colleagues have developed a non-linear FEA computer model using PAM-SHOCK™software from ESI Group (New South Wales, Australia).
It turns out that a tire is much more complex than a simple, spinning rubber tube. PTI's tire model includes 7,880 shell elements, 4,200 solid elements, 1,680 membrane elements, 120 beam elements, and two rigid body elements to simulate the rim and the road obstacle.
One discovery made with the system is that the wave happens at lower speeds in flatter tires. The engineers can also use the simulator to test speeds up to 280 mph, and determine energy consumed by the tire, the forces acting on the tire spindle, and the pressure at the patch meeting the road. Conditions duplicated include virtual bumps that can estimate ride comfort.
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