Tuesday, October 17, 2000
Drive on under-inflated tires and you're flirting with disaster,
as every Ford Explorer driver in America now knows from Firestone's Wilderness
But what Americans don't know about tire failure is that it's
caused by a phenomenon called the "standing wave."
"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,
To date, testers have determined this maximum speed by rotating
prototype tires in contact with a drum on a testing machine. But El-Gindy and
his colleagues have developed a non-linear FEA computer model using PAM-SHOCK™
software from ESI Group (www.esi.com.au/PAM/shock.html).
It turns out that a tire is much more complex than just a simple,
spinning rubber tube. Their tire model includes 7,880 shell elements, 4,200
solid elements, 1,680 membrane elements, 120 beam elements, and also two rigid
body elements to simulate the rim and the road obstacle.
One discovery of the system is that the critical standing wave
happens at lower speeds, in proportion to lower tire inflation pressure, below
the manufacturer's recommended value. The engineers also use the simulator to
test speeds up to 280 mph, the energy consumed by the tire, the forces acting on
the tire spindle, and the pressure at the patch where the tire meets the road.
They can also simulate road vibrations, and estimate ride comfort by adding
virtual bumps to the program.