Why do some trucks have a set of wheels that don't touch the ground?

Next time you're near a construction site, take a look at the dump trucks or cement mixers bustling around: Somewhere toward the center of a truck you might see a set of wheels that seem to serve no purpose. They're a foot off the ground, goofing off while the other tires do all the work.

What's up with that?

Those tires are part of what's called a "lift axle" and they have what you might call a part-time job. The driver can push a button on the dashboard that lowers to the ground the tires and the lift axle (the metal rod that joins the tires).

The lift axle helps spread the weight of an especially heavy load. Instead of 60,000 pounds of dirt and rocks pressing down on three sets of tires, the load is now pressing down on four sets of tires. The road underneath won't take such a pounding. And by raising the axle when it's not needed, the driver can save wear and tear on that set of tires.

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More and more trucks have the extra wheels because more and more states are requiring them. Maryland, for example, allows dump trucks to carry 55,000 pounds if they have three axles and 70,000 pounds if they have three axles plus a lift axle.

Snowy Doe, who works at the Mack Truck Museum in Allentown, Pennsylvania, says drivers really like the system. "If you don't put it down and the load is heavy, the tires are screaming at you. You can see that there's too big a load by just looking at the tires. They bulge way out."

Time to throw the switch and lower the lift axle.

-- John Kelly

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The tires connected to the lift axle stay up until needed for a heavy load.

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