Why Do Some Trucks Have an Extra Set of Wheels?

When you drive by a dump truck with a couple tires hovering just above the road, seemingly doing nothing, you might assume they’re spares. In a way, they are—but they weren’t installed to act as replacements for flat tires.

Instead, as Jalopnik reports, those wheels are there to help spread out the weight when a vehicle is carting a heavy load. Basically, they’re attached to special axles known as tag axles or lift axles that truck drivers can automatically drop down as needed (typically by way of springs or air bags). Some vehicles, like dump trucks and concrete mixers, may even have extra axles that rest on top of the truck and unfold onto the road behind it. These trailing axles serve the same purpose.

For lighter loads, drivers retract their extra axles so that the wheels don’t suffer unnecessary wear and tear. It may save them some money, too: The number of axles a vehicle has is one of several factors often used to calculate toll rates.

As HowStuffWorks explains, the number of axles is also used to determine how much a vehicle is legally allowed to weigh. These laws vary from state to state, but there are certain trends among them. A single axle, for instance, generally isn’t permitted to bear more than about 20,000 pounds; a tandem axle often maxes out around 34,000 pounds; and so on. (Again, there’s quite a bit of variation and several exceptions to these restrictions.) If trucks habitually exceeded weight limits by not deploying their extra axles, they wouldn’t just be putting extra pressure on their own overloaded wheels—they could also wear out the roads below them.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.