Buzz
When you pass by a dump truck with a couple of tires seemingly floating just above the pavement, you might think they're spare tires. In a sense, they are—but they weren't installed to serve as replacements for flats.
As Jalopnik explains, those wheels are actually there to help distribute the weight when the truck is carrying a heavy load. They're connected to special axles called tag axles or lift axles, which truck drivers can lower automatically (usually through springs or air bags). Some vehicles, such as dump trucks and concrete mixers, may even have extra axles that sit atop the truck and unfold onto the road behind it. These trailing axles serve the same purpose.
For lighter loads, drivers retract these extra axles to avoid unnecessary wear on the wheels. It also helps them save money: The number of axles on a vehicle is one of the factors used to determine toll rates.
As HowStuffWorks outlines, the number of axles on a vehicle plays a key role in determining its legal weight limit. While these laws differ across states, some general patterns exist. For example, a single axle is usually capped at about 20,000 pounds, a tandem axle typically maxes out around 34,000 pounds, and so on. (There are, of course, various exceptions and some flexibility within these rules.) If trucks consistently exceeded weight limits by neglecting to deploy their additional axles, they wouldn't just risk damaging their own overloaded wheels—they could also accelerate road wear beneath them.
