Why Do Some Truck Tires Not Touch the Ground?

One clever design saves thousands

Truck. Image Source: DALL·E

Ever see a big dump truck or cement mixer on a highway and think, “Is something wrong here? The huge wheels are turning, the frame is shaking, but when you get closer, you see that the tires are not rolling.

One entire row isn’t even touching the ground.

They’re just floating around there, spinning in the air, ghosting along the pavement.

On the surface, it seems like some kind of mechanical error — the suspension may be collapsed, or perhaps a glitch in a video game.

However, this floating wheel is not a type of error. It is in fact a brilliant engineering solution that saves billions of dollars in road repairs, saves the trucking company a lot of money on fuel, and prevents the truck from wrecking itself during a turn.

Let’s first take a look at the road under the floating wheel. Concrete and asphalt are hard, but they can reach a breaking point. If you put too much weight on a single point, you don’t just drive over the road, you crush it.

That’s the reason for the strict axle weight limits. The weight of the entire truck isn’t the only thing that matters. It’s all about where that weight is placed.

It’s like walking on a deep snow. You sink right in a pair of stilettos. But if you wear snowshoes, your weight is spread out over a larger area, and you stay on top.

That additional set of wheels, officially known as a lift axle, is for exactly this purpose. In case the dump truck is completely loaded with 20 tons of gravel, this weight is excessive for only two or three regular axles.

So, the driver flips a switch in the cab. That hovering axle is forced to the ground by the action of large airbags inflating. Suddenly the truck is equipped with snowshoes. The load is spread over more tyres, the pressure on the road is reduced and the truck becomes legal to drive without damaging the road surface.

So, why not put them down for all of the time, if they’re supposed to help with weight distribution and keep the roads safer? Why carry them up to begin with?

As in most things in history, this comes down to money.

Tires are expensive. Big rig tires can be expensive — up to hundreds of dollars per tire — and they wear down easily. Friction is created with each mile a tire rolls on the road.

This is known as ‘rolling resistance’.

The more tyres that are in contact with the ground, the more effort the engine needs to put in to move the truck, and the more fuel burned.

Suppose that a truck has just dropped off what it has been carrying. It’s now empty in light. The driver is wasting fuel and working very hard on expensive rubber for no reason if he puts those extra axles down.

They lift them up and at once decrease rolling resistance. For heavy equipment, it’s an eco mode!

Furthermore, there’s the issue of tolls. Numerous highways and bridges levy tolls according to the number of axles on the ground for trucks. A driver can then legally assert that the truck is a smaller vehicle, sometimes half the toll.

The last important role of the lift axle is maneuverability. Trucks are long. The rear wheels don’t just roll when a fixed axled truck attempts to make a tight turn, they are dragged across the pavement.

This is referred to as scrubbing. It screams, it tears down the tyres and it severely stresses out the truck’s structure and suspension system. More than three wheels on the ground bassically means the truck wants to drive in a line forever.

It refuses to turn.

This raises that additional axle, effectively reducing the wheelbase of the truck. This makes the truck easy to handle around corners, avoiding damage to the tire tread or the pavement surface in tight construction areas or urban streets.

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