The Extra Wheels That Don’t Touch The Ground: What is it?

If you’ve ever been stuck in traffic on the highway and noticed a huge tractor-trailer with those additional wheels that don’t touch the ground, then you may have wondered what they’re for. In this article, we’ll explore these extra wheels and see how they are beneficial.

Big truck with 3 wheel off ground
Source: TruckPR / flickr

Truck wheels that do not touch the ground are commonly referred to as “high boys.” These wheels provide several advantages for specific industries. A high boy can be used when hauling heavy loads, large loads, or when the truck is transporting materials along curvy roads.

Types of truck wheels

Different tire types
Source: ALCAR Holding

Truck wheels can be divided into several categories, according to diameter and material. The three main types of truck wheels are steel, aluminum, and alloy.

In general, alloy wheels have a better quality than steel ones due to their better resistance to corrosion and other issues that may occur because of weather conditions. Alloys also have a much longer lifespan than steel or aluminum types of truck wheels. However, alloys are prone to dents if not properly taken care of.

Truck tire pressure and load capacity

Person checking tire pressure

Truck tires are not all created equal. They differ in construction, size, and tread patterns. Each aspect impacts the amount of weight a tire can support before it fails.

For example, a truck tire with a lower profile has more rubber touching the road than one with a taller sidewall. This creates more friction between the road and the tire, which means that less weight is required to make it fail under pressure. A good rule of thumb is that each inch of sidewall height adds 10 pounds per square inch (psi) to the load capacity rating for any given tire type.

Why do some truck wheels not touch the ground?

If you have ever seen a truck carrying something light, you may notice that the truck’s wheels are not actually touching the ground. This is because trucks need to carry different heavy loads. To be able to do this, trucks are fitted with multiple axles.

Steerable axles
Source: Nimbus Logistics
Steerable axles

Trucks are built with three different types of axles: steerable axles, trailer axles, and tandem axles. The combination of these axles determines how many wheels will touch the ground when carrying heavy loads. It’s better to remove unnecessary tires from the ground than to keep them spinning while the vehicle is in motion and consuming extra gasoline.

Trailer axles
Source: My Little Salesman
Trailer axles
Tandem axles
Source: Fleet Equipment Magazine
Tandem axles

On some trucks, there is a suspension device called an air spring or “airbag.” It looks like a giant metal donut, and it extends out from between two tires, holding them up and keeping them off the ground. They provide extra load capacity and minimize wear and tear on the suspension components below them.

Truck air spring or airbag
Source: Hendrickson

So why do some trucks have wheels that don’t touch the ground and instead float on top of the axle?

The short answer is that it makes a truck with a heavy load more stable, but there’s a little more to it than that. The primary function of those wheels is to reduce wear on the other drive components. Since they don’t move when the truck is in motion, they can help prevent damage caused by dirt and rocks being thrown around when driving over rough terrain.


When it comes to the question of why some trucks have extra wheels that don’t always touch the ground, there are many answers. However, no one answer applies to all trucks. The real answer depends on the truck and its intended use. For example, suppose you’re hauling a lot of material over rough terrain. In that case, you might consider getting tires with an additional set of smaller wheels on top of your regular tires. This helps distribute the weight more evenly for better traction.

There are many theories about the purpose of the extra wheels. Some people think that they are there for balance or stability, while others believe that they are used to keep the truck from tipping over. The truth is that those wheels were actually designed for carrying heavy loads. What do you think is the real purpose of those wheels?

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