Why Do Semi-Trucks Have So Many Tires?

Semi-truck with a full-sized trailer driving on the highway

Have you ever wondered why semi-trucks have so many tires? The fact that almost all of them have exactly 18 wheels has always been a mystery to me. I did some research to figure out why this is the case, and I’ll explain it all in this article. I’m going to explain why 18-wheelers have so many tires.

Semi-trucks need more tires because they’re carrying a lot of weight over long distances. The additional tires help distribute the weight so the tires blow out less often. It also helps the rig stay stable, prevent flipping due to a weight shift, and allow the semi to go off-road as needed.

What Is a Semi-Truck?

A semi-truck can go by a few different names: 18-wheelers, big rig, or tractor-trailer. Despite the different options, they’re all referring to the same type of vehicle.

A semi-truck is a large, long vehicle used specifically for commercial freight hauling. They have a comparably short “truck tractor” then a long box-looking piece on wheels behind the cabin, called the “semitrailer”.

To make things simple, people call the front the “tractor” and the rear the “trailer”. Put it together, and you get a tractor-trailer.

Side view of a semi-truck with a full-sized trailer

That’s right, these two parts are completely separate. The tractor has the engine in it and drives the rest of the vehicle. It also has the sleeper cabin where the trucker is positioned all day.

The long trailer is connected to the rear of the tractor and is loaded and offloaded as the trucker picks up and delivers the cargo. You can think of it like a trailer you pull behind your pickup and latch into the hitch.

Inside the container, there could be just about anything. It could be filled with stuffed animals, furniture, or art.

Big rig semi-truck transporting cargo in a refrigerator semi truck driving on the road along the Columbia river

There is a federal weight limit of 80,000 pounds of gross weight that can go into the trailer.

The truck will take cargo from one area of the country to another. Your computer monitor could have been made in Ohio but sold in Washington state. At some point, a semi-truck might have had a whole load of monitors that they took across the country to be sold.

I just want to be clear that these are completely different from pickup trucks. I’ve had friends loosely use the term “truck” to talk about these two vehicles interchangeably. Instead, you should use the terms “semi” and “pickup”.

The Number of Tires

If you look at the base of the 18-wheeler, you’ll see a ton of tires. If you’re a math whiz, you might count 18 of them.

Close up of the semi truck tires on axles of a big rig with the horshoe visible parked in a parking lot

This isn’t universally true. The semis can have either 10, 18, 26, or 34 tires. A standard 18-wheeler is set up like this, moving from the front to the rear:

  • Two wheels on the very front axle, under the hood of the tractor.
  • Four wheels under the front of the trailer. The axle is attached to the tractor.
  • Four wheels directly behind those, still attached to an axle on the tractor.
  • Four wheels further back the trailer, closer to the rear, attached to an axle on the trailer.
  • Four wheels at the rear of the trailer, attached to an axle on the trailer.

Add them together and you get 18. Swap out the four-wheel axles with two-wheel axles to get 10. Take a semi and add 2 or 5 more axles and you’ll get 26 or 34 tires, respectively.

Why Do Semi-Trucks Have So Many Tires?

Why is there so much variability and what’s the need for so many tires in the first place? It has to do with carrying the load safely from one point to the next.

Physically Support the Semi-Truck

The first role is to physically carry the weight of the vehicle. When the trailer is loaded, the weight is transferred directly into the tires and down to the road.

Semi-truck driver stepping on inside the semi truck to begin driving

If the tires aren’t strong enough to support the weight of the semi-truck, they’ll pop. One way to prevent this is to make bigger, stronger, more expensive tires. Still, they can only handle so much weight.

Another way to do this is to use a number of tires. By having 18 tires working together, the weight is distributed 18 ways.

It Meets Loading Standards

The U.S. Department of Transportation (DoT) has standards when it comes to loading semi-truck. Specifically, they only allow 12,000 pounds for the steering axle, 20,000 pounds per individual axle, and 34,000 pounds for tandem axles.

Semi-trucks checking into the weight station checkpoint on the insterstate highway

Using fewer axles means that a truck can legally carry less weight. This forces the driver to take less product on each trip, making less money.

Saves the Semi-Truck During a Blowout

When it comes to over-the-road trucking, blowouts are inevitable. Tire blowouts are caused by too much weight or pressure on a tire that has some failure. A tiny fracture on a tire can lead to a blowout under the right conditions.

Semi-truck trailer tires stored on the ground ready to be mounted

By doubling up the tires on each side of the semi, the driver can save their 18-wheeler during a tire blowout. The other tire next to the blown one will allow the vehicle to hobble to safety on the side of the road and repair the blown tire.

For Stability

Even if the laws didn’t exist and blowouts were ignored, the semi-truck still needs to be stable. These vehicles are driven hundreds of thousands of miles at highway speeds. Instability means the truck will either spin out or flip over easily.

An asian man holding a clipboard checking the safety of a semi-truck by inspecting the tires

With all these extra axles and tires, you’re left with a vehicle that’s highly stable. Compare riding a tricycle to a bicycle to a unicycle. As you have more tires in contact with the ground, you have an easier time balancing.

If semi-trucks only had a set of tires in the very front and back, you would see them on their sides constantly, especially as they’re making turns.

Better Performance Off-Road

There are plenty of times when a big rig driver has to take things off-road. In more remote areas of the country, paved roads are a luxury.

Semi-truck driving on a dusty dirt gravel road in the mountain landscape

This is when it helps to have so many tires. The semi can still balance on unpaved roads, support the weight of the load, and get through the trip.

To Support a Cargo Weight Shift

There are no laws surrounding how the cargo needs to be secured within the trailer. Things can be stacked and piled on top of one another, creating a mountain of goods.

When the semi makes a turn or needs to quickly correct their vehicle, it can result in the weight shifting within the trailer. It might not seem like a big deal, but shifting weight is a huge problem.

Big red semi-truck with a trailer turning on the round highway exit loop with green trees on the side showing the effects of weight shift on the tires

If you take a standard pickup truck and suddenly shift the weight in its bed to one side, it’s likely that the pickup will have two wheels off the road and can flip.

Weight shifts can be very common with 18-wheelers. Thankfully, they have plenty of wheels to keep the semi-truck planted whenever this happens. As a result, the rig can stay balanced even when the cargo shifts around.

What About the Wheel That Don’t Touch the Ground?

You might also see some semi-trucks that have a set of tires just floating over the ground. No, these aren’t the spare tires for the vehicle, this is a “drop axle”.

As you know, more tires equate to more allowable weight for the semi-truck. The trade-off is that the vehicle is less maneuverable. This means taking larger turns, getting a worsened fuel economy and more difficulty driving through parking lots.

To make semi-trucks easier to drive, they install these drop axles. When the axle is up and in the “stored” position, those four wheels aren’t touching the ground. In this position, the semi has the best maneuverability.

Semi-truck turning with the drop axle lifted up wheels in the air

If the load is too heavy, then the driver will drop the axle and it will make contact with the road.

In other words, the driver can decide whether they want 18 or 14 wheels in contact with the road by raising or lowering this axle.

Since the law sets the weight limit based on how many axles are in contact with the road, the driver still needs to adhere to those standards.

The next time you pass an 18-wheeler with an axle in the air, know that their cargo is lighter and they’re choosing a more maneuverable rig.

But, Why Exactly 18 Tires?

In theory, it makes sense why semi-trucks have so many tires (per the reasons I listed in a previous section). But, why do they have exactly 18? Would a truck flip with 16 and refuse to turn with 20?

There’s no reason why there need to be exactly 18 tires. The best explanation is that the number 18 just stuck. Having a set of axles at the front and rear of the trailer makes sense, so 18 would be the logical outcome.

Big classic orange well-maintained semi-truck driving on the interstate highway

Plus, 18 seems to give a good combination of maneuverability and strength. It’s worked for this long, so why change it now?

To be clear, there are no laws or standards forcing these tractor-trailers to have 18 wheels. Just the axle law explained earlier, but that doesn’t limit you to only 5 axles.

Conclusion

In this article, I discussed why semi-trucks have so many tires. It keeps the rigs more stable and allows them to carry plenty of weight. If you have more automotive questions, you might find the answer on my site. Be sure to see what products I recommend that will make your life easier as a car owner.

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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