How Many Axles Does a Semi Truck Have?

By Max Anthony •  Updated: 12/08/22 •  6 min read

How Many Axles Does a Semi Truck Have?

What Is A Semi Truck?

“Semis”, “semi-trailers”, “tractor-trailers”, “big rigs”, “semi-trucks”, “18-wheelers” are all terms used to describe combination vehicles in North America that consist of a motorized truck plus one or more semi-trailers.

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The typical tractor has three axle points; the ones used for hauling large heavy-weight construction equipment may have more than five, and some of the axle points may be lift axles.

One axle for the steering and one for the drive, plus a cab housing the engine, are the common elements in a tractor-cab configuration. Most cars have a fifth-tire connection that can be moved outward and back to adjust the amount of weight that is put on the rear axle(s).

How Many Axles Are There on Semi-Trucks?

Semi trucks are very long vehicles with many wheels. They need to carry a lot of weight. Semi trucks have five axles. and depending on the model, the trailer can hold three to four axle systems.

One axle on a truck is able to be removed by lifting the whole thing; if the weight of a load is less, that axle may be removed, in order to save energy and to wear the brakes, tires, and axle. Many manufacturers now use this method to help them cut down on greenhouse gasses.

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What Are the Components of A Semi Truck Axle?

It takes several components to produce an axle. From large shafts to small shafts, axle components are important.

There are four main drives: a gearbox, pinion gearbox, input wrench, and side gears profile. The drive mechanism is driven by a helical gear.

The gear gets geared by using a helium pump that drives a pinion shaft, and it is geared by using a bevel gear that drives the gear. Additionally, the motor drives a spindle and a shaft.

How Many Axles Does a Semi Truck Have?

What Are the Types of Semi Truck Axles?

Air Ride Suspension

A type of suspension that uses compressed natural gas is becoming the commonest in the trailer industry. Air ride suspension is often chosen by fleet managers as a more cost-efficient option to accommodate heavier loads.

Spring Ride Suspension

Spring suspension is much older than air suspension and was originally intended for horse-drawn carriages but was eventually adopted as a feature in automobiles. Today, walking floor trailers are sometimes fitted with spring suspension units that allow them to roll forward like wagons. But spring-biased suspension is less comfortable and more harsh compared to air ride suspension, so it isn’t as popular as air ride.

Tandem Axles

Semi-trailers are generally equipped with tandem axles. This is the opposite of a conventional axle layout in that it has 2 sets of axles that are positioned right next to each other. The distance between two axles in tandem is 5 feet. Trucks can safely stop their work while the tires are still on the trailer. Typically, people move tandem axles to the side of the trailer to move weight around. New vans have a system where air struts slide down over the rear axle to assist in shifting the axles.

Spread Axles

Trailers with spread axles are equipped with a minimum of two axles to space the vehicles a minimum of 10 feet. Spread trailers have four single axles and are flexible, but they are notorious for the damage that they cause to tires.

Normally, trailers that are more than 48 feet wide must be secured by a kingpin that is mounted on the right side of the axle at a height of 41 feet. It may be difficult to do this with a semi-trailer that has two axles that are positioned apart. Spread axles are generally good for long hauls. They are sometimes referred to as “split” tandems.

Leaf Spring Axles

Many semi-trailers have an axle that uses leaf springs, and they are usually found on cars and trucks. This design allows drivers to steer independently and without having to change direction while turning over bumpy roads.

Leaf spring axles are used to put more weight on the axle by connecting two or more springs that are connected to form a leaf. Springs are available as either two or three-wheel designs, so if a semi-trailer is hauling a lot of cargo, it can be difficult for the spring to distribute properly over the axles.

Torsion Axles

A torsion axle is a type of suspension system in that it has rubber straps that hold the axle in place and give it some kind of spring when it moves. Torsion axles are more stable than those that use a heavy set of rubber cords to compress the axle tube, which is similar to a car that uses a heavy set of bolts to secure the chassis. They can be attached to the trailer frame easily by welding or bolts to provide extra stability.

Lift Axles

This type of axle is also able to reach a high point when it’s attached to a semi-trailer. They do not have any power and are able to raise and lower the axle without disabling the drivetrain. Lift axles are usually equipped with airbags to distribute weight, and springs or airbags are often positioned to allow the axle to be raised and lowered. Drivers are able to quickly operate lift axles by adjusting switches on the steering wheel or by opening air valves on the air box.

Triple Axles

Some trucks with good payload capacities are equipped with three axles. Some heavy trucks have tandem axles with an air-lifted wheel. This triple axle provides a larger load capacity for large trucks and a better capability for heavy trucks to move over sandy areas.

When hauling large loads to a construction site, some dumpers automatically load up to three axles on their hauling trailers, allowing them to drive over lawns or across soft ground.

Quad Axles

Quad axles are some other kind of semi-trailer axle, while being particularly market-specific. Typically, they have two motorized and two unpowered axles. Quad axles allow big vehicles and semi-trailers to go on soft terrain without sinking or creating significant ruts, similar to the triple-axle layout. Though it’s not always the case, the two non-powered axles are often lift axles.

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Max Anthony

Max is a gizmo-savvy guy, who has a tendency to get pulled into the nitty gritty details of technology and cars. He attended UT Austin, where he studied Information Science. He’s married and has three kids, one dog and a GMC truck and a Porsche 911. With a large family, he still finds time to share tips and tricks on cars, trucks and more.

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