Different Types of Semi-Trailer Axles

Tags: Anatomy of a Trailer, Fleet management

Semi-trailer suspension systems minimize harsh movements and protect the semi-trailer and its freight. Suspension systems include the parts that connect the trailer and truck to the wheels to facilitate better ride quality and road handling, such as tires, axles, and affiliated hardware.

Each of these components plays an integral role in a trailer’s performance. They’re also important when looking at semi-trailers for sale or rent because the suspension system determines a trailer’s load capacity, maintenance costs, and driving stability.

As with most vehicles, there are many options to choose from when selecting the right suspension system for your trailer. For heavy haul trailers, axles play a prominent role in trailer operation. They not only allow wheels to move but also bear the weight of the load.

There are different types of suspension systems, and there are also different types of trailer axles. Here you’ll learn about the most common trailer suspension systems and the various semi-trailer axles that go with them.

Air Ride Suspension

Air ride suspension has grown to be the most popular in the trailer industry, with about 75 percent of semi-trailers using a form of air suspension. Air suspension has garnered a reputation as a cost-effective option for fleet managers because it requires less maintenance and repair and has a higher resale value. 

The purpose of air ride suspension is to accommodate the amount of weight being hauled, resulting in a smoother, more efficient ride. As the trailer’s weight changes, the height control valve senses movement between the axle and chassis. When the movement is large enough, the system sends more air pressure into the air springs, bringing the axles to the ideal height, AKA ride height.

Spring Ride Suspension

Spring ride suspension dates much farther back than air ride suspension. It was initially used for horse-drawn carriages and continues to be used in automobile production. Today, spring ride suspension is found on moving floor trailers, flatbeds, and older dry vans.

Semi-trailers with spring suspension have several layers of flexible steel strips that join together to act as a single unit. These steel strips are referred to as a “leaf pack,” and they attach to the trailer’s frame and rest above the axle. The semi-elliptic leaf springs cushion the semi-trailer’s load from shock. However, spring suspension can result in a harsher ride than air ride suspension, so it’s not as common as air ride suspension.

Tandem Axles

Most semi-trailers have tandem axles, which refer to the placement of two sets of axles spaced one right behind the other. Typically, tandem axles are approximately 5 feet apart, and this proximity offers trucks certain benefits.

Tires may occasionally be damaged. Tandem axles allow you to stop your trailer before serious damage is done. The additional tire is close enough to alleviate the burden of weight from the damaged tire.

Tandem axles also impact load distribution. More axles allow your load to be distributed over a larger portion of the frame, helping your trailer bear the weight.

It is common practice to slide tandem axles forward or backward to support weight where it is needed most. Newer dry vans often have air-slide suspension to aid in moving tandem axles.

Pros of Tandem Axles

• Extra tires enable the trailer to carry more weight
• Greater stability at high speeds and in highway conditions
• Easier to maneuver and back up

Cons of Tandem Axles

• Towing must be level
• Reduced tire life if the load is unlevel
• Higher initial investment

Tandem axles are suitable for: Flatbeds, Lowboys, Reefers, Dumps, Dry Vans, Tanks, Tippers, Yard Tractors, Moving Floors, Hopper Bottoms

Spread Axles

Spread-axle trailers have single axles spaced at least 10 feet apart. While they’re known for flexibility, they’re also infamous for the wear and tear they inflict on tires.

Each spread semi-trailer axle is treated individually, allowing a single axle to carry 20,000 pounds. This means spread axles can carry 40,000 pounds, which gives you more leeway when placing a heavier load.

In certain states, like California, trailers over 48 feet long need a kingpin to the center of the rear axle at 41 feet. This can be challenging with a semi-trailer with spread axles.

Spread axles tend to be better for long hauls and are also called “split” tandems.

Pros of Spread Axles

• Increased stability when loading and unloading the trailer
• Easier to balance on scales
• A longer wheelbase makes spread axles more stable

Cons of Spread Axles

• More difficult to turn because of a greater distance between wheels, while the kingpin is in the same place
• Not as capable of tight turns
• Increased wear and tear on tires

Spread axles are suitable for: Flatbeds, car haulers, refrigerated trailers.

Leaf Spring Axles

Leaf spring axles are one of the more commonly used types of axles. They are part of a spring suspension system that flexes to let the wheels on a semi-trailer move independently from the truck body. This is especially useful when driving on uneven roads.

Leaf springs are overslung (straight axles) or underslung (straight axles and drop axles) on the semi-trailer’s axle as a group of curved and stacked springs, also known as leaves. They come in single, tandem, and tridem variations depending on the number of axles and load weight. Tandem and tridem are connected with an equalizer that aids in better weight distribution over the axles.

Springs are rated by their stiffness, known as the spring rate, which is the amount of force it takes to compress the spring one inch, expressed in pounds per inch (lb/in). A lower spring rate provides a more comfortable ride but may not always be the best for trailer/truck handling, especially commercial trailers that haul heavy loads. Thus, a higher spring rate may be more ideal for semi-trailers.

Pros of Leaf Spring Axles

• More even wear on tires
• Less expensive to repair and replace
• Durable

Cons of Leaf Spring Axles

• Rougher ride on uneven roads
• May need repairs more often
• Metal parts may cause additional noise

Leaf spring axles are suitable for: Flatbeds, Lowboys, Reefers, Dumps, Dry Vans, Tanks, Tippers, Yard Tractors, Moving Floors, Hopper Bottoms

Torsion Axles

Torsion axles consist of rubber cords inside the axle tube, which aid in suspension and shock absorption. In contrast to leaf spring axles, torsion provides suspension support by compressing the rubber cords. To create more stability, they are installed directly to the trailer frame by welding or bolts.  

Pros of Torsion Axles

• Quiet, smoother ride
• No wearable components
• Independent wheel suspension

Cons of Torsion Axles

• Costly to replace
• More tire wear in multi-axle configurations
• Cords can stiffen in cold conditions

Torsion axles are suitable for: Light-duty trailers that need a cushioned ride.

Note: These are not found on heavy-duty and most medium-duty trailers as they are not designed for those weight loads.

Lift Axles

Lift axles are another type of semi-trailer axle. They are non-powered and installed ahead of or behind the driving tandem axles. Most lift axles use airbags to carry weight and either airbags or springs to raise and lower the axle. Drivers easily control lift axles with electric switches or air valves.

Pros of Lift Axles

• Minimizes tire wear and tear
• Lowers rolling resistance
• Reduction in toll costs

Cons of Lift Axles

• Extra expense
• Requires additional maintenance
• Complicates the system

Lift axles are suitable for: Flatbeds, Dumps, Concrete Mixers

Triple Axles

Certain large trucks and heavy haul semi-trailers have triple axle configurations. A common configuration includes a tandem drive axle with an air-lift third axle. This third axle allows for greater axle load capacity and enables heavy trucks to drive across soft ground.

It’s common to see dump trucks raise the third axle when driving to a construction site, only to lower it when crossing lawns or soft soil. The third axle distributes the weight over a greater area and prevents the truck from sinking into the ground.

In the same way, heavy-capacity semi-trailers use third axles to displace load weight across more axles. This enables the semi-trailer to comply with roadway weight restrictions. As with dump trucks, third axles in semi-trailers allow heavily loaded trailers to drive across soft ground successfully.

Pros of Triple Axles

• Increased safety due to many states allowing brakes on triple axles
• Improved stability with an anti-sway feature
• Able to carry a larger load

Cons of Triple Axles

• Higher price point
• Decreased mileage due to increased weight
• Increased maintenance and repair costs
• May be limited in where they can be used

Triple axles are suitable for: Flatbeds, Lowboys, Dumps, Step Decks

Quad Axles

Although very market-specific, quad axles are another type of semi-trailer axle. They generally consist of two powered axles and two non-powered axles. Like the triple-axle configuration, quad axles enable heavy trucks and semi-trailers to operate on soft ground without sinking or causing deep ruts. Usually, the two non-powered axles are lift axles, but that isn’t always the case.

If non-powered axles are lift axles, tandem drive axles are placed in the rear, and two air-lift axles are positioned ahead. If a semi-trailer is carrying a light load or no load at all, the two air-lift axles will operate in a raised position. This lessens the effect on fuel consumption and spares tires from unnecessary drag.

Pros of Quad Axles

• Able to carry heavier loads than a triple axle
• Better choice for longer hauls and interstate work

Cons of Quad Axles

• Less maneuverability
• Potential for higher on-the-road costs

Quad axles are suitable for: Market-specific trucks

Additional Axles

Certain semi-trailers, such as lowboys, give you the option of adding additional axles to take on more weight. These other types of trailer axles come in the form of flip axles, pin-on axles, or stingers.

Flip Axles

The flip axle is a complete axle assembly that flips up, down, or is removed depending on when it is needed. Hinge plates and pins connect this axle to the rear of the frame. Flip axles come in handy when a trailer’s axle load exceeds the gross weight limit. Depending on trailer specifications, flip axles can either connect to the trailer’s leveling valve or be used with a hand control valve.

Pin-On Axles

A pin-on axle is another type of semi-trailer axle that can be added to accommodate more weight. It is attached to the rear of the semi-trailer and held in place with pins.

Stinger Axles

Designed in 1995, the stinger axle is an extendable axle that can be installed farther away from the rear drive axle. While a conventional tag axle is mounted 52 to 60 inches behind the rear drive axle, a stinger axle is installed up to 150 inches farther back. This extension is what allows the axle to spread a heavy load and meet federal bridge laws.

Pin-On Axles with a Spread Bar

A spread bar/pin-on combo is similar to a stinger axle in that the axle can be extended. However, it allows for greater flexibility in configuration because you can use the pin-on without the spreader. This comes at the cost of increased weight.

Trailer Axle Width

You’ll need to know your trailer axle width when ordering new or replacement axles. This trailer axle width chart illustrates how the overall width is measured, as well as various other measurements that you may need when replacing parts or ordering axles for a semi-trailer.

1. Overall Width – measured from the outside edge of one tire to the outside edge of the other tire, or the hub face distance plus the tire widths
2. Hub Faces Distance – distance from outside of hub mounting flange to outside of another hub mounting flange
3. Spring Centers Distance (for leaf springs) – measured from the center of one spring to the center of the other spring
4. Overhang – difference between hub face measurement and spring center measurement
5. Frame Width – measurement of the outside dimension of frame rails
6. Tire Clearance – distance between frame and tire
7. Ground Clearance – determined by the drop axle or straight axle and the tire outside diameter
8. Tire Width – measured across the front or back of the tire

Understanding Semi-Trailer Axle Load Capacity

Knowing a semi-trailer’s axle load capacity is vital to ensuring heavy shipments are hauled legally. Axle load capacity is the weight rating that manufacturers specify as the maximum weight each axle can carry. Axle capacity usually translates to the total capacity of the trailer. Semi-trailers with multiple axles will require you to add each “axle” load capacity together to determine the total load capacity.

While different types of trailer axles have limits to the weight they can carry, states also have weight limits for axle configurations. There are maximum legal weight limits for any axle configuration without a permit and maximum legal limits with a permit. The average load capacity on a tandem axle without a permit is usually 34,000 pounds. With a permit, the average load capacity on a tandem is around 44,000 pounds.

Adjustments are made to distribute weight and ensure a semi-trailer’s load doesn’t exceed its axle capacity. Product can be moved forward or backward, the kingpin setting can be changed, and axle spacing also affects axle load capacity. It’s best to check with individual states to be sure your axles are compliant.

Contact Our Trailer Experts

Do you have additional questions about semi-trailer axles? Our friendly, knowledgeable team is ready to assist you. Call us at 877-780-6730 or visit one of our convenient Hale Trailer locations today!

Whether you’re looking for semi-trailers for rent or sale, Hale Trailer has an extensive inventory to meet all of your trailer needs. We’re your trusted source for heavy haul trucking equipment.