Semi-trailer suspension systems do the important job of minimizing harsh movements and protecting 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 the performance of a trailer. They’re also an important factor 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.
Just like there are different types of suspension systems 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 they require less maintenance and repair and have a higher resale value.
The purpose of air ride suspension is to accommodate the amount of weight being hauled. This lends to a smoother, more efficient ride. As weight changes on the trailer, 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 axles to the ideal height, also known as ride height.
Spring Ride Suspension
Spring ride suspension dates much farther back than air ride and was initially used for horse-drawn carriages and continued to be used in automobile production. Today, spring ride suspension is found on some walking 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 frame of the trailer and rest above the axle. The semi-elliptic leaf springs cushion the semi-trailer’s load from shock. But, spring suspension can result in a harsher ride than air ride suspension so it’s not as common as air ride.
Most semi-trailers have tandem axles. This term refers to the placement of 2 sets of axles spaced one right behind the other. Typically, tandem axles are approximately 5 feet apart and this proximity offers trucks certain benefits.
From time to time, tires may incur damage. Tandem axles allow you to stop your trailer before serious damage is done. The additional tire is near enough to alleviate the burden of weight from the damaged tire.
Load distribution is also affected by tandem axles. More axles allow your load to be distributed over a larger portion of the frame helping your trailer bear the weight.
Common practice is to slide tandem axles forward or backward to support weight where you need it most. Newer dry vans often come with air-slide suspension to aid in moving tandem axles.
Pros of Tandem Axles
- Extra tires enable 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 load is unlevel
- Higher initial investment
Tandem axles are good for: Flatbeds, Lowboys, Reefers, Dumps, Dry Vans, Tanks, Tippers, Yard Tractors, Walking Floors, Hopper Bottoms
Spread axle trailers have single axles spaced out a minimum of 10 feet apart. While spread axle trailers are known for their flexibility, they’re also infamous for the wear and tear they inflict on their tires.
Each spread semi-trailer axle is treated individually which allows 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 to have a kingpin to the center of the rear axle at 41 feet. This can be challenging to do 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 trailer
- Easier to balance on scales
- Longer wheelbase makes spread axles more stable
Cons of Spread Axles
- More difficult to turn because of greater distance between wheels while kingpin is in same place
- Not as capable of tight turns
- Increased wear and tear on tires
Spread axles are good for: Flatbeds, car haulers, refrigerated trailers
Leaf Spring Axles
As one of the more commonly used types of axles, leaf spring axles are part of a spring suspension system, which 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 weight being carried. 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 1-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
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 good for: Flatbeds, Lowboys, Reefers, Dumps, Dry Vans, Tanks, Tippers, Yard Tractors, Walking Floors, Hopper Bottoms
Torsion axles are made up 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 the compression of the rubber cords. They are installed directly to the trailer frame by welding or bolts to create more stability.
Pros of Torsion Axles
- Quiet, smoother ride
- No wearable components
- Independent wheel suspension
Cons of Torsion Axles
- Costly to replace
- More tire wear if multi-axle configuration
- Cords can stiffen in cold conditions
Torsion axles are good 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 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 air bags or springs are used 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 good for: Flatbeds, Dumps, Concrete Mixers
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 the third axle 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 successfully drive across soft ground.
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 good for: Flatbeds, Lowboys, Dumps, Step Decks
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. Much 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.
In the case that non-powered axles are lift axles, tandem drive axles are placed in the rear while 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 good for: Market-specific trucks
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.
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.
Another type of semi-trailer axle that can be added to accommodate more weight is a pin-on axle. This type of axle is attached to the rear of the semi-trailer and held in place with pins.
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
Similar to a stinger axle in that the axle can be extended. However, a spread bar/pin-on combo allows for greater flexibility in configuration, as you could use the pin-on without the spreader, but it comes at the cost of increased weight.
Trailer Axle Width
When ordering new or replacement axles, you’ll need to know your trailer axle width. 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.
- Overall Width – measured from outside edge of one tire to outside edge of other tire, or the hub faces distance plus the tire widths
- Hub Faces Distance – distance from outside of hub mounting flange to outside of another hub mounting flange
- Spring Centers Distance (for leaf springs) – measured from center of one spring to center of the other spring
- Overhang – difference between hub face measurement and spring center measurement
- Frame Width – measurement of outside dimension of frame rails
- Tire Clearance – distance between frame and tire
- Ground Clearance – determined by the drop axle or straight axle and the tire outside diameter
- 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 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 as well as maximum legal limits with a permit. The average load capacity without a permit is usually 34,000 pounds on a tandem axle. 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
Have additional questions about semi-trailer axles? Our friendly, knowledgeable team is ready to assist you. Give us a call at 877-780-6730 or visit one of our convenient Hale Trailer Brake & Wheel 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.
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