Different Types of Semi-Trailer Axles

Semi-trailer suspension systems play a crucial role in minimizing harsh movements and protecting the semi-trailer and its freight. These systems connect the trailer and truck to the wheels, facilitating better ride quality and road handling through components like tires, axles, and affiliated hardware.

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Each component is integral to the performance of a trailer, influencing its load capacity, maintenance costs, and driving stability. When selecting the right suspension system for your trailer, it's essential to consider the different types available. Here, we explore various types of trailer suspension systems and their corresponding semi-trailer axles.

Air Ride Suspension

Air ride suspension is the most popular in the trailer industry, with about 75 percent of semi-trailers using some form of it. It's known for being cost-effective for fleet managers as it requires less maintenance, repair, and has a higher resale value.

Air ride suspension smooths and optimizes the ride by adjusting to the weight being hauled. A height control valve senses movement between the axle and chassis, adjusting air pressure in the air springs to maintain the ideal ride height.

Spring Ride Suspension

Spring ride suspension pre-dates air ride, originating from horse-drawn carriages, and is still found on some moving floor trailers, flatbeds, and older dry vans. It includes several layers of flexible steel strips called a "leaf pack" that cushion the semi-trailer's load from shock.

However, spring suspension generally results in a harsher ride compared to air suspension, making it less common.

Tandem Axles

Most semi-trailers have tandem axles, which consist of two sets of axles closely spaced. They provide certain benefits, such as allowing a trailer to continue moving even if one tire incurs damage, and distributing the load over a larger portion of the frame.

It's common for tandem axles to be adjusted forward or backward to support weight distribution optimally. Newer dry vans often feature air-slide suspension to facilitate this movement.

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 feature single axles spaced out a minimum of 10 feet apart. They are known for their flexibility but can cause significant wear and tear on tires.

Each spread semi-trailer axle handles up to 20,000 pounds individually, allowing for a total of 40,000 pounds, which is advantageous for heavier loads. However, some states have specific requirements that can be challenging for spread axles.

Spread axles are often better for long hauls and are also called "split" tandems.

Pros of Spread Axles

• Increased stability when loading and unloading
• Easier to balance on scales
• Longer wheelbase makes spread axles more stable

Cons of Spread Axles

• More difficult to turn due to greater distance between wheels
• Not 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 common in spring suspension systems, allowing wheels to move independently from the truck body, especially useful on uneven roads. They come in single, tandem, and tridem variations, aiding in better weight distribution over the axles.

The spring rate, indicating stiffness, affects the ride quality and handling. Higher spring rates are often ideal for semi-trailers hauling heavy loads.

Pros of Leaf Spring Axles

• More even tire wear
• 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

For more advantages of suspension system information, please contact us for professional answers.

Torsion Axles

Torsion axles consist of rubber cords inside the axle tube, aiding in suspension and shock absorption. The torsion's suspension support comes from the compression of these cords, providing a quieter and smoother ride. They are installed directly to the trailer frame for added 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 in multi-axle configuration
• Cords can stiffen in cold conditions

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

• Note: Not used on heavy-duty and most medium-duty trailers

Lift Axles

Lift axles are non-powered and installed ahead of or behind the driving tandem axles. They utilize airbags to carry weight and can be raised or lowered using airbags or springs, controlled by 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

Large trucks and heavy haul semi-trailers often have triple axle configurations, including a tandem drive axle with an air-lift third axle. This setup allows for greater axle load capacity, suitable for driving across soft ground.

For instance, dump trucks often raise the third axle on the way to a construction site but lower it when driving across lawns or soft soil. This effectively distributes the weight and prevents the truck from sinking.

Pros of Triple Axles

• Increased safety, with many states allowing brakes on triple axles
• Improved stability with anti-sway features
• Able to carry larger loads

Cons of Triple Axles

• Higher price point
• Decreased mileage due to increased weight
• Increased maintenance and repair costs
• Limited in their usage locations

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

Quad Axles

Quad axles are market-specific and usually include two powered axles and two non-powered axles. They help heavy trucks and semi-trailers operate on soft ground without causing deep ruts. The two non-powered axles are often lift axles.

When carrying light or no load, air-lift axles stay raised to minimize fuel consumption and reduce tire 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

Semi-trailers like lowboys offer the option of adding additional axles, such as flip axles, pin-on axles, or stingers, to handle more weight.

Flip Axles

The flip axle is a complete axle assembly that can be flipped up, down, or removed as needed. Hinge plates and pins connect this axle to the rear of the frame.

Pin-On Axles

A pin-on axle is attached to the rear of the semi-trailer and secured with pins.

Stinger Axles

Introduced in 1995, the stinger axle is an extendable axle installed farther from the rear drive axle, allowing it to spread heavy loads and meet federal regulations.

Pin-On Axles with a Spread Bar

This combo offers flexibility in configuration, allowing the use of the pin-on axle with or without the spreader at the cost of increased weight.

Trailer Axle Width

When ordering new or replacement axles, it's essential to know your trailer axle width. The trailer axle width chart below illustrates how overall width is measured and other key dimensions:

1. Overall Width - measured from outside edge of one tire to outside edge of the other, or the hub faces distance plus the tire widths
2. Hub Faces Distance - distance from one hub mounting flange to the other
3. Spring Centers Distance (for leaf springs) - measured from the center of one spring to the other
4. Overhang - difference between hub face measurement and spring center measurement
5. Frame Width - measurement of 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 essential for handling heavy shipments legally. The manufacturer's axle load capacity indicates the maximum weight each axle can carry. This overall capacity usually translates to the total capacity of the trailer by summing individual axle capacities.

While trailer axles have specific weight limits, states also have legal limits for axle configurations. The average load capacity without a permit is around 34,000 pounds on a tandem axle, while with a permit, it can be about 44,000 pounds.

Adjustments can be made, such as moving the product forward or backward, changing the kingpin setting, or adjusting axle spacing to ensure the load doesn’t exceed the trailer's capacity. It's crucial to check with individual states for compliance.

Contact Our Trailer Experts

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 Hale Trailer Brake & Wheel locations today!

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