Double-Eye Trailer Suspension System Review

Suspension is one of those components of your trailer that you often overlook until you run into problems. When functioning correctly, suspension absorbs shock from the road, keeps your trailer level, gives you a smooth ride, and reduces wear and tear on your frame and tires. When your suspension becomes worn down, however, you'll feel the difference in ride quality.

Most trailers come equipped with leaf spring suspension. The two main types of leaf suspension are double-eye springs and slipper springs. Double-eye springs are generally better for lower-capacity trailer weights. They have more moving parts than slipper springs, so there are more components to maintain. However, they typically offer a smoother ride.

To learn more about double-eye suspension systems, what components are involved, and when/how to replace those components, read on below!

You can read more about slipper springs here.

Double-Eye Trailer Suspension Components

There are 7 main components to a double-eye suspension setup. The number of each of these components depends on your axle configuration (single axle, dual axle, or triple axle).

The main components of a double-eye suspension setup are:

Hangers
Leaf springs
Equalizers
Shackle straps
Suspension bolts
U-Bolts/U-bolt plates
Spring seats

Refer to the chart to see how these parts all work together in a suspension system.

Do You Need Trailer Suspension?

In the vast majority of cases, yes, you'll probably want to make sure your trailer has suspension. (The exception is a tow dolly designed for towing vehicles with their own suspension.)

Towing a trailer without suspension can result in an extremely bouncy, uncomfortable, even dangerous ride, especially when the trailer is unloaded. Plus, over time, your trailer will likely experience frame fatigue due to the rough ride, and your tires may become worn or even fail before their time (tires are not designed to act as a trailer's suspension).

So, if you're building a trailer at home and wondering whether or not you need suspension, err on the side of ensuring a smooth ride and add the suspension. If you're considering buying a homemade trailer off someone else, you may stumble across someone's DIY deal with the axles welded directly to the frame. In most cases, you'll probably want to keep looking for a better deal.

Trailer Suspension Parts Needed Per Axle - Double Eye Springs

We'll go over each component in more depth below.

Hangers

HOW HANGERS WORK IN A SUSPENSION SYSTEM

Hangers attach the suspension system to the trailer. They are welded to the trailer frame and attach directly to the leaf springs and equalizers.

There are three types of hangers in a suspension system: front, center, and rear hangers. Front hangers suspend the leaf spring closest to the tow vehicle. Center hangers suspend the equalizers. Rear hangers suspend the rear leaf spring directly or via shackle straps.

There are always two more hangers than springs in a suspension system. For example, if there are four springs in a kit, you will have six hangers.

Note that center hangers are often longer than their front/rear counterparts because they attach to the equalizer between your axles and function as a link between them.

REPLACING HANGERS

When replacing your hangers, it's important to replace them with new hangers of the same height as the old. To replace your hangers, you should know the following information:

**WIDTH (A) ALWAYS = 1-3/4"

HEIGHT (B)

Distance from center of bolt hole to top of hanger. Different heights provide varying levels of clearance for the trailer

BOLT HOLE DIAMETER (C)

Most hangers for double-eye springs require 9/16" diameter x 3" long bolts.

Leaf Springs

HOW LEAF SPRINGS WORK IN A SUSPENSION SYSTEM

Leaf springs act as the shock absorbers of the suspension system. They flex when you hit bumps and rough patches of road, providing you with a smooth ride.

Double-eye leaf springs have an "eye," or bolt hole, at the end of each spring. The steel spring rolls into a curl at the end, making a circle and attaching to the suspension bolts at either end of the spring. (This is one way double-eye springs differ from slipper springs. Slipper springs have just one "eye"; at the other end is a steel arch that is inserted into a hanger or equalizer. Read more about slipper springs here.)

There are always two springs per axle in a suspension system. For single-axle attachments, the spring will bolt onto the front hanger and rear shackles. For multi-axle attachments, the spring will bolt onto the front and rear hangers, with the spring's middle points attaching to an equalizer via shackle straps.

REPLACING LEAF SPRINGS

Replacing the springs when needed protects the trailer and its contents from extra wear and tear, poor trailer handling, and ground clearance issues that can arise when your springs become worn and ineffective. All springs should be replaced at the same time to maintain even support from the trailer.

Replace your springs if they are:

Cracked
Broken
Extremely rusty
Overly worn and sagging
You are upgrading the axle to a greater weight capacity

To replace double-eye springs you need to know the following information:

**WIDTH ALWAYS = 1-3/4"

NUMBER OF LEAVES NEEDED

Varies with weight rating of spring. Different springs can have identical dimensions with varying numbers of leaves; the only difference will be the weight rating.

WEIGHT RATING OF AXLE

Determines weight load for each spring

LENGTH (A) (EYE TO EYE)

Measure from center of one eye to center of other eye (allow for slight variations in this measurement because the springs can sag and elongate as a result of wear)

LENGTH (B) (EYE TO SPRING)

Measure from center of eye to center of spring

HEIGHT (C)

Measure from top-center of spring to center of eye

CAPACITY

Leaf spring capacity ranges between 1,000 and 3,500 lbs per spring for double-eye suspension. Each spring carries 50% of an axle's weight capacity. This means that double eye leaf springs can support axles with weight ratings between 2,000 and 7,000 lbs.

Ex: If axle rating = 3,500 lbs, each spring should be rated for 1,750 lbs.

Note that the overall spring weight rating should not be greater than the trailer's weight capacity. If it is, this can result in a stiff trailer ride. Plus, you will still be limited by the weight capacity of the trailer, so you won't be able to benefit from the full capacity of your springs.

Trailer Suspension Leaf Spring Measurement

Equalizers

HOW EQUALIZERS WORK IN A SUSPENSION SYSTEM

Equalizers are suspended from the center hanger and serve as a link between leaf springs on multi-axle trailers. Equalizers transfer weight from one axle to another, specifically when you drive over a bump. This transfer of weight equalizes the weight being carried by all axles and allows the smooth transfer of the impact through the suspension system. Without an equalizer, driving over a bump would cause one axle to carry a disproportionate amount of weight.

Shackle straps are used to connect the springs to the equalizer. Equalizers swing from front to back, allowing the axles to adjust to bumps. There is always one equalizer between two connecting springs in a suspension system.

Four types of equalizers are available, depending on the trailer application:

Flat (A): Generally used for horse trailers
Tall (B): Designed to allow a trailer to ride low in RV applications
Curved (C): Most common; found on utility trailers and car haulers
Short curved (D): Typically used with short springs, on which front and rear hangers are close together

REPLACING/UPGRADING EQUALIZERS

Equalizers themselves don't typically see much wear and tear, since they do not support the bulk of the weight in a suspension system. However, equalizer bushings are subject to wear and are likely to need replacing. It may be more cost-effective and time-efficient to simply replace the entire equalizer rather than just the bushings.

You can also purchase upgraded equalizers such as the Dexter E-Z Flex or the Lippert Equa-Flex. These upgrades replace your standard equalizers with cushioned components for a smoother ride.

What information do you need to replace an equalizer? It depends on the type of equalizer, but some common measurements include:

**DIAMETER OF LEFT/RIGHT HOLES ALWAYS = 9/16"

LENGTH (A) SIDE TO SIDE

Distance from center of left-most eye to center of right-most eye

LENGTH (B) SIDE TO CENTER

Distance from center of left eye to center of middle eye

DIAMETER (C)

Diameter of center eye hole

HEIGHT (D)

Height from center of bottom hole to center of top hole

Shackle Straps

HOW SHACKLE STRAPS WORK IN A SUSPENSION SYSTEM

Shackle straps are used to link components of a suspension system and to allow small movements in the system. In a single-axle system, the shackles link the springs and hangers. In a multi-axle system, the shackles link the springs and equalizers. There are usually two shackle straps per spring in single-axle systems and four per equalizer in multi-axle systems.

REPLACING SHACKLE STRAPS

Shackles handle the most movement and weight in a suspension setup. Therefore, they are the most likely component to wear and should be checked frequently.

Replace your shackles if they show signs of wear, such as:

Severe rust
Bolt hole openings become elongated or oval shaped
Bolt and nut appear to be moving closer to edge of shackle

To find the correct replacement size shackle strap, measure your current shackle strap length. Length is measured from the center of one bolt hole to the center of the other. All shackles in a system should be the same length.

NOTE: Another way to determine your shackle length is to simply check what type of suspension system you have (single axle, dual axle, triple axle). Shackles come in three standard sizes that typically correspond to the type of setup you have.

3-1/8" - Typically used on single-axle systems
2-5/8" - Most popular; typically used on dual-axle systems and all triple-axle systems
2-1/4" - Typically used in 1 dual-axle kit
Bolt hole diameter (all) - 9/16"

Suspension Bolts

HOW SUSPENSION BOLTS WORK IN A SUSPENSION SYSTEM

Suspension bolts hold together the various moving components of a trailer suspension system, serving as the pivot points in the system. These bolts hold together the springs and hangers, the springs and equalizers (via shackle straps), and equalizers and hangers.

There are three different types of suspension bolts as well as three different nut styles.

Regular shackle bolts (A) - For use between springs/hangers and between slippers/hangers
Wet shackle bolts (B) - Have zerk fittings that allow grease to be injected, which assists movement and halts corrosion. Can be used anywhere in system as long as bolt is appropriate size (eyes with bushing often do not need wet bolts)
Equalizer bolts (C) - Suspend equalizers

Regular nut (D) - Standard nut; reusable
Lock nut (E) - Has mismatched threads to ensure nut cannot twist off bolt; one-time use
Castle nut (F) - Looks like castle wall; includes cotter pin, which threads through small hole at threaded end of bolt (ensures nut cannot come off bolt); most often used on equalizers of systems with high weight capacities

REPLACING SUSPENSION BOLTS

It's time to replace your suspension bolts when they are cracked or severely rusted. It helps to know the following information when replacing bolts:

**LENGTH ALWAYS = 3" (bottom of head to end of bolt)

DIAMETER (A)

Should match the bushing or bolt hole inner diameter. Equalizers can have bolt holes of varying diamters, so make sure you have the correct bolt size for each hole.

BOLT/NUT TYPE

Refer to the information above to determine which type of bolt and nut you have.

PLACEMENT IN SYSTEM

What is the function/placement of the bolt in the system? For instance, is it an equalizer bolt (which is different than a standard bolt)?

U-Bolts and U-Bolt Plates

HOW U-BOLTS/U-BOLT PLATES WORK IN A SUSPENSION SYSTEM

U-bolts secure the leaf springs to the axle, and U-bolt plates are used to attach them.

The U-shaped or square part of the bolt attaches directly to the axle (round U-bolts are designed for round axles; square U-bolts are designed for square axles). The ends extend over the spring seats and springs through the U-bolt plate, where they are secured to the axle with nuts. There are always 2 U-bolts per spring (one for each side of the spring; 4 U-bolts per axle).

Torque ratings:
1/2" bolts: 45 ft/lbs - 70 ft/lbs
9/16" bolts: 65 ft/lbs - 95 ft/lbs
5/8" bolts: 100 ft/lbs - 120 ft/lbs

Trailer Suspension U-Bolts and U-Bolt Plates

REPLACING U-BOLTS/PLATES

There are a few pieces of information and some measurements you will need to know when selecting new U-bolts and plates. Bolts and plates typically come together as a kit, though you can purchase them separately as well. To replace a U-bolt plate on its own, you'll need the diameter of your U-bolt to make sure they match up.

U-bolts and plates should be replaced if they are:

Rusted
Stretched
Bent
Show stress cracks or worn spots
Show other significant signs of wear

Typically, U-bolts and plates are replaced when you replace your springs. This is because the U-bolts are generally rusted and must often be cut off your suspension system in order to be removed.

The following information will help you determine which U-bolt/plate kit you need:

AXLE

Axle type and capacity determine the shape and dimensions of U-bolts.

WIDTH (A)

Between each bolt arm (matches axle diameter).

LENGTH (B)

Distance from lowest point in arch to end of bolt arms. Depends on axle diameter and number of leaves on springs. U-bolts must clear the axle, spring seat, and spring so the U-bolt plate can be attached.

DIAMETER (C)

Diameter of bolt. The larger the diameter, the greater the U-bolt weight rating.

Typical Axle Diameters Based on Axle Capacity

1,000-lb - 2,000-lb Axles: 1-1/2" - 1-3/4" diameter
3,500-lb Axles: 2-3/8" diameter (can have a 3-1/2" diameter, but rarely)
6,000-lb - 7,200-lb Axles: 3" diameter
8,000-lb Axles: 3-1/2" diameter
9,000-lb Axles: 4" diameter
10,000+ lb axles: 5" diameter

Spring Seats

HOW SPRING SEATS WORK IN A SUSPENSION SYSTEM

Spring seats are weld to the axle and are sandwiched between the axle and springs. Spring seats provide a flat surface on which the springs rest. A nub on the spring lines up with a corresponding hole in the spring seat, preventing the springs from shifting and the U-bolt assembly from rotating.

REPLACING SPRING SEATS

If your spring seat shows signs of wear, you'll need to replace it. You only need a few basic pieces of information to do so:

SHAPE

Whether the spring seat is made for a round or square axle.

ARC WIDTH (A)

From one side of the seat arc to the other (designed to fit around axle diameter).

Slipper Spring Trailer Suspension System Review

Where to Position the Trailer Axle?

Trailer Wheels with Leaf Spring Suspension

Trailer Maintenance Schedule

Written by: Amber S.

Updated on: 10/22/19

Questions and Comments about this Article

Xavier 4/12/2021

A lot of good information, unfortunately my trailer is a spring OVER axle. So useless

Reply from Jon G. 4/14/2021

Unless you have a different axle design the same rules/information should apply whether your leaf springs are above or below your axle.

George H. 1/25/2021

Enjoyed your information. Do you provide Plans for 16'x 6'6" trailers with Tandem Axle design please? And costings for the Tandem Assembly.

Reply from Jon G. 1/26/2021

We don't have any plans on trailer builds but we do have some technical info to add. In addition to this article you can check out the "Where to Position the Trailer Axle" article that I'm linking for you to check out.

Helpful Links

faq-where-to-position-trailer-axle.aspx

Contact & Help

What our customers are saying:

"I like the easy of navigation through the site. It only took me a couple of minutes to place my order. Thank You I will shop this sight again in the future."

Kevin

Manchester, MD