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

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.
Double-Eye Trailer Suspension
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.
Double-Eye Trailer Suspension
Trailer Suspension Parts Needed Per Axle
We'll go over each component in more depth below.

Hangers

HOW HANGERS WORK IN A SUSPENSION SYSTEMHangers 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.
Trailer Suspension - Hangers
REPLACING HANGERSWhen 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.
Suspension Hanger Dimensions

Leaf Springs

HOW LEAF SPRINGS WORK IN A SUSPENSION SYSTEMLeaf 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.)
Trailer Suspension Leaf Springs
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.
Trailer Suspension - Leaf Springs
Trailer Suspension - Leaf Springs
REPLACING LEAF SPRINGSReplacing 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 NEEDEDVaries 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 AXLEDetermines 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
CAPACITYLeaf 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 SYSTEMEqualizers 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.
Trailer Suspension Equalizers
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
Equalizer Types
REPLACING/UPGRADING EQUALIZERSEqualizers 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 SIDEDistance from center of left-most eye to center of right-most eye
LENGTH (B) SIDE TO CENTERDistance 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
Equalizer Dimensions

Shackle Straps

HOW SHACKLE STRAPS WORK IN A SUSPENSION SYSTEMShackle 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.
Trailer Suspension Shackle Straps
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"
Trailer Suspension Shackle Straps

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.
Trailer Suspension Suspension Bolts
Suspension Bolts
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
Suspension Nuts
  • 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 BOLTSIt'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 SYSTEMWhat is the function/placement of the bolt in the system? For instance, is it an equalizer bolt (which is different than a standard bolt)?
Trailer Suspension Bolt Dimension

U-Bolts and U-Bolt Plates

HOW U-BOLTS/U-BOLT PLATES WORK IN A SUSPENSION SYSTEMU-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/PLATESThere 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:
AXLEAxle 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
Trailer Suspension Bolt Dimension

Spring Seats

HOW SPRING SEATS WORK IN A SUSPENSION SYSTEMSpring 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.
Trailer Suspension Spring Seats
REPLACING SPRING SEATSIf 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:
SHAPEWhether 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).
Trailer Suspension Bolt Dimension
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Written by: Amber S.Updated on: 10/22/19


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