Double-Eye Trailer Suspension System Review

Trailer suspension is composed of several components that work together to give your trailer a smooth ride. Suspension systems absorb shock from the road and reduce wear and tear on your trailer. They also help to keep your trailer level when you encounter bumpy roads and uneven surfaces.

Differences Between Double Eye Spring Systems and Slipper Spring Systems

• Shape of spring
• Weight capacity of double-eye springs is generally lower than slipper springs
• More moving parts, more parts to maintain on a double-eye suspension
• Generally rides more smoothly than slipper springs

Hangers

Hangers are the system's point of attachment to the trailer.

• Hangers are welded to the frame
• Hangers are used to suspend:
  • Leaf springs
  • Equalizers
• Note: There are always 2 more hangers than springs in a suspension system. For example, if there are 4 springs in a kit, you will have 6 hangers.

Types of Hangers

• Front - always suspends front of leaf spring (closest to tow vehicle)
• Center - suspends equalizers
• Rear - suspends leaf springs directly or via shackle straps

Hanger Dimensions

• Width:
  • 1-3/4"
• Height
  • Distance from the center of the bolt hole to the top of the hanger
  • Different heights provide varying levels of clearance for the trailer
  • Hangers for a single system do not have to be of equal height
• Bolt hole diameter
  • Most hangers for double-eye springs require 9/16" diameter x 3" long bolts

Leaf Springs

Leaf springs act as the shock absorbers of the suspension system. Their flexing action when you go over bumps and rough road smooths the trailer ride.

Features

• Double-eye springs have an "eye," or bolt hole, at each end of the spring, created by rolling the steel of the spring to make a circle
• Each spring is 1-3/4" wide; length varies
• Springs can carry between 1,000 and 3,300 lbs per spring (2,000-lb to 6,600-lb axles)
• Single-axle attachments
  • Bolts suspend double-eye spring from front hanger
  • Spring bolts to shackles at the rear, allowing for movement
• Multi-axle attachments
  • Attach to hanger at front and rear
  • Middle points attach to equalizer via shackle straps
• Note: There are always 2 springs per axle in a suspension system

Weight Capacities

• Each spring carries 50 percent of the axle's weight rating
  • For example, if the axle rating is 3,500 lbs, each spring should be rated for 1,750 lbs
• Number of leaves varies with the weight rating of the spring
  • Typically, the more leaves there are, the more weight the spring can support
  • Different springs can have identical dimensions with varying quantities of leaves - the only difference will be the weight capacity

Replacing Springs

• Springs should be replaced if:
  • Cracked
  • Broken
  • Extremely rusty
  • Overly worn and sagging
  • You are upgrading the axle to a greater weight capacity
• Replacing springs protects the trailer and its contents from:
  • Extra wear and tear
  • Poor trailer handling
  • Limited ground clearance
• To replace springs you need to know the following information:
  • Number of leaves
  • Weight rating of axle (to determine weight load for each spring)
  • Length of spring

• Useful dimensions to have when replacing double-eye springs
  • A = Length from center of one eye to the center of the other eye
    • This is the most important measurement
    • Allow for slight variations in this measurement because the springs can sag and elongate as a result of wear
  • B = Height from top-center of spring to center of eye
  • C = Length from center of eye to center of the spring

• All springs should be replaced at the same time
  • Ideal for maintaining even support for the trailer
• Overall spring weight rating should not be greater than the trailer's weight capacity. If it is:
  • Can result in a stiffer ride for the trailer, especially when unloaded
  • You cannot use full capacity of the springs because you can haul only as much as the trailer is rated for
• New or replacement double-eye springs come with bushings (except for part #TE30, for mobile home suspension)

Equalizers

Equalizers are used on multi-axle trailers to transfer additional weight caused by driving over a bump from one axle to the other. This transfer of weight equalizes the weight being carried by all axles at the time of impact. Without an equalizer, going over a bump would cause one axle to carry a disproportionate amount of weight. Additionally, the equalizer allows for a smooth transfer of the impact through the suspension system under the trailer.

Features

• Suspended from a middle hanger and serve as a link between the leaf springs
  • Shackle straps are used to connect the springs to the equalizer
• Swing front to back, allowing the axles to adjust to bumps
• Note: There is always 1 equalizer between 2 connecting springs in a system

• Types
  • (A) Flat - generally used for horse trailers
  • (B) Tall - designed to allow a trailer to ride low in RV applications
  • (C) Curved - most popular; often found on utility trailers and car haulers
  • (D) Short curved - commonly used with short springs on which front and rear hangers are close together

Replacing Equalizers

• Typically, equalizers do not need to be replaced because they do not support the bulk of the weight
  • Equalizer bushings are subject to wear, resulting from a lot of movement, and are likely to need replacing
  • If bushings are worn, it can be both cost and time effective to simply replace the equalizer
• Measurements required to determine which replacement equalizer is needed vary by equalizer type
• Examples of typical measurements needed
  • A = Length from center of left-most eye to center of right-most eye
  • B = Length from center of left eye to center of middle eye
  • C = Diameter of center eye hole
  • D = Diameter of left and right eye holes (always 9/16")
  • E = Height from center of bottom hole to center of top hole

Shackle Straps

Shackle straps are used to link components of a suspension system and to allow small movement in the suspension system.

• Link between springs and hangers (single-axle system) and springs and equalizers (multi-axle system)
• All shackles in a system should be of equal length
• All shackles have 9/16" diameter bolt holes

• 3 Sizes (length is from center of one bolt hole to center of other bolt hole)
  • 3-1/8" Long shackle straps - typically used on single-axle systems
  • 2-5/8" Long shackle straps - typically used on dual-axle systems and all triple-axle systems; most popular
  • 2-1/4" Long shackle straps - typically used in 1 dual-axle kit
• Note: There are usually 2 shackle straps per spring in single-axle systems and 4 per equalizer in multi-axle systems

Replacing Shackle Straps

• Shackles should be checked frequently because they are the part most likely to wear as a result of handling the most movement and weight
• Signs of wear
  • Bolt hole openings become elongated or oval shaped
  • Bolt and nut appear to be moving closer to the edge of the shackle
  • Severely rusted

• Measurements for replacement
  • Measure current shackle from center of one eye to center of the other eye
  • You can also often determine the shackle length from the type of suspension system you have

Suspension Bolts

Features

Suspension bolts hold together the various moving components of a trailer suspension system, serving as the pivot points in the system.

• Bolt together:
  • Springs and hangers
  • Springs and equalizers via shackle straps
  • Equalizers and hangers

Types

• Suspension bolts
  • (A) Regular shackle bolts - for use between springs and hangers and between slippers and hangers
  • (B) Wet shackle bolts - have zerk fittings that allow grease to be injected, which assists movement and halts corrosion
    • Can be used anywhere in the suspension system as long as the bolt is of appropriate size; eyes with bushing often do not need wet bolts
  • (C) Equalizer bolts - designed to suspend equalizers
• Nuts
  • (D) Regular nut - normal nut; reusable
  • (E) Lock nut - has mismatched threads to ensure that the nut cannot twist off the bolt; one-time use
  • (F) Castle nut - looks like a castle wall; includes a cotter pin, which threads through the small hole at the threaded end of the bolt to ensure that the nut cannot come off the bolt
    • Most often used on the equalizer of systems with high weight capacities

Dimensions

• Diameter should match the bushing or the bolt hole inner diameter
  • Equalizers can have bolt holes of varying diameters, so be sure that you have properly sized bolts for each hole
• Length is 3" (all double-eye leaf springs are the same width)
  • Length measurement provided is the usable length from the bottom of the head to the end of the bolt

Replacing Shackle Bolts

• Replace when:
  • Severely rusted
  • Cracked
• Useful information to have when replacing bolts
  • Bolt diameter
  • Bolt length
  • Bolt's placement in the suspension system (For example, is it an equalizer bolt?)
  • Bolt and nut type

U-Bolts and U-Bolt Plates

U-bolts are designed to secure the leaf springs to the axle of the trailer.

• The U or square part of the bolt attaches directly to the axle, and the ends will extend over the spring seats and springs
• U-bolt ends go through U-bolt plate and nuts to secure springs to axle
  • 2 U-bolts per spring - 1 for each side of the spring
  • 4 U-bolts per axle

• Types of U-bolts
  • Round U-bolts for round axles
  • Square U-bolts for square axles
• Bolt dimensions (depend on axle type and capacity)
  • (A) Width - matches axle diameter
  • (L) Length - varies depending on axle diameter and the number of leaves on the springs
    • Needs to clear axle, spring seat and spring so that U-bolt plate can be attached
  • (D) Diameter - the larger the diameter, the greater the U-bolt weight rating
  • 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

Replacing U-Bolts

• Information needed to choose new or replacement U-bolts
  • Axle capacity
  • A = Width between each bolt arm
  • D = Diameter of bolt
  • L = Length from lowest point in arch to the end of the bolt arms

U-Bolt Plates

• Used to attach leaf spring to axle
• Dimensions vary depending on:
  • U-bolt width and diameters
  • Must match up so that bolts fit securely
• Should be replaced if:
  • Rusted
  • Stretched
  • Bent
  • Show any other significant signs of wear
• Look for stress cracks and worn spots when performing routine checks on your suspension system

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 Or more axles: 5" diameter

Spring Seats

• Welded to the axle
• Sit between axle and springs
• Provide a flat surface on which the springs rest
• A nub on the spring sits in the hole in the spring seat, lining up the springs to the spring seat and preventing rotation of the U-bolt assembly
  • Helps to keep the spring from shifting
  • The U-bolts and plates hold together everything in the system
• Dimensions
  • Arch width - designed to fit around axle diameter
    • This is the only relevent dimension
  • You also need to know whether the spring seat is made for a round or square axle

Parts Needed to Complete a Trailer Suspension System

Single-Axle Suspension

Dual-Axle Suspension

Triple-Axle Suspension