Slipper Spring 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 Slipper Spring Systems and Double-Eye Spring Systems

• Shape of spring
• Weight capacity of slipper springs is generally greater than double-eye springs
• Fewer moving parts on slipper spring suspensions, fewer parts to maintain
• Slipper spring systems ride a little more roughly than double-eye spring systems

Hangers

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

• Brackets are welded to the trailer 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 end of leaf spring (the end with an eye)
• Center - suspends equalizers
• Rear - always suspends rear end of leaf spring (the end with the slipper)
  • Slipper spring hangers have a space where the rounded, slipper end of the spring rests
  • Slipper is supported by:
    • A cylinder welded into the hanger
    • A bolt inserted through the hanger (bolts do not come with hangers)

Hanger Dimensions

• Width
  • Should be matched to the width of the spring that will be used with the hanger
  • Widths
    • 1-3/4": 1,000-lb - 3,500-lb axles
    • 2": 3,500-lb - 9,000-lb axles
    • 2-1/2": 9,000-lb - 12,000-lb axles
    • 3": 10,000-lb - 12,000-lb axles
• Height
  • Distance from the center of the hole to the top of the hanger
  • Different heights provide varying levels of clearance for the trailer
• Bolt hole diameter (where applicable)
  • Each hanger accepts bolts of a specific diameter only
    • Varies from 9/16" to 1-1/4" bolts
  • Depends on springs and equalizers

Leaf Springs

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

Features

• One end of the spring has an "eye" and the other is an arched, open end
• Springs can be 1-3/4", 2", 2-1/2" or 3" wide; length will vary
• Springs can carry between 500 and 11,200 lbs per spring (1,000-lb - 22,400-lb axles)
• Eye hole diameter can vary
  • Most springs have an eye diameter of 9/16"
  • Only a few high-capacity leaf springs require larger bolts (up to 1" diameter)
• Attachments
  • Front eye is bolted to hanger
  • Slipper end is inserted into a hanger or equalizer
• Note: There are always 2 springs per axle

Weight Capacities

• Each spring carries 50 percent of the axle's weight rating
  • For example, if your 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, 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:
  • Number of leaves
  • Width of spring
  • Length of spring
  • Weight rating of axle (to determine weight load for each spring)

• Useful dimensions to have when replacing slipper springs
  • A = Length from center of eye to end of the spring with the slipper
    • 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 = Length from center of eye to highest point of of slipper end
  • C = Height from top center of spring to center of eye
  • D = Length from center of eye to middle of the spring

• New springs come with a bushing in the eye of the springs
• All springs should be replaced at the same time
  • Ideal for maintaining even support for the trailer
• Overall spring weight rating should be 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

Equalizers

Used for multi-axle systems, equalizers are designed to transfer from one axle to another the additional load resulting from bumps. The transfer of load serves to equalize the weight being carried by all axles at the time of impact. Without an equalizer, going over a bump would cause one axle to have a disproportional amount of weight on it. Also, 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
• Swing front to back, allowing the axles to adjust to bumps
• Types
  • 2" Width - used on systems with axle weight ratings up to 8,000 lbs (three 2" equalizers can be used with up to 9,000-lb axles)
  • 2-1/2" and 3" width - used on dually-wheel and tandem-axle trailers with axle weight ratings of 8,000 lbs and greater
• Note: There is always 1 equalizer between 2 connecting springs in a system

Replacing Equalizers

• Typically, equalizers don't need to be replaced because they do not support the bulk of the weight
• Measurements required to determine which replacement equalizer is needed vary by equalizer type
• Necessary information
  • Width of equalizer
  • Weight capacity
• Examples of typical measurements needed
  • A = Overall length of the equalizer
  • B = Length from center of front bolt hole to center of middle bolt hole
  • C = Length from center of middle bolt hole to center of rear bolt hole
  • D = Diameter of rear bolt hole
  • E = Diameter of center bolt hole
  • F = Diameter of front bolt hole

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
  • Equalizers and hangers
• Can also create a platform on which the slipper end of the spring rests in the equalizer and hanger

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 the 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 bolt hole inner diameter
  • Equalizers can have bolt holes of varying diameters, so be sure that you have properly sized bolts for each bolt hole
• Length varies depending on the width of the spring and the total thickness of all the items being bolted together
  • 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 U-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
  • 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 weight 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 fo 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 Slipper Spring Suspension System

Single-Axle Suspension

*Note: Number of keeper bolts necessary can vary depending on the type of rear hangers being used.

Dual-Axle Suspension

*Note: Number of keeper bolts necessary can vary depending on the type of rear hangers being used.

Triple-Axle Suspension

*Note: Number of keeper bolts necessary can vary depending on the type of rear hangers being used.