Slipper Spring 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. Slipper springs generally have a greater weight capacity and fewer moving parts, so there are fewer components to maintain. However, slipper spring systems generally ride a little more roughly than their double-eye counterparts.

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

You can read more about double-eye suspension systems here.

Slipper Spring Trailer Suspension Components

There are 6 main components to a slipper spring 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 slipper spring suspension setup are:

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

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

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 (the end with the eye). Center hangers suspend the equalizers. Rear hangers suspend the rear end of the leaf spring (the end with the slipper). Slipper spring hangers have a space where the slipper end of the spring rests. Slippers are supported via a cylinder welded into the hanger as well as a bolt inserted through the hanger.

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.

REPLACING HANGERS

WIDTH (A)

The width of your hangers should always be matched to the width of the spring that will be used with the hanger.

Width options:

1-3/4" (axles 1,000 - 3,500 lbs)
2" (axles 3,500 - 9,000 lbs)
2-1/2" (axles 9,000 - 12,000 lbs)
3" (axles 10,000 - 12,000 lbs)

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)

Each hanger accepts bolts of a specific diameter. Bolt diameter varies from 9/16" to 1-1/4" and depends on your springs and equalizers.

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.

Slipper springs have two different ends; one end has an "eye," while the other is an arched, open end. The front eye is bolted to a hanger, and the slipper end is inserted into a hanger or equalizer. This is the main difference between slipper springs and double-eye springs, which have two eyes. You can read more about double-eye springs here.

There are always two springs per axle in a suspension system.

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 slipper springs you need to know the following information:

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. Slipper springs can support between 500 and 11,200 lbs per spring (1,000 to 22,400-lb axles). Each spring carries 50% of the axle's weight rating.

Ex: if your axle capacity is 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.

LENGTH (A) - EYE TO SLIPPER END

Measure from center of the eye to the end of the spring's slipper. (Allow for slight variations in this measurement because the springs can sag and elongate as a result of wear)

LENGTH (B) - EYE TO HIGHEST SLIPPER POINT

Measure from center of the eye to the highest point of the slipper end

LENGTH (C) - EYE TO MIDDLE SPRING

Measure from the center of the eye to the middle of the spring

WIDTH (D)

Measure across spring. Slipper springs can be 1-3/4", 2", 2-1/2", or 3" wide.

HEIGHT (E)

Measure from top-center of spring to center of eye

DIAMETER (F)

Most springs have an eye diameter of 9/16", but eye hole diameter can vary. A few high-capacity leaf springs require larger bolts (up to 1" diameter).

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 swing front to back and 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. There is always one equalizer between two connecting springs in a system.

There are two types of equalizers that vary by capacity:

2" wide: For 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" wide: For dually-wheel and tandem-axle trailer with axle weight rating of 8,000+ lbs.

Standard equalizer for slipper springs

Equalizer for triple-axle kit

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. If you do need to replace an equalizer, the information you need varies by equalizer type. However, there are a few common measurements to look for:

CAPACITY

Weight capacity of the equalizer

WIDTH (A)

Equalizers can be 2", 2-1/2" or 3" wide

LENGTH (B) - Overall

Overall length of the equalizer

LENGTH (C) - CENTER TO FRONT

Length from center of front bolt hole to center of middle bolt hole

LENGTH (D) - REAR TO CENTER

Center of middle bolt hole to center of rear bolt hole

LENGTH (E) - FRONT HOLE

Diameter of front bolt hole

LENGTH (F) - CENTER HOLE

Diameter of center bolt hole

LENGTH (G) - REAR HOLE

Diameter of rear bolt hole

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, as well as the equalizers and hangers. The bolts can also create a platform on which the slipper end of the spring rests in the equalizer and hanger.

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:

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.

LENGTH (B)

Varies depending on width of spring and thickness of items being bolted together. Length measurement = the usable length from bottom of the head to end of the bolt.

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).

Double-Eye 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

Alex

10/27/2023

I have a dump trailer with 7k Dexter axles and slipper springs. What would cause the rear axle U-bolts to hit the frame? The trailer can be unloaded and level, and the U bolts still hit the frame.

Dawn K.

10/31/2022

Thank you for mentioning what information and measurements are needed to replace U-bolts for your trailer. I want to go snowmobiling with my family this fall. I will find a great snowmobile trailer first for us.

Mario B.

8/15/2022

I have a 2016 infinity car hauler trailer with Dexter 15k slipper spring set up. A few months ago one of the original front axle leaf springs broke near the bolt eye. My local trailer supply checked Dexter parts catalog for replacement part #072-045-01.I replaced left and right front axle springs and after a short trip the new leaf spring broke right below bolt eye. Long story short,i have since broken 4 leaf springs on the front axle. Front hangers. Equalizers and rear springs bushings are good. Now according to dexter parts catalog, there is only 1 15k leaf part# to choose from, looking at your site, you list 2 15k leaf springs with flat end and radius end. The leaf springs with dexter part are flat end. I did not look at the original springs that were replaced, but judging from the rear axle leaf they appear to have a radius end. Could this be a case of bad quality or batch and i just happened to get all 4 springs the shop had in stock, or my tandem Dexter kit requires radius leaf and by installing flat ends on the front its binding/and or stressing the front leaf under load. Thank you

David B.

8/15/2022

Hey Mario, I only have two 15k leaf springs like you stated. They are the same thing EXCEPT for the eye diameter. Without bushing the dexter is 2-1/16 inches and the universal group is 2 inches. If you had double eye springs you would know because the hangers would be set up for them. I really can't speak to the damage/failures that happened. There are so many variables and situations we could speculate all day and not come up with the right answer. I want to make sure that you know the slipper springs need to face a certain direction too. The eyes of the springs should always face to the front.

Helpful Links

faq-slipper-spring-trailer-suspension-review.aspx

Angela B.

1/20/2022

nice

Douglas S.

7/31/2020

i have a karavan dual axle pontoon trailer mid size 400 miles on it we have to land the toon all the time lately we have alot of noise in suspension when on ruff roads or going over speed bumps could it be the spring inside the slipper equalizer

Jon G.

8/10/2020

Slipper springs are actually noise because you have metal scraping on metal. If you'd like to go with something quieter then you'll want to switch over to a double eye spring setup but this will require you to swap out some hangers (which involves welding).

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