Trailer Axles 101 – What You Need To Know

I will just drop by the parts store and grab an axle . . . . Well, trailer axles are not quite that simple, but knowing a few important details, we can certainly get the right one.  Here is a quick guide to trailer axles— how they fit a trailer, and the features that make them serve the various applications well.  They are not complicated, but fitting the needs is a little more than picking a coffee mug to fit your personality.

For most DIY trailer builds, we recommend that you order the right axle for your needs.  Sure, sometimes you can find an axle off-the-shelf at a store (or online), but, will it have all the options you want?  Will a “standard” size axle fit the functions of the trailer you want?

Trailer Axles — The Specifications

Axles (including the wheels and tires) have several key bits of defining information.  The list below has the big ones, while the trailer axle diagram gives a visual to go with the words.  There are also links with some items to other articles where more detail is available.  Items in Bold are in the dimension illustrations.

1. Suspension Type

   We often categorize axles by their suspension – Leaf Springs, Coil Springs, Torsion, Air Ride, Rubber Compression, Axle-less, Trailing Arm, Walking Beam, and the Odd or Unique trailer suspension, etc..  There are so many types of suspension for axles (or rigid mount with no suspension at all).  Follow the hyperlinks for more information on each type.

   Yet, just a few are really common.  In this article we will discuss the Leaf Spring and Torsion types since they are most common.  While there are many similarities, the info in this article is roughly applicable to all.  Some things do not matter for one type or another, so if you are considering one of the others, just ignore those items that do not apply.  Read more in the Choosing Tandem & Triple trailer axles article.

2. Load Capacity

   How much weight can the axle carry?  Load capacity is the weight rating for the axle.  That is a max, and that number usually defines the total capacity of the trailer.  For multiple axles (like tandems), assuming trailer axles are the same, simply add the capacities for a sum total.

   Generally speaking, look for trailer axles with more capacity than you need.  Also, be sure to match springs and tires, because the real capacity is the lesser of Axle Capacity, Spring Capacity, or Tire Capacity.  Don’t get caught short.

3. Mounting Position

   For this discussion, Mounting Position means the measured location where axle connects to the trailer frame.  For Leaf Springs it is the Spring Centers distance.  That defines the mounting.  For positioning the axle fore and aft on the trailer, the article on Calculating Axle Position may interest.

   Because Torsion Axles do not have springs to mount, it is the bracket width (usually meaning the outside dimension of the frame members or Frame Width) that defines the Mounting Position.  Also, see below because the bracket can fit in different ways.

4. Hub Faces Distance

   This is the distance – from side to side – between faces where the wheels mount.  See the illustration.

   This distance is sort of meaningless by itself, but super important relative to other things on the trailer like the clearance to the frame, the tires, and the Overall Width.  It is also restricted by axle manufacturers with respect to the Mounting Position (left and right) so you do not get too much or too little Axle Overhang.  Read more in the calculations section below.

5. Track Width

   Looking at a front or back view of the trailer, this is the distance across the trailer between the centers of each tire track.  In the illustration above, it is the same as the Hub Faces Distance.  If the wheel mounting is centered in the rim, then the Track Width and Hub Faces Distance are the same.  In some cases they are different.

6. Axle Overhang

   In the trailer axle diagram above, there is a dimension for Overhang.  This trailer axles dimension is technically the difference between the Spring Center and Hub Face distances (divided by 2, since half the distance appears on each side.)  However, there is a lot more to it – especially for things like building an off-road trailer.  It has to do with width, and spacing, yes, but also with axle strength and trailer stability.  More information is below on this page, then even more in the Axle Overhang article.

7. Bolt Pattern (for Hubs and Wheels.)

   Axle hub style is partly set by the number of lugs (bolts) for the wheels, and partly by the choice of brakes.  The pattern of the lugs including how many and how far apart they are is the “Bolt Pattern”  Of course, these must match the trailer wheels you will use.

   There are many bolt patterns available, so be sure the pattern of the wheels match the pattern of the axle.  We call these patterns by the number of holes and the bolt circle diameter.  For instance, 5 on 4.5″ means 5 holes on a 4.5″ bolt circle.  Patterns with more lugs are for heavier loads.  Read this article on bolt patterns for a lot more information.  Of course, every bolt pattern has bolts, so check out this article discussing frequency in tightening trailer wheel bolts.
   

   Trailer axle diagram, side view, showing tandem trailer axles with more variables and important things to know about.

8. Brakes (or not)

   Some trailer axles have brakes, some do not.  Check your local laws and this article about brakes to know if you need them.  There are a few types to choose, like Drum Brakes, or Disc Brakes, and the motive as Electric, or Hydraulic, or mechanical actuation.  Electric are most common, and have good control, though surge hydraulic types are arguably easier to adapt a variety of vehicles (illegal in some areas).

9. Drop or Straight

   See the illustration.  A Straight Axle is what you might expect – a straight beam with hubs at the ends.  A Drop Axle just allows the trailer to sit lower, usually 4″ for a leaf spring style.
   
   Torsion style trailer axles handle the drop a little different.  A lift or drop happens because of the No-Load Arm Start Angle.  For some axles this is a factory setting.  Others adjust.  Either way, it is best to specify the desired angle when buying a torsion axle.  When the angle is UP, then it accomplishes a small “drop” for the trailer frame.  Conversely, if the angle is “DOWN”, it is like giving the trailer a “lift”.
   

10. Spring Type (for Leaf Springs)

    With leaf springs it is primarily Eye-Eye, or Slipper type springs.  This has to do with how the springs interact with the frame for mounting.  Typically Eye-Eye styles are best with lighter trailers (both single and multiple trailer axles).  Then, use slipper springs for heavy, multi-axle applications.  We normally choose Eye-Eye whenever practical.  Obviously, with no springs, this item is not for Torsion Axles.

    On this topic, it is worth noting that not all trailer axles or mounting hardware are equal.  Be cautious with springs and axles that are made for a specific purpose – like mobile home axles.  Wrong assumptions about these will easily lead to big problems.

11. Spring Length (for Leaf Springs)

    Yes, it is the distance for supporting the load – like the distance from Eye to Eye on double-Eye style spring.  In general, longer springs give a better ride, but that is not absolute.  We recommend the longest springs that are practical for an application.  For a lot more about spring length and choosing the right springs, read the article “The Value Of Spring Length“.  And of course, this does not apply to Torsions, air bags, axle-less, etc..

12. Axle Spacing (for Multiple Trailer Axles)

    For Multiple Axle trailers, the distance from axle centerline to axle centerline is the axle spacing.  See the side view dimension illustration above.  While mounting hardware has an effect, Spring Length will largely dictate the axle spacing.  Some standards exist, like 33″ or 35″.  Generally it is good to stick with the standards, and the recommended parts – springs, spring hangers, equalizer, etc..  Follow the manufacturer’s instructions – and make sure you get the right stuff for proper Tire Space.

    See this Equalizer Action Article for an animation of tandem axles in action.  Also, to learn more about various equalizer link styles.

    While leaf springs are the most common for multiple trailer axles, they are not the only way.  We do recommend they link in some way — mechanically, hydraulically, pneumatically, or other — to share the load.  (See Independent suspension below.)

13. Overslung or Underslung

    Since it is easier to illustrate with another trailer axle diagram than to explain, the image shows a pair of trailer axles – one with the springs mounted on top (or Overslung), and one with the springs mounted underneath (or Underslung).  This makes a big difference in trailer bed height.  It also affects stability in a smaller way.  Read more about the differences and the effects in this Article on Overslung and Underslung mounting.

    While these terms usually mean Leaf Spring axles, they sometimes apply with less common suspension types too.

    For torsion axles, the option that is somewhat similar is “High or Low” brackets and “Inside or Outside” brackets.  See the trailer axle diagram near the end of this article for more info on Torsion Axle brackets.

14. Wheel and Tire Size

    Though technically not an axle part, choices about trailer wheels, including the Tire Type are very interrelated.  See Bolt Patterns above.  Also, the Tire Width and Capacity must fit the application.  Both Wheel and Tire load capacities should exceed that of the axle.  Read the post about Tires.

15. Axle Camber

    Camber amounts to a slight bend in the axle which helps the trailer track the highway a little better.  The little tweak helps tracking and tire wear.  In general this is good if you will be on the highway a lot, but not necessary if the trailer is for short, local trips.  The trailer axle diagram below shows a grossly exaggerated illustration of axle camber.  Order trailer axles either with or without camber.
    

16. Do I Need Shock Absorbers?

    The question of shock absorbers comes up fairly frequently, especially in DIY.  Of course, if you are building a trailer, you want the best, so asking the question is a good thing.  But, we do not usually see shocks on trailers.  We cover this topic in a lot of detail in the Trailer Shock Absorbers Article.

17. Independent Trailer Suspension

    By definition, independent suspension means each wheel responds to road conditions without connection to another wheel.  There are many styles, including the torsion axle.  For single trailer axles, there can be some small advantages.  For multiple axles – While this is good for cars, it is not for trailers.  For more information, read this article about Torsion Axles in Tandem or Triple, and see the videos and tech explanation in this one talking about Independent Suspension for both single and multiple trailer axles.

There are more trailer axle details, but the list above covers the big ones.  With this info, we can make the measurements for the trailer – or maybe more accurately – take measurements FROM the trailer (before ordering trailer axles).  That way it all matches when we get them.

Measurements and Calculations for Matching the Trailer

To get some of the fitting info, we need calculations.  Here are some more definitions (with explanation) for key pieces to match and properly mount the new trailer axle.

1. Mounting Position

   Mentioned above – either Spring Centers or outside Frame Width.  This is a measurement from the trailer directly.  Note the difference in the measurement for Leaf Springs versus Torsion Axles.

2. Hub Faces Distance

   Also from above.  This measurement is explicit on the axle, but must actually include other factors as shown in the trailer axle diagram.  Think of the Hub Faces distance as a mathematical calculation:  Spring Centers + 2 times the needed Overhang.
   

3. Overhang

   The distance from the axle supports, out to the hub face.  Calculation:  Overhang = (Hub Faces Distance – Mounting Position)/2.  (Yes, that is a circular reference from above.)

   Axles are specified with a Max and a Min Overhang.  (True for both leaf spring axles and Torsion axles as in the tables.)  If the overhang is too short, the frame will interfere with the tires, or the mounting becomes ridiculous.  If the overhang is too large, it reduces the actual axle capacity.  Axle specifications are available from vendors (like this data tables from Dexter) showing the allowable Overhang.

   Read this article for more about trailer axle overhang and what happens if you want more than the manufacturer allows.

4. Tire Clearance

   Like it sounds, this is the space between the moving tire and everything else.  We must have Clearance between the frame and the tire as in the (front view) trailer axle diagram, and also radially around the tire (side view diagram), especially vertical for suspension travel.  Usually 2″-3″ all around the tire is sufficient, then 3″-4″ vertical Clearance to the Fender for suspension movement.  Finally, 3″-4″ or more as Tire Space between the tires for multiple trailer axles.  Do not skimp on clearance.  In a battle between steel of the trailer frame and rubber of the tire, the tire loses.  Unfortunately, trailers do not operate so well without tires.

5. Overall Width

   The total distance from side to side including the tires.  This is a summation of everything between.  It is the Hub Faces distance plus the Tire Width.  Check local laws if you want to go wide, because most jurisdictions limit width.

6. Ground Clearance

   Needed clearance over the ground is really in 2 parts.  First, the clearance of the axle itself over the ground; and Second, clearance for the main portions trailer.  Clearance for the axle is set by being a Drop or Straight axle (No-Load Arm Start Angle for torsion axles) and the tire outside diameter.  Bigger tires give more ground clearance.  Trailer ground clearance also adds in Overslung or Underslung spring mounting as well as any spacers used for frame lift.  These two combine to define ingress and egress angles for the trailer.

These items are really important when specifying the axle to buy for your trailer.  Or, as the case may be, to know as you build a trailer for the axle(s) you just scored from your uncle.  (Hey, I know, I have been there.)

Ready To Order?

When you order an axle, here are the details you must provide.  We think it is best to make a list, and perhaps even a sketch.  Refer to the information above, because we have covered the basics.  That said, it is not bad to research trailer axles by following the links or Googling these terms.  Oh, and don’t just take someone’s word, because there is a lot of misinformation around . . . like this guy.

For a Leaf Spring Style Trailer Axle

• Load Capacity
• Spring Centers
• Hub Faces
• Straight Or Drop
• Underslung or Overslung
• Hub Bolt Pattern
• Brakes or not, and what type – Drum or Disc – Electric or Hydraulic
• Spring Type and Length
• Axle Camber – Yes or No

For Torsion Style Trailer Axles

Take note of this trailer axle diagram as a reference for torsion axles, then variations in the smaller axle diagrams below.

• Load Capacity
• Frame Width – and Direction of Mounting Brackets
  
• Hub Faces Distance
• Hub Bolt Pattern
• Torsion Arm Start Angle –  Angle Up, or Angle Down (see images in “Straight or Drop” section above.)
• Height of Mounting Bracket – High or Low Mount (see above).
• Brakes or not, and what type – Drum or Disc – Electric or Hydraulic
• Axle Camber – Yes or No

Typically, trailer wheels and tires order separate from the axles.  For those, they need the Bolt Pattern, the Size (like 225-75R15) and the required Capacity.

Learning More

The above is basic info for trailer axles.  Please follow the hyperlinks to other articles with more detail on the linked topics.  While this info does not include specifics for your trailer, we are confident you can handle it from here.  Bookmark the page to come back to review again when you are ready to order your trailer axles.

Just so you know, the Mechanical Elements Trailer Plans have all the necessary information listed to order the right axle(s).  The information is in the instructions document, in the list of materials.  Or, if you want to switch the trailer axles – convert from springs to torsion (for single axles) , we have Torsion Axle Conversion plans available.  It is part of why our trailer plans are the best.  Enjoy!

Good Luck With Your Trailer Axles!