Calculating Axle Position

There’s always more to it.  Our previous post titled “Where Does The Axle Go?” has spawned a lot of great questions.  Here is one in particular asking some very relevant info about calculating axle position.  It’s worth sharing as a follow-on to the previous article because it deals with an existing trailer.

The Customer Question:

“Hello, I recently purchased an enclosed deck over snowmobile trailer with a 31′ cargo box length and three axles.  The manufacturer put the axles in the wrong spot.  Way to far forward.  Now the dealer has to move them.  How do I make sure he puts them in the correct spot this time?  Can I use your formula with a complete trailer with the axles and wheels already installed?  How do I calculate L4 & L5?  Thank you very much for any help.”

Note:  The equation mentioned is from the previous article, “Where Does The Axle Go?” — along with the variables L4 & L5.

Subject for Calculating Axle Position
The answer, is yes, we can calculate proper axle position for an existing trailer.  That said, for new trailer designs, we recommend using the expanded formulas in the previous article.  With existing trailers, like this, we can simplify things to make it a little easier since the trailer and all the components are already there.  This article addresses existing trailers.

Start With What You Have

Calculating axle position — the correct position — is in 2 steps.  First, we establish what we have, Then calculate where the axle SHOULD go.

The easiest way to establish what we have and get the needed numbers is using a vehicle scale.  Load the trailer as it will be going down the road, then go to a vehicle scale.  (Call around.  Scales are often at garbage dumps, scrap yards, highway weigh stations, and more.)  If you can’t drive it, maybe this DIY method to Weigh Heavy Things can help.

Example Procedure:

Drive onto the scale with your vehicle so that just the trailer wheels are on the scale.  Take the measurement.  For example, 2250#.  Now back up a little, then unhitch the trailer leaving it on the scale so it measures the full trailer weight.  For example, 2600#.

Measured Weight at the Wheels:  Ww = 2250 Lbs
Total Measured Weight of the Trailer:  Wcg = 2600 Lbs

Tongue Weight:  Ft(a) = Wcg – Ww = 2600# – 2250# = 350#  (Actual tongue weight for the existing trailer.)

Measure Trailer Loading
Labels and Dimensions in Measuring for Actual Trailer

Now calculate the center of mass (aka Center of Gravity or CG).  We can simplify calculating axle position by knowing the CG directly, then ignore some of the lengths and weights from the previous post.

Length from the Ball to Center of Wheel Set:  L6(a) = Actual, as measured.  (For a triple axle trailer that is probably the center of the middle axle.)
Length from the Ball to the Trailer CG:  Lcg

Lcg = L6(a) * Ww/Wcg

This Lcg tells us where the Center of Mass for the trailer is located.  This is according to the actual weight measurements, so it includes the axles weight (at the wrong position).  This is worth noting, but usually it is not worth worrying too much about.

Calculating Axle Position – Where It Should Be

With the numbers above, we can then calculate where we want the axles to go.  Start by selecting the desired tongue load, maybe 15% of the total trailer weight.  (While this is good in theory, make sure to verify that it is OK for your tow vehicle.)

Ft = Desired weight on the hitch.
Fa = Desired weight at the wheels.

Note that Ft + Fa must equal the total trailer weight, Wcg.

Ft = Wcg * 0.15  –  This is 15% of the total trailer weight.
Fa = Wcg – Ft

Values for Calculating Axle Position
Labels and Dimensions for Calculating Axle Position – Where it SHOULD go.

Now looking at where the axle should go . . .

L6 = Desired axle position.  This is where you want the center of the axle set to be — based on % tongue load you chose above.

L6 = Lcg * Wcg/Fa

That’s it.  That’s the answer to the customer question above.

NOTE:  The images show a single axle trailer.  For tandem axles or triple axles, use the center of the axle set.  With Tandem’s that’s between the wheels.  On Triples, that’s probably the center of the center axle.

Understanding the Numbers

I want to point out that these calculations are based on accurate weight numbers.  If you cannot load the trailer complete as it will be driven on the road, then these calculations become guesses.  That is not all bad, because we know the trailer is not loaded exactly the same every time.  However, the better we set things up, the more accurate calculating axle position will be.

Secondly, there is nothing magic about exactly 15% tongue weight.  Anything in that area + or – a couple % is good.  So, take this all in stride if the numbers don’t come out perfect.  One key point:  We don’t recommend going below 10% tongue weight.  15% works a lot better if you can.

You must make some judgments about distances and loading as you go.  If your vehicle is capable of a lot more tongue weight, then bias the axles back a little bit.  On the other hand, if your vehicle is limited at these tongue weights, then use a lower percentage when calculating Ft.  Higher is better for stability, but not always practical with your tow vehicle.  These are judgments you must make.

Considerations In Calculating Axle Position

Things are never as simple as they seem, so here are some other considerations when making your axle position calculations.

First, as mentioned above, the axles in the wrong place are baked into the Lcg calculation.  For the trailer description in the customer question, as the axle set moves back, the Lcg also gets slightly longer.  Make a judgement about this based on 1) the weight of the axles set compared to the total, and 2) how far the axles need to move.  If the axles weight is small compared to the total trailer weight, and if the placement position difference is small-ish compared to the total trailer length, then don’t worry about the change in length Lcg.

Second, trailer tongue length plays a role here.  It won’t change the Wcg or Lcg numbers, but when making judgements about axle position, a longer tongue is often more forgiving than a short one.  Again, this is just a consideration for calculating axle position, not a defining factor.

Third, loading variability.  If your trailer will be unloaded or lightly loaded much of the time, then biasing the axle back can be mildly beneficial.  If your trailer is loaded different every time you use it (most utility trailers), then go with an average for the calculations.  Certainly you can bias a smaller load toward the front for those times when needed.  Loading the trailer with heavier items to the front is always a good idea.  (Here’s a video showing some instability from loading.)

Back To The Start – A Practical Solution?

Going back to the the start of this article, the reader asked about calculating axle position because he needs to move the axles on his trailer.  That’s a tough one.  If you have to cut them off just to weld them back on . . . . that’s a lot of tear-up.

Here’s a thought.  Make the measurements and calculations as above.  Then, look at the numbers and see what happens if you simply take the front axle off, and put it at the back.  It’s the same as shifting all the axles back one position.  Perhaps then, you only need to add new brackets farther back rather than cutting off everything.  Again, it depends on the numbers.

Good Luck With Your Axle Moving Project.

4 thoughts about “Calculating Axle Position”

  1. How can you measure this if you have no way of knowing what the weight of the trailer is?
    Unfortunately, I can not haul a trailer I am building to a scale, without permanently mounting the axle first.
    Thank you!

    Reply
    • Well, measuring or estimating weight is a pretty big part of finding weight proportions. You can do some tricks like setting up a lever system so you can weight portions using a bathroom scale. I does require more math and careful measurements. For instance, measuring the weight of one side of the trailer (then double it) can give a good estimate. Good luck.

      Reply
  2. Hello,

    I recently acquired a 4 x 10 utilty trailer that had a rotted wood structure that I ended up removing. Based on how it looks and was constructed, I would say it is old; what is funny about it, from my perspective, is the axle placement, as it is located dead center in the bed length. So I have a few questions please as I determine proper axle placement please:

    1.) I cannot load anything on it, obviously, because it is stripped down currently, but I believe I can still weigh it on a scale. Unloaded, are the weight readings still helpful for this determination?

    2.) If they are, and it appears that the axle is not placed correctly, my inclination would be to remove length from the rear to correct the problem, no more than two feet. Would the weight readings make it possible to figure this out with some degree of accuracy?

    3.) For the trailer reconstruction, I plan to add reinforcement metal to the frame along with a trailer tongue toolbox, which, inevitably, will increase the weight on the trailer, hence altering the calculations of the trailer empty. Obviously, this axle determination will need to be made before any reconstruction, so would these additional items (extra metal, trailer box, etc.), just need to be weighed separately and then added to the calculations?

    In the end, maybe I should just leave it as such, but since I am at this point, I’d rather set it up as well as possible, or just rehab as it is. Thank you in advance for your assistance.

    Reply
    • That can be a fun project. 1) Once you have weights of the individual pieces, you can assign them a distance then make the calculations. Perhaps the equations on this related page may be more useful. 2) Estimate the load and adjust position as needed. The length of the back does not matter, it’s the weight balance. If you always load it to the front, you don’t need to cut the back. 3) See previous. If you load the trailer wrong, it will tell you by being squirrely down the road like in this trailer stability video. We talk a lot about theory which is fun, but don’t forget that reality has a lot of tolerance.

      Reply

4 thoughts about “Calculating Axle Position”

  1. How can you measure this if you have no way of knowing what the weight of the trailer is?
    Unfortunately, I can not haul a trailer I am building to a scale, without permanently mounting the axle first.
    Thank you!

    Reply
    • Well, measuring or estimating weight is a pretty big part of finding weight proportions. You can do some tricks like setting up a lever system so you can weight portions using a bathroom scale. I does require more math and careful measurements. For instance, measuring the weight of one side of the trailer (then double it) can give a good estimate. Good luck.

      Reply
  2. Hello,

    I recently acquired a 4 x 10 utilty trailer that had a rotted wood structure that I ended up removing. Based on how it looks and was constructed, I would say it is old; what is funny about it, from my perspective, is the axle placement, as it is located dead center in the bed length. So I have a few questions please as I determine proper axle placement please:

    1.) I cannot load anything on it, obviously, because it is stripped down currently, but I believe I can still weigh it on a scale. Unloaded, are the weight readings still helpful for this determination?

    2.) If they are, and it appears that the axle is not placed correctly, my inclination would be to remove length from the rear to correct the problem, no more than two feet. Would the weight readings make it possible to figure this out with some degree of accuracy?

    3.) For the trailer reconstruction, I plan to add reinforcement metal to the frame along with a trailer tongue toolbox, which, inevitably, will increase the weight on the trailer, hence altering the calculations of the trailer empty. Obviously, this axle determination will need to be made before any reconstruction, so would these additional items (extra metal, trailer box, etc.), just need to be weighed separately and then added to the calculations?

    In the end, maybe I should just leave it as such, but since I am at this point, I’d rather set it up as well as possible, or just rehab as it is. Thank you in advance for your assistance.

    Reply
    • That can be a fun project. 1) Once you have weights of the individual pieces, you can assign them a distance then make the calculations. Perhaps the equations on this related page may be more useful. 2) Estimate the load and adjust position as needed. The length of the back does not matter, it’s the weight balance. If you always load it to the front, you don’t need to cut the back. 3) See previous. If you load the trailer wrong, it will tell you by being squirrely down the road like in this trailer stability video. We talk a lot about theory which is fun, but don’t forget that reality has a lot of tolerance.

      Reply

Leave a Comment

We Found These For You . . .

Article
What Tires For My Trailer?
There are a million tires out there with a ton of classifications and designations.  What tires do I choose for my trailer?  That is not a silly question at all.

Read The Article

Product
6'10" x 16' Flatbed Trailer Plans

A large utility trailer, at full (legal) width and 16’ length -- with options for 12,000 lbs or 16,000 lbs capacity.  Trailer plans include many options for you to configure it to your specific needs.

Article
Metal Working Files In Action
Often misused and not very well understood — so goes the life of many metalworking files.  Yet, they still do things other tools can’t.  That makes them a tool worth knowing!

Read The Article

Article
Trailer With Attitude
When we think about airplanes flying, or boats sailing, it’s easy to imagine how they move.  An airplane rolls some as it banks into a turn.  A boat pitches up and down with the waves.  So, how do these factors…

Read The Article

Article
Trailer Axle Choices
In the discussion of trailer axle leaf springs versus a torsion axle, let’s put some engineering behind the debate.  I don’t want to change opinions, but I would like to offer a practical perspective.

Read The Article

Product
Wider 12 x 32 Tiny House Trailer Plans

For a little more interior room, our widest Tiny House Trailer plans also include full engineering as a mobile foundation.  Low 12’ x 30’ or 32’ top deck.  Up to 18,000 lbs total capacity.

Article
JTP Building A Trailer
We’ve been working with the great folks at Johnson Trailer Parts for a while.  Their business is primarily selling trailer parts, but they understand the greasy hands-on Do-It-Yourself paradigm!

Read The Article

Article
Trailer Instead of a Truck
That sounds like a good idea.  Right?  There are a million good reasons to need the capacity of a pickup truck, but actually, very few people use a Truck as a Truck that often.  Can we get the function we…

Read The Article

Article
Review of the Folding Trailer Tongue
On a recent trailer, I experimented with a new folding tongue design.  Now it’s time to review — What do I like?  What would I do different if building another?

Read The Article

Article
Welding Trailer Spring Brackets
There is some disagreement in trailer building about how to weld-on trailer spring brackets (also known as spring hangers).   Some of you have sent questions about which techniques are best, especially

Read The Article

Article
Trailer Wheel Bolts
A friend asked me this question, along with the companion “How Tight?”  The last thing we want is to lose a wheel — because it will not only delay our trip, but a wheel coming loose can do serious damage…

Read The Article

Product
Hoist Winch Pole Plans

Meet the "Winching Pole".  It’s a simple, mechanical, Out-of-the-Way Hoist Winch to make lifting with a gantry crane easy.  No chains in the way.  No climbing up for a come-a-long.  Plans also include a simple load leveler.

Article
Applied Force vs Movement With Shop Tools
Is there anything more annoying (painful and frustrating) than busting a knuckle when working on things in the shop?  To many of us, blood sacrifices are just part of getting a project done, but it doesn’t have to be that…

Read The Article

Product
Mobile Gantry Crane Plans

Adding a crane is a game changer in the shop.  It’s perfect for lifting . . . anything.  Use the free standing mobile gantry crane - from plans - for pulling engines, to lift the boat, the car, or other…

Article
Spiraling the DIY Project Plans Back Online
January 7, 2016.  Mechanical Elements is back online with a new look, a new logo, new functionality, and a better shopping experience. — With — an expanded offering of the Same Great DIY Project Plans and so much more.

Read The Article

Product
6'x12'-6000 lbs Trailer Blueprints

DIY Blueprints for a beefy 6 x 12 single axle Utility Trailer.  This is a heavy duty version of our 6' wide trailer that has a lot of options - including a 6000 lb or 7000 lb Axle capacity choice.

Product
16K by 20' Flat Deck Trailer

A great mid-size Tandem Axle, 10,000 lbs Flat Deck Trailer is waiting for you in these plans.  At 8.5' wide x 20' deck length, the plans include many options to achieve the function needs.

Article
Makeshift DIY Truck and Trailer
The coolest thing I saw last week was this guy with a very creative DIY approach to a pack trailer — and, of course, his tow vehicle!

Read The Article

Product
5x8 Trailer Plans, 3500# Capacity

Start building your own utility trailer with these 5x8 Trailer Plans.  Blueprints are fully engineered and include options for various needs.  These plans are a perfect map for a great project.

Article
Low & Lean Trailer Suspension
A unique trailer suspension offering a low profile, tuning possibilities, and more. By design, the concept offers complete tandem axle load sharing via a common coil spring which is a big plus. Though different than a torsion axle suspension, this…

Read The Article

Leave a Comment