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.

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