Trailer Build: Where Does The Axle Go?
Trailer axle position? This is a good question. Unfortunately, there is some popular, but misleading information around. So, how do you know proper trailer axle position? Here’s the whole answer, from The Mechanic.
There once was a boy who came to his mother and asked “How do rockets get to the moon?” She replied “Ask your dad.” To this, the boy mumbled “I don’t want to know that much about it.”
If you’re looking for a simple — maybe misguided or incomplete answer — this is not the right article. If you really want to know about trailer axle position, then you’ve come to the right place.
Trailer Axle Position Goals
For this discussion, we will focus on trailers with a traditional tongue attached to the rear of the tow vehicle. 5th Wheel (and Gooseneck) trailers are mentioned, but are not the focus.
To determine trailer axle position, we must first understand what drives the decision. The Goals:
- Stable, predictable towing.
- Proper weight balance for both the tow vehicle and the trailer.
There is a lot that goes into these goals including construction (straightness, flatness, perpendicularity, etc.), proper stiffness and more. However, with respect to axle position, we follow some well proven guidelines.
- Follow manufacturer specifications for the tow vehicle. Note: there are 2 different limits — trailer weight, and tongue weight. Don’t exceed tow vehicle limits. Enough said.
- In general, more weight on the tongue is better for stability. Example: Over-the-road trucks (ie., a Semi or Lorry) trailers have roughly half the trailer weight on the hitch. It works because the tow vehicle is built for it.
- For a 5th wheel (or gooseneck) trailer, weight at the tongue should be in the 20% – 30% range of total trailer weight. (Perhaps we’ll dive into this in a future post.)
- For rear connecting trailers (traditional), weight of the tongue should be in the 10% – 15% range of total trailer weight. (10% minimum, 12% is OK, and 15% is great.)
Stable Predictable Towing
The guideline above for 12-15% of trailer weight on the tongue is time tested for dynamic towing stability. If tongue weight is too low, the trailer will buck more over bumps, and wag around corners.
I won’t go into all the engineering, but the summary is inertia. If there is not enough tongue weight, a change like a bump or turn or steering correction leaves the trailer mass pivoting, and it takes additional energy to get things back to stable. What you, as a driver, feel is a bucking or a wagging of the trailer. In bad cases, it is very unnerving. In severe cases, a crash can result.
If there is significant tongue weight (above 10ish %) gravity serves to settle things back to stable. The more tongue weight, the faster stability is achieved, because the Center of Gravity is further from the axle.
Measuring Tongue Weight
Of course, the easiest way to know tongue weight is to measure it. Most of us don’t have a scale that will go high enough, so take the trailer to a vehicle scale.
For an existing trailer, load it for travel. (This is difficult for utility trailers because you never know what will be hauled.) Drive onto the scale with your vehicle enough that just the trailer wheels are on the scale. Take the measurement. For example, 2250#. Unhook the trailer on the scale so it measures the full trailer weight (tongue and axle). For example, 2600#. Subtracting: 2600# – 2250# = 350# tongue weight. For percentage, divide: 350/2600=13.5% which is great.
When building a trailer, one of the easiest ways to measure is to set the axle in place under the trailer, but don’t permanently mount it. Clamp it in place, then measure as above. Move the axle position forward or back as needed, then verify loading. This technique works well when you have a defined load for the trailer — like a boat, for instance.
On the other hand, for something like a Tiny House Foundation or a Utility Trailer, this method for trailer axle position doesn’t help much because you don’t have the actual load when constructing the trailer. See the Calculation Method below.
That said, using the measurement technique can help when setting bigger loads like a battery pack or water tank for a Tiny House. You can set an axle position that suits the overall design, then place the bigger loads where they measure out to give proper tongue weight.
Calculating Tongue Weight And Trailer Axle Position
We can also calculate tongue weight and trailer axle position. I’ll give an example, then if you need more detail, please contact us, or do some searching on the internet. For existing trailers (if you want to check the axle position or maybe move the axles), please read this follow-on article also about Calculating Axle Position.)
We use a balanced lever approach to calculate loading. First, we sum forces in a vertical direction. There are only 2 points that support the vertical loads (tongue, FT; and axle(s) FA). Then there are several “loads” (depending on your trailer):
- Trailer Weight — WF — Weight of the trailer including frame, sides and flooring can be measured or calculated pretty easily. You will also need to know the center of that weight. (Measure this by placing a board on edge under the trailer frame, then move it till the frame balances on the board.)
- Evenly Distributed Load — WD — Loads that can be assumed as even along the length of the trailer bed. For a utility trailer this may be rocks, or firewood. For a Tiny House, this is the walls and roof. A good estimation is OK, but more accuracy gives a more accurate final answer. The location of this load, L5, is the center of distribution.
- Points of Specific Load — WT — These are big loads at specific locations. For a utility trailer this may be an ATV, or lawn tractor, or tank, or toolbox. For a Tiny House, this may be water tanks or battery packs or kitchen cupboards.
Measurements are center of the hitch ball to the load centers. If any of these are not present, just leave them out. If you have more point loads, just add them in as illustrated. Obviously, Tongue Length L2 has a big effect, so it’s worth reading this article too about Choosing the Right Tongue Length.
Example Calculation:
Let’s use a utility trailer rated for 3000 Lbs. Also written 3000#. We will assume an evenly distributed load of rocks, lumber, or whatever. It also has a mounted toolbox.
Weight of the Toolbox: WT = 300 Lbs @ L1 = 30″
Length of the Tongue: L2 = 42″
Trailer Bed Length: L3 = 96″
Trailer Frame Weight: WF = 450 Lbs @ L4 = 83″
Distributed Load: WD = 2250 Lbs, Center @ L5 = 90″
If we want the Tongue Weight at 12% of the Total . . .
Now we know the loading if the tongue has 12% of the load. If that works for the tow vehicle, then we can move to the next step.
On the other hand, let’s say our tow vehicle can only handle 300# tongue weight. By dividing the max tongue weight, 300#, by the total trailer load, 3000#, we get 300/3000 and that yields 10% tongue load. That is on the margin, but can work.
Summing Moments (Load by Position)
Now we know the force values, the next step is to figure out where the axle goes so all those forces balance. We do that by summing the moments — basically, to sum the loads multiplied by their distance from the tongue load. By setting the moments with forces up equal to the moments with forces down, we can solve for axle position.
Back To Our Example:
Using the values and solutions from above . . .
So, our Axle position is 94.3″ from the hitch. Of course the numbers don’t have to be exact, because loading will vary. With fewer rocks, the load is less. Or, if there are not 300 pounds of tools, then that makes a small difference too.
From the example above, if we remove the toolbox, the calculated axle position changes to 100.9″
Or, using the example, if WT is 15% (instead of 12%), the axle position becomes 97.6″
When building your own trailer, run the calculations a few times with different load scenarios. Design for the maximums. After evaluation, make a judgement call for the final trailer axle position.
That’s it. Now you know how to calculate proper trailer axle position.
Trailer Axle Position for Multiple Axles
What is different for multiple axles? Nothing. Use the central position of the axle group for all the measurements and calculations. Treat them as a single axle. Note: This simplification works for loading and trailer axle position on load sharing multiple axles, but not for stress or stiffness calculations.
What about torsion axles? Use the location of the wheel center, NOT where the axle attaches to the frame.
Proving Or Disproving The Common 60% Rule
On the internet there are several websites and YouTube videos saying to place the axle at 60% of the trailer bed. What do you think?
In our example above, the trailer axle position is ~55% of bed length. If we put it at 60%, the tongue weight becomes 501 Lbs, ~16.7%. Or, if we take the toolbox off and went to 15% tongue weight, the desired trailer axle position is 65% of the bed. These examples show that the 60% guess-of-thumb is not always best.
In some cases it makes the tongue load below 10% which borders on dangerous. Other situations it can make the tongue load too high.
My Opinion? Setting the axle at 60% is the lazy way. In many cases it gives a reasonable answer, but why settle for a lazy guess when you can simply run the numbers and gain understanding. Sometimes the answer is 60%, but calculate and be confident.
All that being said, I recommend biasing the trailer axle position a little farther back for trailers where the load will change — like utility trailers. It improves towing stability, maneuverability, and safety. You can place the load appropriate for the conditions. Just make sure you have the right materials and design safety factors for the loads you intend to carry.
And now you have the trailer axle position figured out, we invite you to read Trailer Axles 101 and some extra tips on mounting a leaf spring suspension.
Good Luck With All Your Trailer Building Projects!!
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February 6, 2019 @ 6:39 PM
Thank you so much for sharing your information!
March 14, 2019 @ 3:23 PM
I am a big fan of this website. The knowledge you share here is very valuable to me. Thank you!
I have a question that is more rooted in academic curiosity than practical application. Everyone discusses “tongue weight”. However, in a physics sense, is actual weight on the tongue the objective, in order to achieve stability? Or is the objective more about locating the trailer’s centre of mass (including the load) relative to the major pivot points (axle & coupler)? (If the latter, I understand that it would be translated to tongue weight rules of thumb, given average tongue lengths, so the layman can easily approximate the centre of mass and achieve a stable condition.)
If it’s more about centre of mass location, is the primary factor the distance from the axle, or distance from both primary pivot points (axle & coupler)?
As a thought experiment to illustrate my question, let’s imagine an 8′ utility trailer with an axle set a little back from the centre of the bed, with a tongue length of 4′, loaded evenly, with a result of 15% of the weight being on the tongue at the coupler. Now, ignoring the weight of the tongue material itself, let’s imagine we extended that tongue to be 12′ long. If I remember my math correctly, the weight on the coupler would be cut roughly in half; probably around 8% now. However, the centre of mass would still be the same amount forward of the axle. Would the second case be likely to tow stably, despite being outside the normal rule of thumb? (Let’s also ignore the other inherent stability benefits offered by the longer tongue, for the sake of this thought experiment.)
March 14, 2019 @ 6:01 PM
Oops, now you’re thinking. We can’t have that! — But these are great questions. How much do you want to know?
Let’s play with your experiment. Let’s take your trailer and and move the load back so there is Zero tongue load. Now, as you drive your car goes over a bump. That pushes the tongue up fast, which throws the load back, which makes the trailer want to tip back (because the load is now behind the axle). As your car comes down from the bump, the process goes the other way. That back and forth switching causes some weird dynamics for the driver and he feels like it is “bucking”.
So making the tongue (your 12′ experiment) long does work like you say, and actually it makes the bump feel (to the trailer load) like it’s half as high. A long tongue has other nifty advantages too — within reason — though your 12′ tongue will create other issues.
All of this is part of why we avoid trailers with really short tongues.
That’s the straight line dynamic. There’s much more to it. (Edit, new article on tongue length is https://mechanicalelements.com/correct-trailer-tongue-length/ )
March 20, 2019 @ 10:06 PM
Leading me further down that thought experiment was helpful. By continuing to follow that though, I think I got some clarity: it’s more about the centre of mass and its location relative to the trailer axle, at least for straight-line dynamics. Brilliant, thanks!
I relish the idea of an article that delves deeper. I’ll keep my eye out!
July 27, 2019 @ 5:01 AM
Having built several trailers, a 6 x 4 single axle, an 8 x 5 tandem load sharing axle, a 10 x 5 tandem load sharing axle and two car trailers with 14 1/2 x 6 1/2 feet decks, 6 1/2 feet draw bars, tandem load sharing axles, all performed to expectation all axles were set no more than 1 inch aft of the deck centre, the difference between mine and others was that all of mine had considerably longer draw bars. All but one had no more weight on the draw bar than i could lift, for two very good reasons, excessive draw bar weight destroys the jockey wheel over time and renders the trailer almost impossible to manoeuver single handed and buries the jockey wheel in your just laid lawn. The trailer with more draw bar weight was my first car trailer I built to take my sprint car around, the reason for the excessive draw bar weight was that a cover was built from the front of the first axle up to half of the draw bar length, in the covered area were about 500kg of spares and tools. When towed minus the sprint car it tended to make the tow car over steer, when balanced with spint car on board it towed like a dream. My point being , have a good long draw bar, just enough weight on the ball to prevent bucking and ease of handling when empty, draw bar weight has little to do with handling, it is more the distance from the load centre to the tongue centre verses road speed. Incorrect weight distribution caused by placement, load shift, aerodynamic factors are the X factors and are probably responsible for as many if not more catastrophes than axle placement. Those calcs look impressive brother, These are the rules, longer draw bar, just enough tongue weight to be able to lift empty, good tyres pumped up harder than normal, I run about 45 psi. and place the load just forward of centre of the axle group, it’s just a trailer, KISS.
June 24, 2019 @ 6:15 AM
I’m trying to build 48’ gooseneck trailer would like some information on this build
July 23, 2019 @ 9:23 PM
Related to the earlier post about tongue length. I was looking at a trailer design with a 36 degree A frame coupler and calculating that changing the A frame coupler angle to a commonplace US 50 degree version reduces the length significantly but only has a small impact on tongue weight. From 10.7% to 11.5% in this case. (Ignoring the impact of the reduced mass of the shorter A frame for simplicity’s sake). (By contrast we are familiar that moving an axle (positioned close to the C of G) a matter of inches generates a significant change in tongue weight). I have read that the longer A frame of a narrower angle version gives greater dynamic yaw stability. Does this stability benefit outweigh the negative impact of the associated slight relative deterioration in hitch weight?
Autorama-7
July 25, 2019 @ 8:38 PM
Interesting question. Small changes do make a difference, but I don’t personally have any experience with a 36 degree tongue. I doubt the angle has as much to do with it as the length. Just a guess.
July 28, 2019 @ 4:54 PM
Yes I believe it is simply the consequential extended length of the narrower angle A frame that is claimed to give the alleged benefit to yaw stability, assuming the width of the trailer main frame rails is maintained. I don’t think there is any magic claimed in the angle used per se.
September 5, 2019 @ 3:03 PM
Great article on axle loading. Thank you for very clear information.
I understand what the end result needs to be in these calculations, but i am curious about the relationship of major weight items loaded on the trailer in relation to their distance from the axle.
For this conversation lets assume a 30′ trailer with large permanently fixed water tanks that contain exactly the same amount of water.
I see to extreme possibilities that could result in exactly the same tongue weight and i am curious if this matters.
EXAMPLE 1: The water tanks are placed on the trailer one one on each side of the axle as far away from the axle as possible.
EXAMPLE 2: The water tanks are place on the trailer one on each side of the axle as close to the axle as possible.
If the tongue weight is exactly the same in both examples, how will the trailer handle in both examples.
Thanks,
Gary Ford
Texas
September 9, 2019 @ 5:46 AM
Some of your answer is in this article about tongue length. In straight line action handling won’t be much different, but in dynamics, Example 2 will “feel” heavier because of the inertia moving the load in teeter-totter, and turning. Example 2 will be more pleasant to pull.
September 18, 2019 @ 6:54 AM
Thank you so much for these formulas. I work for a company that builds trailers. They did the lazy man way and used the 6″ to the rear from center as their go to setting. I will be designing trailers more accurately and safely now using your method. Cheers
November 7, 2019 @ 5:23 PM
I have been lead to believe that the axle should be set back on the left side about a half inch from the right side to make turning better. What’s your thoughts on this.
November 7, 2019 @ 10:18 PM
Do you race NASCAR? I think it would work great if you always turn Left. Otherwise, the trailer will run behind you like a dog. If you’ve ever watched a dog run, their back end is off to one side from the front. You don’t want that with your trailer because it adds to instability.
October 18, 2019 @ 2:20 PM
Axle placement for tandem axles on a gooseneck or fifth wheel trailer
October 19, 2019 @ 8:33 AM
For tandem axles, use the center of the axle group with these same equations. Likewise, the process is the same for 5th wheel and gooseneck trailers except that the target tongue weight is higher — like 20% or 25% depending on your tow vehicle, trailer, and load.
November 19, 2019 @ 8:05 PM
Thanks for the article. I’m currently building a twin axle utility trailer 10′ by 5′. The tongue length 7.5′, I don’t know the frame weight is yet, but I estimate my maximum load (WD) is going to be around the 4,500lb. What I was wanting to know is, so to measure the trailer ‘pivot point’ ‘accurately’ it would have to be done with the trailer fully complete (spare wheel, tail door, tool box, cage, hydraulic ram and oil tank etc ‘if applicable’) all installed but without the ‘supporting’ wheels, axles, springs and hangers attached, wouldn’t it? Otherwise it’ll badly influence the ‘pivot point’s’ measurement doing the final sums at the end, thus null and voiding everything we stand for, right? For example, if you mounted the wheels as far to the rear of the tray/deck as possible, that would surely give a much different pivot point to, if you were to mount the wheels as far forward of the tray as possible, right? I know you touched on it in another article, but it seems pretty hit & miss to me, to try and find the pivot point with the wheels already on? Thanks.
November 20, 2019 @ 4:42 AM
Calculations prior to completing a trailer are a little nebulous (“hit & miss”?) if you really don’t know the weights, loads or positions. The beauty of calculating — you can try many scenarios to see the effect before building. Then, you can make an educated judgement for the actual build.