What is the best beam design for my aluminum tandem axle trailer?

• IdBdan
In summary, Dan is testing beam sizes with online calculators and needs some help. He's using a "section properties" calculator to find the Moment of inertia. Dan is using a "cantilevered beam" span calculator for the span distance from back axle to the rear. And a "simple span" for the distance of front axle to trailer tongue. His beam span calculators require Length (L) - Youngs modulus (E) - and Moment of inertia (I). His section properties calculator is giving him Moment of inertia in (Ix) and (Iy) and (Iz). Dan is using the (Ix) number in the beam calc box for (I). His questions are: 1. Is

IdBdan

TL;DR Summary
Building a trailer. Beam calc's ask for (I) - section calc's give me (Ix) & (Iy)???? Which do I use?
I'm building an aluminum tandem axle (7k total capacity) trailer. Boat (18'/1300#) on the rear and a Ryker Can Am 3 wheel (750#) on the front. It's a 23'-6" x 6'-8" bed. Max live load will be approx 2250#. 900# frame weight. 230# axles.

I'm testing beam sizes with online calculators and need some help.

I'm using a "section properties" calculator to find the Moment of inertia. https://calcresource.com/cross-section-channel.html
I'm using a "cantilevered beam" span calculator for the span distance from back axle to the rear. https://calcresource.com/statics-cantilever-beam.html
And a "simple span" for the distance of front axle to trailer tongue. https://calcresource.com/statics-simple-beam.html

My beam span calculators require Length (L) - Youngs modulus (E) - and Moment of inertia (I). My section properties calculator is giving me Moment of inertia in (Ix) and (Iy) and (Iz). I'm using the (Ix) number in the beam calc box for (I).

My questions: 1. Is using (Ix) correct? Or do I add Ix and Iy together? 2. Do these calculators typically use member self weight in their calc's?

I'm mainly looking for the deflection criteria. I'm trying to keep that under 0.15" (with load). The boats CG is over the axle center so tail bounce should not be a problem. Torsional movement (Iz) is almost non-existent with the cross bracing member size, spacing, and diagonal bracing I've used. The axle system is sliding/adjustable. So I'll be able to tune the tongue weight close to perfection by adjusting the axle location. That should greatly reduce any major lateral instability while towing.

My current choice is an Aluminum Association Channel 8x3.75 - 0.41tf/0.25tw.
Cantilever = 106 in - (69MPa) - MoI 52.04 in^4 - 40# uniform load = (0.101 deflection @ 116")
Simple Span = 144 in - (69MPa) - MoI 52.04 in^4 - 35# uniform load = (0.031 deflection @ 82")

The 32" between axles was ignored. Channel bottom flange is being reinforced with a welded 3/8"x3" sliding axle plate for the center 86" of the total bed length. Both spans are getting that extra moment support and it's not incl in calc. Adds an additional safety factor. The tongue is an A frame that goes under the side channel adding support for another 20% of the front span.

If anyone thinks I'm over designed please let me know why. Aluminum prices are crazy high right now.

Thanks
Dan

Last edited:

I'm curious why no one can answer my question? Did I do something wrong here?

We’ll, it’s a bit to take in, but personally I didn’t see it until today. 3D loading on a system of beams is a trickier to deal with computationally by hand. There are also dynamic loads also to consider. I’ll see if I can come up with a reasonable approach.

Ignore the torque on the beams from the eccentric loading and the tie down rigging to get a first order approximation of the loading.

From your diagrams this looks to me like:

##R'_t## is the reaction at the tongue divided by 2
##M_t## is the moment from the reaction at the tongue applied to the beam ##M_t = 2 R'_t d##, where ##d## is the perpendicular distance between the tongue and the frame.
##R_a## is the reaction at the axle (solve for ##R_a## in the most forward position)
##P## the weight of the ATV.
##w## is the distributed load from the boat (load per unit length), (with gear etc...)
##W_b## the weight of the beam
##W_c## weight of the crossmembersBy the way, these are all "half loads". (except the weight of the beam ##W_b## and the moment ##M_t##)

You need to solve for the reactions ##R'_t, M_t,R_a## using ##\uparrow \sum F = 0## and ##\circlearrowright \sum M = 0##.

Last edited:
Will you ever tow your boat without the 3-wheeled ATV loaded? If so, the trailer looks like it will be fairly unstable...

berkeman said:
Will you ever tow your boat without the 3-wheeled ATV loaded? If so, the trailer looks like it will be fairly unstable...
Oh, that’s a trike…not the motor. Whoops.

berkeman
It does kind of look like an outboard motor the way it is drawn...

erobz
After you get the reactions, the graphical approach is to plot Shear/Moment diagrams, with the goal of determining the moment as a function of spatial coordinate ##x## acting along the length of the beam.

Then, the elastic curve is given by:

$$EI\frac{d^2y}{dx^2} = M(x) \tag{1}$$

The maximum deflection in the beam is found by setting ##\frac{dy}{dx} = 0##, solving for ##x##, and substituting that into ##y(x)##.

*Note* It's best to use singularity functions ( as opposed to a graphical solution ) for this because of the many discontinuities in loading along this beam. If doing it graphically you would need to solve for a different ##M(x)## between each discontinuity (point load), and solve (1) over each domain, using initial conditions from the last domain. It would be tedious us work to say the least.

Last edited:
Obviously concerns about the stability of the trailer should be addressed (I don't have much experience with hauling/trailers and what makes them stable/unstable going down the road), but the methods of analysis for the beam won't change.

erobz said:
Ignore the torque on the beams from the eccentric loading and the tie down rigging to get a first order approximation of the loading.

From your diagrams this looks to me like:

View attachment 324006

##R'_t## is the reaction at the tongue divided by 2
##M_t## is the moment from the reaction at the tongue applied to the beam ##M_t = 2 R'_t d##, where ##d## is the perpendicular distance between the tongue and the frame.
##R_a## is the reaction at the axle (solve for ##R_a## in the most forward position)
##P## the weight of the ATV.
##w## is the distributed load from the boat (load per unit length), (with gear etc...)
##W_b## the weight of the beam
##W_c## weight of the crossmembersBy the way, these are all "half loads". (except the weight of the beam ##W_b## and the moment ##M_t##)

You need to solve for the reactions ##R'_t, M_t,R_a## using ##\uparrow \sum F = 0## and ##\circlearrowright \sum M = 0##.
Thank you very much. I apologize that you went to that much effort. I took a shot at completing that calc but it's beyond my abilities. I've called a few local engineers to see if I could pay them to size the beam for me. No seal, just a verbal recommendation with no liability. They were hesitant and I understand why.

Can you tell me if using LX is correct in my question?

And is locating my 2" oc bolt pattern in line with the beam centroid a problem. (see drawing) Would it be better to offset from the centroid?

With personal projects in the past I've just checked span capacities with a uniform loading and added a CYA size up to obtain minimum deflection. I know that's not the best approach but all I have right now. The front span doesn't worry me. It will have an aluminum deck that will lateraly tie all the framing in the first 7'.

It's the tail end cantilever that has my concern. In "half load" the beam will have to carry 40#/LF over 9.25 LF. Cross members only add 5# every 3'. My axle system is sliding. If need be I can slide the axle farther to the rear to compensate.

I don't think moment is going to be an issue when the trailer is loaded. With boat trailers the boat tie downs turn the boat hull into a frame stiffener. And the boat tie down at the bow actually creates a counterweight using the boat as a beam. It's not a signifigant addition but it reduces tail bounce. Moment when it's empty is what I looked at and it's more than acceptable.

I have a 3/8"x3"x 86" axle plate making my bottom flange thickness 7/8" ttl and it extends 46" fore and aft of the axle location. I'm hoping that and the web stiffeners I've added will give it additional strength.

Thanks again! Greatly appreciate the help.

Attachments

• TRAILER AXLE.pdf
34.5 KB · Views: 99
berkeman said:
Will you ever tow your boat without the 3-wheeled ATV loaded? If so, the trailer looks like it will be fairly unstable...
That was the toughest part of the design. Running multiple scenarios of loading and finding the the right match for a 10 to 15% tongue weight. https://www.engineersedge.com/calculators/trailer-weight-balance.htm

I designed this trailer for my motor home. I want my boats when I travel and I need transportation when I park the motor home.

When the flats boat comes off the trike is loaded from the rear and moves to where the boat is. The bunks for the boat will match the trike wheel base.

When the trike is removed the winch post for the boat slides forward up to 26". 21" is what I calc'd for a 12.5% tongue weight. I'm using airline track with 1" incremental slots.

In odd conditions or different loads my axles can slide 22" forward or backwards from center position in 2" increments. I did that so I can leave the salt water flats boat home on cross country trips. I'll carry my inflatable skiff and the trike. That load requires moving the axle location.

berkeman said:
It does kind of look like an outboard motor the way it is drawn...
It does now that I look at it. lol

It's a Can Am. Conventional trikes steer one wheel. The Ryker puts the two wheels up front for steering.

IdBdan said:
Can you tell me if using LX is correct in my question?
If the beam is loaded like:

You are considering bending moments about axis ##{x-x}##, thus ##I_{xx}## is the appropriate moment of inertia to use.

IdBdan said:
With personal projects in the past I've just checked span capacities with a uniform loading and added a CYA size up to obtain minimum deflection. I know that's not the best approach but all I have right now. The front span doesn't worry me. It will have an aluminum deck that will lateraly tie all the framing in the first 7'.
Applying a "CYA" a.k.a. "Factor of Safety" is indeed good practice, but first we try to get the load approximated as accurately as possible. Sometimes it takes a little effort.
IdBdan said:
It's the tail end cantilever that has my concern. In "half load" the beam will have to carry 40#/LF over 9.25 LF. Cross members only add 5# every 3'. My axle system is sliding. If need be I can slide the axle farther to the rear to compensate.
So is the weight of your boat 740 lbf?
What is the weight of the Trike?

You state the frame is 900 lbf.

You say the "live load" is 2500 lbf. is that just the total weight of everything?

I'm thinking the weight of the boat is supported by the last 4 cross members, 4 point loads not necessarily evenly distributed. Presumably there is an outboard motor hanging off the back, hence the last support is bearing more load. Just ignoring the small weight of the cross members:

If you can just help me reason out the loadings ( the ##T## from the trike, and the ##F_b##'s from the boat), by providing reasonable weights/distributions/dimensions from the leading edge of the beam, this isn't a terrible computation. You can check it against whatever you had planned.

Thanks for taking your time to do this! I've attached a PDF for a better look at the loading.

My objective was to use rectangular tube. Either 2x5x.25 or 2x6x.25 (6061 T6). The 8" channel was what fit the load numbers I came up with while 'fumbling' my way through this design. It's not a good match for the welded joints at mid span of the web, but it met the load.

Tube gave me a stronger and cleaner weld condition. And it simplified my sliding axle design. I was also using the tube frame as a water reservoir for my brake wash down system. Salt water eats brakes.

The trike loading 'T' is mostly concentrated in the front center of the trike body. So I would say each front wheel is just under a half load. About 285# ea wheel. They are sitting approx 5" in from the beam edge and directly over cross members. The rear wheel isn't carrying any more than 75# and sits directly over a cross member. The area the trike sits on is an extruded aluminum decking that is perpendicular to the cross members. It's clipped to the cross members at the butting edges. And it gets a 1/4" bolt mid span of the cross members in the butt joint of the vertical leg. I'm assuming it's adding to weight distribution with it's lateral attachment?

The rear Fb would be 240# uniform half load with a 120# point half load for the motor?

Then forward of your "Ra" the boat hull would be a 345# uniform half load up to the trike location. I'm not sure how the curve of the hull (not on the frame) works in loading? I'm thinking the area forward of Ra to the point where the hull leaves the frame would carry the full forward load? Because that load falls over the axles I was ignoring any drastic effects in deflection. The 'V' forward bunks don't carry any weight. Just there to align the hull to the winch.

Thanks again! I owe you one.

Attachments

• TRAILER LOADING.pdf
182.3 KB · Views: 84
How much does the boat weigh without the motor? How much does the motor weigh? You've said 1300 lbf (I assume boat plus motor) in the OP, but in the diagram show 720 lbf (including motor)? Don't cut any loads in half, I'll do all that and figure out the distributions. There is a lot of numbers to juggle and It's getting confusing on who is doing what to the loads.

I think you also need to consider dynamic loading caused by acceleration, braking, hitting kerbs at speed and cornering.

Nik_2213, Lnewqban and erobz
What sort of damping on those wheels ? As drawn, look like classic 'Indesuspension' with inner and outer square sections set ~45º off. Spacing looks bigger than 'close coupled' so, IIRC, you may need to go up a size / load rating. You must allow for considerable impact loads from potholes, a lot of them torsional.

I've seen a big trailer 'come apart' when A-tongue's welds failed. IIRC, trailer had been sorta 'dancing', rocking from side to side, so may assume metal fatigue. Fortunately, the 'breakaway' cable held, so trailer did not escape, and sturdy 'jockey wheel' kept frame from digging into road...

FWIW, may I suggest provision for two (2) spare trailer wheels plus a pair for your trike-ish runabout ??

Tom.G
erobz said:
How much does the boat weigh without the motor? How much does the motor weigh? You've said 1300 lbf (I assume boat plus motor) in the OP, but in the diagram show 720 lbf (including motor)? Don't cut any loads in half, I'll do all that and figure out the distributions. There is a lot of numbers to juggle and It's getting confusing on who is doing what to the loads.
Sorry for the confusion. You had mentioned half load in a prior post and I thought you were treating the frame 'per side'.

The (down arrows) 720 / 690 / 650 figures are the weights and locations I used to calculate the axle location and the tongue weight. I thought they would apply.

I've made some changes since my OP to the Ryker platform material. I changed to an aluminum decking and added a battery to the bow/front of the boat. So my max weight has changed slightly.

Boat
Dry hull weight = 740#
Motor weight = 239#
Accy's / batteries/ fuel/ etc = 411#
Total = 1390#

Front Platform
Ryker = 650#
Aluminum Deck = 110#
Total = 760#

TTL = 2150

Cross members = 2x4x.25 rectangular tube - 6061 T6 round corner (3.3# / LF)

Frame weight estimated total remains the same 900#.

Thanks again for all this help.

Nik_2213 said:
What sort of damping on those wheels ? As drawn, look like classic 'Indesuspension' with inner and outer square sections set ~45º off. Spacing looks bigger than 'close coupled' so, IIRC, you may need to go up a size / load rating. You must allow for considerable impact loads from potholes, a lot of them torsional.

I've seen a big trailer 'come apart' when A-tongue's welds failed. IIRC, trailer had been sorta 'dancing', rocking from side to side, so may assume metal fatigue. Fortunately, the 'breakaway' cable held, so trailer did not escape, and sturdy 'jockey wheel' kept frame from digging into road...

FWIW, may I suggest provision for two (2) spare trailer wheels plus a pair for your trike-ish runabout ??
I'm using tandem 3.5k torsion axles at 22.5 degree down. They are mounted directly to the frame and better than leaf springs when it comes to reducing flex and stiffening the frame. They eliminate bounce typically caused by standard leaf spring axles. They are also independent of each other and ensure one wheel is always on the ground as the other is lifted from extreme bumps. And the tire reaction to a bump is not directly vertical. It's rotational and to the rear.

Trailers fail because people think they can load them any way they want. Tongue weight to load weight is essential if you want it to track properly at variable speeds.

One spare tire mounts to the tongue on an operable axle hub. It's 5 bolted. If there's a break away, like you said, in lieu of relying on the jack stand wheel the spare hits the ground. The main axles also have disc brakes that automatically apply with a break away.

I couldn't change a tire on the Ryker if I wanted to. It takes special tools and torque spec's to change a front tire. And the rear tire is a direct drive system. It has an automatic transmission. Bad tires on that bike are a tow truck call.

tech99 said:
I think you also need to consider dynamic loading caused by acceleration, braking, hitting kerbs at speed and cornering.

It would not be a bad thing to consider regulatory safety in this case. If this is in the United States, the federal Department of Transportation has some things to say about trailer design, materials of construction, braking requirements, hitch designs, tow vehicle capability, and signal needs.

I expect states to have similar concerns. If this will be a licensed/tagged vehicle for regular use, you may want to talk to an insurance professional about whether or not it (or its contents) will be insurable without professional design and approved capacity rating.

The manufacturing of trailers has a great deal of liability associated with it--People crash. They also overload things.

If you have an original design, you are taking on both the design and manufacturing liability. Designing, and, more importantly, properly welding aluminum trailers so that they don't fail, fatigue, etc. is worth considering.

tech99, berkeman, erobz and 1 other person
I have no expertise in structural engineering and or trailer design and the dynamic loads ( @tech99 post #17 points out) they might experience under normal operation, and this is not a validation of the overall trailer design in any way. This kind of structure and 3d loading is much better suited for FEA software. You know how to draft, so perhaps just get a license and learn to the basics of that software would be better comprehensive analysis. As others have said, you'll have to ensure your design meets regulations (federal and/or state) and whether or not the trailor and the load can be insured given self design/manufacture. See @ChemAir response #21.

That being said, here is what I get:

#Note#: The beam is actually statically indeterminant due to the dual axle system. That would require more computational analysis that I'd like to go through. I've combined the tandem axle reactions into a single reaction at the central location between axles.

Here are the parameters I used: I gave a dynamic load factor of 120%

Here are the resulting Shear/Moment/Deflection Diagrams:

Here are the equations that produced them:

\begin{align} V(x) = R_t - \lambda x - P_{atv}\left< x-a \right>^0 - P_{atv}\left< x-b\right>^0 + \tag*{} \\ R_a \left< x-d\right>^0 - w \left< x-e\right>^1 -\frac{P_{motor}}{2} \left< x-f\right>^0 \tag{shear} \end{align}

\begin{align}M(x) =M+ R_tx - \frac{1}{2} \lambda x^2 - P_{atv}\left< x-a \right>^1 - P_{atv}\left< x-b\right>^1 + \tag*{} \\ R_a \left< x-d\right>^1 - \frac{1}{2}w \left< x-e\right>^2 \tag{Moment} \end{align}

\begin{align} y(x) = \frac{1}{EI} \left( \frac{1}{2}Mx^2 + \frac{1}{6}R_tx^3 - \frac{1}{24} \lambda x^4 - \frac{1}{6}P_{atv}\left< x-a \right>^3 - \frac{1}{6}P_{atv}\left< x-b\right>^3 +\tag*{} \right. \\ \left. \frac{1}{6}R_a \left< x-d\right>^3 - \frac{1}{24}w \left< x-e\right>^4 + C_1 x \right) \tag{Deflection} \end{align}

The deflection was solved with initial conditions ##x=0,y=0##, ##x=d, y=0##. Which yields for ##C_1##

$$C_1 = \frac{1}{d} \frac{P_{atv}}{6} \left( (d-a)^3 + (d-b)^3 \right) + \frac{1}{24}\lambda d^3-\frac{1}{6}R_td^2-\frac{1}{2}Md$$

Here are the reaction equations:

Last edited:
Thanks! Can't tell you how much I appreciate your time on this. And I agree that anything you're helping me with is totally and unquestionably hypothetical.

One last question. In your 'parameters', is the M.o.i. (46.13) what I need to exceed or is that the M.o.i of the member you tested? The reason I ask is because your deflection graph show's -1.4" at the tail end? Wouldn't that be considered excessive? I've been trying to stay under 0.25". Am I being to strict with that requirement?

Thanks again.

IdBdan said:
One last question.
No need for it to be the last question.
IdBdan said:
In your 'parameters', is the M.o.i. (46.13) what I need to exceed or is that the M.o.i of the member you tested?
The M.o.I is for the C channel that you specified( 8x 3.25x.41x.25). I just rough calculated it though, if you have an exact value from the manufacturer, we can use that. I didn't notice that you had given 52 in^4 in the OP, is that the manufacturer spec?
IdBdan said:
The reason I ask is because your deflection graph show's -1.4" at the tail end? Wouldn't that be considered excessive?
It's hard to tell. The normal stresses in the beam are not very high ( S.F. ##\approx## 5 on material yield stress ignoring forces from the second axle, and combined torsional loading effects present). The stresses are what determine the cycles until fatigue failure for the beam...so having the average stress low is a good start.

As for the deflection, I would try to tame it by boxing the beam with some flat plate in a good location. My gut is telling me about a few feet on either side of the axles would help ( if you could imagine taking your finger and pushing the curve down toward zero close to the 150 inch range it seems like you could do some damage to the tail deflection issue), but I haven't computed anything (its tedious business to compute with the moment of inertial changing along the beam).

It's likely that the assembled frame is stiffer than just the beams. How much...going to have to consult FEA on that. The issue with not trying to stiffen the beams is if it is deflecting under that load, then whatever is tied to it is going to act like a stiffener i.e. your boat in this case. So, while it may not deflect as much, that trades off with the stress of the load being carried partially by the boat. I don't know if that is a good trade?

## \text{M.o.I} = 50 ~\rm{in^4}##

"The M.o.I is for the C channel that you specified( 8x 3.25x.41x.25)." The channel is a 3.75 flange not 3.25.

Supplier sent standard spec's on 6061 T6 Architectural Structural. I also used this link for 6061 T6 spec's. ASM Spec's

I used a online calculator online for MoI. (I think I'll buy the basic version of that calculation software. I emailed them with questions because it has limited database & use for Aluminum members)

The calculators I used for deflection and moment did not have the option for combined loads. I had to do it in 3 sections. Front, Middle, Rear. Then I took a WAG on the combination. I'm calling it my "Frankenstein Design Theory". lol Came up with approx 0.42" deflection.

I kept in mind the ^ force between the axles (32" span). I can't box the beam like you suggested but I have an idea shown below. I need access to the axle bolts.

An option, in lieu of boxing, I added a 3/8" x 3" plate to the top and bottom flanges (inside the channel) for 86" (centered on the 32" axle span). That means the flange is stiffened for 27" past the axle/beam connection. Weak axis stiffening ... but it's all I had.

After adding the (2) 3/8" PL's I did an MoI on a 8x3.75 .785/.25 Channel. That gave me an 83 Ixx / 10.8 Iyy / 93.8 Izz. Taking that into consideration ... I "think" the ^ force & moment at the axles and the rear cantilever are going to be ok?

I'd really rather box that channel like you said and leave the 3/8" plate off the top flange. It gets in the way for welding the top of the cross member. I'm considering a truss type 1/4" plate to box it with the 3/8" axle PL on the bottom flange for the axle bolts. That truss configuration will give me wrench access to the bolts. It's such a bastard design that I can't put it into an online calculator.

As a guess??, your thoughts on the truss & bottom flange PL combo being too weak to actually add substantial stiffening?

ChemAir said:
It would not be a bad thing to consider regulatory safety in this case. If this is in the United States, the federal Department of Transportation has some things to say about trailer design, materials of construction, braking requirements, hitch designs, tow vehicle capability, and signal needs.

I expect states to have similar concerns. If this will be a licensed/tagged vehicle for regular use, you may want to talk to an insurance professional about whether or not it (or its contents) will be insurable without professional design and approved capacity rating.

The manufacturing of trailers has a great deal of liability associated with it--People crash. They also overload things.

If you have an original design, you are taking on both the design and manufacturing liability. Designing, and, more importantly, properly welding aluminum trailers so that they don't fail, fatigue, etc. is worth considering.
The only reason I'm building this myself is because it has been impossible to find a manufacturer to build it. Tried for 3 months. Searched the entire east coast from New jersey to Florida. Even the custom trailer builders won't build it because they have so much standard work they don't need it. The big manufacturers all have a standard line of trailers and they don't do custom. And if they will do it, it's a 6 month wait. Right now torsion axles are a 24 week wait. There's one manufacturer that will give you 6 weeks but at twice the cost. And reason #2 - Aluminum builders are scarce.

The other thing I found is that most don't even have an engineer on staff? Their working off 30 year old designs in some cases. And when they fit bunks for boat trailers they use the 60/40 rule for axle location & tongue weight ... and that's it. The most important part of trailer safety is the tongue weight ratio!

Any trailer under 26000# is virtually unregulated. They don't fall under the Title 49 Code of Federal Regulation, Parts 500-599. They have a handbook with 100's of pages and 2 paragraphs on trailers. If you call, you're told to contact the National Association of Trailer Manufacturers and buy a membership for help. A non-Government entity.

And all the important parts of a trailer (axles, brakes, hoses, lights, receiver, etc.) are the part manufacturers safety responsibility. The only responsibility for safety in a trailer build is engineering and the fabrication processes.

So in reality, I believe I'm doing a better job of 'regulatory safety' than they are.

erobz
IdBdan said:
"The M.o.I is for the C channel that you specified( 8x 3.25x.41x.25)." The channel is a 3.75 flange not 3.25.
Whoops...easy fix. It looks like using ##I_{xx} = 52.7 \rm{in^4}##:

IdBdan said:
The calculators I used for deflection and moment did not have the option for combined loads. I had to do it in 3 sections. Front, Middle, Rear. Then I took a WAG on the combination. I'm calling it my "Frankenstein Design Theory". lol Came up with approx 0.42" deflection.
Why that doesn't work is because the beam can have discontinuous shear and moment, it cannot have discontinuous slope and deflection. The online calculators you will likely encounter have built in constraints on slope and deflection for a particular set of loading/end conditions.

For instance a simply supported beam, there is not a constraint on slope at the supports, there is a constraint on the deflection ( i.e. at the supports y= 0 ). For a cantilever beam there is a slope constraint and a deflection constraint at the fixed end.

All hope is not lost though. You don't have to solve the equations as I did. Because of the linearity of the governing differential equation, the principle of superposition can be applied(partial solutions can be added to get the entire loading solution).

You analyze the entire beam under each load (by itself).

https://awc.org/wp-content/uploads/2021/12/AWC-DA6-BeamFormulas-0710.pdf

In the linked PDF figures 19,20, and 21 describe your loads ( figure 18 as well if you include the weight of the beam as a uniform distributed load )

So in order to get the free end deflection of a combined loading you add all the ##\Delta x_1##'s from each loading.

Again, this is applicable to beams of constant section properties ##E## and ##I##, if you start changing them mid span the equations don't hold.

IdBdan said:
An option, in lieu of boxing, I added a 3/8" x 3" plate to the top and bottom flanges (inside the channel) for 86" (centered on the 32" axle span). That means the flange is stiffened for 27" past the axle/beam connection. Weak axis stiffening ... but it's all I had.

After adding the (2) 3/8" PL's I did an MoI on a 8x3.75 .785/.25 Channel. That gave me an 83 Ixx / 10.8 Iyy / 93.8 Izz. Taking that into consideration ... I "think" the ^ force & moment at the axles and the rear cantilever are going to be ok?
My gut says increasing the M.o.I where the slope of the unreinforced beam is changing the most, has to be a good thing for decreasing that free end deflection. However, you won't be able to use the equations in the linked PDF to analyze the effects of the reinforcement. To do that, you would have to break the beam up in to sections containing/not containing the reinforcement, and get the integration coefficients for each section simultaneously subject to slope/deflection criteria (meaning no sudden changes in slope or deflection along the beam).
IdBdan said:
I'd really rather box that channel like you said and leave the 3/8" plate off the top flange. It gets in the way for welding the top of the cross member. I'm considering a truss type 1/4" plate to box it with the 3/8" axle PL on the bottom flange for the axle bolts. That truss configuration will give me wrench access to the bolts. It's such a bastard design that I can't put it into an online calculator.

As a guess??, your thoughts on the truss & bottom flange PL combo being too weak to actually add substantial stiffening?
View attachment 324264
I can't be precise with that complex geometry, its certainly not going to hurt if that is what you prefer. Would you just be trussing the 86 inches centered over the axles?

EDIT: Just for a ball park, if the plate was solid 0.25", the M.o.I only goes up to about 60 in^4. Paneling that over the whole beam only reduces the deflection by a 0.1 in. That juice isn't worth the squeeze.

You need to add material above and/or below the neutral axis as far as you can. Is there going to be any short side rail on top of the beam?

Last edited:
IdBdan said:
The only reason I'm building this myself is because it has been impossible to find a manufacturer to build it. Tried for 3 months. Searched the entire east coast from New jersey to Florida. Even the custom trailer builders won't build it because they have so much standard work they don't need it. The big manufacturers all have a standard line of trailers and they don't do custom. And if they will do it, it's a 6 month wait. Right now torsion axles are a 24 week wait. There's one manufacturer that will give you 6 weeks but at twice the cost. And reason #2 - Aluminum builders are scarce.

The other thing I found is that most don't even have an engineer on staff? Their working off 30 year old designs in some cases. And when they fit bunks for boat trailers they use the 60/40 rule for axle location & tongue weight ... and that's it. The most important part of trailer safety is the tongue weight ratio!

Any trailer under 26000# is virtually unregulated. They don't fall under the Title 49 Code of Federal Regulation, Parts 500-599. They have a handbook with 100's of pages and 2 paragraphs on trailers. If you call, you're told to contact the National Association of Trailer Manufacturers and buy a membership for help. A non-Government entity.

And all the important parts of a trailer (axles, brakes, hoses, lights, receiver, etc.) are the part manufacturers safety responsibility. The only responsibility for safety in a trailer build is engineering and the fabrication processes.

So in reality, I believe I'm doing a better job of 'regulatory safety' than they are.
I have some friends that put a smoker (converted from an oil tank) on wheels and passed state inspection...so yeah, regulations on "light trailers" are apparently scant (at least in structural design).

Last edited:
erobz said:
All hope is not lost though. You don't have to solve the equations as I did. Because of the linearity of the governing differential equation, the principle of superposition can be applied(partial solutions can be added to get the entire loading solution).

You analyze the entire beam under each load (by itself).

https://awc.org/wp-content/uploads/2021/12/AWC-DA6-BeamFormulas-0710.pdf

In the linked PDF figures 19,20, and 21 describe your loads ( figure 18 as well if you include the weight of the beam as a uniform distributed load )

So in order to get the free end deflection of a combined loading you add all the ##\Delta x_1##'s from each loading.

Again, this is applicable to beams of constant section properties ##E## and ##I##, if you start changing them mid span the equations don't hold.
What a great reference! Thanks. With all the engineering notations defined (that was my pitfall) it's a simple matter of putting "square block in square hole" and do the math. I'll be using that reference a lot.

erobz said:
I can't be precise with that complex geometry, its certainly not going to hurt if that is what you prefer. Would you just be trussing the 86 inches centered over the axles?

EDIT: Just for a ball park, if the plate was solid 0.25", the M.o.I only goes up to about 60 in^4. Paneling that over the whole beam only reduces the deflection by a 0.1 in. That juice isn't worth the squeeze.

You need to add material above and/or below the neutral axis as far as you can. Is there going to be any short side rail on top of the beam?
I've been reviewing my material list and I need 1/2" x 3 1/2" plate for my tongue. And the stock lengths in material will leave me with long drops. I looked at the price and it's cost effective to just use 1/2" everywhere and change out the 3/8" to 1/2" and go 144" from the rear end forward instead of 86" centered.

If I add the 1/2 x 3 1/2 to the top and bottom flanges I get a 91.5 MoI.

I've come to realize I can only get 'close to' with a calc on this build. I worked in the pre-engineered metal building industry for 15 years. This thing is a pre-engineered beam candidate. I'm going to use my "call a friend" life line and see if they'll run a pre-engineered beam calc for me and then I'll just build a beam that fits this quagmire of loading parameters.

If that isn't possible i'm just going to build the trailer with the bottom flange 1/2" PL reinforcer and go from there. Load the boat check the deflection and add mod's as necessary. Start strong and build up. My height limitations have me held at an 8" tall member so I need to work around what I have with the 8 x 3.75 channel as a base.

I'll be flipping the channel with the open side out so I can get to reinforcement mods easier.

If that doesn't work I'll have a useless pair of $1500 beams and ..... well .... let's just say my neighbors kids are going to get a lesson in some very loud cussing. IdBdan said: If I add the 1/2 x 3 1/2 to the top and bottom flanges I get a 91.5 MoI. View attachment 324318 If that doesn't work I'll have a useless pair of$1500 beams and ..... well .... let's just say my neighbors kids are going to get a lesson in some very loud cussing.
If the entire beam had a M.o.I of 90 in^4, the deflection comes to about 0.7 in. Is there some reason you aren't placing the 1/2" plate on the exterior faces ( top and bottom ) of the channel? It's going to get you more bang for your buck in M.o.I. increase.

Also, keep in mind that your frame should be more rigid than that beam once constructed. So the apparent deflection of the beam may not be as big an issue as it currently appears.

Last edited:
erobz said:
If the entire beam had a M.o.I of 90 in^4, the deflection comes to about 0.7 in. Is there some reason you aren't placing the 1/2" plate on the exterior faces ( top and bottom ) of the channel? It's going to get you more bang for your buck in M.o.I. increase.

View attachment 324336
Also, keep in mind that your frame should be more rigid than that beam once constructed. So the apparent deflection of the beam may not be as big an issue as it currently appears.
Another quagmire in this trailer!

I have a max deck height restriction and that 8" member just squeaks by. The Trike has a very low 'break over' angle. (3.5" road clearance) Long story short, every inch up gives me an exponentiation add to ramp length and the need for over 25' of load space to get in on and off the trailer. Trailer + Trike + Ramp Length.

Also, a 27' trailer (@ current height) going down a standard 14 degree boat launch needs 22' of ramp to get the tail height parallel with the ramp. The higher I go with the deck the more distance I need to hit water that will float the boat off. At low tides I'll run out of hard ramp area and chance getting the trailer wheels stuck on the drop off with a 32' motor home attached to it.

I think your correct on the complete frame with cross members being much stiffer than the beam by itself. That's why I'm going to do the build and mod method. When I first started designing the trailer a builder at the coast told me a 2x5x.25 tube would do the job? Just goes to show what the "professionals??" are putting on the road today?

I've added another cross member to shorten the spacing's between them under the boat area. I had cut drops in the 2x4 material anyway. I'm also doubling the tube at the rear and adding a diagonal tube to that for bracing to the beam. The boat transom tie downs connect to the 8"beam at the rear. I want that stiff.

Thanks for all your help! I owe you one. Let me know if you ever need CAD work done.

Last edited by a moderator:
erobz and Lnewqban
IdBdan said:
The only reason I'm building this myself is because it has been impossible to find a manufacturer to build it. Tried for 3 months. Searched the entire east coast from New jersey to Florida. Even the custom trailer builders won't build it because they have so much standard work they don't need it. The big manufacturers all have a standard line of trailers and they don't do custom. And if they will do it, it's a 6 month wait. Right now torsion axles are a 24 week wait. There's one manufacturer that will give you 6 weeks but at twice the cost. And reason #2 - Aluminum builders are scarce.

The other thing I found is that most don't even have an engineer on staff? Their working off 30 year old designs in some cases. And when they fit bunks for boat trailers they use the 60/40 rule for axle location & tongue weight ... and that's it. The most important part of trailer safety is the tongue weight ratio!

Any trailer under 26000# is virtually unregulated. They don't fall under the Title 49 Code of Federal Regulation, Parts 500-599. They have a handbook with 100's of pages and 2 paragraphs on trailers. If you call, you're told to contact the National Association of Trailer Manufacturers and buy a membership for help. A non-Government entity.

And all the important parts of a trailer (axles, brakes, hoses, lights, receiver, etc.) are the part manufacturers safety responsibility. The only responsibility for safety in a trailer build is engineering and the fabrication processes.

So in reality, I believe I'm doing a better job of 'regulatory safety' than they are.

While I commend your enthusiasm, I am a metal fabricator that does build in aluminum, and engineer that, after doing our regular HVAC design, and custom ductwork fabrication, among other things, does operate and repair DOT inspected trailers, so....I think I may have some liability perspective, here. I've actually talked to insurance people about trailer manufacture and repair (and performed repairs). This is a large area for potential litigation. I have that risk when I work on trailers, even though I have welders certified against my AWS and ASMEIX welding procedures for processing carbon steel, aluminum, and a variety of stainless and Nickel alloys.

I know, for instance, that if you attempt to weld an aluminum trailer without knowing what you are doing, you can have fatigue failure problems at the welds. These can be caused by lack of heat treatment, poor welding, bad electrode selection, poor metal selection, and just bad practice. Most aluminum trailers are bolted after forming, and that is because the welding is very difficult. If you would like, talk to a couple of aerospace guys about welding aluminum wings. I expect they will tell you that is a bad idea.

If you build a trailer, and it is properly ID'd and tagged, when there is a deficiency in an accident, the manufacturer WILL be involved in any lawsuit. If you are the manufacturer AND the driver/hauler, you will be sued in spades if you made bad decisions, or at least decisions that aren't best manufacturing practice. Blaming the guy that sold you brakes will not absolve you.

As I said before, I'd suggest talking to your insurance representative and tell them that 1) you will be building a trailer for over the road service, and 2) it will be a welded aluminum trailer. If you deliberately mislead the insurer for short-term approval, you may have trouble 'down the road' so to speak.

If you choose to not take my advice, that's fine. But I have had experience with the legal system in the US, and I think that you are optimistic.

There's enough potential liability that I am uncomfortable telling you how to do it right, because, as a professional with expertise in this area, I could inherit some liability, simply by doing so.

If you want a deep dive into this, I can help you navigate, but this is an area I have had no desire to accept any further liability.

erobz
And with the very good post by @ChemAir it is probably a good time to close the thread. It has covered a lot of good ground, but the liability aspect is something that we cannot assume here at PF. Thanks all for your contributions.

erobz and ChemAir