Small Travel Trailer Frame Design Question

[jerlands]

I'm attempting my first trailer frame build and am concerned my design has a couple of weak spots but don't see a good solution. The intent is simply to enlarge cargo space with a 4' W, 3' D, 2' H cargo box that can carry camping supplies and the like. The overall length is just over 7' and the frame is 4' wide. It is also designed to carry a medium sized tongue box. The tongue consists of a single member extending the entire length of the trailer and two side rails to stiffen it. My concern is the joint where the side rails join the box frame. I've changed the original design to where i'm mitering a single length of tube to extend from the nose back to form the rail the axle spring will mount to. My thought is to plate the mitered joint on three sides with 1/4" plate (shown in illustration.) I also intend to put 14 gauge sheet as the floor in the trailer and the box will be built on top of it. For the box i'm using 1" X 1" .063 square tubing with the addition of 1" X 2" at corners and for the lid framing. the following images are the work in progress. i'm guessing the weight of the trailer around 450 lbs with a 1000 lb. payload max. any thoughts are appreciated..

[GTS225]

Well, I'm only an "eyeball engineer", so this is all opinion.

If it's an all welded structure, and the gussets and plating aren't bolted on, I wouldn't hesitate to pull it behind my truck. I think your plating could be reduced to .120" and still have the strength needed for the joint, especially as you're pie-cutting that rail, instead of a full angled cut and (essentially) butt welding the ends back together.
Personally, I don't care for the rather short length, as it's going to get critical for weight and balance loading. Get a bit too much aft of the axle, and your tongue weight is going to go away pretty quick. Backing up with that short of a trailer is going to get tricky, too. It'll jack knife on you in a hurry.

Don't scrimp on the lighting. There's too many "drivers" out there today that aren't paying attention to what's going on around them.

Roger

[jerlands]

Well, I'm only an "eyeball engineer", so this is all opinion.

If it's an all welded structure, and the gussets and plating aren't bolted on, I wouldn't hesitate to pull it behind my truck. I think your plating could be reduced to .120" and still have the strength needed for the joint, especially as you're pie-cutting that rail, instead of a full angled cut and (essentially) butt welding the ends back together.
Personally, I don't care for the rather short length, as it's going to get critical for weight and balance loading. Get a bit too much aft of the axle, and your tongue weight is going to go away pretty quick. Backing up with that short of a trailer is going to get tricky, too. It'll jack knife on you in a hurry.

Don't scrimp on the lighting. There's too many "drivers" out there today that aren't paying attention to what's going on around them.

Roger

Thanks for the feedback... the design is really limited by a 1500 lb. vehicle towing capacity. my attempt is to make it as small as possible while affording about 24 cubic feet of cargo area. it'll probably be a little like towing around a small generator or the like...

[RJ Howell]

I do agree the tongue is habit short, but you center bar should extend forward to fit the hitch anyway, so extend 1-2ft. I have a short utility trailer that I have added a center bar to extend, it was a bear to back up!

I am a fan of the tongue assembly being under the box. By design you have a hybrid tongue, but the A portion doesn't support as it should. The A should go back to the second crossbar and be under it so no breaks in either the tongue or the crossbar.

That leads to my third point of what I like and that's the crossbars on the box are solid, not sections.

My 2 cents and worth each penny... Have fun and enjoy it!

[jerlands]

I do agree the tongue is habit short, but you center bar should extend forward to fit the hitch anyway, so extend 1-2ft. I have a short utility trailer that I have added a center bar to extend, it was a bear to back up!

I am a fan of the tongue assembly being under the box. By design you have a hybrid tongue, but the A portion doesn't support as it should. The A should go back to the second crossbar and be under it so no breaks in either the tongue or the crossbar.

That leads to my third point of what I like and that's the crossbars on the box are solid, not sections.

My 2 cents and worth each penny... Have fun and enjoy it!

Tongue length has now become a real concern to me. I know it will affect the trailer stability so i'm going to need to know exactly what proportion it should be. The tongue itself is 4' in length but the distance from the tow vehicle to the center of trailer axle is only going to be about 6'. Right now I'm not seeing how that will adversely affect anything though? Part of my thinking on tongue design came from https://mechanicalelements.com/correct-trailer-tongue-length/ where it's stated:

Many designers make tongues at 30″ to 36″. I personally think that’s too short. When the tongue gets to the 40″ to 46″ length, that seems to be the sweet spot.

My other concern is the length from the center of the trailer axle to the hitch because that would seem to be the fulcrum affecting everything. I'm just not completely envisioning how it will behave.

[Aguyfromohio]

If you plan to weld on a standard hitch coupler for the A frame know that they are built to a standard 50 degree angle.
That will determine how your A frame tongue looks.



The 1/4 inch thick parts with 0.120 wall thickness tubes seems overly heavy for a little trailer like this. Should last forever, but almost half your total weight is committed to the frame and axle. You can probably get some of that weight back out of the frame and use it for payload.

The earlier comment about balance is important. Move the wheels as far back as you can to get some weight on the ball so it's stable. Consider making the whole trailer a bit longer just to allow you to get the the wheels back and the weight forward of the axle for stability.

Stable towing does not come from the shape of the trailer or tongue, it comes from having 15% of total weight on the ball, which in turn comes from placing the wheels far enough back on the trailer. A short tongue causes different problems - hard to control while backing up, and easier to damage the tow vehicle when the rig gets near jackknifed in a tight turn.

[tony.latham]

...the center of trailer axle is only going to be about 6'. Right now I'm not seeing how that will adversely affect anything...

I think it'll be a bugger to back. Perhaps that's something you can live with. (And unless you find a way to add weight in front of the axle, things could get action-packed.)

I too am a fan of tongues being attached under the frame.



Not to be too critical but those gussets are overkill. One in each corner would be fine. If you put the tongue under the frame, you can eliminate them altogether.

But keep cranking!

Tony

[jerlands]

...the center of trailer axle is only going to be about 6'. Right now I'm not seeing how that will adversely affect anything...

Not to be too critical but those gussets are overkill. One in each corner would be fine. If you put the tongue under the frame, you can eliminate them altogether.
But keep cranking!
Tony

My problem is putting the tongue under the frame without enlarging the trailer by 24" or so... I'm trying to make this thing as light as I possibly can within my means...

[jerlands]

If you plan to weld on a standard hitch coupler for the A frame know that they are built to a standard 50 degree angle.
That will determine how your A frame tongue looks.

Yes, I had adopted the 50 degree (a-frame) because it appeared to me to offer more support than say running 45 degree side rails to the tongue to support it or just having a single tongue. I do have the a-frame coupler but now I think i'm going to extend the center tongue rail out at least a foot.

The 1/4 inch thick parts with 0.120 wall thickness tubes seems overly heavy for a little trailer like this. Should last forever, but almost half your total weight is committed to the frame and axle. You can probably get some of that weight back out of the frame and use it for payload.

I originally had .083 planned after having read information from this forum but speaking with others I'm questioning my ability to successfully weld it. I plan to use stick because a sufficient machine is available to me. Two migs are available but they're both light weight 90 amp.. I gained 40 lbs. on the frame overall with the 1/8" tubing and i'm still gnawing away at this...

The earlier comment about balance is important. Move the wheels as far back as you can to get some weight on the ball so it's stable. Consider making the whole trailer a bit longer just to allow you to get the the wheels back and the weight forward of the axle for stability.
Stable towing does not come from the shape of the trailer or tongue, it comes from having 15% of total weight on the ball, which in turn comes from placing the wheels far enough back on the trailer. A short tongue causes different problems - hard to control while backing up, and easier to damage the tow vehicle when the rig gets near jackknifed in a tight turn.

I do have a 50 lb. tongue box and it will probably haul another 50 lbs or so. unloaded the tongue should have in excess of 20% but I may be miscalculating. My plan is to build the trailer with cargo box and then mount the axle. my ability to move it however is limited somewhat by 21" springs. My concern has actually been too much tongue weight but some trailers seem to be designed in excess of 30% tongue weight..

[jerlands]

Two alterations I've made so far in the design. 1.) I increased the tongue length 1.5' beyond where it had originally formed but also shorted the back end so I could have one continuous member across the frame at the center of the cargo box. The shaded area's highlight this. I've also moved the axle back 2" tentatively until I can measure tongue weight. It is however about as far back as it can go due to the fender intruding on the bumper.

[Philip]

Using 2x3 square tubing. Your not going to need all those gussets.

With expected loads under #1500 total weight. The 2x3 tubing is over kill also. For a trailer with that lite of weight load. I would be looking for 1x2" tubing.

The cross member at the front of your frame box. Notch it and slide it forward just a little to cover the weld of the tonque notch on your outer rail. That will provide all the support you need there. No need for the gussets.

Remember those gussets will be a point for road debris to pile up under the trailer. That will be the first place for the frame to rust out.

[jerlands]

Using 2x3 square tubing. Your not going to need all those gussets.

With expected loads under #1500 total weight. The 2x3 tubing is over kill also. For a trailer with that lite of weight load. I would be looking for 1x2" tubing.

The cross member at the front of your frame box. Notch it and slide it forward just a little to cover the weld of the tonque notch on your outer rail. That will provide all the support you need there. No need for the gussets.

Remember those gussets will be a point for road debris to pile up under the trailer. That will be the first place for the frame to rust out.

i hear and understand the potential for building the frame lighter. I'm at around 130 lbs. on just the frame. 40 lbs. difference between .120 and .083 2X3. Going as light as 1X2 would have to be proven to me someway though. I haven't handled a lot of tubing but I will be building the cargo box from .065 1X1 and 1X2 and i'll get a better feel for it. 1X2 for the frame and I see difficulty with the springs but that might just be my imagination. Anyway, I am going to keep the gussets in plans for now anyway. Once the frame is together I'll make my final decision on it. The cross member for where the side rails join the center rail at the tongue was inadvertently left out of that image but I do have it planned as a .125 2" strip across both top and bottom joints...

[Philip]

At the attachment point of the axle on that smaller tubing. Use a piece of angle iron wide enough for the spring mount or torsion box mount. Then use a couple of gussets on the inside of the frame to stiffen it.

[jerlands]

At the attachment point of the axle on that smaller tubing. Use a piece of angle iron wide enough for the spring mount or torsion box mount. Then use a couple of gussets on the inside of the frame to stiffen it.

I'm considering adding a strip of 1/8" at hanger attachment points on the 1/8" 2X3 I'm planning on using now. I've looked briefly at beam stress and deflection calculators but am still boggled by it. But comparing weights... the trailer design currently weights 130 lbs. using .083 2X3 the weight is at 91 lbs. using 1X2 .120 the weight is at 75.4 lbs and .083 1X2 is at around 50 lbs. so about 80 lbs difference between the two extremes. I'd think though I'm more prone to add 15 lbs. and use .083 2X3 over the .120 1X2 but don't really have and engineering insight into the strengths of each individual tube.

[drhill]

The moment of inertia of those HSS sections will give you an idea of the relative strength and stiffness.
1x2x0.083 wall the moment of inertia is 0.2379 in^4
1x2x0.120 wall it is 0.3214 in^4
2x3x0.83 wall it is 1.021 in^4
So the 2x3 is more than 3 times the strength and stiffness of the 0.120 wall 1x2

It is your trailer for your use, but if you are going to the trouble of building, it costs little more to make it a couple feet longer. Even when I was tent camping I kept finding the list of gear growing all the time. So what you think might be big enough, may be too small after a few trips. Also - you might want to rent or borrow the smallest trailer you can find for an hour or two and find out how painful it can be to have to back up a short trailer. Longer is so much easier.

And just in case you wanted to know.
2x2x0.083 has a moment of inertia of 0.3905 in^4 and
2x2x0.120 wall it is 0.5337 in^4

So the 2x3x.083 would be lighter than 2x2x.120 wall but almost twice as strong in the vertical direction.