Gauging frame strength - opinions requested

[mnswamp]

Actually, real knowledge input would be helpful, but opinions are good too:

OK, I'm committed. Motorhome sold, Timbren 2K suspension with brakes is on order, I've read Tony's book cover to cover, watched about a zillion hours of youtube videos, and I've figured out enough tricks in FreeCAD to make me at least dangerous.
It looks like I'll build the frame over Thanksgiving weekend, so getting ready to order steel. Which is where I could use some experienced help:
Thought #1: 2¨x2¨x1/8¨ tubing throughout - 120¨ long rails with 5 cross-bars, and a tri-Y front triangle, welded on receiver tubes front and rear. Advantage: 1/8 inch steel is pretty easy to weld, relatively hard to screw up

Thought #2: same as above with 14 gauge steel on the main frame, and 1/8¨ on the tri-Y, and gusset plates in corners.
Advantage: cheaper, lighter (by about 40 lb, $100)
Disadvantage: 14ga thinner, more flex, easier to burn thru with torch.

How much strength/rigidity would I give up to save $100 and 40lb off the chassis?

Thanks

marsh

 

[mountainminded]

Weighing in with my Opinion here... Since you’ve chosen a 2K axle it would suggest that you are not planning to build a 600# ultralight. Go with the .120 wall tubing— the 40# and $100 can be saved elsewhere if necessary.

 

[John61CT-TNT]

Yes of course all depends on the weight

Not so much just overall, but if anything dense is concentrated in one location

tanks, batteries, propane?

 

[DrewsBrews]

It depends on the intended purpose... Mostly onroad (pavement, dirt, gravel) or banging boulders offroad?

Crossbars that are just offering floor support definately don't need to be tube. Angle should do fine and be half the weight of same size square tube.

Many utility trailers are built out of angle iron mostly, with tube realy only used where it is most needed and convenient. Remember, you will be building a self supporting structure on top of it too. You might be surprised how flimsy commercially built RV trailer frames can be. They rely on the rest of the structure to stiffen it all up.

 

[mnswamp]

Great questions/comments
1. Intent is to put ~15gal water tank between the wheels (tucked up into the frame and secured. I have not decided on propane location - it would be preferable to have that up front on the tongue. Batteries will also be located central - over the axle in a bay behind the cooler and kitchen.
2. I've yet to design anything that I didn't manage to over-engineer... Goal #1 was to build a trailer frame that did not rely on the trailer body for stiffness. The latter seems like a recipe for open seams and leaks (IMHO). Goal #2 would be removing unnecessary Weight - definitely a secondary concern. I'm shooting for an empty weight somewhere in the 900-1100lb area, with 600lb cargo capacity, including 2 mountain bikes on the back.
3. Gravel road to dispersed BLM/FS is as far as I plan to go. My tow vehicle doesn't even have all-season tires on it, definitely not going bouldering!

 

[saltydawg-TNT]

Okay i preach this a alot of face book group. If you can weld use 2x3 085 for everything if your doing a 3 part tongue. Ie a center and 2 wings.

The 2x3 085 is lighter and stronger, even 1x3 085 flexes less than 2x2 120.. I have the load calcs if you want to see em. well screw it here they are. A 5 x 8 or 10 made from 2x3 085 with a center tongue of 2x3 120 center tongue with the wings made of 2x3 085 will carry 3k with no issue, and the tongue would be fine for 4500 braked. If your going to be under 2k make the whole tongue from 2x3 085, and the cross supports from 1x3 085.

the tongue is what sees the most bending force, take it back at least 2 cross member if not 3.

edit this weight/calc is based on a 1000 lbs loaded body on 100 lbs of frame, so not including wheels/axle or tongue. also mild steel like used in rect tube has a yield strength of 30-50k psi

 

[mnswamp]

Scott,
Thank you for a totally contrarian viewpoint. i see it. I hadn't considered going this way, but the additional stiffness and lower weight is spot on. A couple of questions for you.
1. How do you make a smooth transition from the 2x3 to a 2.5^2 weld-on receiver?
2. I am planning on a 5 foot tongue - 24¨ box on the front, + 3 feet in front of that to the ball. I assume the wings should always come in pretty much immediately behind the hitch?

Thanks for the idea.

marsh

 

[saltydawg-TNT]

Scott,
Thank you for a totally contrarian viewpoint. i see it. I hadn't considered going this way, but the additional stiffness and lower weight is spot on. A couple of questions for you.
1. How do you make a smooth transition from the 2x3 to a 2.5^2 weld-on receiver?
2. I am planning on a 5 foot tongue - 24¨ box on the front, + 3 feet in front of that to the ball. I assume the wings should always come in pretty much immediately behind the hitch?

Thanks for the idea.

marsh

the wings dont need to be behind the hitch, on frames I do it is about 2 feet back. This is my frame

Two ways, one just bolt on a normal receiver or what I did for removable couplings. For the two I did with a removable receiver I cut out the center of the rect tube and welded in the 2 inch id tube. So basically a 1/4 strip top and bottom. Mine is like this and weighs 2300 lbs, no issues.

 

[Tony Latham]

I think Scott, aka Saltydawg is spot on with his .083 rectangular tubing. The reason I didn't push it in the book is that it is harder to weld than .120".

including 2 mountain bikes on the back.

We really didn't like ours back there. It's a lot of weight hanging and no matter what we did there was movement.

They never did cause the trailer to sway but sometimes it felt like it was on the edge.

This over-the-tongue system works much better. You can keep an eye on them and there's not much movement.

Tony

 

[mnswamp]

Arghh... I really hate it when knowledgeable people try and interject reason into my fantasy

My initial thought on putting it on the back was to keep them out of the wind. I use a tray style rack (Yakima Dr Tray or OneUp (https://www.1up-usa.com/product-category/bike-racks/), which should eliminate most of the bouncing, but... It does put them where you can't see them without a camera, and there is the weight question, especially if we eventually follow the herd and get e-bikes at some point (H*ll might freeze over first!).

OK, I'll futz with an adapter idea or 3... The front tongue may have just gotten a few inches longer here!

And for anyone who is interested, here's the start of the build blog. https://swampwerks-teardrop.blogspot.com/

I'll eventually put a build link on this site too.

marsh

 

[saltydawg-TNT]

Scott,

2. I am planning on a 5 foot tongue - 24¨ box on the front, + 3 feet in front of that to the ball.

On thing, if your going that long on the tongue you may want to use the 2x3 120, and I would not go past 2.5 feet past the box. That will allow you to jack knife 95% of vehicles, and take the wings to the front of the box or a little past it. Personally I would take the wings 6 inches past the box, and make the center section 2.5 past the box. You wont have true 90% jack knife, depending on the hitch and vehicle but you will have more than enough.

edit I just ran the calc, a 24 inch section of 2x3 120 with a 150 lb load will have internal psi of 3800 psi, and a flex of .016 inches. That is assuming the wings stop at 24 from the end of the tongue and a tongue weight of 150 lbs. It is not a perfect calc, as the actual weight on just that section wont truely be that much. But it still gives you 8-10 times safety margin at a min.

 

[mnswamp]

@saltydawg - I was planning to order my steel tomorrow and start cutting/cleaning/drilling this week. And then I read the install instructions for my Timbren axle, carefully and again, and Timbren strongly recommends .125 as a minimum... I could probably mickey mouse doubler plates over the attachment, under the attachment, and squish tubes inside the tubes, but at this point I think it's back to 2x2x.125 for the frame. Too bad - I had come around to 2x3x.085. It would make for a light strong frame with few hassles.

@Tony - I've redesigned the tongue stuff to put bikes up front as you've kindly suggested. I'll post a full drawing later in the week showing the bike mounts. Space for 2 MTB, fatties/or even electrics.

~marsh

 

[John61CT-TNT]

Timbren strongly recommends .125 as a minimum

...

Too bad - I had come around to .085.

Stick to 2x3 .085 if you like for the main inside rails and outside framing, i. e. the bulk of it

2x2 or even 2x3 @.125 for the rail that the Timbren bolts to, two rails side by side sitting on the horizontal plate

Adds 4" to the outer width / hub faces width, or to keep that the same you could bring the main rails in

Ideally the "outer main rail" .125 would also be used by the A-frame and strongly tie it back into the suspension.

That said the overall difference in weight going all .125 will likely be well under 100lbs

and the argument for simplicity is even stronger than for overkill strength.

 

[John61CT-TNT]

Sorry doop, SQL error messages

 

[John61CT-TNT]

Sorry doop, SQL error messages

 

[saltydawg-TNT]

Timbren strongly recommends .125 as a minimum... I could probably mickey mouse doubler plates over the attachment, under the attachment, and squish tubes inside the tubes, but at this point I think it's back to 2x2x.125 for the frame.

~marsh

Its easy to add thickness where you want it, the added weight of going thicker and with more flex I dont like. Do the side frame rails out of 2x3 .120 if you the extra thickness, no need to do the whole thing with with it.

But personally I would just weld on some plates where the axles goes.

 

[KCStudly]

If you do decide to do the double (or "scab") plates, consider making them with tapered ends (i.e. diamond shaped). If you just scab on rectangles the edges can form stress risers, concentrating fatigue.

See halfway down this page under the heading "fatigue".
https://www.steelconstruction.info/Desi ... uction#top

 

[mnswamp]

Heh. Talk about 'over-engineering'... It's an interesting technical read, well above my pay grade. What I did glean is that they are talking about kNewtons of force on a bridge deck. I suspect that to build to the same specs, a TD would have to weigh 20T or more!

All kidding aside, it's a very valid point. The best way to avoid stress risers in anything is to avoid sharp points, especially in stressed areas. An arched surface is almost always better, be it on a welded gusset, a bridge or the fillet on a 3D print.

At the end of the day, I will apply the KISS principle with a few extra do-dads. Axles are in, lights are in, steel is on order. Next up, measure, measure, cut, deburr and wash.

BTW, jackstands on Craigslist are the way to go. Typ $20/pair around here for the better ones.

Thanks everyone

 

[KCStudly]

No need to read or understand the engineering, just use the taper outline ratios shown in the picture. The little wedge bevel at the tip of the diamond probably isn't needed. More better than a blunt transition. The theory of flexure absolutely applies to our little trailers, even if not the same magnitude as rail cars.