1/8 2.5x2.5 or 3/16 2x2 for tongue

[Southern Oregon Bob]

I'm building a 4x10 TD that will spend some time off road with a tongue that won't be braced for the last 36-40 inches. Will either of these sizes work and is one stronger than the other? Thanks Bob

[Arne]

Half way down the page:

http://www.erareplicas.com/factory5.htm

[David Grason]

So Arne, if I'm understanding it correctly, the Cobra page is saying that square or round makes no difference if all other things are equal?

[Southern Oregon Bob]

So if I did the math right and what I was looking for was the polar moment of inertia that tells me the 1/8inch 2.5x2.5 is stronger then the 3/16inch 2x2 and the 1/8 2.5x2.5 is twice as strong as the 1/8 2x2. If thats right thanks for the link, if it is not I guess it's over my head and I'll need a simpler explanation. Still learning Bob

[Arne]

I suspect you did the math right. Putting more space between top and bottom (or side to side for that matter) is more important than another 1/16" in material thickness.....

The new beams used in houses are nothing but 2x4's separated with plywood vertical sections, but can span longer distances than solid wood beams with the same external dimensions..

So, it is the top and bottom and separation that is most important.

[angib]

I wrote a spreadsheet section properties calculator for the tongue strength discussion, so here's the results from it for the section modulus of some square tubes.

2" x 2" x 1/8" = 0.511 in3
2" x 2" x 3/16" = 0.701 in3 (+37%)
2.5" x 2.5" x 1/8" = 0.845 in3 (+65%)

I've included the effect of a realistic corner radius (1/4") in all cases. Strength is proportional to section modulus for a given material, if you ignore buckling.

Hope this helps.

Andrew

[purplepickup]

.....just to throw another option into the fray.

This is a picture of my tongue. It's made from 2 pcs of 1x2x1/8 wall welded together. I never figured the weight to strength ratio but I think the extra 1/4" web in the middle gives it a lot of verticle strength.

[Southern Oregon Bob]

Thanks everyone, I came to the forum to help get ideas for my TD and I'm getting that as well as second education. Now if someone could just teach me how to spell Bob

[angib]

OK,

2" x 1" x 1/8" = 0.292 in3
2" x 2" x 1/8" = 0.511 in3
so 2 No. @ 2" x 1" x 1/8" = 0.584 in3 (+ 14%)

Sideways, the double-1x2 has a Z of 0.513 in3 - which shows that extra material on the neutral axis doesn't make it stronger, which is what you would expect.

Now if someone could just teach me how to spell Bob

No need, Bob - you can already spell Bob perfectly.

Andrew

[Southern Oregon Bob]

Thats interesting. How about if you welded four cubes together. Is there anyway you could get a strength to weight improvement. I've already welded my 2.5x2.5 on but I was just curious. Bob

[angib]

Bob, I didn't include weight in the previous info, so here it is:

2" x 2" x 1/8": Z = 0.511 in3 (+ 0%), weight = 3.05 lb/ft (+ 0%)
Twin 2" x 1" x 1/8": Z = 0.584 in3 (+ 14%), weight = 4.40* lb/ft (+ 44%)
2.5" x 2.5" x 1/8": Z = 0.845 in3 (+65%), weight = 3.90 lb/ft (+ 28%)
* excludes the weight of the weld to join the twin tubes.

So you can see how much better the strength-to-weight of your 2-1/2" tube is, compared to the twin 1x2 tubes. Though whether that's important depends on whether you're trying to build light.

I'm working on putting my tube-strength spreadsheet on my web site, so everyone can play to their heart's content.

Andrew

[Nitetimes]

Southern Oregon Bob - With that much tongue unbraced you normal shouldn't have any major concerns with the materials you mention. But, if you are going to be offroad, consider the extra bouncing and jerking around, I personally would go with either 2.5" or 3" X3/16" wall tubing as long as a little extra weight isn't a problem.

[purplepickup]

This is an enlightening thread. I assumed (incorrectly) that the two 1x2 pcs was much stronger and a weight advantage over a larger, thicker wall square tubing. My plan of a 10'L x 5'W x 5.5'H trailer may prove to be heavier than some and I travel long distances over uneven roads so I want as much strength as I can get. It's not to late to change.

I thank you for your calculations Andrew. Access to your spreadsheet would be helpful to some of us "engineering challenged" builders for sure.

[SteveH]

I guess I'm just to dense to understand this arithmatic thing..... My tongue is made of two pieces of 2" X 1/4" angle with a 2" X 1/4" strap welded to the bottom of them to form channels. So, is it strong enough?

[Arne]

Mine is 2 x 2 x 1/4" thick... 2 pieces butt welded together.. it's what I had....

I'm not concerned about it, but will not overload it.. The tear itself is pretty much balanced in the middle with only 75 pounds on the tongue end..

But, I won't be putting a heavy box on it...