Tongue Strength Measure

[Roly Nelson]

Andrew, I don't understand all of the structural calcs, and I am wondering if I have violated some tongue-drilling rule by drilling a hole for the chain support vertically about 4 inches away from the hitch, as well as a vertical hole where the innner tube slides within the outer tube right in the front of the tear. (the hitch assembly is connected to the end of the tongue with holes drilled from the side and vertically as well).

Suppose we were to drill holes in the center of two identical 3 foot long square tube sections of tongue stock, one drilled vertically and the other horizontally, with welded connections at each end holding them parallel to each other. Then insert a hydrolic jack between them and start jacking them apart. I find it hard to beleive that the section with the vertical hole would bend before the other. I am but an old commercial construction superintendent, somewhat aware of the shear moment of the sides and the compression and tension created on the upper and lower sections of the square tube. Wouldn't horizontal holes violate the strength of both of the sides, more than holes in the upper and lower sections, one in tension and the other in compression?

You may be right, but it makes me want to do an actual field test to see what happens. Structural engineering is not my strong point, and I can only look to Dave for pointing out this potential problem at the Spamboree. Thanks Dave............

Roly ~~ all of this thinking makes my head hurt!

[angib]

Heck, Roly - a man who knows what shear is!

I'll start by saying that I am assuming that the important load on a trailer tongue is vertical bending and that most other loads are not significant. By vertical bending, I mean what you get if you could clamp the trailer to a big block of concrete and then move the coupler up and down. I've never seen a failure or cracking of a trailer tongue that didn't look like it was caused by vertical bending, so that's all I've looked at for strength.

I can't see how your hole for the safety chain can be any problem - there's precious little load there, so the hole doesn't matter.

A vertical hole to lock a remoevable tongue into a receiver is a different case as that's the point along the tongue where the bending moment is the greatest - assuming the hole is near the end of the receiver (if it was say 6" in from the end, that would be different). A vertical hole cuts through the top and bottom 'flanges' of a tube tongue and so reduces its resistance to bending quite a bit.

A horizontal hole passes through the point where the vertical bending stress is close to zero, so it has very little effect on bending strength. While you are quite right that it does reduce the shear strength of the tongue, that isn't critical - I've just run the numbers on a 30" long tongue of 2" x 2" x 1/8" tube and the vertical load on the coupler that causes the tongue to yield in bending will only use up 6% of the shear strength. So even if a horizontal hole took away a third of the shear area of the tube, that 6% would only go up to 9%.

BUT saying that a vertical hole reduces the bending strength of your tongue is NOT saying that it's now too weak - that depends on how strong the tongue is to start with. Given the small size and weight of your new trailer, I would doubt very much if it would be possible to make a tongue that wasn't strong enough. If you want to either post or PM the data for your trailer to me, I'd be happy to look at it.

And congratulations on building such a light trailer - that one-handed photo is excellent - Arnie will want to know your secret!

Andrew