Tongue Strength Measure

[angib]

The problem with determining the strength of tongues is that we mostly only know of tongues that haven't broken, so they're either just strong enough or way too strong - and we never know which. So we should all thank Dave Nathanson for this useful post on his broken tongue shown above.

I've tried creating a Tongue Factor that attempts to measure how strong a tongue is. My suggestion is made up like this:

Tongue Factor = Load Factor / Section Modulus x 1000
Load Factor is a measure of how high the maximum bending moment on the tongue may be. It is some combination of total trailer weight, static tongue weight and the length of the tongue. My first shot is:

Load Factor = (1/4 total weight + tongue weight) x Sqrt of tongue length

The balance between total trailer weight and tongue weight (both in pounds) is my guess - it says a trailer with a 25% tongue weight would create twice the bending moment of a trailer with a zero tongue weight.

Tongue length (in inches) affects the bending moment fairly strongly - a longer tongue creates more bending moment - but I doubt it's linear, so I've used the square root of tongue length.

Section modulus is a calculated (and frequently published) property of a particular steel section, in in3 (inches cubed). The figure is doubled for an A-frame tongue 'cos it's got two sections/tubes.

The 1000 divisor is added to keep the result a useful size.

The Tongue Factor ends up in strange units of lb/in^2.5, so in honour of its inspiration it will be measured in 'kilodaves' - so once we've divided by the 1000, the result is in units of 'daves'.

The calculatuion for Dave's desert trailer looks like this:
Trailer weight = "about 1100 lb"
Tongue weight = "about 120 lb"
Tongue length = 40" (estimated from photos, hitch to tongue receiver)
Tongue section: 2x2x1/8" square tube with 3/4" horizontal hole.
Section modulus = 0.491 in3

Tongue Factor = 5.1 daves

Now that Dave has increased the tongue to 3/16" wall thickness, the section modulus has gone up to 0.672 in3 giving a new Tongue Factor of 3.7 daves, a 37% improvement.

Now this is valuable data because we know that 5.1 daves is in danger territory - although from Dave's descriptions of travelling on washboard roads, I suspect this was a fatigue failure and that in 'normal' road use it might be OK to get nearer 10 daves.

I have no idea what can be done with this idea, but maybe some of you will offer (polite) suggestions.

Andrew

PS For Bev, the Widget 6 with two 1/8" wall tubes has a Tongue Factor of 3.9 daves, so I reckon that's safe.

[mikeschn]

Very nice Andrew,

I'm sure you've flattered Dave

Your Dave calculations are based on a tongue with no major holes thru it, but I suppose that's good enough. Most tongues won't have holes.

I would say this is a good rough guess of tongue durability. I would publish a tiny spreadsheet, and give an approximate safe range... That'll prevent someone from building a 4' long tongue with .03125" wall tubing.

I would prepopulate the spreadsheet with some sample values, and then let the user adjust it to see how dangerous he can get.

Mike...

BTW, what's the safety factor on Dave ratings?

[angib]

Your Dave calculations are based on a tongue with no major holes thru it, but I suppose that's good enough. Most tongues won't have holes.

Actually, I went into full egg-head mode (took up pipe smoking, etc) and wrote myself a section modulus calculator (di-lithium crystal-powered) to take account of the corner radius on square tubes and included a term for holes.

Dave very sensibly put the locking hole for his tongue receiver horizontally - in this position it reduces the vertical section modulus of the tube by less than 2%. If he had drilled the hole vertically, it would have reduced the section modulus by 33%!

I'll see about putting up a spreadsheet - the problem being the amount of descriptive notes it would need for anyone else to use!

Andrew

[beverlyt]

Andrew,
It is good to know that my "Dave" factor is within safe limits.

[fornesto]

So what you're sayin' is...if I've got a single-tongue and its 1/8", I would be well served to yank off the hitch coupler and shove some square tube down the middle to strengthen it? Would that work if I covered the whole distance and redrilled my holes? Yes, it would increase tongue weight a bit, but I'm driving a '53 Caddie or a '99 Expedition...not an issue

[Guy]

Dear All,

The easiest way to retrofit a weak tongue is with a tongue stiffener

This should be sufficient for the typical teardrop.

[BrianB]

Guy, I can't make heads or tails of your picture. Could you describe what we're looking at?

[Guy]

Sorry, Brian,

It is a 2"x2" tongue with three flat strips of 2"x1/8" steel welded like a bridge or ramp to the underside, with two pieces of 2x1/8" pieces of flat steel 1 1/2" long lowering the middle section from the tongue. This will make it the equivalent of greater than a 3" tongue and not only prevent the Dave incident but also the swaying problem that many folk experience.

It is a standard technique used in the design of trailer tongues for much higher weight ratings than teardrops. Unfortunately this highly effective technique is not something that HF and its competitors would use when they are attempting to lower price. Sometimes the way these companies design something is taken for the "gospel" and we forget to look at time-tested, proven safety features to throw our DIY arms around.

[asianflava]

It is just the toungue with a piece of flat stock that is welded to the bottom with 2 risers. It is put in tension when the toungue is stressed downwards.

[BrianB]

Aaaah, I see it now. It looked like a perspective picture where I'm looking at it from the backside.

[bdosborn]

The Tongue Factor ends up in strange units of lb/in^2.5, so in honour of its inspiration it will be measured in 'kilodaves' - so once we've divided by the 1000, the result is in units of 'daves'.

Hmm, interesting. What's the safe Dave range? Do you want a high Dave or a low Dave. Also, since there are a ton of folks using the Harbor Freight trailer (myself included), whats the Dave value of that trailer?
Bruce
P.S. Now we need an Angib value. Perhaps the average height the trailer will bounce over bumps at 45 mph. Maybe the number of days it takes to get gorilla glue out of your hair.
P.P.S. Hi Dave

[Dave Nathanson]

WOW! Well, I don't know what to say Andrew! :-)

I guess somebody's got to do the research! !

Oh, and I suppose I ought to mention that I had 120 pounds of gasoline on the TD roof rack! Yes, that's 3 gas cans (5 gal each). I bet that added a little tongue weight when the thing was bouncing all over the road. But I remain convinced that the constant pounding of the washboard is what really made the damage happen. We might have gotten away ok except for the pounding. Most of the Trailer is now "Baja Proven"! But some parts are "Baja Broken"! Hey, it was a fun trip & I'd do it again in a heartbeat.

I'm about to keel over from lack of sleep and hard work - um, uh, you know what I mean.

I'm working on the new tongue design, and based on Andrews modolus of 5.1 dave's being a hazard (even 1 Dave can get everybody nearby into trouble!) I'm going to jump to 1/4" wall thickness materials instead of 3/16".

And a 3" sleeve over the existing 2.5" square tube. not only does this reinforce the whole arrangement quite a bit, but it also gives me a chance to fix up the pin hole that got ovaled. Sort of square it up, so to speak. I can explain it better later, but here's an early glance of it.



It's approx 29" of 2x2 (blue) from the (old) frame receiver (brown) to the tip of the 2x2 where the hitch goes. Actually, the Lock N' Roll hitch extends another foot or something. So Andrew is quite close with his guess of 40" length.

I was thinking it would be good to extend the 3" tube and the 2.5" tube out an extra 7" or so from where it is now. Partly to get a good spot to drill a new hole. But maybe that doesn't matter. The old spot is probably ok, but the hole needs to be made "square" and truly round (not oval). So if the hole is reamed out to be too big, then that bushing can fit in there. If the new hole is too big on purpose, then the bushing can be aligned just right before it is welded in place. That is one way to fix up that hole and make it straight & round. But I'm open for other ideas as well. What do you think is better?

(although it seems like a good idea, I'm not going to go with Guy's stiffener because I need all the ground clearance I can get.)

(edit 9/6/2010: fixed broken picture links)

[angib]

Oh, and I suppose I ought to mention that I had 120 pounds of gasoline on the TD roof rack!

I guess that a third of that weight would go on the tongue, so increasing the tongue weight to 160lb. That would put the Tongue Factor at the time of collapse up from 5.1 daves to 5.6 daves*.

But I still think the important thing is that this was probably a fatigue failure and values greater than 5 daves may be acceptable if the trailer isn't driven on washboard roads!

* Please people, a 'dave' is a unit, not a name, so it doesn't get capitalised (or capitalized either). That's my 'hot button', Joseph.

I was thinking it would be good to extend the 3" tube and the 2.5" tube out an extra 7" or so from where it is now.

Extending the outer tubes will certainly reduce the maximum bending moment and increasing the thickness will increase the strength. My inclination would be to stagger the ends of the 2.5" and 3" tubes by several inches to avoid creatng a real 'hard spot' for the 2" tube at that point, but that may be a bit obsessive!

Andrew

[angib]

What's the safe Dave range? Do you want a high Dave or a low Dave. Also, since there are a ton of folks using the Harbor Freight trailer (myself included), whats the Dave value of that trailer?

Too many daves and your tongue may be overloaded. Fewer daves is good.

Dave's experience is that a Tongue Factor around 6 daves is bad in heavy off-road use. So maybe from 2 to 4 daves is OK. More is risky and less is overbuilding.

But I want to stress - this is not a 'rule', it's only an idea for research.

Someone please measure their Harbor Freight tongue - shape (U or C?), width, depth and metal thickness - you'll probably need calipers to measure the last one and you need to deduct an estimated paint thickness. Oh, and include which HF trailer (1800, 1175 or 990 lb?).

Andrew

[angib]

I have added the results from two commercial trailers to my Tongue Strength page:

- The Northern Tool 1180lb trailer gets a Tongue Factor of 3.2 daves.

- The Tractor Supply 1250lb (payload) trailer gets a Tongue Factor of 3.3 daves. But the tongue is attached to two angle cross-members that will fail long before the tongue does, increasing the Tongue Factor to 14.3 daves.

I'm presuming the Tractor Supply trailers are not collapsing as soon as they're used, so that 14.3 daves result is important.

Thanks to Dale and Walter for the data. Some of this info is repeated on Walter's thread about his Tractor Supply trailer.

Andrew