Help with tongue design

[citylights]

I just want to confirm that my tongue is ok... My teardrop trailer tongue that is...

My teardrop is 1750# dry, so maybe 2000# loaded. I will drive miles and miles on wash boarded gravel roads.

Tongue is single tube 54" long (nose to first crossmember) 3 vert x 2 Tall x 3/8 thk, then 18 inches more to second crossmember.

First crossmember is 2 inch angle iron, second is 2 inch angle separated by 18 inches. My entire trailer frame is topped by 2x4 bolted through to frame. 2x4 topped by 3/4 plywood.

I think the 3/8 thick tongue is fine. I am not so sure about the 2x4 reinforced angle iron cross members.

See pictures of bare bones trailer frame and unfinished deck, finished deck, and finished trailer.

What do you think?

[KCStudly]

Your wooden deck frame looks to be fairly substantial and is probably helping you out quite a bit. How is it fastened to the xmbrs (number of and size of bolts/screws, type of fastener and joint configuration, proximity of fasteners to the tongue member, thickness of the xmbr angle, etc.)? If the 2nd xmbr is well tied into the beefy wood with bolts and washers on top, you might be fine. Lags or screws up into the wood might pull out with time.

On the tongue member, is that 3 (flat or vertical?) x 2 Tall (or wide?) x 3/8 thick? 3/8 thick, really? That's battleship territory.

I'm guessing that the xmbr's are only 1/8 thk. The 18 inch separation does seem kind of close. There is a lot of leverage on the 2nd xmbr trying to bow it down in the middle. I was going to suggest considering a truss, like what Bob Henry did on The Barn, but your basement storage box would make that difficult. You could add a truss across the xmbr from side to side, like a wide short inverted 'A' with a post in the middle. It needn't be any deeper than your basement box. When the rear of the tongue member tries to pull the 2nd xmbr down, the force would be transferred down thru the post in compression and into the long sides of the 'A'. The sides of the 'A' would be in tension transferring that downward force back up to the side frame rails. The xmbr would act in compression to keep the side rails from pulling in, provided that the xmbr does not buckle (although L shape profiles aren't very resistant to buckling, if it is stabilized, such as by being screwed to your wooden frame, you should be okay).

To summarize; if the second xmbr is connected to the wood frame well, you are probably okay. If you see any signs of buckling or bending in your 2nd xmbr (either downward, or front to rear) you need to reinforce.

With it hooked up to tow, if someone beefy squats over the 2nd xmbr and bounces up and down can you see any relative motion between the xmbr and floor? That might help you decide.

[bobhenry]

1st try backing up a tight driveway!

You will find you need to add 12 to 18 inches to the length.

2nd you have the exact problem I had with the Namco trailer I put under the barn. It had been overloaded and the 1st cross member was bent downward. If your step down floor prohibits extending the tongue do yourself a big favor and add two more members to make a vee nose reinforcement. I had the barn out this weekend and in the mirror I can see the side to side flex of the tongue when in transit. The truss I placed under the tongue took care of the up and down flex but there is still some side to side flex. Had I reinforced the tongue by adding the diagonals it would have eliminated the side to side flex.

There is nothing worse than setting by the side of the road with a broken trailer tongue.

Copied from my 07 posting....

Had a major engineering rebuild on the tongue. It had bent as some of you said it would and I jacked it back into position but it needed more so we trussed it by using 1 1/2 flat stock and a 4" piece of 1 1/2 square tube. Placed the tube at the front rail of the trailer frame under the tongue and the flat strap was welded to this tube and 3 feet forward and another 3 feet to the rear. A saddle was placed over the square tube and welded to each piece of flat stock to connect them and relieve some of the strain on the tube weld. Then we tensioned the strap by mashing with a monster "c" clamp and welded the edges at both ends. This effectively makes a truss out of my extra long tongue As the tongue load increases the front strap tensions and actually lifts the leading edge of the frame while the rear also tensions to assist the front strap. I know clear as mud but it worked and you can play a tune on the straps.

Here is the best picture I could find of the truss. You need to remember the trailer frame ends 2 foot back from the front of the trailer body.

[citylights]

Answer to questions...

Yes, the tongue is 3/8 thick. Here is a picture of the original 1/4 inch and the 3/8 inch that I replaced it with



The tongue is 3 inch vertical (tall), by two inch horizontal (wide).

The trailer frame was originally tied to the 2x4 frame by 6 - 1/4 inch through bolts with washers top and bottom. Angle bolted to 2x4 with 2 bolts on each side of the teardrop. Then I added 8 - 1/4 inch through bolts with washers at stress points (at axle spring connections front and back, at rear of teardrop, and at tongue cross member front of teardrop). The 8 added bolts are through the side of the angle through the side of the 2x4.

Anyway, with a night to think and sleep on it, I think I have my answer.

With the trailer hooked to the truck, I can stand on the tongue (I am 200# before breakfast) and bounce up and down with very little flex in the tongue. I think the flex is at the 2nd crossmember.

I am going to get some metal straps and reinforce. I will strap the tongue to the wood on its length and bolt through. That will eliminate the flex and effectively shorten my tongue by 30-inches. My teardrop overhangs the tongue by 12-inch and my tongue box by 18-inch.

Sorry for the panic attack after my teardrop is supposed to be done. I am going on my first trail run with it in a couple of weeks. 4 days of ghost town hopping in the Nevada back country with a group of about 10 vehicles. It is supposed to be all wash boarded dirt or gravel trails, not really 4WD or off-roading. Most will be tent camping. A couple with tent trailers, and me with the teardrop.

[mezmo]

FWIW, oriented as shown, the width is 3 inches and the height is two inches.
The thicker tube walls help, but it would be more rigid if you turned it 90 degrees
so that the 3 inch side is vertical and the 2 inch side is horizontal. It's the
same as with wooden dimensional lumber. Always have the longest side of
the cross section on the vertical for maximum rigidity. Try it with a 2x4
laid flat, then on edge and note the differences in the flex levels.

Cheers,
Norm/mezmo

[bdosborn]

Did you look at this in the design library? It's pretty good:

Tongue Strength Linky

Bruce

[citylights]

FWIW, oriented as shown, the width is 3 inches and the height is two inches.
The thicker tube walls help, but it would be more rigid if you turned it 90 degrees
so that the 3 inch side is vertical and the 2 inch side is horizontal. It's the
same as with wooden dimensional lumber. Always have the longest side of
the cross section on the vertical for maximum rigidity. Try it with a 2x4
laid flat, then on edge and note the differences in the flex levels.

Cheers,
Norm/mezmo

Installed it is 3 vert, 2 horiz. It is installed the strong way.

[citylights]

Did you look at this in the design library? It's pretty good:

Tongue Strength Linky

Bruce

Yes I did see that. The simple table did not have 3x2x3/8 tube and I can't run the spreadsheet from my ipad.

Extrapolating between the 2x2 and 3x3 tube in the simple table my tongue should be fine. The question in mine is the weak trailer frame reinforced by the wood teardrop structure. The tongue strength linky tool can't help with that.

[angib]

Men have this great faith in thick steel, but it just isn't that helpful. 3x2x3/16 is about 40% stronger than 3x2x1/8 - the thicker steel is mostly on the inside where it doesn't do that much good.

But the rest of the trailer looks like a Tractor Supply or Carry-On model and your query about the second cross-member is supported by a couple of guys here who have reported failures with this design of trailer - even the original tongue was stronger than angle ironpart of the frame that it fixes to.

It's not that hard to reinforce the second cross-member and simply fixing it to the floor framing is probably good enough. But that would need to be done with some sort of clamp and not by drilling through the horizontal leg of the angle - otherwise you risk weakening the angle still further.

[ctstaas]

Hi John and Go Wildcats,
I'll put in my $.02 and let you figure it out.
Any tongue requires two strengths, Shear and pull. Shear strength covers vertical loading. How much load would be required to cause the tube to fail and where would it fail? In my experience, tongues fail where the load intersects the tongue. In your case, where the front of your cabin meets the tube is the most likely place for the tube to bend. That tube is not going to bend without some heavy duty force or an oxy-actylene torch.
The other strength, pull strength is very important. When starting or stopping, the force exerted on the tube is equal to the weight of the load. I don't have the engineered numbers for that steel but would feel safe with 10x the weight of your TD in either compression or stretching.
The tube itself is way overkill, nice job, but the stuff attached to it is suspect. Your X members are looking mighty thin. I would "sister" some thicker angle iron to your origional framing.
If you choose to extend your tongue you may want to consider diagonals. A diagonal will not contribute much to the shear or pull strength of the tongue but it will eliminate any side to side movement of your tube. Without movement, there is no "moment". The "moment" is the movement that causes failure of steel.
Your work looks great and I have some questions for you later. I can't ask to early or I'll forget.
Enjoy, Chris

[KCStudly]

Lots of technical inaccuracies in that last post.

Shear strength is a material's resistance to cleaving, such as by two opposing forces (i.e. the weight of your camper pushing down on your tongue at the front wall, and the tongue pushing back holding the front wall up). If your tongue were to split at this location w/o bending it means that there was insufficient cross sectional area for the material to handle the load.

Moment of inertia is a value calculated from the distribution of a member's cross sectional area that relates to its ability to resist forces and is used in engineering calculations to determine a member’s ability to resist bending over a given span. If your 2nd xmbr has insufficient moment of inertia (not enough material at its outer fibers, and/or the outer fibers too close to the center of the materials cross section, then it will bend in the middle due to downward force applied by the rear end of the tongue. The 2nd xmbr can be thought of as a simple beam supported at both ends by the trailer frame with a downward force applied by the tongue in the middle. If the span (width of the trailer) is too great, the moment of inertia is too low, or the load is too great, the member will bend in the middle.

The amount of load at the back of the tongue can be easily calculated using a force moment diagram. It is essentially a teeter-totter. We would have to make some assumptions to account for dynamic situations.

Since the trailer is not rotating in space we know that the torque load from the tongue trying to rotate about the front xmbr is resisted by the 2nd xmbr under most normal loading scenarios. The ball hitch is a reaction point holding the front of the tongue up, the front wall and 1st xmbr is a load point trying to push down on the tongue (assumed to be the centroid of a force moment diagram and is therefore assumed to be stationary) and therefore the 2nd xmbr must be holding the back of the tongue up, otherwise the whole thing would be spinning around the front xmbr. Now if we calculate the sum of the moments equal to zero (balance the torques so that there is no rotation) we can determine the force acting downward at the 2nd xmbr.

Let’s assume that the tongue weight is 200 lbs, that the tongue sticks out 4 ft and extends under the cabin 2 ft. Then 0 = (200 x 4) – (X x 2), solve for ‘X’. X = 800 / 2 = 400 lbs (static). The closer the 2nd xmbr is to the front of the trailer the higher the leverage is acting on it. The more dynamic the situation, the higher the loads become due to acceleration and deceleration forces.

Tensile strength is a materials ability to resist being stretched, and is given as the value at which a test sample fails.

Yield strength is the point that a material will no longer go back to its original shape when a load is removed, or the point at which it becomes plastic. If you know a materials tensile strength, a good rule of thumb is that the yield strength will be about 2/3 of this, and it is a good practice to allow a conservative 2/3 safety factor on top of yield. Mild steel has a tensile strength of 36 kips, so yield strength of 24 kips, and design max of 16 kips. YMMV.

Using this mechanical properties calculator entering 1.5 inches long and 1/8 inch thick for both legs of an angle shaped section we get a puny moment of .07776

Using this simple beam calculator with inputs of .07776 for moment of inertia, 2.5 ft for ‘a’, ‘b’ and ‘x’ (the center point we are interested in) and 400 lbs for the load (leave Young’s modulus as the default given for steel), we get .46 inches of deflection just sitting still. L/300 (length divided by 300) is a good rule of thumb for max beam deflection, so that’s pushing the limit right up to the margin under just static conditions. Take a big hit on a pothole or whoop and the dynamic load can easily triple, if not quadruple! Hit washboard all day long, hit over hit, and it is easy to see how all of those light, too close to the front xmbrs are giving up and bending down.

An easy way to strengthen the 2nd xmbr would be to turn it into a simple upside down truss with the peak under the rear of the tongue and the sloped members in tension running from the underside of the tongue to the points where the 2nd xmbr joins the top of the frame side rails. A piece of flat bar welded on should do the trick.

I won’t go into the engineering of trusses, but you can learn more about that here.

[ctstaas]

Hi John and KC, Go Cats,
KC thanks for the tech updates I wanted to KISS for John in the lay-est terms possible. I forget more rules of thumb cause I don't use them everyday. The rule I do remember is the shear strength of a 1" A36 round, 138,000 #, right?. Its based on surface area. The surface area of a 1" bolt is 0.785". The vertical surface area of the 3"x0.375 tube is 2,25 inches. Almost triple the surface area of a 1" bolt. That would give the tube a shear strength of over 400,000 #. That's a big Guillotine.
Moments of inertia are complicated to easily explain. I just wanted to state, no movement-no inertia.
Enjoy,Chris

[ctstaas]

Hi John and KC, Go Cats,
KC thanks for the tech updates I wanted to KISS for John in the lay-est terms possible. I forget more rules of thumb cause I don't use them everyday. The rule I do remember is the shear strength of a 1" A36 round, 138,000 #, right?. Its based on surface area. The surface area of a 1" bolt is 0.785". The vertical surface area of the 3"x0.375 tube is 2,25 inches. Almost triple the surface area of a 1" bolt. That would give the tube a shear strength of over 400,000 #. That's a big Guillotine.
Moments of inertia are complicated to easily explain. I just wanted to state, no movement-no inertia.
Enjoy,Chris

[KCStudly]

Cross sectional area.

I need to correct myself. 36kips is the yield strength of mild steel, not the tensile strength. 24kips is a good number to use for 1.5 safety factor.

A 1 inch diameter has a cross sectional area of .785, true. Not so much a 1 inch bolt. Bolts usually have a slightly smaller shank diameter than nominal and if the threads are loaded in shear, the equivalent section of the thread is significantly smaller (about 60/ct of nominal). However, most Gr 5 and higher bolts are stronger than mild steel.

.785 in^2 x 36000 psi is only 28260 lbs.

3x3x3/8 HSS has a CS of 3.39 inch^2, (http://www.cim.mcgill.ca/~paul/HollowStruct.pdf pg 21) allowing for the radius corners, so 122040 lbs to yield in shear, but at that point we would also want to look at a collapse scenario in buckling.

So your terminology and math is off, but yes, we are all in agreement that the problem area is the 2nd xmbr; it's small size and location that is too close to the front.

Edit: Oops. I forgot that it was 3x2 tubing, not 3x3, so 2.35 in^2 is about right. 84600.

[NathanL]

I haven't looked lately but the only thing you can still buy with a 36kip yield is plate even tho it's still listed for structural members nobody rolls it anymore or stocks it. The minimum you are likely to get in a tube etc..is 50 now.