Oh no! Cracked frame!

[ntsqd]

I'm the same user name there @ Expi.

[basicfish]

Originally Posted by ntsqd

I'm the same user name there @ Expi.

I am basicfish over there too.

It’s a great site, one of the best sources of diy camping/over-landing or anything to help you get out in the great outdoors

[redbarron55]

On most of these type frames the steel tube (1 1/2" X 3") has already failed when bent it just hasn't shown up yet.
The stresses in bending the square tube have already bent the top, bottom, inner side and outer side.
This is also where a lot of the stress from road use is focused and also since it has also already failed the weakest point in the frame.
I feel that if your frame has not already shown signs of cracking that the inner bend be boxed and the outer be plated with a double tapered plate. A plate over the bottom would be a good idea as well.
The top has already been weakened by the hole drilled to hold the body on and is currently rusting to weaken things more.
To help out more weld a cross member across the trailer from the new boxed inner plates and then you should be home free for a while.

[KarlB]

Yes as you have seen some photos of the damage. I was complentating buying a new frame. But a good welder friend suggested repairing and reenforcing the frame with additional braces and putting on longer fish plates on both sides and under.
I think that it should be safe and good for another few decades.
I’ll post pictures once I get the frame back.

[ntsqd]

Originally Posted by redbarron55

On most of these type frames the steel tube (1 1/2" X 3") has already failed when bent it just hasn't shown up yet.
The stresses in bending the square tube have already bent the top, bottom, inner side and outer side.
This is also where a lot of the stress from road use is focused and also since it has also already failed the weakest point in the frame.
I feel that if your frame has not already shown signs of cracking that the inner bend be boxed and the outer be plated with a double tapered plate. A plate over the bottom would be a good idea as well.
The top has already been weakened by the hole drilled to hold the body on and is currently rusting to weaken things more.
To help out more weld a cross member across the trailer from the new boxed inner plates and then you should be home free for a while.

Any failure from the bend isn't necessarily directly due to the bend itself, but rather how the bend was made (crushed rather than mandrel which preserves cross-section) and where it is located in the overall frame design.

I don't see a cross-member at those bends adding anything except weight and more on-site castings known as "welds".

[redbarron55]

The idea is to control the twist that occurs every time the frame loads and unloads at the bend.
I do't know of any of the trailers we are talking about with a mandrel bend and they all have "failed" already in making that bend and have very little rigidity across it.
The inside wall has bulged inwards and both the to and the bottom are bowed outwards.
Every bounce of the trailer flexes the area a little until it reaches its fatigue limit and cracks.
The frame has little rigidity to spare and most eventually crack in the bend.
Of course it takes some road traveling to crack and if they live in the driveway they will last until they rust out.
You are correct that the problem is in how the bend is made and the geometry that places bending and twisting loads on the already weak point.

[ntsqd]

Looking at this pic: http://www.fiberglassrv.com/forums/a...1&d=1531436401 There are cross-members or lateral structure pretty close to both sides of the bend. And a dropped floor section in the way of a proper cross-member.

It was not a tensile cycle induced flexing that caused the crack. It was bending/flexing in a vertical plane that caused the crack. This is a common failure mode in this part of any trailer's frame and given the low Moment of Inertia of this frame in this region it was an entirely predictable failure. M. M. Smith's book on trailer design (vol. 2) talks about this extensively. Adding a cross-member at the bend will not change anything. The correct cure is to square off the front of the frame to the existing front cross-member and build a new tongue that is under the frame. This doubles the Moment of Inertia in the critical zone. See the M.M. Smith book, it really does lay this out in simple to understand language.

[redbarron55]

Correct, bending in the vertical plane is what causes the cracking and not tensile or pulling.
Also correct is squaring the frame, which is what the cross member actually does and causes the stress to be shared across the frame ans not be focused in the bend.
When the bend is boxed and the cross member is welded to the inside of the box it also strengthens the area.
If the trailer "box" is mounted on a trailer frame that is a pure triangle back to the axle this also removes the stress in the bend since it is not there.
The stress on the driver side is amplified by the door passthrough on the street side which has more flex than the other. This extra flexing (in the vertical) is why the driver's side tends to break first.
Right behind the cracks on the driver's side are the attach points where the sideways tubes under the door transfer the load to the front of the frame.
My guess is that many more are broken out, but harder to see since this is on the top of the part of the door tubes where it is hard to see with the floor in place.
This is a view of this ares in my modified Scamp frame note the tubes coming under the added cross beam.

Here is the outside view. In the stock version there is just a piece of angle and a sheetmetal cross piece. The area where the angle was welded was completely broken out.

As you can guess a bit of weight was added when the frame was extended and reinforced. I also added 1 1/2" square tubing to reinforce the floor all the way to the sides of the shell tying it into the structure of the overall support of the rig.
In the original Scamp the only real structural attachment of the shell was where it passed over the frame in the front and back.
I also added a 1 1/2 X 3 tube under the area of the original bend.

All of this was to add the space for the front bath square floor pan.


Here are some of the added side floor supports and new steel wheel wells to accommodate the larger wheels and tires.
The new 3/4 " exterior plywood epoxy glass covered on all sides is epoxy glassed to the shell all the way around the trailer making the structure a part of the frame instead of riding on OSB sheetmetal screwed to the frame and just stuck to the shell with no real structural strength added to either the frame or the shell.
Here is the rest of the reason for the extended frame.

[ntsqd]

We're going to have to agree to disagree. The only way to reduce the stress in the frame in that region is to A) increase it's Moment of Inertia in the region, B ) remove the bend. The added cross-member does not increase the MoI.

[redbarron55]

I don't mean to be argumentative and I don't disagree about the stress not being lowered by the cross member, but rather helps strengthen the area so that the level of stress will not damage the bend.
However the geometry of the bend with no local stiffening will cause the added flexing in the bend without the support from the other side through the cross member.
Every time the force is bending the frame up or down since the bend is not in a straight line with the frame the result is a twisting of the bend area. The twisting is still there, but mitigated to some extent by the arm of the cross member attached to the equal, but opposite twist on the other side. The cross member tends to make the force a vertical force in line with the major axis of the tube.
But to each his own,
I do know that this is a major failure point for Scamps, Casitas, and others with simillar type frames.

[ntsqd]

That is where we disagree. The cross-member adds no bending stiffness or strength at the bends in the frame rails. A Free Body Diagram of the trailer will show a large increase in Moment at the front of the trailer body. This requires more bending strength in this region of the trailer frame. The only way to get more bending strength is to increase the cross-section in the vertical direction. The easiest way to get that is to put the tongue under the frame and extend it back some distance from the front edge of the body. I personally like to tie the tongue into the front spring hangers, but on longer trailers that is excessive.


https://www.amazon.com/Trailers-How-...dp/0914483323/

[stevebaz]

The problem is the frame is bent and reduces its load capacity. The next problem is that the floor is attached close to the bend. The floor stiffens the frame where it is attached and keeps the stress from being distributed down the frame member and concentrates the stress at the weakest point where it was bent.

Since the frame is bare steel inside the tube it is able to rust going unnoticed and the factory in many cases further reduces the frames capacity by drilling holes in the frame, not counting what owners do to the frame and lack of maintenance to the exterior.

The frame is designed around street travel since its low to the ground. traveling on poorly maintained roads and unpaved roads add greater shock loads to the stressed frames and an old dried out/worn out axle allows more road shock to be transferred to the structural member too.
Add it all up and it is amazing that the frames last as long as they do.

But as designed what was the expected life expectancy? Surly no one expected or calculated or designed them to last 35 years or so. That would be an unrealistic expectation. Unfortunately when they fail they are for the most part catastrophic.

It is wise to inspect old frames after every trip so you have time to fix them before the next trip or reinforce them before they fail. Or you can deal with the issue when and where it fails.

I don't know if Scamp makes and sells rolling frame assembly's. I live to far for them to be of use for me. This discussion has me thinking more about my 32 year old axle and frame condition.

[redbarron55]

You are correct, it add no additional stiffness in the vertical plane, but it does keep the area from twisting along with the bending in the vertical.
Bend your elbow and take your opposite hand and push down and you will see what I mean.
If you push up your elbow goes down and if you push down it goes up.
The force is not in alignment with the attach point of the frame at the axle along the tube, but rather to the inside of it. It is not a pure bending moment.
Either way correct or not the bend is where the frame will fail.
This is due to the poorly fabricated bend and/or the twisting moment generated by the geometry or more likely both.
If there is a cross member then the force at the bend is more of a pure bending moment at the point of the bend.
The cross member triangulates the front of the trailer which in the horizontal plane is the strongest and in the vertical plane is at least the definition of plane where the force is applied.
One could consider the force as the distance of a lever arm of a straight line from the hitch to the axle attach point, probably close to a foot and a half.

[stevebaz]

Here's something to think about. In most of the failures in these types of trailers seem to start on the drivers side. Why? I suspect because the step box in the frame passengers side frame is less stiff and more flexible transferring added load to the drivers side frame. Does adding the cross member at the bend counter act the transfer of this load? Will it move the failure to the step box joint? and out of the bend area. Since the Frames seem to get close to 30 years to fail the structure has to be very close to where it needs to be. It is my understanding that the later models have increased frame strength.

[ntsqd]

I've linked a book that explains all of this twice with no comments or even a nibble. All of these assumptions are just that, assumptions. And most of them sound good and reasonable, and are erroneous. I'll encourage anyone who wants to learn how to properly and effectively design a trailer frame or design a repair to an existing trailer frame to learn from a degreed Engineer who designed and built trailers for a living rather than taking all of these assumptions as gospel. All of his exercises and explanations followed exactly as my Engineering classes taught me to do, only his were trailer specific rather than generic as is taught in Engineering Statics & Strengths of Materials.

I know the price listed on amazon isn't cheap because the book has been out of print for some time. I didn't link it there to be bought there. I linked it so that people would know what it is that they're looking for when trying to find it elsewhere. None of the other trailer books on the market that I have reviewed had the same content or were worth buying in the place of this book.

[redbarron55]

Here's something to think about. In most of the failures in these types of trailers seem to start on the drivers side. Why? I suspect because the step box in the frame passengers side frame is less stiff and more flexible transferring added load to the drivers side frame. Does adding the cross member at the bend counter act the transfer of this load? Will it move the failure to the step box joint?

You are correct on the whole thing.
My frame was cracked on both sides and where the tubes under the door attached to the front part of the frame as well.

I think the first to show up is the driver's side for the reason you mentioned.
The early units were thinner tubes later one were thicker, but still have cracking.
It may be related to the amount of travel and the roughness of the roads and perhaps the condition of the rubber in the axles and the tire pressures.
My frame had been "repaired" several times in the bends, but never where the reinforcement that ties the front side of the tubes that go under the door attached.
The metal ties were cracked out of the under tubes.
I added a reinforced cross member at the front of the door both for the hanger for the bathroom floor pan and to tie both sides together better.
I don't really mean to argue with anyone as these trailers have survived with repairs quite well considering the age.
Mine is a 1985, but take a look at the front section I cut off of mine.


Through the years before I bought this trailer it had many repairs of questionable quality.
Note that the driver's side bend was plated as a repiar and the cracking continued to every place where the stresses were concentrated and not helped by the mounting holes where corrosion did not help any either.
However all this being said the trailers probably hold up better than the average stick built.

[redbarron55]

As an heavy industrial maintenance manager for 45 years I can appreciate the engineering involved as I have spent most of those 45 years fixing the under designed work product from engineers.
The theories are good and can be proven, but often they do not take into account the actual loads imposed as the models exist in a purer world that the real one.
If the stresses were withing the limitations of the structure there would be no localized failures.
A root cause failure analysis points to poorly distributed loads being concentrated at a point without the strength to carry the flexing loads and failing from fatigue.
The method of bending and the stresses add up to failure, possibly after a considerable time and reasonable life, but not as long as the shell.
The floor probably will rot out about the same time or sooner if not maintained.
I remember when I was working in the NASCAR racing as we found a weak point and fixed that we just moved the point of failure someplace else.
My new frame will easily outlast me with the trailer at 33 years old (3 since rebuild) and me at 70.

[Rivenwood]

Looks like a good crack. Congratulations and join the club.
As long as the trailer is empty and the welder is not too far away and the roads are fairly smooth, it should be ok to tow it to the welder.
I had my Boler welded with gussets on either side of the frame and it is still rolling. Good luck, Ron

[Burro1]

Same thing happened on my burro same exact place I had a welding shop weld 2x4” xquarter inch angle iron extending in both directions and still sturdy that was about 10 years ago still working