Attaching the cab to the frame

[PSBreen]

Finally past the required repairs around the house and ready to get back on the trailer build. My plan to attach the cab to the frame is to use 6 ea, 1/4" dia bolts with large washers and lock nuts. They would be in the 4 corners and 2 along the rails in the center. The wife is worried that it won't be enough. Any thoughts?

[CarlLaFong]

I have four 1/4 inch bolts with washers and locknuts holding the fender on my motorcycle. Why not jump up to 3/8 inch just to be on the safe side?

[bobhenry]

That's all that has held the body on Chubby, my teardrop , since 07 and it hasn't fallen off yet.

I would ONLY bolt it on and NEVER rely on screws into wood only.

I also used hugh fender washers on the wood side it is very hard to pull a saucer size washer thru a 3/4 plywood floor.

[PSBreen]

Point taken. Using 3/8" or 1/2" would be a bit safer

[KCStudly]

1/2 inch would be overkill. It would be better to spread out more fasteners than to just go with a few large ones.

I will be using sixteen (16) 3/8 inch bolts; 4 each along each of the front, rear, and two intermediate xmbrs. They will pass up through tabs ("clips") at the front and rear, and directly through the intermediate angle iron xmbrs; through blocking or 2x2 framing in the torsion box floor; into T-nuts recessed into the top surface of the floor frame, but hidden under the top floor skin. My trailer should not weigh more than 1500# loaded for bear. (We'll see.)

Here's a chart showing the cross sectional area (tensile stress area) for coarse and fine thread bolts. The cross sectional area of a bolt at its threads is significantly smaller than the nominal size would suggest. A 1/4-20 bolt has a tensile area of 0.0318 in^2. Multiply this times your 6 bolts and you get 0.1908 in^2 total. Assume the bolt is low grade mild steel with a tensile strength of 36kips (a kip is 1000 #/in^2), multiple this by 2/3 for a factor of safety on yielding and the steel is good for 24kips. Times how much steel you have (24kips x 0.1908 in^2) and you get 4579 lbs. capacity. That's a static load capability applied gradually and assumed to be applied evenly over your whole trailer.

In reality, your trailer frame will be trying to flex putting greater strain on either the front and rear bolts together or the middle ones alone. Also there is shock loading to be considered, such as when hitting a pothole or bumping a curb. The braking forces of your car will cause the bolts to see a shear load as the cabin attempts to slide forward off of the trailer frame.

So for me and my 16 bolts, 3/8UNC and 1500# camper it goes like this...

0.0775 in^2 per bolt x 16 bolts x 24,000 #/in^2 all divided by 1500# = 19.8

10 is a good conservative dynamic load factor (your trailer would have to hit something pretty hard to develop 10g's), so I am almost double that. In my "book" (not really a book, just my TLAR) that is an equivalent safety factor of 2.

Purist engineers can most certainly show me where I have strayed from the science of static and kinematic design theory, and how my safety factor is not being calculated accurately due to conservatism, but I'm okay with that. I know just enough about this stuff to be considered dangerous.

[Wolffarmer]

Get some 3/8 elevator bolts. That is what I am going to use in my next trailer.

They have a big flat head.

What an elevator bolt is and looks like

The stainless steel ones are expensive, stay with the galvanized ones.

Fastenal has them. But I got mine from a local place across the street for less. Keep the local boys and gals going.

Randy

[angib]

1/2 inch would be overkill. It would be better to spread out more fasteners than to just go with a few large ones.

So true - it is never the strength of the bolt that is the limit, but the strength of the bit of wood floor that they are holding on to. Some folk proudly say they used only Grade 8 bolts - when they are attached to Grade 0 plywood and guess which is the weakest link in the chain?

However, it would seem that worrying about the floor connection to the frame is completely pointless - the guys restoring old Scamp and Boler fibreglass trailers find they are held on with less than ten severely-rusted self tapping screws into the frame, and yet no-one has ever reported a body detaching from the frame. I think this is a job where a half dozen dabs of well-chewed chewing gum would actually provide enough strength.

[Lancie49]

and maybe a coupla ropes over the top to hold the chewy in place..............

[Forrest747]

1/2 inch would be overkill. It would be better to spread out more fasteners than to just go with a few large ones.

I will be using sixteen (16) 3/8 inch bolts; 4 each along each of the front, rear, and two intermediate xmbrs. They will pass up through tabs ("clips") at the front and rear, and directly through the intermediate angle iron xmbrs; through blocking or 2x2 framing in the torsion box floor; into T-nuts recessed into the top surface of the floor frame, but hidden under the top floor skin. My trailer should not weigh more than 1500# loaded for bear. (We'll see.)

Here's a chart showing the cross sectional area (tensile stress area) for coarse and fine thread bolts. The cross sectional area of a bolt at its threads is significantly smaller than the nominal size would suggest. A 1/4-20 bolt has a tensile area of 0.0318 in^2. Multiply this times your 6 bolts and you get 0.1908 in^2 total. Assume the bolt is low grade mild steel with a tensile strength of 36kips (a kip is 1000 #/in^2), multiple this by 2/3 for a factor of safety on yielding and the steel is good for 24kips. Times how much steel you have (24kips x 0.1908 in^2) and you get 4579 lbs. capacity. That's a static load capability applied gradually and assumed to be applied evenly over your whole trailer.

In reality, your trailer frame will be trying to flex putting greater strain on either the front and rear bolts together or the middle ones alone. Also there is shock loading to be considered, such as when hitting a pothole or bumping a curb. The braking forces of your car will cause the bolts to see a shear load as the cabin attempts to slide forward off of the trailer frame.

So for me and my 16 bolts, 3/8UNC and 1500# camper it goes like this...

0.0775 in^2 per bolt x 16 bolts x 24,000 #/in^2 all divided by 1500# = 19.8

10 is a good conservative dynamic load factor (your trailer would have to hit something pretty hard to develop 10g's), so I am almost double that. In my "book" (not really a book, just my TLAR) that is an equivalent safety factor of 2.

Purist engineers can most certainly show me where I have strayed from the science of static and kinematic design theory, and how my safety factor is not being calculated accurately due to conservatism, but I'm okay with that. I know just enough about this stuff to be considered dangerous.

His math is solid. If i recall i went with 1/4 bolt. i also went every two feet for bolt placement. with three in teh fron and two at teh rear. i think your corners and center will be fine, if you want just had one more in teh center at teh front. add a large fender washer to add more surface area for teh head and nut. If you want something really scary to think about the Airbus A380 has three 1/2 inch bolts holding each engine on the wing.




Readers warning. I am a huge fan of mythbusters. this is from thier snow plow myth:

How much energy does it take to flip a car?
They found a car with a high center of gravity to flip. They towed it with a truck from one side until it started to tip over and measured the force from the truck at 1000 lbs. Kari measured the surface area on the side of the car and figured out that it would take 0.21psi to flip the car from the side.

In order to match 0.21 psi to the force actually generated by a passing vehicle, they hooked up a tube with water to the side of the M5 Industries truck that measured air pressure. At 65mph, the M5 truck managed to generate between 0.018-0.036 psi, or about a sixth of what was required. Tory felt that this put doubts into the myth already.

Now how does this relate to bolting a trailer down. well the forces they were getting was from a full from blast. Q pressure. now a trailer is going to have less Q pressure due to it drafting behind a tow vehicle. Think of the MB drafting behind a semi. you are talking a 20sqft area for a 4x5 front profile. would be intresting to hook up alot of pressure sensors to a tear drop and see what we get. hmmm future project.

[Forrest747]

Ok this has been bugging me all afternoon. so after talking to some freinds and looking at some high school and college U-tube mannometer readings and finding those matched from teh Mythbusters readings. based upon those reading and using a frontal surface area of 20 sqft square and assuming we were pushing the trailer and not towing then the force upon teh face of the trailer is 2880 square inches (20*144) times the pressure of .25 psi you get a pressure of 720 pound acting full on the trailer.

Now using a table i found on gap between tractor trailers that the pressure drop is 12 percent for a 1.25 meter gap. using this pressure drop we get 633 pounds acting on the full frontal profile of a square trailer being towed at 65 mph. the closer the trailer is to the tow vehicle the less force acting on the trailer.

[danlott]

Ok this has been bugging me all afternoon. so after talking to some freinds and looking at some high school and college U-tube mannometer readings and finding those matched from teh Mythbusters readings. based upon those reading and using a frontal surface area of 20 sqft square and assuming we were pushing the trailer and not towing then the force upon teh face of the trailer is 2880 square inches (20*144) times the pressure of .25 psi you get a pressure of 720 pound acting full on the trailer.

Now using a table i found on gap between tractor trailers that the pressure drop is 12 percent for a 1.25 meter gap. using this pressure drop we get 633 pounds acting on the full frontal profile of a square trailer being towed at 65 mph. the closer the trailer is to the tow vehicle the less force acting on the trailer.

This has been bugging me also. So I combined it with this information.

1/2 inch would be overkill. It would be better to spread out more fasteners than to just go with a few large ones.

I will be using sixteen (16) 3/8 inch bolts; 4 each along each of the front, rear, and two intermediate xmbrs. They will pass up through tabs ("clips") at the front and rear, and directly through the intermediate angle iron xmbrs; through blocking or 2x2 framing in the torsion box floor; into T-nuts recessed into the top surface of the floor frame, but hidden under the top floor skin. My trailer should not weigh more than 1500# loaded for bear. (We'll see.)

Here's a chart showing the cross sectional area (tensile stress area) for coarse and fine thread bolts. The cross sectional area of a bolt at its threads is significantly smaller than the nominal size would suggest. A 1/4-20 bolt has a tensile area of 0.0318 in^2. Multiply this times your 6 bolts and you get 0.1908 in^2 total. Assume the bolt is low grade mild steel with a tensile strength of 36kips (a kip is 1000 #/in^2), multiple this by 2/3 for a factor of safety on yielding and the steel is good for 24kips. Times how much steel you have (24kips x 0.1908 in^2) and you get 4579 lbs. capacity. That's a static load capability applied gradually and assumed to be applied evenly over your whole trailer.

In reality, your trailer frame will be trying to flex putting greater strain on either the front and rear bolts together or the middle ones alone. Also there is shock loading to be considered, such as when hitting a pothole or bumping a curb. The braking forces of your car will cause the bolts to see a shear load as the cabin attempts to slide forward off of the trailer frame.

So for me and my 16 bolts, 3/8UNC and 1500# camper it goes like this...

0.0775 in^2 per bolt x 16 bolts x 24,000 #/in^2 all divided by 1500# = 19.8

10 is a good conservative dynamic load factor (your trailer would have to hit something pretty hard to develop 10g's), so I am almost double that. In my "book" (not really a book, just my TLAR) that is an equivalent safety factor of 2.

Purist engineers can most certainly show me where I have strayed from the science of static and kinematic design theory, and how my safety factor is not being calculated accurately due to conservatism, but I'm okay with that. I know just enough about this stuff to be considered dangerous.

This doesn't even take into account if you used drywall screw or wood screws, if you used the black goo on the bottom, if you have 1 door or 2 doors, if your trailer is 4 feet or 5 feet wide, if you used a fantastic fan or some cheaper fan. Also don't forget the con artist trying to get rich selling 2 teardrops in Missouri, you never know what he would try using to attach his floor with.

What I calculated out is to use whatever you want. I am going with the chewing gum and straps.

Dan

[Forrest747]

What I calculated out was to use whatever you want. I am going with the chewing gum and straps.

Dan

Well given those parameters I would say easily 15,000 lbs to torque to get the body seperated from the frame or equal to 3 trexs and a raptor pulling at teh same time while distraced by a goat. SQUIRELL!!!!

I personally wanted to use duct tape baleing wire and snot but was overruled by a 6 year old at the time, snot was too gross.

[KCStudly]

To be honest with you, when I wrote that it was the first time I had bothered to do any calculations on the subject. My design was based on TLAR (That Looks About Right) Engineering.

Maybe I'll wrap a piece of bubble gum around the head of each bolt to act as a sealing washer when I bolt the cabin down. The added strength would bring my numbers up to at least 11 or 12 SF and I won't ever have to worry about rot.

Hope no one was offended by my honest attempt to help a fellow teardropper put some logic to solving a design question (where's the pointy eared emoticon?). Never claimed to be an expert, just self confident like you.

[PSBreen]

Wow! Did I open a can of worms or what? After all the input, I'm going to stay around the middle of the road. We're going with eight ea 3/8" bolts with very large washers and locking nuts. Thanks for all the help.

[Wolffarmer]

I have 6 3/8 grade 5 bolts in shear, not tension holding mine. Don't ask for description, something you gotta see what this old farmer did. I sort of made it like I am use to in the way we use to attach truck beds to the truck frame long time ago. But 6-8 3/8 bolts in grade 5 will be plenty. I would stay away from grade 2 though they will probably workj and grade 8 is throwing money away. 5/16 would probably work also but they are not as common on the equipment I have and I have a mess of 3/8 in my bolt bins. But I have my elevator bolts now for #2.
Randy