I will say that the frame is going to be the least of your worries if your truck is overloaded that much. I do think the frame will handle much more than anything else than any other component in your truck.

 

So do I have this right that a late 70's 150 frame, and 250 frame are basically the same frame?

 

No! F-150 & 250 are not the Same.

F=150 & F-250 are NOT Universally or mutually Interchangeable from a structural or a loading standpoint. These frames are based on Gross Vehicle Weight Rating or carrying capacity. Having said that, dimensionally these frames are virtually the same in wheel base for wheel base & model for equivalent model terms. But again NOT in Terms of max loading.
An F-250 with a GVWR rating below 6900 lbs is roughly equivilant to an F-150 but an F-250 with a GVWR over 6900 is more like a light duty F-350 through 1979 anyway.
But the problem isn't so much at what point will a frame catastrophically fail, as much as it is, at what point does the max load exceed the frames ability to flex under that load without permanently distorting or being damaged by weight dynamicly speaking.
I would agree that the 80-95 style frames DO NOT have the flex, resiliency, or basic tensile ability of the earlier 70s frames. It is this difference that allows damage to happen. Here is a point to consider about "Plating" frame channel sections to box the frame.
1st heat from welding changes characteristics of the original metal, plus by boxing or "plating" the open side shut, the flexion rate becomes modified. This introduces a potential for a catastrophic failure rather than a catinary event where rails sag out as the frame becomes damaged.
I agree there is a fudge factor within the GVWR or Max carrying Cap and 5K will not destroy a vehicle rated at 1.25 or 1.5 times that amount. But as somebody else mentiioned terrain and duty req's will have a bearing on the outcome.Also basic vehicle performance, like handling, stopping, ride, fuel consumption etc etc. will most likely be negatively impacted by such overloading.
Most importantly to me however, is if I am 2500# or 4000# over my GVWR and am involved in a wreck I could be held liable for negiligence and that's what really gets heavy IMHO.

My $.02 in this thread. . .
FBp

 

That is what I was saying about my ford manual. It appears the frames are the same material, but that's where the similarities end.

 

Ted I "thought" you said that the 100- 350 SRW are the same 36,000psi Frame(light duty) then the 400-larger series rigs got a diffrent frame. I GUESS nobody knows for sure I even took it as far as measuring with a feeler gauge on the frame thickness and other measurements and Between The 74 F250 8,000gvW rated truck and the 6200lbGVW f150 were the same. But I will not claim that they are the same a a DRW 350 they clearly are not. Question if the frame was used to determine the GVW of a truck then ford would have 85 "DIFFRENT" frames. I think not, they just used one frame for a reg cab F100-350SRW and one frame for ext cab 100-350SRW ya see it now. They did this till 96 then yes they are diffrent between a F150 and a 250SD, the SD was taller frame rails along with diffrently place cross members
My point is that why would a company make many diffrent frames with the diffrences so minute that no one here can give a text book answer. It would be a serious waste of money. Because they would have a stamper to stamp out frames for trucks with gVWR of F100 -4450, 5000,5570, 6000, f150- 6050, 6100, 6150 6200, 6300, F250- 6250 6300, 6400, 6600, 6750, 6900, 7300, 7500, 7750 , 8000, 8400, 8550, F350 SRW- 8850, 8900, These are just a few of the Gross weights of trucks in the 73-79 era That would be 24 DIFFRENT frames just to name a few. Anyone have a Textbook answer to my question???? or does my piont put some of you back to the drawing board.

 

The frame is hell for stout...I have never heard of a frame failure. All of the
other safety and drivability issues previous posters mentioned are
correct. Also..overloading will tweak the wheel bearings.

 

I heard that. I understand the overloading issue on the truck but I'm more concerned for those of you who say the are not the same but why would ford create virtially identical frames that are diffrent??? You guys go me steered up on this one. I'm wanting the posters that say if I'm overloaded then I hope you (my)tires blow out before you(I) get on public streets to look at the issue I'm trying to address and forget the overloading issue altogther

 

Alright, you guys have me wondering now. I will check tom. and post my measurements. I have 6 trucks to check and compare. I have three hear and there is 5 at my brothers. We are collectors.

 

I don't know if the frames or the same or not. And I don't think it matters that much because they are both really, really, strong. But, I will reiterate that the frame strength is going the least of your worries if you overload your truck a huge amount of over GVW. Something else will break or fail long before the frame even breaks a sweat. I do believe that the frame will be last thing to fail unless there is something wrong with it. Steel supports unbelievable amounts of weight - rolling weight however, is going to be less due to physics involved. For those that don't believe just how strong steel is, go out to your garage and put weight on a couple of 16 penny nails that are supported in your vise. Please let me know how much weight you put on it before it breaks. Also, let me know how your workbench holds up....I'm curious as to what its shear strength is. Anyway, once again, your frame is the least of your worries.

 

Still seems like much ado about nothing, Dustin.... GVW-Vehicle Weight was still a higher number then the payload weight, right????? Or am I not reading your numbers correctly? Anyway, by now I'm sure you've hauled whatever it is you were hauling and you and your truck both returned home safely, much to the chagrin of all those who wished you such ill will!!!!!

 

I'm surprised that everyone is so set on the frame being the last thing to worry about. Sure these things are tanks, but they're not indestructible. I've got a couple cracks in my frame, and the truck's never been overloaded.

 

Cracks in the frame are most likely due to fatigue, not overloading. If you overloaded a frame to the point of damage, it would result in either a deformation of the rail or rails or possibly a catastrophic failure rendering the truck undrivable. But as mentioned in many other posts, something else would probably fail first. On trucks this old, fatigue sets in, even if you can't see any evidence of it, and can lower the frame's original strength. Not to mention rust. Frame cracks are a very serious matter IMHO.

 

Usually a cracks on frame will appear, near the steering gear box.
This results from a loose steering gear box not being mounted properly, thus fatigue and undue stress on frame, resulting in cracks.

I have hauled and done some very hard four wheeling and off roading with some very heavy loads on my trucks, and have not had any problems with the frame cracking or twisting.

These older Ford Truck Frames are the best frames built of any other truck frame manufactured in this time period.

 

I don't doubt these frames are strong; I'm just saying that they're not as indestructible as most would like to believe. Like snowdog79 said, "On trucks this old, fatigue sets in, even if you can't see any evidence of it, and can lower the frame's original strength." I couldn't have said it better myself.

 

The frame and truck body are made to flex.
Mostly, the trucks body will crack before the frame breaks.

It seems newer vehicles, are having problems with the welds. http://www.aws.org/cgi-bin/mwf/topic_show.pl?tid=5289

Can someone describe fatigue of frame?