is the f150-350 2wd frames the same? i say this because i was working on a 1997 f250 2wd and the frame looked pretty thin for a f250. looked like my f150 frame. whats different about it?

 

I haven't seen much difference myself either as far as thickness.

 

Even the 2wd F-250's had stronger frames than the F-150.

 

this is true, but how are they stronger? what kind of places get extra metal or a bend to increase rigidity?

 

The whole frame is stronger. It's not just reinforced in a few places. When you are towing a big load, say 8000#'s or more, every time you hit a bump with the trailer you transfer an enormous amount of energy to the truck's frame. Hitting a pothole with a trailer that weighs 8000#'s at 35mph may be a momentary weight of 80,000 or 100,000 pounds. Your entire truck frame has to absorb all that force. The whole frame has to be stronger, or the energy would attack the weak point and that's where it would fail.

Every time you tow a load, forces are being applied to the frame in every direction. Compressing, stretching, twisting, and bending in all directions. This is the overlooked thing that makes a 3/4 ton truck able to tow more weight than a 1/2 ton truck. This is why people who put 3/4 ton axles on a 1/2 ton trucks aren't happy with the results. They tow their fifth wheel a few times and everything seems fine, but the frame has stress cracks all over it and when they hit a big enough pothole, CRACK.

You won't necessarily see the differences, but they are there. The steel might be produced in such a way to make it more able to absorb energy without damage (I don't know much about steel production).

 

thanks for clearing that up, ive been wondering how that frame can be rated at 3/4 ton.

 

Theoretically this is true, but in reality I have never seen much if any difference. I have never heard of any "special steels" used in a heavier truck either. I understand were you are coming from, but I like hard evidence.

Someone who has a bunch of these trucks in the backyard could probably mic the frame thickness and report back the results.

I do have experience comparing the 250/350 trucks with the 350 chassis/cab trucks. The chassis cab truck frame rails are narrower in the back, the dually rearend is narrower with narrow leaf spring packs, and it has what looks like a normal frame, with a reinforcement layer of flat steel riveted to the top and bottom frame rails, running front to rear.

 

I certainly agree the 3/4 and 1 ton have stouter work to do, but that doesn't say the frame differs. It could be one stout frame for all. I'll look it up in the parts catalog if someone will give me the group number.

 

You can't measure tensile strength with a mic, and there is much more to frame strength than the thickness of the metal. There is simply way more to this that either of you guys realize.

Yes, I HAVE done my research on this.

 

I agree with you on different alloys and tensile strength in steels, and frames have their combos of these to achieve the desired strength. But I would like to see hard evidence of this being different between a Ford f150 frame and a f250 frame.

 

It all has to do with the SECTION MODULUS which refers to the frame rail depth.

I cannot give you the section modulus after 1979, because this info is no longer listed in the parts catalogs...but it was prior to 1980.

1979 F150 2WD 133" wheelbase: Section Modulus: 3.95"

1979 F250 2WD 133" wheelbase: Section Modulus: 4.33"

The frame rails are also thicker on an F250 over an F150.

The frame part numbers will also be different.

Pick an F150 & F250 frame from any year after 1980...I'll betcha the part numbers will not be the same.

If the part numbers aren't the same, the frames are not the same.

 

Bill, do you any yield strength numbers on 1/2 ton vs. 3/4 ton frames? I can't seem to find much in a google search.

 

I know the part numbers will be different, because they are setup different for leafs versus coils in the front.

It stands to reason if they made the frames different in the 70's, they continued this practice in the 80's. And you would probably not catch the difference between 3.95 and 4.33 by eye, so unless someone has any more info, I would agree there is probably a actual size difference between the lighter trucks and the heavier trucks.

 

I measured my F250 ('83) chassis rails today - .190" thick
I understood F350 rails to be the same, but I don't know for sure.
About 9 years ago I welded a crack in an Australian-built '79 F100 frame for a customer - it was a lot lighter than mine - probably 1/8" thick - & for memory, I think it was shallower, as well

 

I've never compared a F250LD frame to a F250HD or F250 frame, but, F250 HD(over 8500 GVWR) and F350 frames are built to the same spec, same thickness, height and rail width, all of which are considerably larger than an F150 chassis. IIRC, in most places the frame has a 2" higher section and is around 1" wider, this varies along the length of the chassis due to the different bends etc, but, for the most part those numbers are useable. They are also double if not a little more the thickness of an F250 chassis.