Frame Strength, Who to Believe! The RV Sales Guy or the New Truck Dealer? Maybe Neither?

[rruff]

Page 19, F350 chassis, 36 ksi frame, 8.7 sec modulus. 26,100 lb-ft bending strenght, believe it or not, lower than a modern Ram pickup. Thickness by itself can be deceiving.

Good work, but... a single # for section modulus doesn't make sense since we know this is highly variable at different parts of the frame. In the Ford video above he says the chassis cab has a much thicker frame even on the boxed section, while the pickup is "closer to 1/8 inch". This would correspond well with what what NoDak said above, that Ram boxed pickup frames are "3-4mm thick". I suspect the chassis-cab frame is more stout.

Also... a couple other factors like tie-downs creating a localized stress right at the point where the failure occurred, and the inability of boxed frames to flex torsionally... if he did get adventurous at all offroad. And he probably did bottom out a few times; that's very easy to do when over the load limit.

 

[RoyJ]

Good work, but... a single # for section modulus doesn't make sense since we know this is highly variable at different parts of the frame. In the Ford video above he says the chassis cab has a much thicker frame even on the boxed section, while the pickup is "closer to 1/8 inch". This would correspond well with what what NoDak said above, that Ram boxed pickup frames are "3-4mm thick". I suspect the chassis-cab frame is more stout.

Also... a couple other factors like tie-downs creating a localized stress right at the point where the failure occurred, and the inability of boxed frames to flex torsionally... if he did get adventurous at all offroad. And he probably did bottom out a few times; that's very easy to do when over the load limit.

The video shows an F450, which is much stronger than a 350 chassis cab. But good question, I too wondered where they measure it. Took some more digging:

https://madocumentupload.marketinga...189f4d99779c27fea908a96bf7d7983e96a4&v5=False

Pg 14, left side. F450 and 550 frames vary by wheelbase. Makes sense - longer WB causes more bending under normal loading. i.e. not reverse bending like tow truck or rear heavy camper.

Long wheel bases use up to a 0.6" thick frame! Massive 17.2 sec modulus (51,600 lb-ft bending).

Best part - fine print 3) says it's measured at back of the cab, reinforced section. Guess where all the frame bends have occurred...

 

[rruff]

Best part - fine print 3) says it's measured at back of the cab, reinforced section. Guess where all the frame bends have occurred...

I haven't seen bent chassis-cab trucks though, just the fully boxed frames.

 

[Bama67]

Say what you want about the old flexy C-channel Fords but they take some insane abuse and it's very rare to see a frame fail.

But about the Tundra, it always amazes me how people overload those so badly. My full-size Tundra 4x4 had a lower payload than my little Gladiator by 300lbs.

Replaced the Tundra with a Ford F350 6.2 that gets the same MPG, just as reliable, and 3 times the payload capacity.

 

[stevo_pct]

I still don't understand how, in some of those pictures above, there are bent frames from towing a teardrop camper. How is that possible?

 

[WOODY2]

It seems there might be more to this whole "truck camper thing" than:

A: "the salesman said"

B: "I think we are under GVWR"

C: "so far I have had no problems"

D: "It's a full size truck"

CG?What the hell is that? Hurry up and load the rest of the stuff Martha. Ican't wait to see if this drives like one of those Trophy Trucks.

 

[WOODY2]

I still don't understand how, in some of those pictures above, there are bent frames from towing a teardrop camper. How is that possible?

Just a tad too much speed thru some serious whoops.

 

[stevo_pct]

Just a tad too much speed thru some serious whoops.

Those things only have 200-300 pounds of tongue weight. I understand that the weight is situated all the way back behind the axle but its not much different from having 200-300 pounds stacked in the back against the tailgate. That shouldn't be a problem for a mid sized or full sized truck.

Maybe the trailer has sort of a whiplash effect or dynamic loading when going over some of the whoops? In that case I would think the hitch receiver would bend before the frame but I also don't know much about that stuff.

 

[rruff]

Maybe the trailer has sort of a whiplash effect or dynamic loading when going over some of the whoops?

Ya, like... when the trailer gets air and slams down right when the rear of the truck is already bottoming out...

I'd like to know more about the provisions for crash standards. My brief search indicates that 1 ton pickups are crash rated, while 1 ton cab-chassis are not. So what exactly is done to the pickup frames to make them collapse on impact... and is this what is making them fail under load?

It also appears to be a "shortbed phenomena"... maybe where the weak part of the frame and the highest stress location coincide?

And... I wonder about fully boxed frames. They are torsionally stiff which is great, but they may not be strong enough to stand high twisting forces with a big load. In this case the initial damage to the frame was probably caused by twist, but it didn't completely fail until later.

 

[jbaucom]

It also appears to be a "shortbed phenomena"... maybe where the weak part of the frame and the highest stress location coincide?

Except the Ram 3500 in the original post is a long-bed.

 

[rruff]

Except the Ram 3500 in the original post is a long-bed.

That's right! I think... it was called a shortbed in the article, but it must be 8.

 

[JaSAn]

I still don't understand how, in some of those pictures above, there are bent frames from towing a teardrop camper. How is that possible?

These kind of failures usually happen over time: stress fractures start and propagate over time until the frame is too weak for a shock load. Don't know the history of other vehicles that failed. OP had ~25,000 miles carrying that load.

 

[stevo_pct]

These kind of failures usually happen over time: stress fractures start and propagate over time until the frame is too weak for a shock load. Don't know the history of other vehicles that failed. OP had ~25,000 miles carrying that load.

Yeah but I would think a full sized or mid sized truck could tow a tear drop camper (even a heavier off-road one) for the lifetime of the truck with no issues. Unlike the load on the OP's truck, a small trailer is well within all towing specs.

 

[tacollie]

Yeah but I would think a full sized or mid sized truck could tow a tear drop camper (even a heavier off-road one) for the lifetime of the truck with no issues. Unlike the load on the OP's truck, a small trailer is well within all towing specs.

There was a thread about it. The biggest take away is that off-road creates dynamic forces that are harder on the vehicles. Hence vehicles with off-road pack just tend to have lower payloads and tow ratings.

You read the whole thread or you could just read this post. Articles they link are really good.

Colorado / Canyon bent frame

I know that I have never used trailer brakes when I towed up to 5k lbs with my 4runner. The 1k limit for no trailer brakes is listed in the Tacoma's manual, not sure if it is also listed in the 4runner's. But as you point out, most Toyota owners likely aren't aware of that restriction....and I...

expeditionportal.com

 

[jbaucom]

There's a reason that Yakima rates their truck bed racks for only 60% of their on-road capacity when used off-road and Leitner rates theirs at 50% when used off-road. The forces generated off-road are unpredictable. Folks would be wise to consider this when selecting a vehicle for carrying loads off pavement. There's a big difference in the occasional overloading for a brief time on-road (or off-road) to accomplish a task versus a state of perpetual overload for years on end. Some people may never have a problem with a 4WC on a Tacoma or a giant Eagle Cap on a Ram 3500 because they rarely venture out on rough roads and/or slow down considerably when they do. Another could break their frame towing a 3,000 lb teardrop for the first time because their truck's suspension can swallow up the bumps without spilling their coffee, so they think they don't have to slow down off-road, but they don't realize or understand what's going on back there at the trailer hitch.