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

[RoyJ]

Some math for those curious

The Ram 3500 has a frame section modulus of 9.3 in^3, tensile strength is 50,000 psi (or lb/in^2). Multiply the 2 and we have a max frame bending moment of 465,000 lb-in, or 38750 lb-ft.

In the wrecker example (3k lbs hanging 6' out back), bending moment was 18k lb-ft. To bend the frame, you'll need a bump generating 2.15g

If the camper was loaded so COG shifted a foot back (which is a LOT), you'll need a bump generating 6.46g. A 200 lbs bike 6' back on a hitch carrier is only 1200 lb-ft, or 3% of the Ram's frame strength - negligible.

The math is simplified, assuming bump stop contact, and all weight rests on the bump stop. In reality, a dynamic impact would load the spring hangers, shock mounts, and bump stop, resulting in less bending moment (multiple fulcrum points)

 

[calicamper]

If you take a close look that truck kinked at the front cross bar / camper Tie down. Not by the rear suspension. It’s possible that tie bar was mounted in a manner that created or developed a weak spot in the frame, but that section of the frame is also where modern crash rated trucks bend ie the cab/bed location. The discussion regarding lever effect on load numbers vs frame strength definitely is in play with how this rig bent.

 

[ramblinChet]

Too many assumptions here.

Who stated and confirmed that the camper tie down were welded to the frame? The tie downs I see are manufactured by TorkLift and are bolted on to existing holes and also use a large clamp.

If the tie downs were welded to the frame, who stated and can confirm that the frame is heat treated? Many of the larger trucks (Mack, Volvo, etc.) do have heat treated frames and are marked "HEAT TREATED - DO NOT WELD TO RAIL OR DRILL FLANGES." I have no knowledge regarding heat treating for 3/4 or 1-ton frames and have not seen a similar marking on mine.

Again, why is everyone discussing welding on a heat treated frame if none of this has been confirmed?

 

[dstefan]

This was posted recently on the “Overlandy Meme Thread“. Having seen a video of his in the past and thinking it was a well-built Tundra, I found these two videos about how this happened that are quite detailed and pretty interesting as to the cause and results of the event (along with being typical Youtuby clickbait epic journey stuff).

The first video (chapter four, part two) shows the actual incident that bent the frame starting shortly after the 17 minute mark. The second video (chapter five) recaps the issue, but then goes on to show him limping this screwed up Tundra up over Lippincott Pass (during the night and with a fast leaking, radiator!) into Death Valley where he gets picked up.

The owner appears to have built this rig to be a combination overland vehicle, and a desert racer. Early in the chapter 4 video, he speaks about the suspension allowing him to bomb over the road, and mentioned he’s going 65 mph in one stretch. You’ll see the frame bending incident was him mis-gauging a dip in the road because he was going so fast. He also mentions that he can’t believe how well the truck handles the camper despite the camper’s 2200lb weight! I suspect this guy is/was more than double his trucks payload, not to mention it’s a crew cab and the camper sits farther back.

He sure made a lot of questionable decisions, and he does admit to everything that happened being fully his fault, but he doesn’t address the role speed and weight played in the problems he had. I pretty much think he’s an idiot, but I have some grudging admiration for how he got himself out of the fix he got into, and the toughness of the poor Tundra.

 

[rruff]

...that section of the frame is also where modern crash rated trucks bend ie the cab/bed location.

And 3500 pickups are crash rated. Basically made to collapse on impact, saving the occupants from high G forces. Obviously this goal will be somewhat at odds with making the frame strong.

The big unknown here is exactly what sort of forces this truck was subjected to. If he was in the habit of going to out of the way places, that means dirt roads and ravines/ditches, undulating bumps, etc with many opportunities to greatly amplify loads and get crossed up. Since all the manufactures have gone from C-channel to fully boxed frames on 1-ton pickups, the frame needs to be strong enough to handle that big load, and teeter on two diagonal wheels... repeatedly. But it might be an area where Ram simply didn't design enough strength margin for this kind of loading.

Here is some marketing BS slamming C-channel (older Ford) from both Ram and GMC.

 

[rruff]

This was posted recently on the “Overlandy Meme Thread“.

17:30 in the first video is the best part. He's flying down a rough dirt road, gets air through a ditch/wash, and then smacks into the other side. "Estimated dynamic force of 18 tons".

 

[RoyJ]

If the tie downs were welded to the frame, who stated and can confirm that the frame is heat treated? Many of the larger trucks (Mack, Volvo, etc.) do have heat treated frames and are marked "HEAT TREATED - DO NOT WELD TO RAIL OR DRILL FLANGES." I have no knowledge regarding heat treating for 3/4 or 1-ton frames and have not seen a similar marking on mine.

Again, why is everyone discussing welding on a heat treated frame if none of this has been confirmed?

Just hearsay passed around forum to forum. If it really is a Torklift, then it is a bolt-on.

Heat treatment on these modern pickups probably falls in the gray zone. Older ones like c-channel Fords were near certainly not heat treated, 36 ksi steel is pretty much rebar grade. Ram uses 50 ksi, GM uses 60 ksi, and believe modern boxed Ford SDs use 60 as well. Half tons use up to 70 ksi on some sections (under the cab). We're getting into high tensile range.

MD trucks use mostly 80 ksi, and Class 8 trucks around 130 ksi. Those like you said are definite no-nos for standard welding. Still, if I were to weld on a modern pickup, I'd use pre-heat and post-heat after weld just to be safe.

 

[RoyJ]

View attachment 759525

Crew cab short beds seems to be the worst for frame bending.

1) COG is almost always behind the rear axle

2) The bump stop is the closest to the cab to bed transition point. That point appears to be a weak-link, thus all the bend examples we see happen here.

The frame under the cab is the thickest / tallest, and under the bed it's the thinnest. Furthermore, the bed and cab adds significant strength to the frame, whereas at the transition point, all bending stress is solely carried by the 2 frame rails.

They're also most likely to be driven like a trophy truck by some bonehead...

 

[RoyJ]

Pictures to show why frames always break where they do, look how much thinner it gets past the cab:

Now hang a 2000 lbs camper out back, hit a big bump, full bump-stop contact (just inches past the the cab to bed transition), not hard to bend the rails.

I just learned this about the Tundra frame, the front part is boxed (that I know), the cab section is the tallest, but also reinforced (double flanged):

The bed section don't even have a full flange, it's literally a C channel. I hope manufacturers makes future frames stronger under the bed section.

 

[plh]

In the wrecker example (3k lbs hanging 6' out back), bending moment was 18k lb-ft. To bend the frame, you'll need a bump generating 2.15g

2.15g isn't much.

 

[rruff]

Crew cab short beds seems to be the worst for frame bending.
They're also most likely to be driven like a trophy truck by some bonehead...

I don't know about others, but on the Tundra 8' bed, the most forward bed mount is a stout pillar that attaches to the beefiest part of the frame (the double-thick C-channel that runs under the cab). But the 5.5 ft bed is entirely attached to the thin C-channel on the aft end. He then mounted a stiff flatbed and camper to a flexible frame, which stiffened the rear, but amplified the stress at that forward mount where the stiffness suddenly terminated. Then to make it even worse, he had someone weld a cross brace stiffener to the frame itself... which also terminated right behind the cab!

 

[RoyJ]

I don't know about others, but on the Tundra 8' bed, the most forward bed mount is a stout pillar that attaches to the beefiest part of the frame (the double-thick C-channel that runs under the cab). But the 5.5 ft bed is entirely attached to the thin C-channel on the aft end. He then mounted a stiff flatbed and camper to a flexible frame, which stiffened the rear, but amplified the stress at that forward mount where the stiffness suddenly terminated. Then to make it even worse, he had someone weld a cross brace stiffener to the frame itself... which also terminated right behind the cab!

Sounds like the absolute worst case for stress concentration!

Based on your info, I feel the ideal truck for high speed desert type offroading is a reg cab long bed. Load the COG ahead of the rear axle (easy on a long bed), so during a full compression impact, it's actually bending (not literally) the frame upwards and not downwards.

Since that bending moment occurs at the double framed and tall section, it's unlikely to actually bend.

Setup similar to this:

 

[JaSAn]

. . .
The bending moment of 465,000 lb-in, or 38750 lb-ft.

In the wrecker example (3k lbs hanging 6' out back), bending moment was 18k lb-ft. To bend the frame, you'll need a bump generating 2.15g . . .

Where are these numbers from?

Wreckers are a different animal. The Class B I drove (Ford 650) had:
- a straight boxed frame from behind the engine.
- frame reinforced with L channel from under cab to tail.
- wrecker box is heavily reinforced.
- boom/sling superstructure is engineered for weight and bending placement.

Manufacturers publish maximum GVWR/GVAR/payload numbers. Those have been tested. Not hard to find pictures of all makes with this failure when overloaded.

 

[Metcalf]

A few notes on the 'welding to the frame' thing that is going around....

In general, most all factory frames ( these days ) are welded assemblies that use good quality steel that is either hydroformed or die formed to create sub-sub-assemblies that are then WELDED together. As far as I know, they do not use any post heat treating process. Weld filler material may vary based on material. Most of these welds are done with a robot. OEM quality control on those weld processes, in my experience, are hit or miss. I've seen good strong welds and I have seen crap welds.

Welding to a frame should be taken seriously, but doesn't require anything more than good welding practice. Bad welding practice, like if you get undercutting or whatever, can create serious fatigue issues. I don't suggest welding to the frame if you don't have to, but there are a ton of OEM level conversions in the aftermarket that weld things to the frame. There are also a lot of repairs that require welding to the frame as far as suspension or collision repair.

 

[RoyJ]

Where are these numbers from?

Wreckers are a different animal. The Class B I drove (Ford 650) had:
- a straight boxed frame from behind the engine.
- frame reinforced with L channel from under cab to tail.
- wrecker box is heavily reinforced.
- boom/sling superstructure is engineered for weight and bending placement.

Manufacturers publish maximum GVWR/GVAR/payload numbers. Those have been tested. Not hard to find pictures of all makes with this failure when overloaded.

Section modulus listed here (under Measurements):

https://www.carhp.com/ram/3500-2014/specifications

50 ksi is published data from Ram. 3000 lbs is a conservative minimum based on light wrecker bodies:

https://www.millerind.com/products/light-duty/century-411-412

Most that I've seen do not have frame reinforcement:



6' is typical distance from wheel center of truck to towed axle. Wreckers come in all shape and sizes; many older Class 3s running around daily where I live, like these:

http://secondlifetruck.com/trucklist.php?make=33&model=1548&type_id=7

You Class 6 wrecker is MD, different animal.

GVW takes many things into account, tire / axle / brakes included. If you're saying the same thing wouldn't happen at under GVW, what about those near empty Raptors and Gladiators?

I'm saying there something beyond overloading - big bump off road, blew through suspension travel