Don't confuse a long string of good luck with success.

Not to mention, if the vehicle manufacturer says "don't", and you do, and something bad happens on the road, very expensive fingers may be pointed in your direction by insurance companies and lawyers and who knows.

Quote:

However, I have heard from two different experienced welders that "welding behind the axles is fine".
Both have extended truck frames - by welding - numerous times over about 20 years. Anecdotally, neither has ever had a report of failure. One even pointed out that the pictogram shows the failure in front of the axles.

Both NASA Space Shuttle disasters were an outcome of such attitudes, resulting in the deaths of 14 astronauts and traumatizing a generation of school children when their hero teacher Christa McAuliffe perished in the launch of Challenger. In both cases, it was the attitude that "nothing bad happened before when we did this" that allowed both missions to continue to fly when limits were exceeded.

Certainly, there may be some latitude due to conservatism on the part of the manufacturer, but it only takes one mistake to cause a disaster.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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https://tfltruck.com/2022/12/this-huge-truck-campe...

... "but welding extensions on frame rails isn't the same as putting in a slide-in camper"

You're adding cantilevered weight behind the rear axle, though. In addition to beam-strength considerations for the frame, there are also axle loading and vehicle-stability considerations.

... "but the truck already has holes drilled in the frame and the crossmember is welded in"

Yes, and someone in the manufacturer's engineering department has taken all that into consideration in the design, the holes and welds have been carefully placed and accounted for in the published load capacity ratings and, for commercial trucks, in a document that Stellantis/Chrysler calls the Body Builder's Guide https://www.ramtrucks.com/ram-commercial/body-buil... and other manufacturers offer something comparable.

The original vehicle manufacturer provides what in Canada is called the National Safety Mark which is part of the declaration that the vehicle conforms to applicable motor vehicle standards. There are many cases where the original manufacturer (e.g. Ram trucks) provides a partially-built vehicle (e.g. a frame with a so-called "cutaway" cab) to another manufacturer who completes it (e.g. by completing the construction as a school bus) and also affixes the National Safety Mark because they have gone through the process with Transport Canada that authorises them to do it, and that includes whatever engineering and/or re-engineering of the original partially-built vehicle is involved.

So, are those "two different experienced welders" working for a company authorised to apply the National Safety Mark or whatever the equivalent is elsewhere?

If so, then they can speak with the engineering department of this company to see what's involved in cutting or welding a truck frame.

There is another possibility of welding or drilling on vehicle frames, and that is as part of collision repair or corrosion repair process. The vehicle manufacturers also provide information on how their vehicles are to be repaired after a collision or when repairing corrosion damage.

If neither of these cases apply, then they are on questionable legal and insurance grounds.

And yes, I know there are businesses involved in restoring classic vehicles or customising vehicles, which are probably not following procedures in repair manuals (or where such manuals don't exist), and are probably drilling holes and welding stuff together, perhaps even in ways they're not s'posed to.

Even in those cases ... if something bad happens ... which way does the finger point?

Thanks for the responses MintJulep, IRstuff and BrianPetersen. Your comments all reinforce good engineering practice.

Brian, thank you also for the link. The truck in the article - and the one in the comments - both broke 'in front' of the driving axle.
Is it possible that those failures are reinforcing the myth that 'welding behind the axles is fine'?

I guess I'd like to rephrase my question to this: "How did the myth of 'welding behind the axles is fine' start?"

Quote:

I guess I'd like to rephrase my question to this: "How did the myth of 'welding behind the axles is fine' start?"

As a general, and biased, view, welders are mostly men and men tend not to follow instructions. I would guess someone decided that since aft of the axle wasn't specifically structural, they could extend the carrying capacity of their truck by extending the bed. Such an operation will not result in immediate catastrophic failure, so it must obviously be OK, and the manufacturer is just being a wimp. And, to make it worse, increasing the load capacity of a truck changes its resonance properties and bending of the frame; something that was relatively stiff likely is flexing between the axles and that's probably where the fatigue failure will occur, so again, aft-axle welding is obviously safe, since the frame broke elsewhere. However, the root cause is that someone welded and extension to the frame, which changed the failure behavior of the truck.

That's my guess, based on pure conjecture.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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I have worked with a company that makes their own super-heavy-duty wreckers.
They weld on frame rails a lot.
With lots of controlled pre-heat, qualified weld procedures and welders, PWHT, and lots of other details.
And they only do it for their own equipment.
Can it be done, sure.
Should it be done, not unless you are well aware of the issues.
In many trucks there is plenty of excess capacity so the risk is low.
But say they extend a bed to allow carrying more volume of a low density load.
Likely no harm.
But the next owner presumes that he can load whatever he wants to and disaster ensues.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

https://www.ramtrucks.com/BodyBuilder/service/Imag...

Rollback carrier reinforcement instructions.
Sadly no mention of what the frame rails are made of, norany details about weld filler, preheat or ???
Assuming the lengthening is forward of the rear axle, the tension flange is bolted, not welded.
The top (compression) flange and web are butt welded with a beveled weld prep.
The web "side reinforcement plate" is stitch/intermittent welded.

Figures 10-13.

Fig 9 is missing.

The key sentence is the second one in that document. "But the persons modifying the frame must assume complete responsibility for assembly, performance, reliability and compliance to all applicable FMVSS standards."