Frame strength and design?

[Anor4k]

Question that I didn't see quite answered in other threads.

Im not an engineer so hoping to understand...
if you look at the frame chart for regular cab, the HD frame has more thickness, but the yield strength for both is the same. Why doesn't the yield strength change for the HD, and what is the purpose of the thicker frame? What are they not telling us, is it like torsion strength or something?
Also, the specs for the 122 versus the supercrew are the same but the yield strength is much higher. I'm assuming it is a weaker steal used on the regular cab?
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This started when I was trying to figure out why my front gawr is so low (3150lbs) on my snow plow package.
Does frame affect axle rating or just axle, springs, wheels, brakes etc? If not, is the axle rating so low just bc of the wheels? The snow plow package should have strong springs I imagine.

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[Pedaldude]

That yield strength should be for the grade of steel used, not the frame as a whole. I believe their measurement is the tensile strength in PSI and it’s done on a testing coupon of a standard size that is put in a rig called a load cell that measures how many pounds it will take to pull it apart. Steel is remarkably strong in tension, just look at a suspension bridge. Ladder frames are comically weak in torsional rigidity, which is why they have been obsolete for passenger cars for pretty much a century.

It’s about saving weight and costs. The frames also aren’t completely high strength steel. Only certain components. It’s also to make it a little easier to repair the ladder frame if it’s been in a minor collision because Ford sells the replacement horns to weld back on after a bent one has been cut out. Toyota doesn’t and even a fender bender will total the vehicle if the frame is tweaked at all where the bumper mounts.

I have a HDPP truck and while I don’t plan to plow with it, the front GAWR is pretty disappointing.

 

[Anor4k]

That yield strength should be for the grade of steel used, not the frame as a whole. I believe their measurement is the tensile strength in PSI and it’s done on a testing coupon of a standard size that is put in a rig called a load cell that measures how many pounds it will take to pull it apart. Steel is remarkably strong in tension, just look at a suspension bridge. Ladder frames are comically weak in torsional rigidity, which is why they have been obsolete for passenger cars for pretty much a century.

It’s about saving weight and costs. The frames also aren’t completely high strength steel. Only certain components. It’s also to make it a little easier to repair the ladder frame if it’s been in a minor collision because Ford sells the replacement horns to weld back on after a bent one has been cut out. Toyota doesn’t and even a fender bender will total the vehicle if the frame is tweaked at all where the bumper mounts.

I have a HDPP truck and while I don’t plan to plow with it, the front GAWR is pretty disappointing.

This makes a lot of sense, thank you for breaking it down for me!
I wishbthey would publish the tensile strength but I could see why they wouldn't.
Still wonder why my axle ratings are so incredibly low!

 

[JExpedition07]

49,300 psi is good steel, up to 2015 the Super Duty yield strength was 36,000 psi. The frames are sort of gimmicky imo, people get too caught up in it. An open-c with a thick wall section is great for stopping rust and not cracking but Chevy killed that with advertising. Look at any tractor trailer — it’s an Open-C. Now I have to deal with the downfalls of a closed box in NY salt because a bunch of idiots on the internet fell for a ad campaign. Torsional rigidity is NOT good. It’s only good to an extend then shit starts to crack with age and it rides like shit and stresses other components. Flex is good.

I have seen countless closed box frames fail and crack here, as they rust from the inside out, you never see the open-c fail with age like a close box section.

 

[SilverSpur]

Thickness of metal changes by wheelbase. The longer the span the thicker the frame.

 

[Pedaldude]

It’s the EPA that drove this and a good advertising strategy is pretending something was your idea when you really had no choice. The EPA and NHTSA have shaped the design of modern trucks more than anything. GM just had a savvy ad agency.

Ford mentioned that the box section frame saved 60lbs over the outgoing design.

Ram has been forced into more extreme measures because of their crappy engines and for their 2019 frame redesign saved 225lbs and are even using aluminum cross members:

https://www.repairerdrivennews.com/...5-lbs-with-high-strength-steel-some-aluminum/

By tailoring steel thickness and strength to certain elements of your frame you can optimize your load carrying capacity while reducing weight by not using material where it’s not needed. Back before computer aided FEA, auto makers would make models of their unibody frames in lucite because they discovered that under certain lighting and filters, they could actually see the stress in the model frame when under simulated loads.

One of the reasons why the heavy duty trucks are still using straight open C channels in their frames is because they are made in custom lengths and wheelbases. They are crazy heavy and because of their importance to commerce have largely been allowed a pass by federal agencies. It’s all about the almighty dollar!

 

[Buyer2021]

I believe their measurement is the tensile strength in PSI and it’s done on a testing coupon of a standard size that is put in a rig called a load cell that measures how many pounds it will take to pull it apart.

One might hope that Ford design criteria, as suggested by the heading of the table not shown in the previous post, is based primarily on Yield Strength (stress resulting in permanent deformation) rather than Tensile Strength (stress resulting in total failure), since the former would be detrimental to vehicle behavior / performance while the latter would result in traumatic damage. The two should never be confused, they are different properties of steel.

Tensile Strength of Steel vs Yield Strength of Steel | Clifton Steel

From the above brief and easy-to-understand article:

"While tensile strength is important, you shouldn’t make your decision based solely on that. You also want to take into consideration the yield point, the difference between tensile and yield points, and the elongation percentage."​

​

More FYI - the Maximum Side Rail Section and Section Modulus are characteristics of the actual fabricated frame member (whereas Yield Strength is a characteristic of the metal used to fabricate the frame). All of these table data are of limited academic interest, while they may give some indication of some relative differences among chassis, none of them really convey the behavior / performance of the different chassis' in any practical / quantifiable sense.

Im not an engineer so hoping to understand...

I was (now retired), hope that clarification helps increase your understanding.

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