Question for the fabricators amongst us

Do you trust what you know? Can you look at something done a certain way and derive anything useful from how it is done?

Or, do you just throw lots of metal at it and solve the problem that way?

Hypothetical case in point-

Transmission cross member to hold up a 32RH at the end of a 4.0 with a NV241OR hanging off the other end.

What size would you make the cross member?
What size would you make the bushings if you use them at the frame?
Would you try to reuse the OEM trans mount?
What thickness would you make the tabs for the frame side connections?
What diameter and grade bolt through those tabs?

Do you trust what you know?

Bonus question- can you make the 97-02 transmission adapter mount low profile, if so, by how much?

How does one know what they know? If they have the experience of making it wrong and seeing it fail (even better if it's several times in several different ways), then they probably have a good idea of what it takes. Or if they have the math behind the loads and stresses applied to the part and it's material properties, then they probably have a good idea.

Anything beyond that is just guessing, and one guys guess being lucky enough to land closer to the "right" design doesn't necessarily make him better at anything but guessing.
 
How does one know what they know? If they have the experience of making it wrong and seeing it fail (even better if it's several times in several different ways), then they probably have a good idea of what it takes. Or if they have the math behind the loads and stresses applied to the part and it's material properties, then they probably have a good idea.

Anything beyond that is just guessing, and one guys guess being lucky enough to land closer to the "right" design doesn't necessarily make him better at anything but guessing.
So I'm guessing when I look at the TJ axle control arm mounts and understand the loads there?
 
So I'm guessing when I look at the TJ axle control arm mounts and understand the loads there?

I wouldn't call it guessing to leverage billions in R&D spent by a global auto manufacturer to use as a minimum baseline, I'd call that smart, and critical to the process for someone that doesn't have an army of engineers and Elon Musk's net worth. That said, they DO have a non-zero acceptable failure rate in a given time period, and the harder we beat on our rigs the more likely we are that we'll find ourselves part of that statistic. So, if we decide that a design that met Chryslers accepted failure rate in their design conditions returns an unacceptable failure rate in our conditions, how do we know how much material to add? We obviously can't reproduce their R&D, so what do we do?

Clearly, as the inspiration of this thread, there are those that just make it so heavy and overdesigned that it can't possibly fail. Sure, it's sloppy and inefficient, but they're probably making a conscious decision that the profit they're losing out on due to material waste doesn't justify the cost of the upfront testing and design work to do it right. I make no claims to that being a right or wrong approach, because (provided they have data in some form) it can be right or wrong depending on the cost-benefit of the up-front analysis and testing.

I consider designing without data to be guessing. Data can come in different forms, including experience. It's not going to be as precise as the army of engineers and billion dollar testing facilities but it's the best most low-volume offroad parts producers are going to be able to do.

I'm a mechanical engineer, mostly a fluids/thermo/heat transfer guy, and though I don't call myself a fabricator, I've done enough mechanical component design to have an eye for what's necessary for most things if I have the factory part to go off of. I have no idea how anybody would even start designing a part without any of that experience informing their design process; so much so that it's hard for me to believe that there would truly be any significant numbers of people like that in the industry. But I've been wrong before.
 
I wouldn't call it guessing to leverage billions in R&D spent by a global auto manufacturer to use as a minimum baseline, I'd call that smart, and critical to the process for someone that doesn't have an army of engineers and Elon Musk's net worth. That said, they DO have a non-zero acceptable failure rate in a given time period, and the harder we beat on our rigs the more likely we are that we'll find ourselves part of that statistic. So, if we decide that a design that met Chryslers accepted failure rate in their design conditions returns an unacceptable failure rate in our conditions, how do we know how much material to add? We obviously can't reproduce their R&D, so what do we do?

Clearly, as the inspiration of this thread, there are those that just make it so heavy and overdesigned that it can't possibly fail. Sure, it's sloppy and inefficient, but they're probably making a conscious decision that the profit they're losing out on due to material waste doesn't justify the cost of the upfront testing and design work to do it right. I make no claims to that being a right or wrong approach, because (provided they have data in some form) it can be right or wrong depending on the cost-benefit of the up-front analysis and testing.

I consider designing without data to be guessing. Data can come in different forms, including experience. It's not going to be as precise as the army of engineers and billion dollar testing facilities but it's the best most low-volume offroad parts producers are going to be able to do.

I'm a mechanical engineer, mostly a fluids/thermo/heat transfer guy, and though I don't call myself a fabricator, I've done enough mechanical component design to have an eye for what's necessary for most things if I have the factory part to go off of. I have no idea how anybody would even start designing a part without any of that experience informing their design process; so much so that it's hard for me to believe that there would truly be any significant numbers of people like that in the industry. But I've been wrong before.
Is 1/8" thick HR steel flat bar strong enough to hold up both ends of a transmission cross member?
 
Is 1/8" thick HR steel flat bar strong enough to hold up both ends of a transmission cross member?
Depends on your tolerance for deflection and your expected part life. 😉🤣

Hold it up, sure. But I wouldn't take it out of my driveway. Give it some width and some stiffening ribs and it would do fine for most. I wouldn't be able to judge the effectiveness of the ribs by sight though, so I'd probably put it in a press and compare the deflection to a stock skid.
 
Depends on your tolerance for deflection and your expected part life. 😉🤣

Hold it up, sure. But I wouldn't take it out of my driveway. Give it some width and some stiffening ribs and it would do fine for most. I wouldn't be able to judge the effectiveness of the ribs by sight though, so I'd probably put it in a press and compare the deflection to a stock skid.
And that's the difference between an engineer and someone who knows how to build stuff. I guarantee you that it won't fail.
 
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lock a bolt through that and you'll have a hard time bendin it. the load it's gonna carry, and the way it's carried, it should be fine.
 
And that's the difference between an engineer and someone who knows how to build stuff. I guarantee you that it won't fail.

How do you know you're not using excess material or labor to produce the part you'll guarantee not to fail? Do you care? Does it matter?

I'm really enjoying this answering a question with a question thing. 😊
 
See if you can figure out just about how much longer I'm going to enjoy it?

I'm honestly not trying to needle you, I'm truly interested in the answer, because I have no idea how I'd do it without the foundation I have. The "fabricators" in engineered mass production mostly just like to bitch and moan without offering any solutions and are obviously a different breed than the ones that have to design their own stuff as well as fab it. The ones that are good end up in indirect positions making tools and fixtures and I didn't get to deal with them as much. I'd like to learn a perspective that I haven't been exposed to.
 
most engineers can't put together kids toys.
But we can pull em apart!

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Do you trust what you know?
Yes, I trust what I know but don't always trust my ability to do.

Can you look at something done a certain way and derive anything useful from how it is done?
Oh yeah, I make a living from reverse engineering. Matter of fact I am much better at repairing and modifying using reverse engineering than I am at coming up with a design from scratch.

Or, do you just throw lots of metal at it and solve the problem that way?
Sometimes because that is what I have on hand and don't have time or funds to design, purchase, and build correct, but throwing metal or parts that I have available will get it done that night while deferring the spend.

Hypothetical case in point-

Transmission cross member to hold up a 32RH at the end of a 4.0 with a NV241OR hanging off the other end.

What size would you make the cross member?
I have 1.5" x 0.120 and 1.75" x 0.120 wall tubing left over from another project so I would most likely try to use one of those. I would most likely dig around to see which piece was closest to length, bend, etc... and use it to reduce scrap or cutting a long tube..

What size would you make the bushings if you use them at the frame?
I have a set of poly bushings that I bought for another project and never used because I came up with a different design so I would just use those. I believe they are 1.75 OD, 3" long with 1/2" bore.

Would you try to reuse the OEM trans mount?
I would try to reuse. I always try to reuse as much as possible.

What thickness would you make the tabs for the frame side connections?
I have 3/16 laying in the corner of my garage, so that is what I would use.

What diameter and grade bolt through those tabs?
The bushings I have are 1/2" ID, so 1/2".

Do you trust what you know?
Yes, again I trust what I know more than my ability.

Bonus question- can you make the 97-02 transmission adapter mount low profile, if so, by how much? I have actually never seen one in person, so I don't know but I if I had to I would do a search and find this thread and now i know. I have no problem with copying.
 
here's a stress crack for y'all..........this sounded like a lightning bolt hit the building.

this is about a 14" x 4.25" x 4.25"chunk of the bending bed that used to be 12ft long an sat under a monster press brake. look what this fool went and did to it. 12K mistake......BOOOOOM. he's lucky it didn't come loose and tag him.

anyways this piece is mine now and i'ma clean it up and weld it up and use it on my hydraulic press at home.

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131737780_1021742971641225_8944926815099217899_n.jpg


131762563_1599788360228747_377526561641223120_n.jpg
 
here's a stress crack for y'all..........this sounded like a lightning bolt hit the building.

this is about a 14" x 4.25" x 4.25"chunk of the bending bed that used to be 12ft long an sat under a monster press brake. look what this fool went and did to it. 12K mistake......BOOOOOM. he's lucky it didn't come loose and tag him.

anyways this piece is mine now and i'ma clean it up and weld it up and use it on my hydraulic press at home.

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There is a reason they warn against coining stuff.