The Silver Fox LJ Build

[mrblaine]

Understanding stress risers and how they propagate is something I could use a better grasp of when designing fish plates.

We violate so many sound practices by just throwing lots of metal and weld at stuff that it almost doesn't matter. The FSM warns against welding close to the edges of the frame to slow down cracking and then the propagation thereof to failure. If we followed those warnings, it would be difficult to create most of the modifications we routinely do.

The common idea behind a fish plate is to direct the stress riser into the center of the sides of the frame away from the edges. We rarely do and it never fails.

 

[JMT]

We violate so many sound practices by just throwing lots of metal and weld at stuff that it almost doesn't matter. The FSM warns against welding close to the edges of the frame to slow down cracking and then the propagation thereof to failure. If we followed those warnings, it would be difficult to create most of the modifications we routinely do.

The common idea behind a fish plate is to direct the stress riser into the center of the sides of the frame away from the edges. We rarely do and it never fails.

How does a diamond shaped plate direct stress to the center of the sides of the frame Blaine? And should the corners of a diamond shaped plate fish plate be rounded off? I wish I even knew to ask a question before doing anything.

 

[mrblaine]

How does a diamond shaped plate direct stress to the center of the sides of the frame Blaine?

You are increasing the length of the path the stresses follow and then directing them into an area that is less likely to crack due to the orientation of the stressed member.

And should the corners of a diamond shaped plate fish plate be rounded off? I wish I even knew to ask a question before doing anything.

Depends on the size and what you are trying to accomplish. If and the if is the part that matters since it won't happen, but if you were to stress the rear bumper to move it repeatedly in the direction of the arrows, the crack path is the red line. It would crack there for 3 reasons.
1- the fish plate is too thick and you have created a very rigid section that butts into a more flexible section.
2- the heat affected zone is the shortest path of stress.
3- the stress path is as short as it can be and doesn't redirect any of the load along a longer path.

None of any of that matters since you will never have enough load back there that flexes the rear crossmember up and down.

Your plug welds are almost a waste, they add something but if the connection were dependent upon them, there aren't enough.

 

[bbaldwin237]

You are increasing the length of the path the stresses follow and then directing them into an area that is less likely to crack due to the orientation of the stressed member.

Depends on the size and what you are trying to accomplish. If and the if is the part that matters since it won't happen, but if you were to stress the rear bumper to move it repeatedly in the direction of the arrows, the crack path is the red line. It would crack there for 3 reasons.
1- the fish plate is too thick and you have created a very rigid section that butts into a more flexible section.
2- the heat affected zone is the shortest path of stress.
3- the stress path is as short as it can be and doesn't redirect any of the load along a longer path.

None of any of that matters since you will never have enough load back there that flexes the rear crossmember up and down.

View attachment 661279
Your plug welds are almost a waste, they add something but if the connection were dependent upon them, there aren't enough.

As someone that is not doing this type of thing now, but can see myself doing in the future, thank you for that explanation and drawing. When reading it, I was confusing myself, but then the picture made sense to me and made everything else make sense. Like Jeremy, I don't think I would have even known to question it before.

 

[mrblaine]

I don't think I would have even known to question it before.

You don't because it isn't critical for what and how we do things. If they were critical, lots of failures and how to prevent them would be posted up very often. As much as we like to think we are building something badass that can handle big loads, we really aren't. The loads on that rear crossmember are not high or cyclical. Start pulling a trailer with a lot of tongue weight on rough roads, now we have to pay attention. Until then, just throw metal and weld at it and it will be just fine.

The main reason we fish plate is we don't trust our welds and joint prep. FYI, 90% of my fish plates are simply weld backing so I get full penetration on highly beveled joints and I don't want burn or melt through. They are almost never the same thickness as the material I'm working on. I routinely use 1/8" and .150" thick material for frame plates.

 

[JMT]

You are increasing the length of the path the stresses follow and then directing them into an area that is less likely to crack due to the orientation of the stressed member.

Depends on the size and what you are trying to accomplish. If and the if is the part that matters since it won't happen, but if you were to stress the rear bumper to move it repeatedly in the direction of the arrows, the crack path is the red line. It would crack there for 3 reasons.
1- the fish plate is too thick and you have created a very rigid section that butts into a more flexible section.
2- the heat affected zone is the shortest path of stress.
3- the stress path is as short as it can be and doesn't redirect any of the load along a longer path.

None of any of that matters since you will never have enough load back there that flexes the rear crossmember up and down.

View attachment 661279
Your plug welds are almost a waste, they add something but if the connection were dependent upon them, there aren't enough.

Ok, this all makes sense. Thank you for explaining. This is good information. It helped me clarify some threads on welding forums that you stimulated me to go read. Learned some things.

 

[mrblaine]

Ok, this all makes sense. Thank you for explaining. This is good information. It helped me clarify some threads on welding forums that you stimulated me to go read. Learned some things.

 

[JMT]

View attachment 661469

In that pic, would you just fill in the spot welds and it would be structurally sound?

 

[mrblaine]

In that pic, would you just fill in the spot welds and it would be structurally sound?

Possibly. I was just illustrating the more appropriate use of plug welds.

 

[lBasket]

I like the idea of using enough metal that the engineering doesn't really matter that much. Way easier

 

[sab]

I like the idea of using enough metal that the engineering doesn't really matter that much. Way easier

The downside to that strategy is that, if you do enough modifications that way, you end up with a heavy vehicle. Weight really creeps up on you, if you aren't diligently paying attention to it, and weight affects so much (accelerating, stopping, turning, fuel economy, and part durability).

 

[JMT]

The downside to that strategy is that, if you do enough modifications that way, you end up with a heavy vehicle. Weight really creeps up on you, if you aren't diligently paying attention to it, and weight affects so much (accelerating, stopping, turning, fuel economy, and part durability).

A couple of things @mrblaine. 1) I wondered while doing the rear frame tuck if a fish plate was necessary. Seems from everything I read that they are pretty standard when you have a cracked frame or when the frame is cut, but is a fish plate necessary for this application? 2) From your description, would a smaller fish plate that is diamond shaped in approximately the following dimensions be sufficient? 4”x2”x1/8”

 

[lBasket]

The downside to that strategy is that, if you do enough modifications that way, you end up with a heavy vehicle. Weight really creeps up on you, if you aren't diligently paying attention to it, and weight affects so much (accelerating, stopping, turning, fuel economy, and part durability).

True, but I don't think in situations like this it is very consequential. The fish plate being discussed probably weighs little and the "perfectly engineered" plate would probably be an even less meaningful reduction in weight

 

[mrblaine]

The downside to that strategy is that, if you do enough modifications that way, you end up with a heavy vehicle. Weight really creeps up on you, if you aren't diligently paying attention to it, and weight affects so much (accelerating, stopping, turning, fuel economy, and part durability).

My answer was a bit simpler.

I like the idea of using enough metal that the engineering doesn't really matter that much. Way easier

Said no race car builder ever.

 

[lBasket]

My answer was a bit simpler.

Said no race car builder ever.

My jeep isn't a race car!

Plus I've compensated in other places by removing other useless fluff, like seats that aren't for my own butt

 

[mrblaine]

A couple of things @mrblaine. 1) I wondered while doing the rear frame tuck if a fish plate was necessary. Seems from everything I read that they are pretty standard when you have a cracked frame or when the frame is cut, but is a fish plate necessary for this application? 2) From your description, would a smaller fish plate that is diamond shaped in approximately the following dimensions be sufficient? 4”x2”x1/8”

I offered an illustration. You declined.

 

[mrblaine]

My jeep isn't a race car!

Plus I've compensated in other places by removing other useless fluff, like seats that aren't for my own butt

Weight is not your friend, it never will be. Build however your sensibilities tell you to do things.

 

[jjvw]

True, but I don't think in situations like this it is very consequential. The fish plate being discussed probably weighs little and the "perfectly engineered" plate would probably be an even less meaningful reduction in weight

My argument is that if you approach things with an effort to be thoughtful in your design choices, the overall result can't help but be improved. A project vehicle like a Jeep is unusual for most because it can encompass an extremely wide range of problems to figure out. All of these problems eventually need to work together and will shape and reflect the driving experience we get from this project vehicle.

 

[lBasket]

I'm under attack here!! I do build with weight in mind and I would say my jeep has a very minimal amount of heavy unnecessary junk. Aluminum for all armor so far (those sliders aren't installed yet that's a cost issue), I'm just happy I can use 8oz extra metal in welding vs. engineering calculations that I really don't even know how to do for the ideal minimal amount possible

 

[jjvw]

I'm under attack here!! I do build with weight in mind and I would say my jeep has a very minimal amount of heavy unnecessary junk. Aluminum for all armor so far (those sliders aren't installed yet that's a cost issue), I'm just happy I can use 8oz extra metal in welding vs. engineering calculations that I really don't even know how to do for the ideal minimal amount possible

The old advice is to add weight slowly