Any engineers or engineering students here?

[Jim Oaks]

I'd really like to work out a way to measure force on a link to compare to buckling. CHow can it be done by using the weight on the link and length and angle of the link?

 

[JohnnyU]

First, I wouldn't even consider a low-carbon steel such as a 10 series, Chromoly alloys would be my choice. Granted they are more expensive, their material properties are more desirable as well as typically more uniform. I DEFINITELY would not use Stainless, not only is it cost-prohibitive, in many cases, there are 'better' materials out there to use.

Those Excel programs are not FEA software, they are really only geometry calculators. I mean that most people don't do much if any Finite Element Analysis on their suspension components. Most of the design is done as "Eh, 1/4" plate oughtta be strong enough for this shit." And as long as the suspension is designed without any binding, it probably will be strong enough.

To be able to analyze the force applied when the link buckles from contacting a rock or similar would require knowing that force. More importantly, it would require knowing the impact load, which can reach up to hundreds or thousands of KSI for microseconds. Those loads can be sustainable, but it depends greatly on the amount of time that these loads are distributed over.

Quite honestly, if you try to engineer something down to only a factor of safety of say 2, you will fail it without a doubt. I could sit here and say that a 1.75" OD 0.25"W tube would hold up to your loads because everyone else used it, but then again, it may not. I understand your desire to better understand the loads applied and the strength of the material. Looking at other trucks and their failures and understanding WHY they failed is perhaps more valuable than trying to determine loads that are VERY hard to calculate properly.


I'm going out to do an engine swap. I'll put more thought into this later on this weekend.

 

[krugford]

I'd really like to work out a way to measure force on a link to compare to buckling. CHow can it be done by using the weight on the link and length and angle of the link?

From the calculations I've done, it backs up the statement that most of the failures are going to be from bending loads (landing on a rock in the middle of the link) than it will be from buckling.

For the buckling load, using that formula of Fa = T/(a+b), you should get Fa = 5500 ft*lbs/(10.5/12 ft) = around 6300 lbs. Divide by the cross sectional area of 1.178 in^2 should give you around 5300 psi. I wouldn't worry too closely about dealing with a ground force. The formula I posted is used for a "trapped tire" scenario. If the vehicle is accelerating, it can never be more than the maximum torque output the axle can withstand. And you are using the maximum rating of the axle anyways.

One thing to keep in mind is that with impact loading, it's true that the applied forces can far exceed the stress limit of the material, but it's only for a extremely short amount of time. In the words of Andy Dufresne, it's all about pressure and time. For instance, if you hit the link with a hammer, the stress at that point is going to be very high and you might "ding" the tube. You will not, however, buckle the entire tube. Take that same force and apply it for a longer period of time, and you'll probably get the truck to break orbit. If I was designing a four-link for my truck, I would probably design that lower link for a 5G impact both for the truck standing on end (buckling) and the link hitting a rock (bending). This means you should be able to withstand a sustained load of 5Gs without failure. As for factor of safety, I would run with an FOS of around 1.2-1.3. It's all really just a play on numbers. You can run a lower FOS (1.5) with a higher expected load rating (5G) or you can run a lower load rating (3G) with a higher factor of safety. There is no reason to design for 5Gs and then run a factor of safety of 8. Because all you're doing then is designing for 40Gs with a FOS of 1. (Factor of safety is the load that the tube CAN withstand divided by the load it WILL actually see)

People are right that it can get complicated when you try to analyze a dynamic situation from a static point of view, but it doesn't have to be. For starters, you could analyze it at ride height, full droop, and full compression. That oughtta get you good nominal values with boundary conditions at both suspension travel limits. It's all in the assumptions. I threw out a number of 5Gs. That number came from design work I've done on "on-road" suspensions. If you want to estimate, you'll need to pick and example or assume a situation.

With some of the buggies you see, I've seen them trying to climb a near vertical bluff that's maybe 20-30 ft high. Then they stall out and land back on the rear suspension with a near vertical vehicle. From that, you can use the good old F=ma. You know the weight of your truck (m), and you can estimate the acceleration by calculating the speed at impact (based on distance above the ground when it started freefall, and the time it took to reach a velocity of zero. Roughly one half of the rebound time of the tire should be a good estimate of the acceleration (deceleration) time. That will give you a loading scenario that you can apply to your truck. Are you going to see something that extreme? Probably not. But it's a start.

Okay, this post is probably long enough.....

-krug

 

[Jim Oaks]

I've been working with psi's. How much force is 5G's?

 

[JohnnyU]

Neither unit is a force, technically. Pounds per square inch is a pressure, while a "G" is a unit of acceleration. In this case, one "G" would be 32.2 feet per second squared.

 

[Will]

The inertia that a 5,000# truck has is great enough to ensure that the time will never be milliseconds--you'll always have plenty to time available for the force to cause a failure.

As a matter of fact, I would be inclined to use impact resistance as me goal. Take a certain size and thickness tube and use your equations to make sure it will survive a 36" drop with a 2,000# load on it. If it can survive that, you can then try the other equations on it and I'll bet it would survive the rest of it.

An example of this type of thought is in a boat hull. If you take a 1" strip of the hull the longest unsupport length and simply apply a hydrostatic load to it, you could have a destroyer hull made out of 1/8" plate. But you don't even think about the hydrostatic load because once you worry about the impact of a dock, slamming loads from heading 40 kts into waves etc. the hydrostatic load is nothing. Somewhere along the line you are going to end up dusting off an old book full of rules of thumb--basically, what has worked for other vessels of this size, how is mine different, what can I get away with. Nobody is doing anything new, you always use what other people have done and work from there.

 

[krugford]

I was using G's in the sense that it would be 5X the weight of the truck. In other words, the force the truck would exert on a planet with five time the gravity sitting still.

 

[351ranger]

1.75 od x .25 wall dom would be plenty. All the math bull shit is a waste of time if you ask me when it comes to making links. All I can tell ya that trial and error will go a lot further then all your calculations.

 

[351ranger]

Remember, you will never have all 5,000 pounds landing on a link at once.

 

[krugford]

Remember, you will never have all 5,000 pounds landing on a link at once.

...says the guy who gets to limp home (or get towed) with a bent or broken link.

I'm pretty sure Jim's not going for a full analysis of his suspension system here, but he is trying to get some numbers together, do some simple calculations, and make sure he's not wasting his time with the wrong size tubing.

He was asking for help from anybody with experience in doing these types of calculations. He doesn't need to be told what size to use. He is more than capable of looking at a dozen trucks and seeing what is out there. He wanted to be able to backup his design with a couple numbers and say "this is strong enough for my needs, and here's why".

They way you're suggesting, what would happen if you broke/bent a link? How much bigger would you make the tubing? Why did it fail in the first place? What happens if you break it again? Guess and test gets old real fast and it also get expensive. IF Jim does fail a link, he'll have a much better understanding of how strong it needs to be the second time if he chooses to make it stronger.

And that "math bullshit" is what gets you to work in the morning and makes the world go 'round. Without it, you'd be riding a horse and buggy to work in the dark. Just because you can't do it, or don't want to do it, doesn't mean others feel the same. I personally love it. You can bet that when I decide to build my own four-link, there will be a reason behind EVERY bolt and tube.


-krug

 

[351ranger]

...says the guy who gets to limp home (or get towed) with a bent or broken link.

I'm pretty sure Jim's not going for a full analysis of his suspension system here, but he is trying to get some numbers together, do some simple calculations, and make sure he's not wasting his time with the wrong size tubing.

He was asking for help from anybody with experience in doing these types of calculations. He doesn't need to be told what size to use. He is more than capable of looking at a dozen trucks and seeing what is out there. He wanted to be able to backup his design with a couple numbers and say "this is strong enough for my needs, and here's why".

They way you're suggesting, what would happen if you broke/bent a link? How much bigger would you make the tubing? Why did it fail in the first place? What happens if you break it again? Guess and test gets old real fast and it also get expensive. IF Jim does fail a link, he'll have a much better understanding of how strong it needs to be the second time if he chooses to make it stronger.

And that "math bullshit" is what gets you to work in the morning and makes the world go 'round. Without it, you'd be riding a horse and buggy to work in the dark. Just because you can't do it, or don't want to do it, doesn't mean others feel the same. I personally love it. You can bet that when I decide to build my own four-link, there will be a reason behind EVERY bolt and tube.


-krug

I don't need to be told I don't know what I'm doing, or don't know how to do it pal. I've probably done more wheeling than you have ever dreamed of doing or ever will do. I've seen what works. All I was saying is trying all the different equations is worthless sometimes because when it fails in a place where you didn't expect, what happens then? Do you have an answer for that? I shouldn't have even replied to this thread. All I was trying to get across is use what works. There isn't a math problem to figure out whats going to happen in every situation on the trail. That's why if something breaks you fix it, that's part of the game. I guess I've gotten sick of engineers or people claiming to be when I have to show them what works and what doesn't out in the work field. That is all. Have a nice day.....

 

[JohnnyU]

I guess I've gotten sick of engineers or people claiming to be when I have to show them what works and what doesn't out in the work field. That is all. Have a nice day.....

I deal with "engineers" who don't even know which direction to turn a bolt, or what a zerk fitting is, every day. I frequently find myself correcting their designs so that the parts can actually be manufactured or cast, or assembled.

Sometimes, however, you do need to do the calculations to determine stress based on a few different load cases. In this case however, I would combine every load case imaginable and compile it into a worst-case type of situation. In my opinion for something like this, you aren't going to over-build a suspension system that will slide over rocks, have a truck land on it, be yanked on trying to pull someone or yourself out, etc. Build it stout, build it once.

 

[krugford]

I guess I've gotten sick of engineers or people claiming to be when I have to show them what works and what doesn't out in the work field. That is all. Have a nice day.....

Don't worry, I'm sick of engineers who don't know which direction to turn a bolt too. But I'm also sick of seeing designs made up by hacks that make me want to take their keys away until it's done right. Trust me, I've seen a lot of both. And both are equally dangerous in more ways than one.

As I more or less said in my previous post and what JohnnyU just stated is right. There's nothing wrong with trying to run a few load cases that encompass the more extreme scenarios this truck will see. If Jim wants to be able to drop his truck onto a rock and still drive home, then there's load case No. 1.

 

[Will]

If Jim designs it to drop on a rock and still drive it home, it will survive everything else he will encounter. He doesn't need a billion calculations performed. That was the point I was attempting to make before.

351 builds nice stuff. We've been seeing it for years. People that work with this stuff all of the time have a good feel for what works and what doesn't.

And still, shit happens. I broke an old style Duff radius arm--something I will still swear is impossible. I also tore the drivers side TTB axle beam--another thing I thought was impossible. That was hopefully my fault for intentionally leaving the shear bolt out (trying to outsmart the engineers?)--but it last 5 years like that, who would've thunk?

I've seen people whaling away on booger-welded tow hooks without a problem, and then people bust what appears to be a well engineered steering system on a little root. Experience is the real key here. Without it, you don't know if you are focusing on the pertinent thing. You might not have a grasp of moduli and calculations and figures and #D modeling software, but if you have the right mind you can see all the things in your head that make something work.

 

[Jim Oaks]

351Ranger, I'm not that comfortable with 1.75 x .25 tubing for the lower links based on some numbers I ran and I'm not about to do a lot of trail and error. Imagine the money we spent going to kentucky and then bending a link on the rocks in the first hour we were there. That would be a lot of wasted money, and tubing ain't that cheap anyway.


I'm not trying to come up with calculations for every aspect. I just want a 'guideline' per say to follow when choosing material.


I think my biggest threat would probably dropping off say a 4-foot elevation and having the middle of the link landing on a rock.

I'm sorry guys, I still don't understand how to factor this in:

If I was designing a four-link for my truck, I would probably design that lower link for a 5G impact both for the truck standing on end (buckling) and the link hitting a rock (bending).