Control arms - typical bends

[RatLabGuy]

The whole "John Fuller boxed-in control arms" debacle got me thinking.
In most cases, when control arms get bent, what direction is it in? Seems most I've seen has been upwards due to getting hung up and banged on, so the force is upwards with the weight of the vehicle pushing down. But my experience is limited compared to many here.

Square tube is much weaker to bend along the flat side than round tube, however at a 45 so that the force is diagonal across the tube, it is very strong. So IF you were wanting to mostly strengthen along one direction, would it be crazy to use square tube rotated 45 degrees? Adapting the heim to teh ends will be interesting of course

 

[John Fuller]

daiiimmm...you really get into this physics of arms and metals lol hey i got it...i see where this is going, now you're gonna tell me to cut my arms and weld them on a 45 degree angle so they won't bend on rocks lol. (btw, i already landed on the right front lower arm on Easy St several times and not a dent but the 2x4 crossmember got a good dent so idk, that kinda says a lot already...ask who was guiding me the first time when i turned the wrong way and landed hard on it, lol...his eyes got pretty big and laughed "Daaammmm he said, but it is fine, crossmember's not though he he)

 

[John Fuller]

Ol Stumpy kinda gave my arms a good beating too....both stock and the 2x4 crossmembers took hits but arms only have scratches where you can see them sliding on rocks so if anything, i need to upgrade the weak 2x4 crossmember not the arms lol

 

[XJsavage]

I made due with 2x2" .250 wall for a few years. Never bent it with 36" links on a 1/2ton cherokar beating the crap out of it. Getting the impact to be dead on the 45* is impossible since Jonny joints and hiems have rotational movement.

 

[John Fuller]

the 2x4 even got bent on the corner...wtf. now that surprised me since the corners would be the stronger side but i kept landing on this one boulder on Easy st LOL until like 4/5th time going up i finally learned how to avoid it or drive over it properly....learning to drive on/over rocks is a lot harder than i would've imagined and i don't like it! mud is easy, rocks suck!

 

[Fabrik8]

If you're worried about the arm buckling and want to turn it 45 degrees, remember that you'll have less suspension travel before you hit the cross member because the tubing will be wider in the 45 degree direction

Buckling is the same for a square whether you rotate it 45 degrees or not. The moment of inertia is the same, the sectional area is the same, and the radius of gyration is the same, so the buckling stress is the same.

If you want to increase buckling strength and still use square tubing, adding a simple doubling plate to the area that the cross member would hit is simple and easy to add. You really just need something to distribute the point load from the cross member impact along the tubing.

 

[catfishblues]

The whole "John Fuller boxed-in control arms" debacle got me thinking.
In most cases, when control arms get bent, what direction is it in? Seems most I've seen has been upwards due to getting hung up and banged on, so the force is upwards with the weight of the vehicle pushing down. But my experience is limited compared to many here.

Square tube is much weaker to bend along the flat side than round tube, however at a 45 so that the force is diagonal across the tube, it is very strong. So IF you were wanting to mostly strengthen along one direction, would it be crazy to use square tube rotated 45 degrees? Adapting the heim to teh ends will be interesting of course

I thought the same thing about square vs. round. Then I did a little research. Apparently, according to all engineering charts I could find, square is stronger than round of the same cross-section and wall thickness. Nothing else to add here. I'm not well-versed in the science of metals and stuff, but that was an interesting tidbit.

 

[GotWood]

I would think the real issue would be then the square tube deformed or deflected into parallelogram, then they would act as flat pieces of steel and bend easily. I guess I'm really not sure since all of that fancy mathmatics stuff would be involved.

 

[Chris_Keziah]

Could always sleeve the square tube with ribs tube on the inside but that kinda defeats the purpose of the square tube. If you wanted to increase the square tube resistance to bend you could weld plate to each of the sides (left and right now top and bottom)

 

[Fabrik8]

I thought the same thing about square vs. round. Then I did a little research. Apparently, according to all engineering charts I could find, square is stronger than round of the same cross-section and wall thickness. Nothing else to add here. I'm not well-versed in the science of metals and stuff, but that was an interesting tidbit.

Yes, that's correct. Square will support a higher load in bending than round tube of the same diameter and wall thickness. You can make a lighter control arm of the same strength with round though, because you can increase the diameter of round tubing for the same weight. A control arm is supposed to be mostly in tension or compression (some torsion is usually inevitable) but all that changes when you put it into bending by contacting a rock or cross member. So making it stronger for the same weight in tension/compression by using round doesn't necessarily make it stronger in bending.

 

[GotWood]

Yes, that's correct. Square will support a higher load in bending than round tube of the same diameter and wall thickness. You can make a lighter control arm of the same strength with round though, because you can increase the diameter of round tubing for the same weight. A control arm is supposed to be mostly in tension or compression (some torsion is usually inevitable) but all that changes when you put it into bending by contacting a rock or cross member. So making it stronger for the same weight in tension/compression by using round doesn't necessarily make it stronger in bending.

Solid aluminum brings a whole new equation I guess? What kind of comparison does equal size .25 wall steel vs solid aluminum?

 

[Fabrik8]

Solid aluminum brings a whole new equation I guess? What kind of comparison does equal size .25 wall steel vs solid aluminum?

Depends on lots of things. What kind of steel, what aluminum alloy, cast versus machined, forged or not, etc. You could make some assumptions about things if you just want to compare spare steel tubing versus square extruded aluminum of whatever alloy, etc.

You'd be pretty hard pressed to want to use a large enough chunk of aluminum to equal a piece of 0.25 wall steel of any appreciable tubing size, and then there's the fatigue life tradeoffs, notch sensitivity, etc. If you're doing solid aluminum, you're wasting a lot of material in the center of the part that isn't going to add anything to stiffness. That's why you use tubing in the first place, or make a casting with ribs, etc. that can increase stiffness in the proper areas but decrease cost/weight. Solid parts may be easy to make, but that doesn't mean they're anywhere close to a good design.
Just like many things, it's not really appropriate to substitute a different material of the same dimensions for a given part, because you're not really taking advantage of the material properties.

 

[Mac5005]

Depends on lots of things. What kind of steel, what aluminum alloy, cast versus machined, forged or not, etc. You could make some assumptions about things if you just want to compare spare steel tubing versus square extruded aluminum of whatever alloy, etc.

You'd be pretty hard pressed to want to use a large enough chunk of aluminum to equal a piece of 0.25 wall steel of any appreciable tubing size, and then there's the fatigue life tradeoffs, notch sensitivity, etc. If you're doing solid aluminum, you're wasting a lot of material in the center of the part that isn't going to add anything to stiffness. That's why you use tubing in the first place, or make a casting with ribs, etc. that can increase stiffness in the proper areas but decrease cost/weight. Solid parts may be easy to make, but that doesn't mean they're anywhere close to a good design.
Just like many things, it's not really appropriate to substitute a different material of the same dimensions for a given part, because you're not really taking advantage of the material properties.

-2.0" .250" wall 1018-1020 HREWDOM

Edit:

-2.25" solid 7075 solid aluminum.

- 2 x 2 x .250 square. Guessing a36, or is it a53, brain shart....


For comparison 36" long. 1.25" chromo heat treated & cryo heims.

 

[shawn]

Seems like AL lowers are usually 2.25" or 2.5" OD, though.

Stock XJ/TJ lowers usually fail when you come down on them. They're just formed sheetmetal, so as soon as they get deformed a bit, they crumple. Uppers fail similarly, but either after a lower has bent or after the lower link bracket has ripped off of the axle housing.