Reduced to crutches, then left in the woods.

[Dishtowel]

Well, I don't want to sound like I'm over-criticizing, but it broke because that part of the arm is
much too small, the fact you "changed" it afterward only helped it along.
Extending them puts a huge bending moment right in that spot. If you don't cut the stock arm
back and weld the tube into where the arm is thicker, you get a concentration
of force right there, and is why it broke.

Now that I see you holding it in your hand, that hex looks even smaller than I originally thought. You
should have something at least 1.75" dia. (preferably 2") to stand up to the load it sees (if you
don't wish to use bigger tubing, then maybe use two tubes so it's triangulated.
Then you could probably get away with using that hex stuff without fear of having this issue repeat itself).

First off I want to say thank for your input.
I dont wish to lock horns or anything. I defiantly respect you and your know-how. But I have a few points for discussion.

that part of the arm is much too small

Not going to disagree, but I believe it would have taken much more to break if I had left it at the original angle.

Extending them puts a huge bending moment right in that spot.

How can the moment change when the distance from the load stayed the same? (I do not believe it can, moments are a function of distance, and the distance is the same. So, I believe it is the same as stock)
How can I have a greater moment "right in that spot"? (I do not believe there is anything special about this spot, concerning moments. Yes it is obviously a stress concentrator, but that does not change the moment or the loading.... This stress concentrator was put here by Ford. Not blameing them, but... they thought it was ok)

You should have something at least 1.75" dia. (preferably 2") to stand up to the load it sees

It's not seeing a torque load.......
I believe that this solid shaft would out perform 2" X 3/16" round tubing. Round tubing is nothing to brag about when it comes to bending loads. And solid material is going to take the compression/shock loads of driving into stuff better.

 

[4x4junkie]

Ford didn't design it with another whole foot of length behind it.

 

[Dishtowel]

Ford didn't design it with another whole foot of length behind it.

moments are a function of distance, and the distance is the same.

(distance to the load)

Or is there something I'm missing? Not trying to be a dink.

 

[CheapThrillB2]

i'm tired and worn out from work but let me try.....

The driverside beam is like a solid leaf sprung rear axle, it wants to wrap (twist) under load. now stock the radius arm is a foot long (just rounding). So spreading the load of the twist over the foot isn't bad. But now extend it another foot and the load wants to move to the center of the arm. When the arm was a foot long then their wasn't much distance between mounting points making it stronger. Now extending it puts more stress on the center of the arm with it having more distance between mounting points,

Take a peice of wood between two cinder blocks, put the short peice between the blocks and you can jump and stand on it at the center with barely any deflection/bend. but now take the longer peice and do the same and it bows and bends or breaks alot easier.

make any sense or i am just that tired and the brain has shut off.

 

[Dishtowel]

i'm tired and worn out from work but let me try.....

The driverside beam is like a solid leaf sprung rear axle, it wants to wrap (twist) under load. now stock the radius arm is a foot long (just rounding). So spreading the load of the twist over the foot isn't bad. But now extend it another foot and the load wants to move to the center of the arm. When the arm was a foot long then their wasn't much distance between mounting points making it stronger. Now extending it puts more stress on the center of the arm with it having more distance between mounting points,

Take a peice of wood between two cinder blocks, put the short piece between the blocks and you can jump and stand on it at the center with barely any deflection/bend. but now take the longer peice and do the same and it bows and bends or breaks alot easier.

make any sense or i am just that tired and the brain has shut off.

Thanks for trying man, but what you described is internal shear. And the rad arm is not loaded in the middle. It's loaded at the ends.

When you get axle wrap that is a concentrated moment. Internal shear in the leaf spring pack comes from the weight of the vehicle. A good visual on internal shear is illistrated useing boards, like you said. The best one I've seen is a bunch of 1 X 4's stack up (the weak way) and then a fat guy stands in the middle. Imagine if they where glued together, the glue has to not shear for the board pack to stay straight, if it fails they bend.

Rad arms see moments. And tension/compression. Moments result in internal shear, but get there differently. Torque wrenches are the most excellent example of moments. Force X distance = Moment.
Eg. torque nut to 100 ft lbs. apply 50 lbs on the end of a 2 ft ratchet. = 100 ft lbs.

Bigger tires and more HP could have been a factor in breaking this. I dont think they where the main reason. I was not on the throttle very much. I think it was the force at which I hit the ditch with.

 

[rickcdewitt]

it does sound like it was the impact not the twisting action of the beam that made it fail

but think of what cheap thrill said like this-take that same piece of wood and hold it a foot from the end and press down on the block.it will only deflect so far.hold it further out and do the same thing and it will deflect more.so the twisting force has more leverage with a longer arm

if you take that longer arm while its curved(like your radius arms-they aren't straight) and press on the ends it deflects easier too(impact)

 

[Dishtowel]

but think of what cheap thrill said like this-take that same piece of wood and hold it a foot from the end and press down on the block.it will only deflect so far.hold it further out and do the same thing and it will deflect more.so the twisting force has more leverage with a longer arm

I dont want to come off as an A-hole, so I'm going to try not to.

What you described with the wood: you are changing the point of loading. The radius arm point of load never changed. The point of interest is still (approx) 1 ft from the front connection. We are all agreeing that things change with distance, and the distance between these two points did not change. That is key.

so the twisting force has more leverage with a longer arm

Sorry to say man, but I believe that is backwards. The twisting force of the axle has the same amount of force the whole way thought the arm and LESS force on the frame.
Because, (if we view it as a ratchet again) if the axle is wanting to twist with a force of 200 ft lbs (made up number, picked out of the air) then the old 1 ft long arm must resist with 200 lbs force at the frame. Now my arm is 2 ft long (approx), so where the arm meets the frame there is only 100 lbs force. There is still 100 lbs force at 1 ft in the arm. This did not change.


(the reason I italicize or underline stuff is not to be rude, but to make sure it gets read/absorbed.)

 

[Dishtowel]

In the above post I wrote This did not change. I mean that the force there did not change due to what I did to the radius arm. (Lengthen it.)

It DID change due to larger tires and more HP.

 

[mhughes165]

the only thing i have to inject is to think about this.....yes the material u used was plenty strong enough for the job being done HOWEVER

think of it this way, u have a 2x4 but at the middle it is narrowed down to 1x4(the weak point like the point where your extension added onto the stock arm) and u have up and down forces aswell as forces from a sudden impact(tire hittin stump or rock or something) then all the force will be concentrated on the weakest point(the attaching of the extension) and it will procede to snap like a twig.

smae thing can happen with sudden up and down movements, when the tire takes a sudden upward plunge the stress will once again be transferred to the weakest link(the union)......

now i know this may not ahve anything todo with exactly how u were discussing it, but im just speakin from my experience, i dont pretend to know technical terms, but i know how shit works on trucks....

 

[85_Ranger4x4]

Just by looking at it you should have carried on with the hex or some other reinforcement farther than where the stock arm tapered down. It looks to me like that was the weakness. That bar will have next to no give, so it is going to take everything out on the stamped arm, unfortunatly the bar ends at the arm's weakest point which would be where it quits getting smaller.

I have never done anything like this so I may be full of crap, but is the feeling I get when I look at it.

Edit: I got interupted before I posted, which is why I say pretty much the same thing as mhughes.

 

[Dishtowel]

the only thing i have to inject is to think about this.....yes the material u used was plenty strong enough for the job being done HOWEVER

think of it this way, u have a 2x4 but at the middle it is narrowed down to 1x4(the weak point like the point where your extension added onto the stock arm) and u have up and down forces aswell as forces from a sudden impact(tire hittin stump or rock or something) then all the force will be concentrated on the weakest point(the attaching of the extension) and it will procede to snap like a twig.

smae thing can happen with sudden up and down movements, when the tire takes a sudden upward plunge the stress will once again be transferred to the weakest link(the union)......

now i know this may not ahve anything todo with exactly how u were discussing it, but im just speakin from my experience, i dont pretend to know technical terms, but i know how shit works on trucks....

Just by looking at it you should have carried on with the hex or some other reinforcement farther than where the stock arm tapered down. It looks to me like that was the weakness. That bar will have next to no give, so it is going to take everything out on the stamped arm, unfortunatly the bar ends at the arm's weakest point which would be where it quits getting smaller.

I have never done anything like this so I may be full of crap, but is the feeling I get when I look at it.

I'm not contesting that the smallest part is the weakest part. That is obvious. Glad we agree. It is to bad for me that I didnt reinforce it. BUT the reason I didn't is because I believed it to be OK, (before I bent it to gain correct wheel caster). My point is that the length had nothing to do with this break, neither is the material I added at fault. I removed the pin that ford had in there and replaced it with an equal part, and more weld. The size of the (ford) material and the bend are contributors to the failure.
I dont pretend to know technical terms either, lol. I only used ones I DO know.



On a lighter note, here is what some dumpster diving at work netted me. 4" channel, 5" channel, 3/4" plate for top gussets (why not? lol), and some 1/2" square bar for a runner along the bottom.

PS: I get a kick out of the use of 'proceed' and 'snap like a twig' in the same sentence.

PSS:

smae thing can happen with sudden up and down movements, when the tire takes a sudden upward plunge the stress will once again be transferred to the weakest link(the union)

The radius arm cares very little about the vertical motion of the tire. The spring and the pivot bracket handle those forces.
EG. When testing flex with a forklift, the radius arm is seeing very small forces.

 

[mhughes165]

looks like ur doin some free advertisement for them lol, with the company name on the steel lol

 

[JohnnyU]

Ok, I wasted 25 minutes reading (ok, skimming) this entire thread. A few thoughts come to mind:

1) Glad you got the truck out, and learned that pulling a trailer through anything other than dry, solid ground is MUCH less than desirable.

2) You got the basic concept behind the extended radius arms, but they were SEVERELY under-built considering their loading conditions. In my opinion, the drastic change in cross-sectional area played a greater role in their failure than putting them in a press.

I'm too tired to type much more than this.

 

[Dishtowel]

I didn't change the cross sectional area....
But I am guilty of under-reinforcing.

 

[JohnnyU]

Yes, it would appear that you did. In fact, it failed right at the transition region.