Reduced to crutches, then left in the woods.

[Dishtowel]

I removed the rad arm.
I ground the weld holding the pin (with threads on the end for your rad arm nut to go over the and hold the rubber bushings) and drove the pin out.
I turned down the hex on the latch so it was a mild interference fit.
Welded the straight, then welded around the hex where it steps up to the arm. I did not change the ford arm.
Installed, discovered bad caster, bent on press.
Broke driving through ditch.

I removed a solid pin, I replaced a solid pin. I do not believe I changed the cross sectional area, If you do, might you show me where? So as I dont do it next time?

 

[4x4junkie]

Or is there something I'm missing?

I think you're missing the fact the forces are infact NOT the same at every given point along the arm. They are much greater as you get closer toward the axle. If the arm tapers down to being straight too quickly, it fails right where the taper stops (such as it did).


Lemme see if I can try an example this way...

Take a 1' length of 2" pipe secured to a fixed object (this is the "stock radius arm body"), and thread a reducer (or two, if needed) onto the end down to 1/2" pipe. Thread a 4" section of 1/2" pipe into it (this is the "threaded stub" of the arm) and try to bend the 2" pipe by pulling down on the 1/2" end. You probably can't do it (it holds up and stays together). The 4" piece of pipe is sufficient.

Now take out that 4" long piece of 1/2" and replace it with an 18" long piece ("extended arm"). Now try to bend it. The 2" part probably still stays intact, but now the 18" piece bends right at the end where it threads into the 2" pipe (with far less force than what the 4" piece was able to support). Failure.


Hopefully that helps to explain what goes on, but if it doesn't, then you'll just have to take everyone's word for it here that the arm needs to be built bigger so you won't have the same failure happen again.



Edit

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

No you didn't change the cross-sectional area, but you DID redistribute the forces in the arm by lengthening it, making that small section fail.

.

 

[JohnnyU]

What you need to do is maintain a constant cross sectional area of material from the axle beam to the flex joint/bushing. By doing so, you will get nearly uniform stress distribution throughout the arm. Boxing in the front factory portion and it's transition into the tube would probably be the easiest method for doing so.

This is extremely complex geometry, more so than a simple beam from an example problem in any textbook. Essentially you have a tapered c-channel transitioning to a solid hexagonal bar. I understand how you fabricated the arm, you don't need to continue explaining that. Where I feel you went wrong was that you did not take into account buckling of the arm. The larger radius of the material (further from the center axis of the arm) the better, hence Sean's suggestion of a minimum diameter tube. The stock arms have approximately 3" of solid rod protruding past the end of the stamped and folded/welded sheet metal of the radius arm. This entire distance is supported by the bushing. Nothing is cantilevered out another 12-14" as it was on your extended arm.

Please stop being so defensive, no one is picking on you for your design. What we are (or at least I am) doing is offering constructive criticism of your design, in an effort to improve said design. Seldom will you ever find an instance in industry where the part remains unchanged from initial design through to production. At the very least, there are other engineers that will all sit down and review the design, and offer some improvements that may not have been initially considered relevant or needed by the original designer. I sit through hours of design reviews/discussions every week. My team has been working on one such type of design since last July, if that tells you anything. Of course, there are hours of FEA involved with that as well. For our products, failure is NOT an option.

On edit, you must also consider fatigue into your failure mode considerations. I doubt you encountered any forces driving through that ditch that would cause this to catastrophically fail as it did.

 

[Dishtowel]

Thank you for your input gentlemen, good example junkie.

I think you guys where looking at like this (?? yes/no ??) "in changing it as I did I produced a stress concentrator" I did not think this was a big of a deal as it grew into. (and broke, lol)

I was looking at it from a different angle. "I changed the arm but did not weaken it" This I still believe to be true. (if we excluded the bending on the press).

Two different, and true statements. If I am off track, could you help me where? I dont want to make this mistake again. Especially in a professional (as opposed to hobby) environment.

 

[CheapThrillB2]

these are were not finished in the pic, just using it to give an idea

 

[4x4junkie]

Even those would concern me a little bit IMO if they didn't have long gussets on the top.

This is how I did mine:

With enough torque put to it, I suppose mine could also fail in the same region, but being that no part of that area has less than a 2" cross-section, I believe a failure in the axle itself would occur before enough torque could build up to cause an arm failure.
I would definitely rather have an axleshaft or something else break instead of having the suspension collapse... at least you can still drive around in 2 or 3-wheel drive this way.

 

[CheapThrillB2]

Even those would concern me a little bit IMO if they didn't have long gussets on the top.

i agree thats why i redid them,

Theirs alot more to the radius arem brackets then whats seen in the pic.

they held up to my normal abuse i give my trucks and survived this, WARNING the last word spoken in the vid is *SHIT*
http://youtube.com/watch?v=2FGUd0flPgw

when the impact occured on the RF it shattered the RF hub, When i hit i thought when i got out of the truck, i was gonna find the whole pass beam ripped from the truck it hit so hard, i almost bounced my face off the steering wheel. I only found that the hub exploded, the whole hat was ripped from it, but found no damage to the arms or the mounts. Sorry to sort of hi-jack.

 

[4x4junkie]

Ah yes, I remember that vid.

 

[bobbywalter]

well i have not been around in a few days.....so this was painful to see.


especially from a farmer.


had we been together, i would have chained the beam back to the frame at the body mount and used the winch to suck the beam forward....or my come along, ratchet strap/chainfall....had you not had your own winch.


then while you helped....i would have pulled you out.












i have had to do this with ttb trucks too numerous to remember. my own rig at least 15-20 times.


































as to the radius arm. no fawking way ever will they hold up to even mild abuse....i dont know how they lasted as long as they did. and i dont know how your math brought you to think they are fine without the bend, as i was skeptical when they were first constructed and i seen pics somewhere....i mean if a guy actually had a locked front axle and a v8 with those things its when not if they will fail.... but they are plenty strong for normal use i have to admit....

there hasnt been any shown so far i wont fold eventually...eventually like in under two years in high traction situations and my powertrain with 35+ tires.. its just a maintenance thing to me....just rework em when i can not align it anymore....bout the 5th or 6th shaft shattering and either the beam is tweaking or the arm is tweaking.


even if your arm is straight like before you bent it, i am gonna bend it with the engine...i fawked up stock 150 ttb arms too.....rancho arms have the lifespan of an ant...the old style duff arms were really the best...just needed repaired a time or two per year. and they are much stronger then what you have here.

 

[Dishtowel]

TRY ME NOW said the little radius arm to Dishtowel.

Thanks for the interest and input guys.

So, I dont think it will break at that spot ever again. I know there is too much weld there. I kinda did that on purpose, so I can grid it down and have it look like I want. But I'm tired of standing on concrete for today, so that gets done tomorrow. 1/2" plate gusset on the top, 3/8" on the inside, just for fun.

Note: bobbywalter, so I dont know if you were referring to me or you when you talked about a the front being locked. Mine is open.

 

[Todd]

Well here is my beef.

 

[bobbywalter]

yes i was referring to you. i realize your not locked yet, and myself went to a brake controller after the quicklock as a selectable locker wasnt justifiable for an axle that shaft fails at some point nearly everytime i run it in a place worth locking the hubs...

considering everytime there is a failure theres a real good chance an arb is going to fail as well eventually with a fragged spider or something.... learned that the hard way

i also run 4x4 on the street alot in winter and on hard pack working....moving laden trailers and shit. so selectable is necessary.



v8 power, even wheezer v8 power and healthy 4.0 power will find all the faults in tractive situations over a 2.9. once i went to real good drop brackets that joel and the gang put together i couldnt keep the arms/beams straight. i had a problem with certain drop brackets needing welding alot



when i was younger and my truck still had its 2.9 i used to jump the shit out of it. where i lived it was about the most fun stuff available....(yeah i drank alot back then too) once i lifted a lil higher, went to a v8 and larger tires the speed/jumping had to stop...but the breaking of front end shit didnt i had to drive it home with the situation you just experienced a few times...and trailered/u hauled home many times as well


if you stay with stock type bushings on the end it seems you will be fine till the next upgrade, but its still thin back there for me. your current design is definatly a huge improvement, but plese look at todds, junkies, and cheaps setups with some detail. especially todds beef at the beam mounts. that has left me hanging a few times.




one other thing, we bust alot of leafs/links onthetrails, and the method i listed works for any location situation(which i know you know)...just putting it out there for the inevitable. ripping links off the frame is a REGULAR OCCURANCE for some of the people i run with

 

[stegomon]

dude that was awsome...i am nomming you for stuck of the year!!!...

 

[Kona]

Here you go,from a couple of pages back a deuce and a half ie: 2 1/2 ton troop transport