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Discussion Starter #1
Possible design flaw with "Radius" style control arms, ala RE Front Long Arms

One of the guys that I work with is a mehanical engineer, and he's not a 4 wheeler ,but a good friend. He's worked for years at some of the mines in CO. I asked him questions often, as he has a ton of engineering books with all types of good information, like steel tubing strength, etc. Anyway he stopped by my desk a day ago just to chat and I happened to have the RE catalog out and opened. I was on the phone at the time, so he picked up the catalog and was just thumbing though it. When I got off the phone he said that there is as "problem" with the front control arm design with the RE Long Arms. The way the front control arms are, is what some call a "radius arm". there's a upper and lower control arm that attach to each end of the axle but the upper arm doesn't go all the way back to the frame, but instead attaches to the lower arm so that the whole thing looks sort of like a 'y' sitting on it's side. It said the "problem" with that is, as the axle cycles, one side up and the other side down, like during typical wheeling, it puts a lot of twisting on the axle tube. The type of twisting that he is talking about is length wise twisting. That is, pretend the axle tubes were threaded and screwed into the diff housing, instead of being pressed in. With the "lazy y' or radius arm type control arms, when the axle cycles, one end up and one end down, one end is twisting the axle tube one way, say right handed, like it's being screwed into the diff housing (if threaded), and the other end is being twised the other way, as though it's being unscrewed from the diff housing. We went back to the shop and he made a little model with a wooden dowel as the axle tube and four pencils taped to the dowel acting as the control arms. All I have to say, is holy crap, he's right. Try it. You hold the ends of the "control arms" and drop one end of the dowel, and raise the other end, and it really binds up and will twist the scotch tape completely off. He said the ideal way is a control arm on both the top and bottom of the axle (as most"conventional" lifts are) that extend back to the frame and are *parallel* to each other, that is where they attach to the frame, the distance between the upper and lower arm is the same as on the axle end, and they are the same length. Again, back to the wooden dowel with pencils scotched taped to it. This time with another wooded down at the other end of the pencils to hold them parallel. Now hold the "frame" end of the pencils and then have someone cycle the "axle" wooden dowel, one end down, and one end up. Wow, not twist at all !!! Anyway, he said that he really doesn't understand how the radius arm setups really hold up in the real world as they place so much twisting stress on the axle tubes and the control arm mounting brackets. Another little model that we made is one like one person out here has done, that is made his own 3 link front suspension. It's long armed but the front only has two control arms. One control arm, on one side of the axle connects to the bottom of the axle, the other control arm, on the other side of the axle, connects to the top. That, via the model,works well. The only problem he saw with it, is that you'll still have some twisting forces once one tire gets up against a rock and is trying to climb over it, which is trying to push that side of the control arm backwards. At this point you have a similiar twisting force on the axle tube. The radius arm style does also, but the radius styles has that twisting force just cycle "dry" so to speak.
He mentioned that another possible problem with the radius style long arm, is, again,
when one tire gets up against a rock and is trying to push over it. You now have a force directed, more or less, back along the length of the control arm, trying to compress it along its length. The force is actually trying to bow it and the radius design actually adds more "bowing" stress at the point of attachment of the upper control armto the body of the lower arm.
Anyway, I was pretty surprised, and just thought that I'ld pass this along for anyone
who is interested, to think about it, and those of you more into it than me, to debate it. <IMG SRC="smilies/wink.gif" border="0">

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Fred
 

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Your friend is correct that there will be some bind to some degree. Ford Broncos, f150s, Land Rovers, Warn XCL, as well as Rubicon use a variation of the Radius arm design. But you have to remember that as with most things you must reach a compromise when it comes to suspension desing. Radius arms give good performance and are easy to design to fit a vehicle. While they have limitations (such as the twisting effect on the housing) the advantages of their simple design more than make up for them.
 

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Discussion Starter #3
As I mentioned, Joe has no interest in wheeling, and doesn't know who RE is, nor Tera, nor....He, nor I, in posting this, is slamming RE in anyway, intentionally. He was just making a comment, and then we talked about it and he drew pictures, made models, etc. I said that it works, as it's out there. He was still pretty surprised, as he said, depending exactly where the upper arm ties into the lower arm, that for every degree of difference there is in axle flex between the ends, they'll be almost a 1:1 correspondence in that amout of attempted axle tube twist, that should really stress the axle tubes, the tubes where they enter the diff housing, the control arm mounting brackets, etc. I just happen to notice the "Big Truck Suspension" tread, and they said the same thing as Joe, about the "est" set up being control arms, top and bottom on each end of the axle and then running parallel to the frame tie in, and, being the same length. I'm not a mechenical engineer (I'm a software engineer). I just wheel <IMG SRC="smilies/wink.gif" border="0">.. But anyway, I thought Joes comments, drawings and models where interesting. If I ever go long arm, no doubt, I'll have it made here by the person who made his own 3 link (front) for his XJ. I'll either have him do a similiar 3 link for me, or a more "conventional" 5 link, with equal length, parallel control arms.

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Fred
 

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Fred. I really appreciated you post. I think your friend makes a good case for possible problems. My guess is that the tubes may weaken over time...but only time will tell.
 

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You/he are absolutely correct. The twisting on the axle housing is inherant in a radius-arm suspension. It is not a problem typically, however. The axle is more than strong enough to resist that force, and the bushings used to mount the arms are large and relatively flexible.

One reason to use this style is *because* of the binding between the arms. It serves to limit flex to some degree, and provide a "sway bar" type of action. Good control of your axle is a good thing.

When the arms come together in a "Y" shape like the RE ones do, there is a large bending force where the two parts join. The arm has to be very strong to resist bending at that point--an easier way is to make the arm more of an "A" shape, but that's harder to fit in the space available.

Everything's a compromise.
 

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Discussion Starter #6
Here's another part. This is a reply from another board, where I posted this same topic:
~~~~~~~~~~~~~
All of your engineer friends observations are correct. He only overlooked one small detail which allows the RE radius arm to let the axle live. The rubber bushings have massive amounts of give in them. This greatly reduces the twisting forces acting upon the tubes.
~~~~~~~~~~~~~


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Fred
 

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RE also uses an "upper" arm that is jointed to the long lower with flexible joints ... spherical at one end and rubber at the other. I would imagine that this extra plane of movement would alleviate some of the previously mentioned torque loading on the axle.

I remember Superlift or Skyjacker, one of those guys, that made a TJ kit that used "Y" arms up front but they did not have those extra joints. The Y's were rigid weldments. I think there's a reason that you don't see those kits being advertised anymore ... or am I mistaken and they're still out there?

cm "Mechanical Enginner by day, dashingly handsome male escort by night" k

[ 11-01-2001: Message edited by: cmk ]
 

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Originally posted by cmk:
<STRONG>I remember Superlift or Skyjacker, one of those guys, that made a TJ kit that used "Y" arms up front but they did not have those extra joints. The Y's were rigid weldments. I think there's a reason that you don't see those kits being advertised anymore ... or am I mistaken and they're still out there? cm "Mechanical Enginner by day, dashingly handsome male escort by night" k

[ 11-01-2001: Message edited by: cmk ]</STRONG>

Superlifts were like that awhile back (3-4 years ago)...They ditched them.
 

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When you install the RE long arm, on the front upper control arms you remove the spherical joints and install the rubber ones, so you end up with rubber joints at both ends. The steel ball/teflon setup is gone.
 

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Originally posted by TornadoTJ:
<STRONG>When you install the RE long arm, on the front upper control arms you remove the spherical joints and install the rubber ones, so you end up with rubber joints at both ends. The steel ball/teflon setup is gone.</STRONG>
... I did not know that ...

cm "thanks bro'" k
 

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Your friend Joe is correct. There is a natural bind in the suspension, although that isn't necessarily a bad thing.

It gives the suspension somewhat the same effect that a swaybar adds. Now the RE does away with quite a bit of this since there is another joint at the lower/upper arm intersection. A true radius arm like Ford or Warn XCL exhibits much more bind.

At any rate, I built my suspension a bit more simply, I like straight links so you just loosen jam nuts and twist the bar to change it's length. I use a 3-link with a trackbar in the front.

CJ
 

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Wow, simply amazing, <IMG SRC="smilies/thefinger.gif" border="0"> I guess thats why they say, you learn somthing new every day. <IMG SRC="smilies/thefinger.gif" border="0"> this wasn't it. <IMG SRC="smilies/biggrin.gif" border="0"> <IMG SRC="smilies/thefinger.gif" border="0"> <IMG SRC="smilies/biggrin.gif" border="0"> <IMG SRC="smilies/thefinger.gif" border="0"> <IMG SRC="smilies/biggrin.gif" border="0">
 

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Same goes for the Tera long arm... long lowers and short uppers, the short uppers have a much tighter radius so as the axel moves down the pinion tries to tilt downwards and vica versa... when flexing they are opposing each other.

Having said that, seems to work OK and I run one!
 

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To anyone that has the radius arm setup and works in or near an engineering department: see if your engineers have access to strain gauges.

It wouldn't be hard to epoxy a gauge on to an axle tube, flex it up, and see what happens. I'd be curious to do this side by side with a vehicle equipped with the stock suspension. Oh yeah, the axles have to be the same also.

cm "why am I suddenly thinking of Chubby Checker?" k
 

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There is a problem of that nature on the Warn YJ XCL, only it's in the rear. The Upper control arms mount to a bracket that goes across the top of the pumpkin, while the LCAs mount to the bottom of the axle tubes, a la TJ. Problem is, with the torque from the axle, tires, and beating it takes wheeling, you can spin the pumpkin from the leverage of the control arms. That happened to one of the guys I was on the 'con with.

They don't make the kit anymore (it's coming back eventually), and I think a lot of the pumpkin spinning issue comes from the fact that it was on a D35, but nonetheless it's something to take into account if you're making a traingulated 3 or 4 link....
 

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Im glad i didnt go to college.

Ignorance is happyness I guess.
 
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