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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">
<IMG SRC="smilies/smile.gif" border="0">
Fred
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">
<IMG SRC="smilies/smile.gif" border="0">
Fred
