J Arms Vs. Mid Arms

[XJJPR]

Ary,

Try this picture, it is a little dark.

http://www.orgsmfg.com/project/WJK026s.JPG

Paul,

You kind of answer your own question. But you do end up with the simular charactoristics of a longer arm.

You almost need to play with this J arm setup to see what it does.

However, I wouldn't agree with zero diff., you're always going to see some type of change, be it a little, as you lengthen the arm no matter where it is attached.

mark
orgs mfg

 

[Safari Ary]

It's a little dark and a little small Is the arm longer than stock? or just built differently 'cause it uses the two joints?

 

[XJJPR]

It's a little dark and a little small Is the arm longer than stock? or just built differently 'cause it uses the two joints?

It is basically a lower arm with 2 2.5" JJs at each end.

I used my conversion bracket for the over axle track bar as the mount on the side of the frame and gusseted it.

mark
orgs mfg

 

[Safari Ary]

It is basically a lower arm with 2 2.5" JJs at each end.

I used my conversion bracket for the over axle track bar as the mount on the side of the frame and gusseted it.

mark
orgs mfg

Got it. Any reason not to retain two uppers with J-arms?? I know the advantages of the 3-link and how they don't bind and such. But I'd feel better running two uppers on my Jeep since it's daily driven and I don't have a full shop to fix things if they break like you do I've already got RE adj uppers with the superflex joints at the frame, so re-using them would save time, money, and headache. Thanks for answering all my nagging questions :angel:

Ary

 

[BrettM]

However, I wouldn't agree with zero diff., you're always going to see some type of change, be it a little, as you lengthen the arm no matter where it is attached.

I disagree


- the point about which everything rotates is the frame mount of the LCA.
- there is a (nearly) constant radius between the center of the axle tube and the frame LCA mount.
- the axle will always travel on this radius as long as the LCA is attached at the frame in the same place.
- yes, the end of your arm follows a larger radius arc, but the axle tube stays (within a few fractions of an inch) on the same radius arc as with normal arms
- you did bring the radius more into the vertical part of the arc by simply raising the mount, but this could be done on either side of the axle
- your 22"(guesstimate) arm will still have the axle traveling about a roughly 19" (guesstimate) radius
- if you had a 22" arm mounted farther back on the frame (kinda like Goatman) and mounted to the rear of the axle, your axle would follow a 25"(guesstimate) radius arc


do you have a good CAD program? or a drafting program? it's pretty easy to draw it up and see how all this works.

 

[XJJPR]

do you have a good CAD program? or a drafting program? it's pretty easy to draw it up and see how all this works.

No, just actually doing it on a jeep. Hands on stuff.

The J arm set up on MY jeep acts NOTHING like the stock arm set up that was on MY jeep.

So are you just picking on terminology or are you really trying to tell me the J arm set up on my jeep is exactly like the one I had on prior using the oem arm mount locations? The axle moves exactly like before,the tire travels in the exact position and there is the same exact pinion movement as before.

If the end result of a arm modification creates a more straight up and down movement than it was before, where it traveled more forward and aft,

is it running in the same radius or arch? of both meaning one and the same? Or has something changed? if so what?

mark
orgs mfg

 

[XJZ]

No, just actually doing it on a jeep. Hands on stuff.


mark
orgs mfg

Mark, how dare you do real world/hands-on testing in this web-wheeling, computer aided, virtual world we live in? It's so retro, it's cool!

OK, I better get back to work.

 

[dennisuello]

effective arm angle changes with j-arms, but the arc the axle moves in doesn't.

 

[BrettM]

effective arm angle changes with j-arms, but the arc the axle moves in doesn't.

exactly! same as with drop brackets

- you did bring the radius more into the vertical part of the arc by simply raising the mount, but this could be done on either side of the axle

the arc/radius (same thing, radius determines arc) is the same as a normal short-arm setup. Yes, your axle moves less fore and aft because you brought it into the more vertical part of the arc. So yes, it basically is the same as a drop bracket, same arc but starting in the more vertical part of the arc.

You could have accomplished the same thing by moving the mount up behind the axle. The one difference I can see is that your pinion will behave differently versus moving the mount up behind the axle.

one concern I would have; do you still have enough seperation between the LCA and UCA axle mounts? what are you running for bushings/joints on the axle end?

 

[XJJPR]

I'll agree with your drawing if we we talking radius arm designed suspension or a track bar design, just two single points.

Maybe you can input a upper arm into your computer and see where the axle tub travels then. It's just not one point affecting the travel of the axle, it's two and the relation between the two, which is what creates the pinion changes. It will suprise me greatly if the stock arm conf. and the j arm conf. run the same arch/radius on the axle tube. Been wrong before.

Maybe I'm just not using the terms right but I know the two set ups are reacting very different from one another, seat time stuff.

Lets not forget this is a thread about j arms vs mid arms and not a school lecture class, not everything has to always be by the book.

mark
orgs mfg

 

[Paul S]

exactly! same as with drop brackets


You could have accomplished the same thing by moving the mount up behind the axle. The one difference I can see is that your pinion will behave differently versus moving the mount up behind the axle.

one concern I would have; do you still have enough seperation between the LCA and UCA axle mounts? what are you running for bushings/joints on the axle end?

This is kind of what I said in my post, but there is a major difference between mounting the arm in front of the tube or behind the tube. Mounting the arm behind the center of the axle tube will cause fairly bad anti dive (if that's what it is called), unless you move the upper axle mounts up. J arms give you the benefits of mounting the arms in the center of the axle tube, but actually space the upper & lower mounts further apart than stock (not in distance, but in function).

Guess it's hard to have a thread on J arm Vs mid arm when there is only 1 J arm & 2 mid arms on the board.
Mid arms are lighter It's all about weight.
Paul

 

[Erik]

And a very good thread it is. Sorry to have created so much work for you Mark, I thought I was the only one who sat around work all day surfing this stuff. What about the queston of running two UCA's? And did you change the UCA mount on the frame, or just the axle? And what exactly did you bend on Rocker Knocker? Enquiring minds want to know.

 

[Erik]

Originally posted by Paul S [/id
Mid arms are lighter It's all about weight.
Paul [/B]

Screw that alfalfa breath, I'm packing beef in my rig. :moon:

 

[Paul S]

Screw that alfalfa breath, I'm packing beef in my rig. :moon:

That is the funniest post I have ever seen, I just can't believe that a simpleton like yourself came up with it.
Paul

 

[XJJPR]

And a very good thread it is. Sorry to have created so much work for you Mark, I thought I was the only one who sat around work all day surfing this stuff. What about the queston of running two UCA's? And did you change the UCA mount on the frame, or just the axle? And what exactly did you bend on Rocker Knocker? Enquiring minds want to know.

Erik,

Running two uppers is jut fine.

My three link involved changing both frame and axle upper mounts to using 2.5" JJ basically for the bolt size but for the thing being bigger than stock size stuff. Done for over kill on my part, I didn't engineer the strength difference just though bigger would be better/stronger.

You had to be there to get that knowledge.

mark
orgs mfg

See who speaks up first about the last one.

 

[XJJPR]

Screw that alfalfa breath, I'm packing beef in my rig. :moon:

I like this too!

hinkley

 

[Goatman]

Alright, since I started screwing with this topic with my questions....just to have some fun, which I'm having , I better make a few more comments to help Mark out.

If you draw this out, you will see that with the axle in a certain position, the longer J arm is higher in it's arc relative to the shorter arm. What I mean by this is that if you draw a horizontal line figuring, say, 8" of droop, and use this as the bolt center for the the arms and for the center of the axle tube, you can see that the arc of the longer arm is less than the arc of the shorter arm. If you draw a line from the frame mount roughly straight through the axle tube, and figure this to be the mount of the J arm, then the arc of the axle is the same with both arms. However, that would be like on a radius arm, not on a four link (or three link). On the link suspension, the axle tube will have a minimum of rotation, so the longer arm, in a mount horizontal to the shorter arm mount, is positioning the axle tube according to a higher relative position in it's arc. You can easily see this by drawing lines from the mounting position of both arms, in front of and behind the axle tube, then back to the frame mount. The longer arm is at less of an angle than the shorter arm, and the longer arm has a better arc of movement than the shorter arm does, and the arm determines the arc that the axle will travel in.

Try it, it's simple to draw. I figured with a 16" short arm, and a 21" J arm, and 8" of droop, the axle will travel rearward 1" less with the J arm than with the stock arm. The key to drawing this out is that the two arm mounts are on a horizontal plane, not both in a straight line from the frame mount.

Now, this assumes that the upper links are configured in a way to minimize axle rotation (pinion change), so this is theroretical......but that's why it would work.

BTW, my single upper arm is also an old lower arm, but mine is attached at the passenger side.

 

[XJJPR]

BTW, my single upper arm is also an old lower arm, but mine is attached at the passenger side.

But that's a whole other thread.

Atleast I'm not the only one to see the upper arm does exist.


mark
orgs mfg

 

[Ed A. Stevens]

Olympic XJ Arm Wrestling?

Alfalfa Breath in dis-corner verses the Beefeater (widdafuzzyhat) in dat-corner.

Arcs -- Smarks, who cares about the difference in arcs as long as the roll axis and anti-dive IC is nearly the same and the pinion angle gain is managable?

The reduced anti-dive with the J-arm improves the compliance of the suspension as much as the length of the Mid-arm system. When you get into a comparison, the relocation of the axle bracket pin height in both systems make a world of difference in the resulting road feel, because the reduced anti-dive minimizes the arm angles defining the roll axis (the roll axis that tends to fight suspension travel through the linkages rather than the springs). Nearly the same result (near the same as drop brackets, too) through different methods. Each method improves the ride quality.

I have not sketched a J-arm in a long time, although IIRC the pinion gain was better with the axle mount leading the axle centerline. The other side benefit was how the bushing was loaded, as the J-arm placed more load on the UCA (something to watch with the 3-link). I knew the Rover J-arms were solid hardened bar stock, and even 1 5/8, 0.30, is some beefy tube to bend in that tight of a radius. Nice work on an alternate solution.

 

[Safari Ary]

So after all that, I'm left with one question....

Should I do this or long arms??

Now I realize that this is the question that pretty much faces everyone to one degree or another. I was all set to do my own variation of long arms(similar to a stock FJ80 or a land rover suspension), and then Mark starts talkin about these J-arms and they seem like a pretty good idea. I like the idea of using the stock LCA mounts 'cause I won't have to build a x-member, and it always leaves the option of going back to the stock setup should I desire.

My main thing is that I drive about 15-25000 miles a year(depending on the gf status ) and 'wheel about one day per week, so it's hard to find a balance. I'm tired of getting beat up on the road with my short arms at 4.5", I want to go bigger(read 6" and 35s), and it really irks the crap out of me when I go to climb a ledge and my tire tries to run away and hide under the Jeep So what do you guys think. Breakover angle is not a huge concern for me over here on the right coast, not nearly as much as the left coast anyway. Any opinions to steer me one way or another?? Thanks

Ary