J-arms/front suspension geometry

T&A-XJ

New member
Same post on Pirate but in searching I found some of you are running this setup, any advice?
On my current XJ I have a radius arm front suspension consisting of a single upper CA on the drivers side. While this works well and the arms are tucked up flat inside the frame rails, I am wanting to try a differnt route for the next XJ. The Currie J-arms are what Ide like to work off of. I have the axle mounts figured out and the panhard bar is already raised to match the WJ knuckles.
I want to utilize a 3 link w/panhard bar. What would be the ideal AS% for the front and how does the roll axis and roll center affect the front suspension with a panhard bar? I would think a close to 100% AS but am unsure of the rest as the panhard bar changes everything right? I am looking for an improvement over my current radius arms both on the hiway and on the trail.
Here is what Ive come up with so far, 1st being J-arm lowers with stock located uppers as all the TJs I've seen do it. Doesnt seem ideal to me...

Here is with the same lowers but moved the UCA down and further from the axle.

Here is my current radius arms but I only run a single upper CA and it is attached mid arm, done right?
 

Goatman

NAXJA Member
NAXJA Member
Hinkley has J arms on his front three link, and Erik has J arms on his front four link. The J arms seems to work well.......Currie took second place in the year long standings in the first year of rock ARCA rock crawling competition with a front J arm four link.

I don't have J arms, but I can tell you a couple of things about a front three link. The UCA on the axle should be pretty high, with the arm as level as possible. The upper mounts need to be stout, I wouldn't trust the stock mounts on the axle or frame, plus they're in the wrong place. When I first did mine, I didn't have enough anti-dive so there was a lot of nose dive on braking, and both sides would lift once in a while when one side was climbing. I raised the UCA mount on the axle 2" and it made a big difference. I think my axle end UCA mount is 4-5" higher than the stock mount.

While you're at it, make the axle end of the track bar as high as you can, it raises the roll center and helps the handling and stability.
 

CRASH

New member
Run scenario #1 with the UCA axle side raised 2 inches. that would give you 9" of separation rather than 7.
 

T&A-XJ

New member
Thanks for the help, my track bar is currently raised to match the WJ knuckles so that should help. So what you are saying is to remove the stock D30 cast mount and build a truss with a higher/stronger mount? I am currently using the cast mount with a solid poly bushing and a 5/8 bolt. I just drilled out the poly to 7/8" and used as .875x.120 as the sleeve. This seems to be working great, and hasnt shown any wear or slop in almost 2 years of use. My plan was to re-use this on the axle end and use either a large Currie JJ or a Summit Machine Jimmy joint at the frame end depending on available space. Will the added stress of less separation be too much or do I need more separation due to handling or both? I am a little worried about raising the axle end UCA mount due to up travel clearence, I will have to crawl under there and take a look.
 
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Skullver

Still Flailing
T&A-XJ said:
Thanks for the help, my track bar is currently raised to match the WJ knuckles so that should help. So what you are saying is to remove the stock D30 cast mount and build a truss with a higher/stronger mount? I am currently using the cast mount with a solid poly bushing and a 5/8 bolt. I just drilled out the poly to 7/8" and used as .875x.120 as the sleeve. This seems to be working great, and hasnt shown any wear or slop in almost 2 years of use. My plan was to re-use this on the axle end and use either a large Currie JJ or a Summit Machine Jimmy joint at the frame end depending on available space. Will the added stress of less separation be too much or do I need more separation due to handling or both? I am a little worried about raising the axle end UCA mount due to up travel clearence, I will have to crawl under there and take a look.
I think they are talking about the psngr side uca/axle mount, not the cast, for the 3-link scenario.
 

T&A-XJ

New member
Using the 3 link w/panhard bar calc, I came up witha few different link locations. Varying between 35% and 70% AD and about 6* roll axis. The calc shows the link forces and when leaving the axle end UCA mount the lb force is about 13,000lbs, when moving the mount location up it lowered the force to 9,000lbs. I still need to check clearances for raising the axle end mount. This might necessitate moving the UCA to the passenger side and building a mount off the track bar mount for support.
 

Goatman

NAXJA Member
NAXJA Member
It's pretty tight in there. My axle side track bar mount and UCA mount can hit the frame on a really hard hit, but only after compressing the bumpstops about 3". Just make it as high as you can and still be comfortable with the clearance at full stuff........keeping in mind that a hard hit will colapse the bumpstop all the way. I wouldn't use the stock axle UCA mounts, besides being in the wrong place, they're not stout enough.

Here's a couple pics of what mine looks like, if you'd like some ideas. I later raised the UCA mount 2" higher than it is in these pics.





Later, UCA mount 2" higher:



Oh, and roll axis means little on the front, since you control your direction with the steering wheel.
 

Weasel

NAXJA Member
NAXJA Member
Goatman said:
Oh, and roll axis means little on the front, since you control your direction with the steering wheel.
Are you confusing roll axis with roll steer?
 

vetteboy

NAXJA Member
NAXJA Member
Weasel said:
Are you confusing roll axis with roll steer?
No...I think he's saying that it's not as much of a design constraint as it is in the rear, because you can compensate for it at the steering wheel.

But yes, indirectly he's referring to roll steer.
 

Weasel

NAXJA Member
NAXJA Member
roll axis pertains to the line between roll centers, which is the line about which your vehicle will yaw. Jacking and body roll are directly related to roll axis. Or atleast thats how I understand it, how is that effected by the steering direction?
 

vetteboy

NAXJA Member
NAXJA Member
Weasel said:
roll axis pertains to the line between roll centers, which is the line about which your vehicle will yaw. Jacking and body roll are directly related to roll axis. Or atleast thats how I understand it, how is that effected by the steering direction?
Because a slanted roll axis results in flex steer. It's not affected by the steering direction, the steering direction is affected by it. In a rear fixed axle you're pretty much stuck with whatever roll-over or -under steer your setup results in, but in the front you can compensate for an inclined roll axis by making up the difference with the steering wheel.

I think you're looking at it as the body moving over the axle, and I'm looking at it as the axle moving under the body.
 

Goatman

NAXJA Member
NAXJA Member
Weasel said:
Are you confusing roll axis with roll steer?
Roll axis angle effects roll steer, and roll steer isn't an issue on the front. Body lean and handling are affected by roll center height, which is different than roll axis angle.
 

T&A-XJ

New member
Thanks for the advise w/pics. Did you raise the axle mount again primarily to get a more desireable AD%, separation, level link angle for handling purposes or all the above? From what I remember reading it is more desireable to have a (from the frame end) down sloping link on the drivers side and an upward sloping link on the pass side although it isnt a huge concern. The roll axis still should be designed if possible to be as minimal as possible though to help with high speed predictablity right? I also plan on lowering the body .75 -1" when I build this so that cuts the roll axis almost in half which will almost be transparant anyway.
 

Goatman

NAXJA Member
NAXJA Member
T&A-XJ said:
Thanks for the advise w/pics. Did you raise the axle mount again primarily to get a more desireable AD%, separation, level link angle for handling purposes or all the above? From what I remember reading it is more desireable to have a (from the frame end) down sloping link on the drivers side and an upward sloping link on the pass side although it isnt a huge concern. The roll axis still should be designed if possible to be as minimal as possible though to help with high speed predictablity right? I also plan on lowering the body .75 -1" when I build this so that cuts the roll axis almost in half which will almost be transparant anyway.
I raised it specifically to increase anti-dive and cut down on torque jacking, which it did. Basically, after discussing all the ins and outs of suspension design and mechanics, it all comes down to making the arms as level as possible. As has been said in other places, what effects handling at speed on asphalt is different than what effects trail performance at slow speeds. A little trial and error, and getting other peoples experience, is what can really help in what we do.
 

T&A-XJ

New member
Finally took a look under the jeep. I have 12.25" between the axle tube and the frame. With my current 4" of uptravel I can get the new mount about 6" above the axle tube to achieve about 2.25" more height than stock. Utilizing the passenger side is much more do-able as the frame end TB mount and brace are already tight. I found this that might be worth building off of with a simple gusset to the axle TB mount.

This setup has me thinking about trying it on my current XJ with radius style long arms(removing the arm mounted UCA). The AD% would be alittle higher than I am shooting for at 112% but is considerably less than my radius arm geometry. Raising the lower mounts flush with the axle tube as in the pics above will lower the AD to about 83% but would have to bend my arms to clear the frame.

Thanks for all the info, Im sure I will have more questions...
 
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