3 link rear MJ coming soon

So for various reasons I have decided to link the rear of my MJ. If you wonder about why I would do something as bass-ackwards as reversing the factory setup to front leafs/coils rear read THIS, then ask any questions you have.

I would have liked to do a dual triangulated 4 link, but I have decided that I want to leave the gas-tank where it is under the driver's side of the bed, forward of the axle. This left me with 2 options, parallel 3 link w/ panhard, or triangulated 3 link offset to one side. After researching both, I have decided I do not want to mess with some of the uneven characteristics of the 3 link w/ panhard, so I will be doing an offset triangulated (wishbone) 3 link. I will also be losing about 4 inches off the wheelbase in the rear, bringing me down from 115" to 111" while still not interfering with the gas tank.

Here is a picture of mine underneath (older picture, ignore the exhaust and basically everything besides the frame, gas tank and axle):



here is one done similarly, but mine would be on the other side:



Build materials

-By the nature of this system it will function in geometry exactly as though the wishbone was the full width of the frame and symetrical if the frame mount is relatively rigid. Therefore I will be making Delrin bushings for those two attachment points rather than Poly. This will attach to a crossmember which will just barely fit between the gas tank and cab. This presents the first question, what material for the crossmember? The three I am considering at the moment are (in order that I would like to use them) 1.25" .250 wall DOM, 1.5" .120 wall 4130 chromo, or 1.5" .250 wall DOM. This crossmember will span 40 inches without support.

-The upper wishbone will be made primarily from 1.75 .120 wall DOM with multiple gussets.

-Both lower links will run parallel to the frame and will be made of 2x2 .250 wall square tubing.

-Both ends of the lower links will be poly bushings, as will the axle end of the wishbone. Rather than use rod ends or cartridge joints (and for adjustability) I will be using a large threaded section to make these "twisty links". They will consiste of 1.25" B7 (chromo) all-thread with 12 tpi, threading into grade 8 coupler nuts, with a total of 20 threads of engagement. This type of link has been suffiecently proven in my mind by a guy named Brook Green from Alaska. He uses the same thing on his Chevy truggy whereas his are 1.5" thread, however his Chevy weighs roughly twice as much as my MJ, has a 513 Caddy motor under the hood, and runs 46" tractor tires. He has been running "twisty links" for several years of serious abuse and when he recently took them apart he noticed no increase in slop.

-some reading on "twisty links": http://www.pirate4x4.com/forum/showthread.php?t=358084
http://www.pirate4x4.com/forum/showthread.php?t=282710
http://www.pirate4x4.com/forum/showthread.php?t=240839



-For coils I will be using stock XJ coils. My MJ is only 3600 pounds and heavily weighted to the front, so I'm hoping the soft rate on the XJ coils will be about right. They will be on adjustable mounts, so that if I need to cut the coils some to increase springrate I can easily adjust the ride-height back. 14" travel Bilstein 5125 shocks to go with.



Suspension Geometry

All these numbers are still approximations because I won't be back at my MJ for another couple days, though they should be fairly accurate based on measurments I've taken before and written down.

-Anti-squat looks to be coming in at 70%, some adjustability in the mounts would allow that to go up about 20% or down 10%.

-Roll center is at 28 inches. Sprung COG is (guessed) at 40.5, making overall COG at 34.5".

-Roll axis is tilted back (oversteer) 8 degrees. This is the only number I don't really like, but sort of a necessary evil of the triangulated 3 link.

-39" upper, 43" lowers.

-68% antisquat at 5" compression, 113% at 10" droop.



This is essentially what I'm looking at:





Coil mounting

I am going to have the coils captured on both ends, and I think that along with a high roll-center and leaf springs up front will keep my MJ very stable. The coils will be mounted about 43" wide, directly under the frame rails.

The top mount will be this style:



but on a threaded adjuster (same 1.25"-12 allthread) like this:




The lower mount will have the coil feed through two small pieces of tube like this, and will have an internal bumpstop like an XJ coil front suspension:







Well, that's all I got for now. It all seems pretty clear in my head, but please give input anywhere you see room for improvement or problems you see

Common, we finally got an advanced tech section and the best you guys can handle is notching tube and attaching radius arms? (we need that smiley)

No input on suspension geometry? No input on material for the crossmember? I was also thinking of using square tubing for the crossmember since the bending force will be at a relatively predictable angle.

Ahh, come awn. You expect us to take you seriously after you do leaf springs on the front?

You can get opinions to support any position you choose..........so we keep our own opinions.

I haven't read all that you posted, but from a quick look with the time I have, it looks like you pretty much have it figured out. Personally, I like panhard bars, but you already eliminated that possibility. Sure, in straight up and down axle cycling there is a slight side to side movement, but it effects little. When articulating, that doesn't happen, and I like how a panhard bar keeps things tracking straight. Panhard bars are very cool for articulation.

I would think the leafs up front would make you take me more seriously because it shows that I am willing to think outside the XJ box :laugh3: besides, who does "mid-arms" anyway? everyone knows longarms are where it's at

I also have nothing against panhard bars, but the decision to not do a parallel 3-link came from the handling quirks of having the upper link offset to the passenger side. Ideally the upper arm would be on the driver's side to counter-act the torque from the engine and keep even weight on the rear tires, but having it to the passenger side accentuates the weight shift from engine torque. I did a bunch of research on this and there is even an equation to figure how far from center to the driver's side you should have the arm based on angles and gear ratio in the diff. This is still a minor consideration however because I've heard on the Pirate board that the effects are negligable, but it's hard to say if that statement is from experience.

"So move the gas tank" is the logical reply, but if I decide to go that route I will just do a dual triangulated 4 link.

You have a good point there. It would be hard to determine the exact effect until you built it, which is a lot of work. It's also a lot of work to move the gas tank.

So, why do you want to switch from leaves in the rear? It works just fine for Fordyce and the 'Con, doesn't it?

jeez, looks like you have covered all your bases. I like that you incorporated the adjustable spring perch and the lower spring retainers. The only thing I notice is the (oversteer) that you mentioned.. but like I said, I think you have covered all your bases without actually testing it in real world...

BrettM said:

"So move the gas tank" is the logical reply, but if I decide to go that route I will just do a dual triangulated 4 link.

Click to expand...

So move the gas tank
Billy

I was worried about a driver's side upper arm on the front of the URF three link. The fear was unfounded. It climbs like a dream.

PaulS talked extensively to Currie about their new rock rig. They have done it both ways, and have felt zero difference.

FWIW....YMMV

CRASH

BrettM said:

I would think the leafs up front would make you take me more seriously because it shows that I am willing to think outside the XJ box :laugh3: besides, who does "mid-arms" anyway? everyone knows longarms are where it's at

I also have nothing against panhard bars, but the decision to not do a parallel 3-link came from the handling quirks of having the upper link offset to the passenger side. Ideally the upper arm would be on the driver's side to counter-act the torque from the engine and keep even weight on the rear tires, but having it to the passenger side accentuates the weight shift from engine torque. I did a bunch of research on this and there is even an equation to figure how far from center to the driver's side you should have the arm based on angles and gear ratio in the diff. This is still a minor consideration however because I've heard on the Pirate board that the effects are negligable, but it's hard to say if that statement is from experience.

"So move the gas tank" is the logical reply, but if I decide to go that route I will just do a dual triangulated 4 link.

Click to expand...

no responces due to only members aloud to see the thread thinks I.

responces of "ooooh and ahhhhhh" would come from black usernames.

put an 8.8 in there and call it good...

I would suggest getting a fuel cell and remove the gas tank or move the tank (or Cell) to behind the cab then making a four link. It would be a lot of work, but in the long run I think it would be worth it. Just my .02

Ron4x4 said:

I would suggest getting a fuel cell and remove the gas tank or move the tank (or Cell) to behind the cab then making a four link. It would be a lot of work, but in the long run I think it would be worth it. Just my .02

Click to expand...

I actually agree with that.
What are you trying to accomplish by doing this? Seems like you're working around stuff to make it fit and compromising the design.
If I were going to go through all the trouble and expense I'd do something like Led did, just cut off everything from the cab back and start with a clean slate.
When are you going to relink the front?

Jes said:

I actually agree with that.
What are you trying to accomplish by doing this? Seems like you're working around stuff to make it fit and compromising the design.
If I were going to go through all the trouble and expense I'd do something like Led did, just cut off everything from the cab back and start with a clean slate.
When are you going to relink the front?

Click to expand...

What am I trying to accomplish?

That is a great question and something I should have addressed up front. One of the main things I want to deal with is axle wrap in the rear. I could add some leafs to the pack to deal with wrap, but my truck is so light in the back it wouldn't flex at all then. I could add a traction bar, but the way I see it, that's half-way to really linking it, and with far less control of geometry, so might as well just go all the way. Another reason is it should actually come out lighter than leafs, and I'll be able to chop more of the frame off the rear. I also (minorly) bent 2 MJ main leafs last season, and I don't feel like dealing with that any more.

I don't think I can do a fuel-cell for california smog reasons. I certainly have thought about moving my tank behind the rear axle or doing the same with an XJ tank. I really don't want to raise the tank since that's about 150 pounds of weight that is down real low right now, and I'm concerned that leaving it at the same height behind the axle would leave it too vulnerable.

(Jes, no plans in the foreseeable future to link the front back up, if I do it will probably be a couple years off until I can do it with a HP housing and probably links very much like yours)

CRASH, thanks for the input on the parallel 3-link w/ panhard... I'm still going back and forth in my mind on this one, and I'm starting to really think of just trying it out; the lower links and coils mounts would all stay if I decided to change it, so it would just be a matter of changing the upper link to a wishbone... hmmmmm....

All the smoggys care about is the gas cap. As long as all your canisters & everything else is there than there will be no questioning. The gas cap could go to nothing, but as long as it passes their little vent tester than you're cool...

CRASH said:

I was worried about a driver's side upper arm on the front of the URF three link. The fear was unfounded. It climbs like a dream.

PaulS talked extensively to Currie about their new rock rig. They have done it both ways, and have felt zero difference.

FWIW....YMMV

CRASH

Click to expand...

Actually, John Currie said that he prefers a front driverside UCA.
I decided that he & the guys at URF had it wrong, I put mine on the PS

Paul

Fishboy living to his name! Going against the flow!


Heres a wet one! :kissyou:


Ok, back to it: My take is to either to a tried and true 3link or a dual triangulated 4 link. Messing around with other ideas work, but why go through the trouble? Honestly when I go to do mine, a 3link will probly be what goes in. A whole lot less math to get the geometry right, and it just works.

Think about what you are able to do, then just do it.

Matt

Brett,

My MJ gas tank fit nicely across the between the uniframe rails..
Like this only with a real mount..

since you still have the actual floor of the bed back there I'd cut a hole to fit the tank into and fab some brackets and straps to keep it secure.

The biggest problem I have seen so far with shortening the wheel base is the unirails get in the way.. Mine is at 117 inches and I would like 110.
After several contemplations with the tape measure I opted to just keep the unirails and leave the wheel base at 117. Eventually I will run 40's or larger so the added wheel base isn't much of a problem in my opinion...
Once the tank is moved a triangulated 4 link will fit nicely.. No track bars or pan hard bars to fool with.

only other input I have is, capturing coils is dangerious as well as premautrely wears out the coils.. I'd suggest leaving one end of the coil uncaptured.

Take it easy
DaveO

captured coils are dangerous?! never heard that one before. I can understand how it would prematurely wear them, but with stock XJ coils I could care less; I posted on the Sierra chapter board that i'm looking for some stock XJ coils and within 24 hours I had offers of 4 pairs for free.

I know what you mean about the rails getting in the way... I'm not totally opposed to chopping the frame right in front of the coil mounts, but hopefully it doesn't come down to that.

BrettM said:

captured coils are dangerous?! never heard that one before. I can understand how it would prematurely wear them, but with stock XJ coils I could care less; I posted on the Sierra chapter board that i'm looking for some stock XJ coils and within 24 hours I had offers of 4 pairs for free.

I know what you mean about the rails getting in the way... I'm not totally opposed to chopping the frame right in front of the coil mounts, but hopefully it doesn't come down to that.

Click to expand...

This project is gay unless you cut the frame at the cab.

Be one with the Sawz-All.

Embrace the tube bender.

Your aura will glow brightly.

CRASH

I have seen captured coils like you pictured launch out from under vehicles.
This would be my impression of unsafe.. Also, stock XJ or MJ coils will not give you the lift you have now... After playing with MJ, XJ, and TJ coils I went with a 3 inch lift XJ coil for the height I needed..

Keep up the Great work Brett.. your Rig is really taking shape...
I just wish I had time to finish mine... lol...

Crash, Be one with the Sawzall.... She's your friend......
Stay tunned to my next project... One fugly 94 XJ

later
DaveO

CRASH said:

This project is gay unless you cut the frame at the cab.

Be one with the Sawz-All.

Embrace the tube bender.

Your aura will glow brightly.

CRASH

Click to expand...

i know... i know... this project may get put off until the fall and then cut it at the cab...

arg