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Coil rear suspension again


NAXJA Forum User
OK I've been reading (and reading and reading) on the subject. My head hurts but I now understand anti-squat (I'm at about 75%CG) and roll axis. I have a few more questions.
Has anyone read the article in the July 4wheel & off road Mag? Opinions?
I managed to design a suspension based on their guidelines.
They recommended a upper link at "no less than 70%" of the length of the lower arms. Why? I know some guys on this board have their links almost the same length. Problems?
Anyone running a triangulated 4 link or 3 link feel the need to use a panhard rod? A local 4x4 shop owner who says he has built a "lot" of custom suspensions said almost everyone who runs a four link on the street ends up needing one. Then again the guy was generally being a Richard Cranium and acted like he was the God of suspensions and I was a total idiot for even questioning him.
Anyone on this board run a 4 link on the street? Bad idea or no?
My XJ is a daily driver. A 4 link would give several advantages, but I don't want to dread my 10 mile commute to work or the occasional 75 mph highway trip.
Matt- Some of the info in the mag was decent, but take it for what it's worth... a short article in a rag. There are some substantial books dedicated solely to suspension design. And, even the "experts" don't agree on the best set-up.

I believe ANY properly designed 4 link will not need a track bar. If the links are triangulated properly, they will control the axle's lateral movement, which will eliminate the need for the track bar. Some soft coils set ups may need a sway bar, especially if street driving is going to be part of the equation. If the suspension is working well, street driving won't be a problem.

I have a lot more experience in the world of drag racing, but some of it crosses over. Most of the drag cars I have seen built, and helped build required a panhard bar if the car was going to be driven on the street. On the smooth confines of the drag strip the cars performed flawless with no panhard. If the owner tried to drive the cars at speed on the freeway the rear-end would take off on them when taking a corner and hitting bumps (think trying to turn a corner on a gravel road with braking washboard in the turn). The 22.5 wide drag tires don't help any but the theory is the same. If you are gonna drive the XJ on the trail at slow speeds I would say no panhard required, but judging from your post it sounds like it is a street driven rig also. I have seen quite a few guys try to make a street drivable drag car without a panhard and they always end up putting one on. This holds true whether it was a four link, ladder bar, or 4 bar setup. Hope this helps at least a little (if not you can throw me back in my cage).:D
Drag car theory doesn't apply to that area of 4WD suspension, not in this case. 90 percent of 4WD suspension will run upper or lower arms triangulated to at least 30 degrees which serves the same function as a panhard. If your space is tight then you can triangulate top and bottom both to lesser degrees. Really, assuming 30ish degrees triangulation upper, which is typical, bringing the lowers in at the front will cut back on flex steer quite a bit, and at the same time give even more side stability.

I guess ultimately you can look to the comps once again. Guys will sidehill well over 45 degrees and the axle does not shift laterally. If you can do that with no trackbar, cruising the mall poses no problem. ;)

Anyhow if you do a typical |\/| setup pay attention to the lowers, mounting directly below the 'frame' rails may be easy, but ideally in my experience you want them inboard more.

Also the reason to keep the uppers and lowers similar in length is to keep the pinion pointed at the t-case throughout travel.
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Most rigs I have seen recently with 4 links and are still very street worthy are not running pan hards, they are running Ati-Rocks. Currie runs this type of set up on there new production vehicle " The Wrangler Strangler". And they now sell them with tubes in different widths and different length arms. I wheeled with currie this year in Moab and they had no problmes and no regrets about thier new rig. I will be installig one on the front of my rig this year to try and get some more flex out of my rear leafs. But I think doing a four link in the rear with Anti rocks in the front and rear would be tits.
Cool. Sounds like I'm finally going in the right direction.
Kaczman, from what I can see, my geometry will likely be pretty similar to yours. Do YOU feel you need a panhard?
JJacobs, my upper links have a 45 degree angle between them. Is the angle between the arms the 30 degrees you were referring to? Also my lowers will be about two to three inches inside the frame rail and about two inches below it. I'd rather they not hang down so far, but it's a sacrifice to reduce anti-squat.
I will also try to run a stiff sway bar of some kind with disconnects. I'm hoping that this will give me some highway stability. The springs will likely be my old Up-Country front springs cut down to fit if needed.
Matt, why can't you raise your lower mounts on your axle so as not to sacrifice more ground clearance?

Matt- I don't think I'll need a track bar, especially since there is NO lateral axle movement now that my junk has taken up full time residence on jackstands. My uppers are ~35 degrees, and the lowers are angled in slightly. My lowers are mounted lower than I'd like, but if I mounted them flush with the bottom of the frame, the anti-squat would jump from ~75% to over 150%. I posted a sketch of my design on another thread, but here's a link if you didn't see it.


The pic shows how the rear is set up, and the front is still in the design phase.

There is a big difference in desired functional performance of a linked suspension for street use and one for offroad use. Or any other purpose built suspension for that matter.

For a street suspension you want something that is very tight, and limits body roll and is essentially a 'rigid' suspension. Long travel, articulation are of no concern or value to a street rodder. That is likely why a street rod suspension guy would steer you to a panhard with a fourlink. You simply cannot get the same lateral rigidity from a long link 4-link as you can with a panhard. If you're a street rodder, you want lateral rigidity. If you're a rock crawler, you want long links (read travel). Can't have both..... Which ONE do you want.

Even something like a trophy truck suspenion is not the best "copy" for rock crawling. Trophy trucks are concerned with long travel but do not want body roll or articulation. They do not need to worry about rear steer. We do.

The same basic philosophies of any linked suspension design will be employed regardless of the application but the subtle differences between each can result in very different performance characteristics.

Like someone said above, you need to figure out what compromises you want to make. You can't make a street rodder (or NASCAR, or trophy truck, low rider. etc.) linked suspension and put it on your buggy or take your buggy's long travel 4 link and put it on your street rod (or yadda yadda ya).

You'll need to be the ultimate judge of what you think you want.

"Richard Cranium" may well know hot rod suspension set-ups like the back of his hand but how many rock crawlers or trophy trucks has he done?
Just like you can't take my advice and run with it, you can't take Richard's either.
Hey Sean, you're going to be at the Crawl in July right? See you there. When you raise the lower frame mount, you increase the ammount of anti-squat force. In other words, when power is applied to the rear wheels, they try to go foreward under the truck while pushing the truck up into the air.
Immagine if the link went through the jeep and was mounted to the roof above the passengers heads. Obviously the axle would just roll foreward while pushing the jeep up a little bit untill the slip yoke on the driveshaft collapsed. This is an exageration of course, but immagine the range of possible mounting positions between horizontal and vertical.
When the link is just above horizontal, the force pushing the truck up is very small, but it increases for every bit you raise the frame mounting point. The problem comes because when the truck is being pushed up, the axle, wheel and tire are pushed down. (Action-reaction) The tire, acting like a spring, compresses. It then must rebound. The end result is wheel hop and broken gears and axleshafts.
Keeping the lower arm frame mount as low as is bearable should keep anti-squat and therefore wheel hop generating forces at a reasonable level.
This is based on what I have read, heard, and my background in basic physics. I have not built a single four link suspension in my life, but this is how I understand the problem. I hope it makes sense and a little lightbulb is going on over some peoples heads like happened to me a few days ago.
I do want a compromise. I want to be able to adjust some spacers, or swap springs and go from streetable 33s at 5" lift to weekender 36" swampers at 7-8" lift in about an hour. I know gearing is an issue. The steering will allow it. The front long arm suspension will do it. Camber and pinion angles will be easily adjustible with the arms. I know it won't be perfect on the street or on the trail, but it should do better in both places than my uneven sagging rear leaves do now. And besides, I love an engineering challenge. It's the whole reason I do this. It's not just a jeep, it's a medium of self expression. Good or bad, well executed or not, I consider it to be a work of art. To quote Tim Allen: "arrgh! arrgh! arrgh!"
Thanks all you guys for your help in the creation of my opus.
JJ, go back and reread Matt's original post and give up on your paranoid delusions :) (unless of course you were the guy at that shop)

Matt, I realize exactly what happens when you raise the frame mount. I don't think you took what I was saying in the correct context. Try doing a couple of drawings while playing with your axle mounts and not your frame mounts and see what that does with your AS and RC values.

JJ13, Dude! Relax! I don't think anyone was refering to you as a Richard. That was about a guy I talked to at a local shop here in Florida. His attitude was basically "my way or the highway" and he didn't want to even hear how I used my rig. Just wanted me to give him $6,000 and shut up. It had nothing to do with you personally. Please refer to my post at the top of the thread.
Sorry Sean. Didn't mean to sound patronizing. I know you know how to build a jeep but wasn't exactly sure where you were coming from. Also I just recently figured all of that out and wanted to explain that to others who, like me two months ago, didn't understand it.
I've done the drawings (too big to fit onto my scanner at 1/4 scale). I assume you mean pushing the lower arm axle mounts up on top of the tube or higher. The problem is I start running out of travel at the top of the housing if I am going to keep 8" between the top and bottom axle mounting points. The upper arm mounts will come close to hitting the tub.
Is that what you meant or did you have other ideas?
My real problem is why the hell am I running a 4-link on an XJ with only 5" of lift and 33's
The other issue is that I am already running Claytons cross-member with my own arms. The thing already hangs down three inches below the rails.
I stand corrected and embarrased. It had been a day since I read your original post and completely forgot about that part of your post. I apologize.

no more posting for me on finals week.

No, just a rumor some one started a few years back to promote the web. Look it up here. urbanlegends.about.com/ :badpc: