Four link and leaves?

[XJJPR]

Just thowing in some :arrowr: "OUTSIDE THE BOX" :arrowl: thought.

Atleast it wasn't a :arrowr: "IN THE BIRDCAGE" :arrowl:thought

IMHO



mark

 

[CW]

OK, my brain hurts from researching all of this, but I am getting a basic understanding of what needs to be done. I don't think using the spring as a link would solve the problem because it would still flex under acceleration causing axle wrap. This is my daily driver and I don't want to cut the floor and remove the back seat in order to position the links in there ideal location, it should be fine as long as I keep it close to ideal, right? Can 1/4 elip work on the road with the edition of a sway, I like the idea I just can't lose streetability. This thread has answered most of my initial questions, but now I have more questions than I started with.

P.S. This is probably a dumb question but what is K.I.S.S. ? :anon:

 

[Ed A. Stevens]

P.S. This is probably a dumb question but what is K.I.S.S. ? :anon:

Keep It Simple Stupid (K.I.S.S.)


The "birdcage" is a nice name for linkage mounts on an axle, and one of the "birdcage" variations (used on older drag racers) is a "full floater" caged axle housing bracket.

I mention the full floater "cage", as it allows the axle housing to rotate (float) inside parallel brackets, with a leaf spring mounted to the bracket. The axle is allowed to float in relation to the bracket cages (and leaf spring), so only the control arms (or traction bar) locate the axle. The axle is located in the cages (right and left side cages) with a tab welded to the axle, located inside each cage. This allows a rigidly welded traction bar (or arms) to always keep the pinion pointed at (or parallel to) the trans output, to minimize driveline problems with a drag racer, and eliminate axle rotation loads from torquing the leaf springs into an "S" shape. It is a very heavy and complicated way to prevent spring wrap. It also creates considerable wear where the axle rotates inside the cages (not too good for a daily driver).

A floating cage can also be modified to allow you to use a traditional leaf spring mount and control arms, without binding the axle (a variation on what Mad MaXJ mentions). The leafs keeps the cage oriented in the same way a lower control arm works, and the upper arms control pinion angle rotation. It has the same problems with wear.

The rigid (pinned) half of the leaf springs will still force the axle to travel in an arc, regardless if the axle is caged to float (with arms bolted to a cage bracket) or controlled with welded brackets (both leaf pad and control arms bolted to brackets welded directly to the axle housing). Cages add considerable weight, and once the wheel travel grows beyond a few inches they lose any real advantage over a true (well designed) linkage suspension.

I would stand back and remove the topic of how to spring the suspension (1/4 eliptic leaf, coil, coilover, semi-eliptic leaf, etc.), until you decide how to link the suspension. When you mention a goal of keeping the back seat and floor, you will limit what you have room to fit any suspension, and the rear leaf springs begin to prove themselves as a good compromise between the space available and simplicity (with a traction bar for pinion angle control).

If you can excuse the penetrations needed for coilover shocks and a cage it can be much more space efficient than any leaf system. The 1/4 eliptic leaf pack can have an advantage of a progressive spring rate, but it will weigh quite a bot. Old Class-8 & 6 off-road racers were forced by the rulebook to use leafs (suspension must use stock spring type: leaf or coil) and 1/4 eliptic leaf packs were a way to bend the rules. The result was good given the rules, but coilover shocks and mechanical linkage systems quickly replaced leafs in any configuration (when Trophy Trucks replaced Class-8 as the premere cubic dollar unlimited class).

Have fun with tossing out ideas (and the ribbing), but the advice to K.I.S.S. is good for a reality check.

 

[XJguy]

Ahhh the voice of reason, my hero. Ed's the man! :loveu:
Thanks for the info Ed.

XJguy

 

[Beezil]

at this point, I don't know how to set this up to avoid flames, but I'm just gonna belt it out there.

I was debating with one ton a long while on this subject, until I finally did the geometry work to see his point, and I eventually surrendered.....

people do 4-links STRATEGICALLY in order to use geometry to thier advantage, so that the jeep performs by design. "locating the axle" is NOT the primary function.

link geometrey directly affects the way the jeep behaves under motion.

that said, heres what I have to say about four links on XJ's....

There is NO WAY I can see that a FUNCTIONAL 4-link can be designed for an XJ with moderate to large lift height WITHOUT CUTTING THE FLOOR.

what do you think of that CW?

 

[macgyvr]

functional....

beez,

can you define what you mean by functional four link? my pipe four linked rear coil long arm conversion is i think turning 2 years old.... and never had the thought/math/brain burning put into it that you guys do....

and i'm not saying that mine is perfect by any means, i'm just curious what makes a four link functional beyond the obvious reasons...

btw, thanks for putting that jeep of your's on its side, got a nice look at some of the angles of the rear links in a pic someone snapped.

mac 'linky logs' gyvr

 

[Beezil]

two words for you to think about when you are anylizing your 4-link

"roll axis"

yours scares me just watching it!

 

[macgyvr]

you should drive it....lol

and what exactly is it about mine that scares you?

remember.....hillbillies built mine....the weekend before winterfest 2002......

now beyond that fact, what scares you watching it?

mac 'jiven better with rear coils than leafs' gyvr

 

[Beezil]

body roll

 

[macgyvr]

ooooooh

well have you seen it on the trails since i ditched the stock coils and went with the TJ rear 4.5 re coils? That and new shocks nearly eliminate the body roll.....

so what your saying is that yours doesn't have body roll cause its set up right? to eliminate the body roll you have to go through the floor with the links? again, not saying mine is perfect, cause you have WAAAAAAAAAAY more thought in yours than I do, i'm just curious, i sent you an email last week asking some questions about your rear suspension as well as some other things, shoot me a reply whenever you get a chance....


mac 'rolled body' gyvr

 

[BrettM]

Beezil, can you give us a quick, simple lesson on roll axis and body roll? I've browsed the God of Suspension and 4-link for Newbies threads, but have yet to sit down and really study them because I may never actually get the time and/or money to 4 link mine.

 

[Beezil]

this is pretty much all I know....

1. links high angle flat

2. point lowers at cg.

3. frame seperation 70% of axle seperation.

4. read stevens' posts more than a dozen times

5. copy someone elses design.

 

[C-ROK]

Here's an excerpt from the SAE pub Fundamentals of Vehicle Dynamics on linked suspensions and roll axis and roll center.
Hopefully you have good eyes or can zoom in to read it.

And Beez is just waaaaaay to anal.

I just didn't design my jeep for high speed cornering capabilities.
The Cummins handles that while the Jeep is on the trailer behind it. Come to think of it..... my Dodge isn't designed for high speed cornering either.
Again, what's perfect and what's practical are just not always synonymus.

 

[Beezil]

you big boner c-rok! I didn't design my 4-link for cornering, I designed it with enough anti squat to give me good ledge climbing ability. I also wanted a more flexy rear to match the front, but at the same time I wanted side hill stability.

I got lucky. I got each of those!

kept my links as horizontal as possibile, they are all above the axle tube.

 

[Kaczman]

B- Have you calculated your anti-squat and roll axis angles?

I figure my roll axis is around 4* and anti-squat ~85%. My scientific jackstand tests show the rear body roll to be less than 2" off level at 12" of wheel travel (full stuff and not even close to full droop) It "appears" the rear will function as I planned. However, I sacrificed 1.5" of clearance at the lower link frame mount to get every thing to work out. I'm just wondering how the numbers compare with the two completely different rear designs.

-Jon (Web-wheeler specialist)

 

[MrShoeBoy]

Beezil do you have any finished pictures of your rear set up? I along with a few others would like to see them too.

Thanks,

Aaron

 

[Beezil]

Kacz and shoe.

I have the jeep at work right now.

I am brining it home monday, and I will take some measurements of my final link locations and do a little drawing of both side view and top view to begin a rigorous critique. While the thing seems to work, I still have a shitload to learn, and it would be real fun to play around with it, since I have multi link bolt hole locations. I'd love an opportunitiy to have some brains anylize the thing, and see what happens.

I am chopping the rear off, and that will change the CG alittle.

this will be fun to mess with.

remind me (stay on my ass) about getting those line drawings posted.

show, I foregt who, but someone posted a pic of my rig on its side.

it'll give you a good viewing of both the wishbone and the rear 4-link.

 

[C-ROK]

you big boner c-rok! I didn't design my 4-link for cornering, I designed it with enough anti squat to give me good ledge climbing ability. I also wanted a more flexy rear to match the front, but at the same time I wanted side hill stability.

I got lucky. I got each of those!

kept my links as horizontal as possibile, they are all above the axle tube.

Except for the "all above the axle tube" you've bascially described my rear suspension. My anti squat is also fairly high due to a shallow angle on my lower links and the near flat angle and high intersection point of my wishbone.
I also have good side hill stability with the addition of my sway bar. And my rear is far more flexy than the front.

My ledge climbing response is not ideal but that is due to features of the front suspension and not the rear.

Yet all my links mount below the floor pan on the frame end and both above and below the axle centerline on the rear.

We've achieved similar results - both by luck and by design for each of us, but with different designs.
Having been in the design field for many years I've well recognized that there are always many "good" ways to achive an end result. I do not believe in the perfect solution.

Utopia is a different place for each person.

 

[woody]

Utopia is a different place for each person.

No Doubt!

Many different tweaks for many different freaks.

I am digging on all the advanced linkology R&D... and liking the 90*+ R departure/climb angle thing too.

 

[4ward]

well have you seen it on the trails since i ditched the stock coils and went with the TJ rear 4.5 re coils? That and new shocks nearly eliminate the body roll.....

Matt, what coils were you using before? I'm betting it only feels more stable now because of the spring rate change. I'm also betting that you are back into the same old XJ problem of the rear end now dictates the attitude of the vehicle. No biggie, wheel it and have fun.

My AS= 82%. I don't remember what the rake of my roll axis is. It's probably around 2-3* downhill towards the front.

Here's a couple shots showing mine during the build.