Design, geometry and functionality of tracbar-less suspension

[Beezil]

I would like to start a conversation and debate on tracbarless front link suspensions.....

in order to avoid this topic being ruined there's some rules:

Leaf spring discussions are irrelevant
"Why?" is irrelevant
Daily driving, on-road situations irrelevant
JNJ's opinions are irrelevant and unfounded.

let's hear something about link geometry and lengths, and mounting locations....

anti-squat / rollcenter info would be great.

[xj4rocks]

3-link?
triangulated 4-link?
radius arms? (can you even do it with radias arms?)
or all of the above?

We you thinking of a specific one beez?


Bob

[FarmerMatt]

What about daily driving?

[Beezil]

I won't answer "why?"

I don't know the answer to "what kind?" yet......

was thinking about a four link........before i did my radius arm set-up, I was going to do a upper wishbone similar to one-tons but decided to do the radius because it was fast and easy, and I was pushing to have my rig ready for moab.....Now I have time to re-explore my options.

I was to figure out the geometry and paramters first in order to see how difficult it will be to build with clearance issues with the exhaust and oil pan in the way.....if this was a rear 4 link question, it wouldn't be an issue as much....

I'm trying to figure out link lengths and placements that will make the most sense in regards to anti-dive and articulation......

Or maybe someone can't point me to a POR discussion that has reliable info, and that ISN'T the "god of suspension" thread.....

[xj4rocks]

TPIJeep on pirate has been doing a lot of link calculations. there are a number of threads that he has started that seem to have some pretty good solid real world calcs.

if you're into calcs that is.

if you triangulate the uppers and bring them together towards the center you'd either be above the diff or next to it but I can't see how one would get around the oil pan/driveline clearance issues very easily.

wanderingwillys (i think Matt is his name) built a 3 link with a WIDE 'U' for the upper so that it would clear the oil pan etc.

I keep thinking about it in my head because i have a real disdain for the tracbar (just don't like it for some reason) but haven't come up with anything great yet. I'll certainly be curious to hear more from others that have fabed or worked on linked fronts with no tracbar.

[Loose_Nuts_Enterprises]

What about a triangulated 4 link where the lowers come from the center of the crossmember to the outside of the axle end and then do the uppers straight (relatively) front to back?

I believe the give away avalanche buggy is done this way.

Dan

[XJJPR]

Beezil,

Here's one to think on, but it does have a TB but only one upper.

I know Goatman is running one, but the one on the MJ is on the driverside. It articulates really well and can still be used with normal steering so keeping it so you can drive around on the street still works. I know what you said, but... (Options) Check out the MJ page on the site you'll see the setup I'm talking about.

As for a 4 link no TB you'll really want to do a inverted 3 link. Wish bone on the bottom, single center link on axle end, 2 arms on top. This gives you big time clearance and has been proven in Comp buggy. (Ron's at ORGStore). He has a two ended ram to make it steerable at high speeds. His new buggy has this front and rear. The lower wishbone is plated across the whole thing so it works as a big stid plate also.

mark
orgs mfg

[Carpenter]

A front suspension without a trackbar is hard to do with conventional steering. The latest failed attempt I've personally seen used what looked like a traction bar on one side of the axle to control pinion wrap with another forked, traction bar-looking link turned on its side to try and keep the axle centered. Needless to say, that setup didn't work too well with conventional steering. You need alot of lateral confinement (for lack of a better term) to withstand the side-to-side leverage placed on the front end by the draglink. Otherwise, the Jeep will just want to "walk" over top of the axle when you steer. But I'm sure you've already considered that. Are you willing to go full hydro?

[Rev Den]

Beezil. having said that daily driving is not relevent. Might I suggest a full hydro steering, no linkage to the frame might make the design easier.

Rev

[Beezil]

hydro steering is highly probable....

and ultimately necessary

[Cresso]

eh, my heart's not in it at the moment, but try to keep in mind the correct terminology. So far, every time someone has said the term "3-link", they've gone on to describe a triangulated four link. A 3-link only has 3 attachment points on the frame. That's how suspension link systems are defined. All of the proposed systems above have 4 frame attachment points (oddly enough, a track bar does not count as a link for some reason).

Unfortunately, I don't have much else to contribute to this. I really like track bars. I like the way the axle shifts when articulated - it'll push to the high side more than most of the non-track bar systems I've seen and I like how it grabs when it does this. My only complaint about my track bar is how short mine is. If I could get it about 33% longer, I'd be extremely happy. I wish you much luck in getting proper geometry and still clearing the oil pan and exhaust. The triangulated 4-link off that giveaway Avalanche rig may actually be your best bet as far as fitting, but I have yet to see enough info on it to get any sort handling impression of it.

[JnJ]

Use leaf springs.

Why?

You wont be able to use it as a daily driver or on road.

Beez, the trackbar is not a big bad monster. really, deal with it you wimp. Do a triangulated upper (want to see that work on an XJ (no response from you Matt)) and some form of hydro or hydro assist steering.

You should just got leafs, you know you want to.

Oh ya, check out the POR thread "God of Suspension". Loser :rolleyes:

[JnJ]

Originally posted by Cresso
eh, my heart's not in it at the moment, but try to keep in mind the correct terminology. So far, every time someone has said the term "3-link", they've gone on to describe a triangulated four link. A 3-link only has 3 attachment points on the frame. That's how suspension link systems are defined. All of the proposed systems above have 4 frame attachment points (oddly enough, a track bar does not count as a link for some reason).


So since my "Y" arms only attach to the frame (Lowers from frame to axle, one upper from lower to axle), this would mean I have a 2 link system............

[Cresso]

Exactly. Ford named their two-link system "radius arms" and that term seems to be pretty popular. EIther way, that's what you have.

[XJJPR]

Originally posted by Cresso
but try to keep in mind the correct terminology. So far, every time someone has said the term "3-link", they've gone on to describe a triangulated four link. A 3-link only has 3 attachment points on the frame. That's how suspension link systems are defined. All of the proposed systems above have 4 frame attachment points (oddly enough, a track bar does not count as a link for some reason).



Well, Not sure of your terminology but what is on the MJ, mentioned above, is a 3-link per your terminology. But it has a 4th link holding the axle in place called a track bar. Why is a wish bone called a 4 link. The axle is only held in by 3 points. Yes it attaches to the frame in 2 places, so who/what makes it a 4 link? The wish bone is also just one arm even though it has 3 mounting points, total. What would keep the front end stable if there isn't a TB link in your 3 link? Jeep calls the XJ front end a 5 link, by definision. Not sure how a wish bone is a 4 link. Which would never use a TB being a 4th link. Why isn't a TB a link? Could you clearify your thoughts for me so I can undersand your terminology a little better, because I must be missing something in your explination.

mark
orgs mfg

[Cresso]

It's not my terminology, it's real old school terminology used in suspension design. It's what you'll find in literature about designing race suspensions, from F1 to dirt track. I have no idea why they don't count the track bar as a link, but the general rule is that links that play no part in fore-aft location and ONLY provide lateral location (like a track bar or Watts linkage) are not to be counted when naming the system. I'm not familiar with the MJ system - does it differ from the stock 4-link system on an XJ? As for Jeep calling their system a 5-link, this is merely an example of a company trying to differentiate what they use at the sacrifice of strict correctness. Technically, the front suspension of a TJ/XJ is a 4-link with a track bar. The link system described in JnJ's post is still a 2-link. There are only two mount points on the frame. The third member of the suspension(that upper link) goes from the axle to the lower link. Since it does not go to a distinct point on the frame, it does not count as another link in the naming convention.

[XJJPR]

Your not familar with the my MJ suspension because you didn't look at it you just said it was all being talk about incorrectly.

I'm not sure of Old School suspension design, I'm not formally trained, just 20 yrs of dealing with suspension guys and race truck guys, not familar with F1 never seen one with a straight axle on one, all refer to links as what is attached to the axles, not what is attached to the frame. Maybe just real world instead of old world. I'm not stating I'm a suspension expert, but certian things just arn't quite clear about that terminology.

mark
orgs mfg

[Weasel]

I was under the impression it was determined by where it mounts to the axle. If the upper link is jointed and has one mount it's a 3 link. 4 link is seperat mounts for upper.

Beezil, you might want to get some books. I would recommend Carroll Smith Tune To Win and Herb Adams Chassis Engineering (I belive is the title)

This should give you all the info you need and weed out the opinion of those that don't know what they are talking about. Not pointing fingers at anyone.

[XJguy]

Beezil, I see the only practical way to go about this is with triangulated top and lower CAs, similar to what is used on solid axle GM cars, but I would take the NASCAR approach and extend them like a long arm system; I believe they are then called trailing arms. I too have contemplated ditching the track bar because to me it just seems like a patch for a badly engineered system. If you need that extra inch of clearance for the CAs..no one said modifying the oil pan is out of the question.

XJguy

[Beezil]

gang, since I don't want to run a tracbar, how bout we just exclude it from the discussion, no matter how many times you want to count it.....

[XJJPR]

Beezil,

Then your looking at a triangled 3 link or a triangled 4 link. Your big questions are are you going to coilovers? Single sided Hydro steering or double sided hydro steering. The coilovers will give you more room for arm locations.

I still like the inverted triangled 3 link design Ron has for keeping all the need clearances. I will eliminate your TB. The triangled lower keeps it out of the way of eveything on your jeep.

mark
orgs mfg

[gearwhine]

I highly doubt this is a good system, but teraflex came out with a bolt on trangulated front 4-link for TJ to get rid of the tracbar. It looks a bit junky to me especially being bolt on, but there was a review in JP magazine on it. They said it wasn't all that bad, but they NEVER give bad reviews. I guess you could take a look at that and maybe get a small idea or two to help you design yours. _nicko_

[C-ROK]

People have pretty much hit on the issues and you seem to already be aware of them yourself, but I'll chime in for the "hey".

If you don't run a track bar/pan hard/lateral tracking bar; you will have big steering challenges so long as you maintain a drag link tie to the steering box.
As the suspension cycles, the drag link will push and pull the steering knuckle along the arc of the drag link. Unless the axle itself is moving along the same arc, the wheels will be forced to turn as the suspension cycles; ie bump steer, etc. This is the reason why the pan hard arc must match the drag link arc.

If you go full hydraulic and do away with the drag link, you do away with this problem but you do introduce some other problems with full hydraulic; namely, a very "wiggy" driving feeling at speed. 60 mph in a full hydraulic requires more than a little attention.

Second isssue can be clearing the oil pan. You should be able to design around it but it's an obstacle none the less.

IMO, a well designed pan hard link system (however many link points you want to count it as) is a very sound and predictable suspension system for front axles.

You can do it, but I'm not sure you'll neccessarily see any beneift of it over a well designed panhard link system.

If you do go w/o a panhard up front, I might recomend a wishbone style. I believe that the wishbone can offer superior lateral rigidity over a four link and also crazy travel.
You could also consider a forward mounted wishbone which would avoid the oil pan clearance issues.

Walker Evans ran this type of setup on the front of his 2001/2002 S10 Compeition truck.:)

[Kaczman]

Beezil- You really need to quit crawling around in my head. I've been working on the design of a front four link for my junk as well. As you know the front is very limited for space, and the final product will be all about compromises, especially if you don't want a "U" design like Sean and Matt. Here is a pic of what I"ve come up with. Warning: No fancy CAD drawings here, but I'm fluent in low tech.

http://home.att.net/~j.kaczmarski/wsb/media/123450/site1044.JPG

The upper link will BARELY clear the pan with uptravel limited to ~4-5". Obviously I still have to work out the bugs, but at least I now have a starting point. I've got a PVC axle and control arms made, so I can cycle and change things without busting a nut.

-Jon

[Beezil]

c-rok

this is a rig that will never see the more than 20 mph......bumpsteer is not an issue, even if I decide to run a sag box style steering, the effects of draglink operating in an arc while the axle cycles vertically is not a concern. My main concern is to not have the axle operating in an arc. THATS THE MAIN POINT. On my existing setup, my short tracbar is the problem. but your points are well taken.

Kacz.....

hold it right there mister! don't go anywhere.......Can you please comment on how you've located your roll center, and how you are organizing the points of rotation on all your links to give you what you are looking for in anti-squat and anti dive? I cannot tell from your drawing if you are locating the U-arm on the lower or upper side of the axle? also, it looks like your u-arm is positioned a little foreward of the axle tube, or is this just an extended construction line?

[Willis]

Sure sounds like a dry sump system would solve a lot of problems :D Have you read Chassis Engineering by Herb Adams? Great book, goes into explicit detail about much of what you are talking about. I have thought about doing the same, but time and budget don't really allow such severe modifications. Hope you find a system that works very well, and keep us updated (like you always do). Hope Matt chimes in soon. He should have some good information.

[Kaczman]

Beezil- The "U" arm mounts will actually be approximately flush with the rear of the axle tube. The extended construction lines are only to aid with the geometry layout (if you look closely, you can make out the points flush with the tube). The uppers will have ~5" horizontal seperation and be ~6" above the tube centerline. The lowers will be just below the centerline. The pictured drawing is only a rough lay out of "what fits", primarily around the oil pan and frame design. I have yet to actually do the work of calculating the roll axis and anti-dive of what's pictured, but by my scientific "eye-ball" method, the roll axis seems to be a bit steeper than I'd like. I plan on moving the LCA frame mounts lower than pictured to get it better. I'd like to move the uppers up a little, but I doubt there will be room.


Keep in mind, I'm just beginning to understand front 4 link geometry. I have a grasp of rear (uh-oh, that ain't right) geometry, but I don't yet understand how front and rear roll axis, anti-dive, and anti-squat correlate. More importantly, I don't know how much room the front of the XJ will allow us to change this. It makes a buggy more appealing every day.

-Jon

[Beezil]

Kacz,

I can dig it.

I have definite plans to build a buggy.

its for sure.

but plan A is to use this free cherokee as a "sketchbook" as well as a test platform to experiment with a couple new suspension designs as well as add new parts while I'm working on driving skill. The whole while I'll be going bigger on the xj until I figure everything out, then I will build the buggy frame while I'm driving an attending events, and when I'm finally ready I'll make a quick switch of the goodies....

sounds like this is kinda what crash and a couple other folks I know are up to, makes sense.....I want to "get it right the first time" when i go to build the buggy.

I don't care so much if I re-work the xj link geometry for the worse......I'm just trying to do something sorta new as a learning experience....

all the responces have helped, including the recommendations on books, to those who have listed titles or authors, thanks. I'll be checking thos out soon.

[C-ROK]

Originally posted by Beezil
c-rok

this is a rig that will never see the more than 20 mph......bumpsteer is not an issue, even if I decide to run a sag box style steering, the effects of draglink operating in an arc while the axle cycles vertically is not a concern. My main concern is to not have the axle operating in an arc. THATS THE MAIN POINT. On my existing setup, my short tracbar is the problem. but your points are well taken.


Careful with the assumption on steering arc. With a long travel the knuckle can pull over far enough that you're left with no steering adjustment; ie the steering is all used up with the arc pull from the drag link. You may have to turn the wheel all the way right to go straight on down travel situations and vice versa on up travel. Just something to consider.

Going full hydraulic is a good thing for a full time Rock Crawler. And it's not that you 'can't' drive it at speed; it's just that you need to be a 'little' more attentive;)

Like I said earlier too; you can use a forward mounted wishbone to avoid the oilpan. Your limited on total vertical travel due to the shorter length allowed of the wishbone but can still have plenty of articulated travel.

Suspension design stuff is fun:D

[Loose_Nuts_Enterprises]

Say you build a link system, triangulated, wishbone, whatever; set up properly so as to contain lateral movement of the front axle. How would the steering issues be any different than say, a front leaf sprung old CJ with crazy flex. I know in my 48 willys, I can score over 1000 on a 30* ramp.( whatever that's worth) with no steering issues.

Am I missing something?

[Beezil]

How would the steering issues be any different than say, a front leaf sprung old CJ with crazy flex

exactly dan.

My cj-6 had an awesome amount of flex with front buggy springs and shackle reversal, and steering was never an issue.

[Loose_Nuts_Enterprises]

articulation is all about TWIST unless you have independent suspension, so the drag link would not have to grow or shrink in length with the fulcrum being in the middle of the axle. That's my take on this.


Dan

[C-ROK]

As did my leaf SOA Scrambler and I think you make good points.

The difference is though what point the axle is pivoting around. On a leaf sprung system in articulated travel, it will roughly be pivoting around the compressed leaf spring. On a drivers side drop, this won't matter since the drag link will remain relatively flat. On a passenger side drop the axle will pivot around the drivers side spring perch which is relatively close to the centerline of the steering box, thus no noticeable drop steer effect.

On a center linked system the axle will roughly pivot about the center link points and the drop steer effects will be more noticeable.

If you have enough turning swing on your knuckle you may be able to negate the affects, but it will be there to some degree. It's unavoidable.

[Loose_Nuts_Enterprises]

Maybe the difference is that the tierod and draglink on my willys are just about parallel, unlke the majority of lifted XJs out there.

Maybe a track bar-less system with drop pitman, and high steer knuckles would help negate this phenomenon.

Or maybe it's just a secret obsession to make something in the front end 'parallel.'

Dan

[XJJPR]

Dan/Beezil,

Greg said it right. The Center link setup twists the axle at the center point across the axle. A leaf spring setup is just like a TB setup the axle twists in a large arch starting from the driver side (or high side) the same place the Steering gear is attached. On a center link your steering gear would need to be attached close to the center link as possible. Then you would have a REAL short drag link which creates many more problems, not the least of how to attach a steering gear to the center of the frame.

mark
orgs mfg

[bgcntry72]

Take a look at the front and rear setup on this buggy I found on Ebay.
Looks to be sans-trackbar with a 4 link setup with the uppers mounted to a brace that runs the length of the axle.
I need to look at this some more.

http://cgi.ebay.com/ebaymotors/ws/ebayISAPI.dll?MfcISAPICommand=ViewItem&item=2419869925&indexURL=3&photoDisplayType=2

[jimk403]

Then you would have a REAL short drag link which creates many more problems, not the least of how to attach a steering gear to the center of the frame
You wouldn't need to attach the steering gear in the center, just the attachment point for the draglink.
Put an idler arm on the passenger side and run a relay rod from pitman to idler, attach the draglink in the center. Maybe you could even run one to each wheel separately, kinda like IFS steering.

[C-ROK]

Originally posted by jimk403
You wouldn't need to attach the steering gear in the center, just the attachment point for the draglink.
Put an idler arm on the passenger side and run a relay rod from pitman to idler, attach the draglink in the center. Maybe you could even run one to each wheel separately, kinda like IFS steering.

Ya..... But I'd just go to full hydraulic before I got myself into a multi-linked steering drag link system.

I'm not disagreeing here. What you described is the same kind of steering geometry that mega lift (mostly street) trucks run. Sometimes called "swing set steering" because of the swing rocker. Like this:
http://www.randyellisdesign.com/FAB%20PICS%20AND%20STUFF/sub7.JPG

While it certainly does work, it is comparitively much more complicated than just two hydraulic hoses, a ram and a tie rod. But I'll grant you that it does solve the drop steer problem.

[XJJPR]

Originally posted by jimk403
You wouldn't need to attach the steering gear in the center, just the attachment point for the draglink.
Put an idler arm on the passenger side and run a relay rod from pitman to idler, attach the draglink in the center. Maybe you could even run one to each wheel separately, kinda like IFS steering.

I was just trying to give them an idea of what needed to be done so they could picture it in their mind. In no way was I suggesting that as a solution. Full hydro is the only way to go, IMO, with a 3 or 4 link type suspension setup.

mark
orgs mfg

[Gojeep]

I hope you dont mind me asking this question in this thread but here it is.
I have wondered about the effects that kits like RE's control arm drop brackets have. Dropping all the control arm 4" also then lowers the point of convergence of the arms by the same amount. But the rear has not been changed but do not know how this point is worked out this point on on leaves. Is there one?
Now I read in a suspension book the the roll centre of a leaf spring is halfway between the spring perch and the lower shackle eye centre. Would you agree with this and if so the rear roll centre would go up 2" on a 4" lift for example. So if you did a tie rod flip and raised the trackbar mount on the axle by the 1 1/2" -2" to keep it parallel with the drap link would this then help balance what has happen in the rear?
Sorry for all the questions but want to learn more about this;)

[Gojeep]

Originally posted by Gojeep
I
I have wondered about the effects that kits like RE's control arm drop brackets have. Dropping all the control arm 4" also then lowers the point of convergence of the arms by the same amount. But the rear has not been changed but do not know how this point is worked out this point on on leaves. Is there one?


Thinking about it now, would it be a line drawn from the axle centre line then through the front spring eye?

[Beezil]

Thanks to all that responded....

to those that suggested books, I have ordered three titles, can't wait to read up and learn.

Mark, i still haven't looked at your MJ to see what's going on there.

it has a tracbar, and that was against the rules in this thread.....shame on you.

[4ward]

Beez, did you get the Millikin& Millikin book? Find someone that's a member of ASE and you get $15 off.

As for the leaf spring question. The answer is over on POR. I've seen it before, but never paid attention to it. Happy searching.

Mark, how about throwing up a good shot of the buggy with the inverted 3 link.

Sean

[Beezil]

What book dat be?

the answer to the leaf spring question is MOOT.

there is no leaf spring discussion allowed on this thread.

[4ward]

Sorry, spelled it wrong.

book (http://www.millikenresearch.com/rcvd.html)

It's pretty much the bible in the suspension fabrication world.

Sean

[Beezil]

amazon has it for 79 bucks.......

exactly how good is this book????

[zjmike]

What about this setup?, this is what Mark H was referring to as the setup on Ron's..

I am assuming you have a HP front end, as it is necessary for this to work..

Ron ran this setup on his COmp buggy for the past year or so and it worked great..


The triangular link, mounts under the axle...and the driveline runs above it..

forgive the crude drawing, but I think you will get the basic idea..



http://www.fototime.com/9C3EA043343539F/standard.jpg

[Beezil]

That's essentially what i want to run, cept there's a couple things.....

the arms cannot be straight, they have to arch around the oil pan......

furthermore, I want to design and fab in such a way that what i end up with is a bit different than what sean r runs.....

I don't want the guy thinking I'm copying off him.

I mean, the guy already had to remove his roof so his head would have a place to go, i don't want to contribute to those problems......

[Flowers]

Would they still have to be arched around the pan if the triangular link was the lowr link? Which is how the picture is represented.

[4ward]

That's not what I have Skippy. The wishbone is on the bottom. I contemplated this on my design but found it to actually be easier to run the wishbone on the top due to driveline problems and where I wanted to put the frame mounts. A triangulated 4 link is what I'm going with next. Probably going to do some slight cutting on the pan too so that I can run longer uppers. I think you should try it first so I can copy you. You could always dry sump it and not have any pan interference problems. Vertical climbs wouldn't drop the oil pressure to 0 psi either.

Dang you Tim, ya beat me by a couple seconds.

Sean

[Beezil]

sean, can you answer flowers question? What the hell were you saying about the lower u-arm???

some crap about rollcenter or something......I wasn't paying attention....

flowers, the driveshaft clearance would be much nicer on an hp 44......don't you think???

[Beezil]

from a functionality standpoint, what is the difference between a u and what you are goping to? two heims instead of one? That would be better for someone who can't weld fer shit.....think: penetration! (flipoff)

I am not opposed to cutting the oilpan.....in fact, it would probably be required to do it right....

dry hump? haven't needed to do that since high school.

[zjmike]

there arent any clearance issues with the oil pan, cause that lower triangle mounts under the axle....the upper arms are straight,

If even clears my V8, although I have a ZJ but essentially the front end is the same

[Flowers]

BJ,

Sean answered my question before you posted asking him to answer my question. The lowers are triangulated, the uppers are straight, unlike you. I'm assuming that is for oil pan clearance. Take some Methylphenidate and pay attention!

Focus you need to young Jedi.

Due to your severe memory deficiency, the price of your axles just doubled. Do you even remember the price I quoted you? Has anyone seen Beezil's keys.

out,


Flowers

[Ed A. Stevens]

Originally posted by Beezil
sean, can you answer flowers question? What the hell were you saying about the lower u-arm???

some crap about rollcenter or something......I wasn't paying attention....

flowers, the driveshaft clearance would be much nicer on an hp 44......don't you think???


I have not read the entire thread, but I believe they steered you to good reference books.

The question you are looking for is how to determine the roll AXIS elevation for a three link system? The answer is that one point of the line that determines the roll axis is always the single centered joint of the A/U-frame third link. The elevation of this joint is on the roll axis for the suspension system.

The other point on a three link system roll axis is determined by the elevation of the intersection of the other two links convergence point (hopefully along the vehicle centerline).

The goal is to design the system with a flat roll axis, to minimize axle steer. This demands the two independent links convergence point to be level with the single centered third link joint (hard to do).

Since one point of the roll axis is the centered third link joint, it makes sense that placing this 3rd link above or below the axle centerline will have a significant impact on the elevation of the resulting roll axis (high 3rd link = high roll axis, low 3rd link = low roll axis).

Remember each suspension system, front and rear, has a distinct and separate roll axis. The individual roll axis impacts the roll steer of the axle in relation to the chassis. More rake in the roll axis and the more the axle steers through the travel range. A higher roll axis tends to make the suspension less compliant in roll stiffness (the impact of the leverage the sprung weight has over the roll axis).

The two, front and rear, axles suspension system's work together (or fight each other) to determine the vehicles combined roll axis. As each axle's roll axis rake and elevation changes, throughout the suspension travel, the vehicle experiences a change in the combined roll axis.

The difference between the roll axis of the two axle systems can significantly alter the handling at speed.

A tall front roll axis (like in a lifted XJ) adds roll stiffness up front that can temper the fairly mild change in roll axis height of high arch rear leaf springs (making the front understeer to match the axle steer change in rear axle geometry). If you matched the same front lift with flat leaves and tall blocks the result would be significantly different (a flatter cornering ride, to a point, but with less feedback and less stablility at the traction limit).

The roll axis height of the leaf system is "somewhere" along a line between points located between the axle centerline and the leaf eyes (the fixed front eye and the rear eye on the shackle). The "somewhere" comes into play because the textbook roll point assumption is that the leaf pack is straight and level (treat it as a hinged link with the roll point halfway between the axle and eye). This assumption fails with high arch leaf packs, and fails with progressive spring rates (the true roll axis moves as the spring rate changes in the travel range). The advantage of a leaf is simplicity to alter the roll axis rake: longer or shorter shackle, spring under or over, inverted shackle, assymetric leaf pack). The disadvantage is use of flexible (spring steel) for the locating links of the system (predicting the actual conditions under load is questionable).

On the steering, just look at full hydro or a combination rack & pinion with hydro assist, fixed to the axle. The bell crank linkages work but have poor steering feel and considerable deflection and slack (play due to wear in the mank linkage points).

[Beezil]

Ed, that was the sexiest post I ever read.

thanks for takin the time you did to type....I got my books today and I'll be getting my learn awn soon.

Your post makes a ton of sense.....I just wish my friends did....

will you be my friend?

Tim, I found my keys thank you.... poster nutbag was curled up on top of them....

[REDXJ4FUN]

Good grief Ed even if i wasn't burnt from working tomany hour My head would still hurt lol. Mybe when my head ckears up i can actualy figure out what you said but as always your knowlage is greatly admired here.

[Crunch]

I can't take any more. I'm sitting here with tears in my eyes from laughing so hard. At about the start of the third paragraph from Ed's post, I realized I had no clue at all what I was reading and continued to read and laugh hoping just reading it would make me smarter :D :D . Ya right. I'm picturing Ed maybe with a beer in one hand and maybe yelling at the dog like its nothing for him to do this. I'm gonna read it again in case I missed something.

[Ed A. Stevens]

Originally posted by Kaczman
Beezil- You really need to quit crawling around in my head. I've been working on the design of a front four link for my junk as well.
<snip>
Warning: No fancy CAD drawings here, but I'm fluent in low tech.

http://home.att.net/~j.kaczmarski/wsb/media/123450/site1044.JPG
<snip>
-Jon


Sorry guys :) for adding to the confusion, but I read the thread and can use Zaczmans drawing to ID the roll axis. Maybe it will help (maybe not ;) ) and thanks for the drawing (I like hand drafting).

Lets isolate and identify the front axle roll axis (and yes, I understand the drawing is of a four link)?

Copy the drawing and print it out (place the curser on the image, right click the mouse, select "view image", and after the image loads as a separate web page, select print).

Look at the top view (upper) drawing and the side view (lower) drawing.

Locate the lines that represent the arms, and locate the ends of the Upper Control Arms (UCA) and Lower Control Arms (LCA). You will need to look at both views to identify the arms.

Plot the ends of each arm on both views (draw in the arms). These are the fixed mounting points of the arms on the frame and axle with the vehicle at rest. I find the UCA mount at the axle above and slightly behind the axle centerline, and the LCA mount directly behind the axle centerline. I find the UCA frame mount inside the frame where the kick-up begins, and the LCA mount slightly rearward and below the frame.

Draw a vertical line between the two views where each pair of arms are pinned to the frame and axle. You should have added four vertical lines on your paper.

Look at the top view. Locate the UCA's drawn, and extend the UCA lines to their intersection that is in front of the axle, along the vehicle centerline (this was done on the original drawing). This point looks to be just barely ahead of the diff cover. This is one point on the roll axis line for the front axle suspension system.

Plot this virtual intersection point on the side view (lower) drawing. This requires you to draw another (fifth) vertical reference line between the two drawings. This virtual point on the roll axis will be lower than the physical UCA connection to the axle mounts, and forward of the physical mounts (as it should appear on the drawing). This point, when plotted on the side view, is on the roll axis for the front suspension. Draw a larger circle around this point on the side view drawing.

If the system was three-link this virtual intersection would not be "virtual" point, but a physical intersection with the axle by a single link (a rod end or bushing) somewhere near the axle (just behind or above the axle). This point, when plotted on the side view, is on the roll axis for the front suspension.

The other point of the front suspension roll axis is found at the virtual convergence of the LCA's near the rear of the chassis (this intersection was drawn in on the original drawing). Look at the plan view drawing and follow the LCA lines back to where they intersect, slightly ahead of the rear axle centerline. This is easier if you draw another (sixth) vertical reference line between the two views to locate this point on the side view drawing (although, I think, it was identified on the original drawing in an effort to identify the Instant Center of the front suspension).

Again, the virtual intersection of these LCA lines needs to be transcribed to the elevation (side) view drawing of the chassis. This point (and the LCA's) does not appear to be drawn in on the elevation view drawing (add it in). This point, when plotted on the side view, is also on the roll axis for the front suspension. Draw a larger circle around this point on the side view drawing.

You should now have two points identified on the side view with larger circles around them? Draw a line between these two points on the elevation view drawing, and you have drawn the roll axis for the front suspension.

The roll axis is (amazingly, and by mere coincidence) almost inline with the UCA's, with a slight forward rake, when this vehicle is at rest (~16-degree, EDIT == 8 degrees, I had to relearn :) how to read my adjustable triangle). I know I said we want the roll axis to be level (one of the rules of thumb in every textbook) but wait, there more ... so don't toss the design just yet.

Note that these points that define the roll axis move when the suspension links move through the travel range. The roll axis elevation and rake change as the suspension travel is exploited.

Change is not good when you want predictable handling performance, and this change is why road race suspensions are limited in wheel travel. Minimizing change from the at-rest (or at-speed) suspension geometry is the goal of the road race vehicle designer/engineerto make the car handle in a predictable manner (keep the wide tires planted firmly with minimal bumpsteer or axle steer). This is the design goal assumption for every "race suspension" book I have ever read, and it fails when you take the vehicle off-road where we design for mega wheel travel.

Rather than be happy plotting the at-rest (or at-speed) geometry, and tie the suspension down, lets take the design off-road (think past the textbook case).

If you were to drop both front tires into a hole the UCA point would lower (slightly) and the LCA point would raise (significantly), increasing the rake of the roll axis. What does this mean to handling?

Drop one tire, and stuff the other tire, and predict how the axle moves. The axle end tire that drops moves down (duh) and also moves back. The axle end tire that stuffs in compression, moves up, and forward. The forward and back movement is axle steer. Axle steer is a byproduct of a linkage suspension.

The rate of change of this axle steer is slight when the control arms are level to the ground (why every "race suspension" textbook says "keep the control arms level"), and dramatic when the arms are angled sharply (when we go off-road).

This forward movement helps the tire with the most traction, the axle end in compression, to bite the obsticle on the trail surface hard. This is a good thing, but like most things it's only good in moderation. Tall short arm systems can alter the suspension geometry to a point that, rather than stuff the tire and bite the trail the suspension quits working and the axle folds under the front end. This is due to the change in location of the front axle's Instant Center (something for a later lesson).

I hope this little doodling exercise helps identification of the front suspension systems roll axis for the axle in relation to the chassis? I also hope it helps you identify why the textbook "race suspension" design goals are a level roll axis and level control arms? The last hope is that you will realize the textbook ideal for a road racer may not be ideal for an off-road rough terrain vehicle?

Now that all this is clear as Alabama mud, the lesson is over. Any questions (pop quiz next week)?

[Kaczman]

Ed- Thanks for the eloquent explanation. My wife calls the hand drafting diagrams "caveman drawings", but I use what works for me.

One more question. I'm trying to build the XJ with a bias for climbing, and I understand the affects of rear squat/anti-squat. However, I'm not clear on how anti-dive relates to keeping the tires planted as the Jeep climbs. It seems the "long-arm" XJ's have a tendency to "unload" as the front weight transfers to the rear. I know a limiting strap can minimize the unloading, but front traction still suffers. Any suggestions on overcoming this?

Thanks,
-Jon

[Beezil]

Kacz, the limiting strap absolutly works, in fact, now that I finally installed one, I consider it to be necessary! Well, at lleast as far as I know on long radius arm set-ups....

the difference is remarkable.......

however, I can't wait to see if there is a better way.....lookin foreward to responces.....

[C-ROK]

I started running a center 'sling' style limiting strap on the front axle several months ago..... huge difference.
Totally agree with Beez's experience; the center limit strap works and the difference before and after on the same obstacles was tremendous.

[Kaczman]

Is the benefit of the strap due to it limiting the unloading of the front, or does it somehow help pull the front back onto terra firma, and help with traction? (i.e. increase anti-dive?)
-Jon

[C-ROK]

By holding the front end down I'd say the answer is both.

Before I had the strap the front would unload to the point I almost went over backward on a couple of occasions and had to back off obstacles. All the time my front wheels would still be on the ground but rather than pulling up, my body would just separate further and further from the front axles until my side limiting straps were tight and it began to pull or untill I had to back off.

With the center strap it can't unload as far. This keeps more of the weight of the vehicle over the axle and thus increases the traction to the front axle. Less unloading = more traction.

[XJguy]

Sounds like a good reason to run active suspension, load and unload whatever wheel you want at will.

XJguy

[Beezil]

try running a boulder garden with active suspension.

if I wanted to fiddle around with devices I'd play nintendo!

[Ed A. Stevens]

Who has drawn out something like this?


http://members.aol.com/Stvns/FRollAxis



I left out the last step of drawing the actual front systems roll axis between the two points on the axis itself (because it would obscure the UCA).

If your doodles look anything close to this, you can scratch out a full size analysis of your factory XJ suspension on the garage floor (and then make changes to learn why it acts the way it does when lifted).

Is this worth the effort?

[Ed A. Stevens]

Originally posted by Beezil
i did i did !!!!

next lesson please!


Quiz:

Draw the control arm system after changing the two UCA's to a centered three link with the 3rd link behind the axle centerline (your original question).

Find the roll axis?

Extra credit:

What are your choices to level the roll axis?
What system makes leveling the roll axis easier: three-link or four-link (remember there is a vehicle & engine in there)?

[Beezil]

Ed, may I please have an extension???

have to attend to some non-jeep matters over the next few days.....

will this affect my end-of-the-semester grade?

I need to spend some time under the jeep with a tape measure and figure out what version A of my planned suspension will be for a group critique....

jnj.....callin me a front row nerd?

yer just sore me and my friends beat you up and stole your lunch money....

band geek loser.

[Ed A. Stevens]

Originally posted by Beezil
Ed, may I please have an extension???

have to attend to some non-jeep matters over the next few days.....

will this affect my end-of-the-semester grade?



No, it's your thread, but the smart boys in the back row will be targeting you for spitballs when you return (and the girls will be armed with soggy peas). Make sure your shirt tail is tucked in.

I'll be teaching a 4x4 class this weekend and also need a break to prepare.

[Kaczman]

Let's bring this thread back to life....

I figure the roll axis in the sketch looks ok at ride height, but at 10" of drop, it gets WAY high (~30 degrees by my crude calculations). It seems this would get very tippy in off camber situations. Do you radius-type long arm guys have this same issue?

I'd like to keep the suspension predictable in all types of terrain, even though it will be used as a trail only-slow speed rig. How can I keep the roll axis flatter given the limited space for the triangulated arms, or am I over-thinking the whole design?

Thanks, Jon

[Rockit]

Sorry for my inexperiance, but has anyone ever experimented with a watt's linkage style track bar? Fairly common on older roadrace cars, could it be adapted to a 4 wheelin application?

[Kaczman]

A watt's linkage in the front would require full hydraulic steering in order to keep bump steer out of the equation. It also complicates the front geometry, which reduces the primary benefit of a triangulated front 3 or 4 link-- simplicity, and NO track bar.

-Jon

[Beezil]

also, the arms on the linkage itself would have to be pretty big to allow for droop and stuff.......

this thread back to life?

cool.

this coming week I GOTTA-HAFTA start designing, cause I wanna hang the new front axle in the next few weeks.......

if this thread dips to the bottom, hopefully whatever I come up with will get Ed's attention.

[COXJ]

Would it be out of the question to just coppy Teraflexes LCG kit? im wondering because it is triangulated and seams to work. read about it in i believe JP and when i met Dan Mick he had one of there chassis waiting for the body on the other side of yard. so im thinking it may simpilify but that may not be what u want to do.

WIll