Check my 3 link

[Justpunchit]

Here's what I have at the moment, the anti squat seems really high however... but I'm using a modified TNT cross member so the lower control arms really can't move much at the frame location... same with the upper due to exhaust routing and such... am I missing something?
It's my first shot at this so be gentle

 

[EndlessMtnFab]

" By the book " it is a little high. It's been generally established that you want to be in the 70s somewhere. And it's a really good idea to have multiple mounting options.


You can make or buy new LCA mounts at the axle. Rotate them up slightly to decreae vertical separation. That will drop your AD numbers a bit. You can also make a new pass side UCA mount (on the axle) with 2 extra sets of holes. At the very least, you'll have 3 possible choices depending on how you want your chassis to react.



Although it's not going to change a whole lot of anything ... i would reduce your tire rolling radius by 1/4 - 1/2 inch. Figure the tire is deflecting under the weight of the vehicle. Unless you plan on keeping it at 65 psi all the time.

Your roll axis angle is a bit high ... you might want to try and spread the LCA mounts a bit at the axle end (you'll need joints at both ends to allow for this becaue of your fixed frame mounts). The closer you get to zero or very low negative numbers ... the more neutral your chassis reactions will be to input.



Defintely have a good starting point ..... just gotta tweak it a bit



Joe

 

[Goatman]

Any idea what all that stuff means?


Personally, I wouldn't bother with the 3 link calculator for the front, no one even knows what good numbers are. I've built a couple 3 links, and mine have been copied a couple times, and they have worked well. My current buggy has a 3 link (front and rear). For the front, make the lowerrs as flat as you have room for, make the upper flat, and mount the track bar as high as you can allowing for bump clearance. That will give you decent anti-dive and a fairly high roll center, as much as you can get building on the front of an XJ.


Oh, and roll axis angle means nothing on a front axle, since roll axis angle determines roll steer, and on a front axle you steer with the steering wheel so it doesn't matter. Use the calculator for a rear 3 link, but forget it for a front 3 link.

 

[MrSimon]

Hey, Justpunchit, do you have build thread for this new project of yours. I'm catching bits and pieces of what you are doing, but what exactly are you putting together? Your old Jeep was pretty darn cool, I'm excited to see what's in store next.

 

[EndlessMtnFab]

Any idea what all that stuff means?

Not a clue ... had to look for my books and saved Web articles. :moon:

Personally, I wouldn't bother with the 3 link calculator for the front, no one even knows what good numbers are. I've built a couple 3 links, and mine have been copied a couple times, and they have worked well. My current buggy has a 3 link (front and rear). For the front, make the lowerrs as flat as you have room for, make the upper flat, and mount the track bar as high as you can allowing for bump clearance. That will give you decent anti-dive and a fairly high roll center, as much as you can get building on the front of an XJ.

Can't really argue with the logic. Just another set of good opinions

Oh, and roll axis angle means nothing on a front axle, since roll axis angle determines roll steer, and on a front axle you steer with the steering wheel so it doesn't matter. Use the calculator for a rear 3 link, but forget it for a front 3 link.

That's where I would like to disagree. If your vehicle is top heavy (say a tire on the roof) ... you're going to have road induced sway which will indirectly feed back to your steering. If you're attempting to control how quickly the chassis reacts to input, then it's my thought that roll axis matters. Don't get me wrong .... much of this can be controlled via other means. One example would be a sway bar. Shocks would also be another example. I see it as another tuning "device"


In the end ... there really is no right or wrong way. It's whatever makes you feel comfortable driving the Jeep !



Joe

 

[Justpunchit]

Thanks Joe, thankfully I'm doing this with a complete stripped D60 so I have a clean slate And I'll try moving my mounts a little further outward to try and help this also along with my lower tire radius and making my upper slightly higher.

And Goat I see what your saying I've been really geeking out with this stuff and thanks for the tips I guess I'll use this as a rough outline instead of following to it perfectly. I'll mess with some things and see what I can get now

 

[kastein]

Probably stupid question (I know nothing about suspension really) - why does no one put the LCAs above the axle, and the UCAs even further up? Seems like it would result in a much better control arm angle. Problems I see with this likely-retarded idea:
* kinda like using blocks on a leaf sprung suspension
* puts a LOT more force on the upper control arms due to leverage, like with blocks
* leaves your pinion and driveshaft hanging out in the breeze being rock magnets

As for what it would fix? My thoughts are that it would fix the control arm angle.

(this is why I generally keep my mouth shut in advanced fab)

 

[Justpunchit]

Hey, Justpunchit, do you have build thread for this new project of yours. I'm catching bits and pieces of what you are doing, but what exactly are you putting together? Your old Jeep was pretty darn cool, I'm excited to see what's in store next.

Not yet! I really didn't think many people would be interested in one.. but some of my plans are going with full hydro steer, D60/10.25 combo with np231/D300 doubler on 42" pitbull rockers and H1 wheels and I'm also gonna stretch the front 5" and the rear 5". And Thanks! it always worked well but change is good

 

[ktm racer 419]

Probably stupid question (I know nothing about suspension really) - why does no one put the LCAs above the axle, and the UCAs even further up? Seems like it would result in a much better control arm angle. Problems I see with this likely-retarded idea:
* kinda like using blocks on a leaf sprung suspension
* puts a LOT more force on the upper control arms due to leverage, like with blocks
* leaves your pinion and driveshaft hanging out in the breeze being rock magnets

As for what it would fix? My thoughts are that it would fix the control arm angle.

(this is why I generally keep my mouth shut in advanced fab)

many people run the lowers flush with the axle tube. the problem with having the links higher up is you can only go so high with the uppers before they start hitting frames andmotor mounts and oil pans. sure you can just raise the lowers, but having vertical separation of the links is a good thing.

 

[Justpunchit]

Here's my second freshly educated shot at it, this revision would mean I'd be putting my frame LCA mounts on the bottom of my belly pan but the numbers work A LOT better....

 

[Goatman]

That's where I would like to disagree. If your vehicle is top heavy (say a tire on the roof) ... you're going to have road induced sway which will indirectly feed back to your steering. If you're attempting to control how quickly the chassis reacts to input, then it's my thought that roll axis matters. Don't get me wrong .... much of this can be controlled via other means. One example would be a sway bar. Shocks would also be another example. I see it as another tuning "device"


In the end ... there really is no right or wrong way. It's whatever makes you feel comfortable driving the Jeep !

Joe

Roll axis angle effects roll steer, which is the axle dropping out far enough to make the wheel turn in and cause a change in steering direction. It has nothing to do with sway. Roll angle axis induced roll steer only happens at the ends of suspension travel when articulating, it can't happen on the road. So, no feed back that would effect steering stability is possible. On the trail, any roll steer in the front is slight and won't even be noticed since you're steering the tires with the steering wheel.

Roll center height effects stability and feel, along with center of gravity, and where you want roll center height is based on what you do with it. A high roll center gets more of the vehicle weight under the roll center and less weight above it, it also makes the axis of sway (rolling side to side) higher for less leverage (in effect). For a trail rig, it's virtually the higher the roll center the better, since it adds quite a bit to stability in turns and off camber situations. Roll center height in a 3 link is simply the height of the panhard bar mounts, which is one reason why 3 links are popular on comp buggies, especially moon buggies. Look at them, they all have very high panhard bar mounts.

A higher roll center also gives more stability at speed when there is a generally high center of gravity, like on our 4x4's. This isn't quite as clear cut as it is at trail speeds, since we're still learning about going fast, but it seems to be having the same effect of better stability. There can be a limit to how high a good roll center is at speed, since actually rolling is less predictable......the car feels OK right up to the point where it actually rolls, where with a low roll center there is a better driver "feel" of it becoming unstable or close to the roll point. Of course, spring and shock design and tuning has a huge effect on this, so there are many variables. Also, at speed, too high a panhard mount puts funny forces on the suspension by loading one side more than the other, but this is another subject applied to buggies with more travel and more panhard height options, and not an issue on most front 3 links and on XJ's.

 

[Goatman]

Here's my second freshly educated shot at it, this revision would mean I'd be putting my frame LCA mounts on the bottom of my belly pan but the numbers work A LOT better....

I would recommend putting the lowers exactly where you want them, and move the upper to get good numbers.........if you know what good numbers are. Anti-squat percentage is different for front or back, in fact in the front it's called anti-dive.......resistance to nose diving under hard braking.

Leave the lowers where you had them and make the upper flat and see what your numbers are. I assume the lowers are mounted even with the axle tubes?

 

[Goatman]

Probably stupid question (I know nothing about suspension really) - why does no one put the LCAs above the axle, and the UCAs even further up? Seems like it would result in a much better control arm angle. Problems I see with this likely-retarded idea:
* kinda like using blocks on a leaf sprung suspension
* puts a LOT more force on the upper control arms due to leverage, like with blocks
* leaves your pinion and driveshaft hanging out in the breeze being rock magnets

As for what it would fix? My thoughts are that it would fix the control arm angle.

(this is why I generally keep my mouth shut in advanced fab)

Easy to do in the rear, but probably not advised in the front. My rear LCA's are mounted totally above the axle tube. In the front you have a strength issue because there has to be enough strength to run into rocks and ditches and not fold the front suspension, better to keep the lower links even with the axle tube.

 

[EndlessMtnFab]

I'm guessing we're having a difference of definition on roll axis.


An inclined roll axis (which I'm referring to as the roll axis angle) also effects the chassis' turn in behavior. The more it is declined towards the rear axle, the slower the turn in response should be since the body is yawing (pitching) out of the turn. I would think this should be called a "negative" roll axis. Kinda like a skateboard, if you will. I interpret this to be a more stable, but slower response. Probably a good thing when you're at least 6-7 inches taller than what AMC designed the Cherokee for.


Therefore (in my mind) I perceive shocks and sways bars as a further fine tuning aid having a slight effect in altering the vehicles response characteristics. Obviously, these parts have limitations as to how much they can band-aid inherent flaws of a poorly designed suspension. Which means they do have an effect and are partially relevant to the discussion.


Stolen from a PDF present by Chrysler (mainly for Independent suspension, but the definitions and concepts should still apply)




As the Roll Axis gets high
•Steering linearity and blending deteriorates
•Vehicle response is quicker

As the Roll Axis gets low
•Blending improves but roll increases
•Vehicle response is slower

As the Roll Axis becomes more positive (high in rear)
•The yaw damping of the vehicle improves

As the Roll Axis becomes reversed (high in front)
•The yaw damping is reduced



Based on this ... my personal thoughts are more than roll axis angle indirectly affects steering & roll steer (2 separate things, obviously). This would also justify (in my twisted mind) why sway is also affected.



Don't get me wrong ... there are so many factors to consider, that it's silly. I don't proclaim to be an expert at this --- this is simply my interpretation of the definitions based on some experience, some reading, and some schooling.


One definition I found online regarding roll steer makes the comment that a poorly designed suspension can move the axles out of their normally parallel position thru spring deflection or link placement. AKA Rear steer or axle walk.


Here's another set of definitions I found via Google:


[FONT=Arial, Helvetica, sans-serif]Roll Axis: [/FONT][FONT=Arial, Helvetica, sans-serif]The line between the front & rear roll centers. In most cases you want this line to be parallel to the ground. In other words, you want the front and rear roll centers to be at or near the same height. If the roll axis is not parallel to the ground, body roll will create a yawing moment as the car tries to twist around the roll axis. Usually this can be felt as the car pitching forward or backwards and twisting as the car rolls. [/FONT]

Roll-Steer: The effect of the rear axle "steering" to one side as a result of the car leaning over (rolling) in a corner. It is caused by the rear control arms pulling the axle backward and forward as they swing through their arc when the axle moves up or down. To minimize roll-steer, the lower rear control arms should be as close to level as possible.




We seem to agree on the concepts of Roll center and it's affects. So I'll just leave it at that. :laugh2:



Cheers ! :wave1:

 

[Goatman]

Well, for one thing, you're talking about the roll axis between the front and rear axles, which isn't calculated in the 3 link calculator, and so in my mind has nothing to do with this discussion.

The roll axis (or roll axis angle) between front and rear that you are referring to is based on what I am calling, and what the 3 link calculator refers to, and what your Google definititon calls the roll center height. The line between the front and rear roll center heights would be that roll axis. Since the calculator only deals with one axle at a time, that roll axis isn't part of the equation. Nor can it be calculated if there is no calculation on the XJ rear leaf suspension.

The roll axis angle that is referred to in the 3 link calculator is based solely on the link geometry of one axle, and is a measurement of the fore and aft axle movement under articulation that determines how much turn in of the tire there is. This can be important in a rear suspension, as too much roll axis angle, and resulting roll steer, can steer a rear tire off of the intended line. This is especailly important when driving a V or wall, where the rear tire can literally drive off the wall and drop into the hole. This is typical on a lifted short arm TJ which happens to have terrible roll steer/roll axis angle. This roll steer is the same as in your Google definition.

In our discussion, and in the calculator, roll axis angle refers to roll steer amount, which can't happen in the front since you're turning the tires and controlling the direction yourself. This is why I said it doesn't matter.

I discussed slightly the effects on handling of a high roll center, which we have the ability to control when setting up a suspension. Roll axis can't be determined on our XJ's (well, I guess if we had the calculation for a leaf spring suspension it could be), but goes along with the general concept of how a higher or lower role center effects road handling. A higher roll center reduces roll tendancy (what we'll call stability) but is less predictable, while a lower roll center height can allow more roll tendancy but is more predictable.........this matches the definition you posted of the effects of a higher or lower roll axis.

Good discussion. Maybe someone is getting something out of this.

 

[EndlessMtnFab]

I see where you are going with this and decided to fart around QUICKLY with my 3 link calculator.

When I adjusted the track bar height, the roll axis angle did change (that's a given). But what I noticed was that it still followed my definition regarding the angle. In this case ... when I raised the track bar height (at the frame) ... the number became even more negative as the axis line pointed downward at a greater rate to the reat axle (at the instant center/axis point).


Now that you mention it ... I have to agree that my definitions are accounting for both axles and the calculators do not. But it still looks general concept still applies here.



I'll have to look into this more later .... right now I must start dinner. :chef:



I just hope this won't cause a stomach ache when I get back to this later. :gag:

 

[EndlessMtnFab]

At this point I'm not really sure what I can add to my last point.



It appears we are getting to the same end point, just in different manners and using differnt theories. While I understand what you are typing .... I can't see how we are just changing the names of terms to suit what we are doing.


Regardless ... the following point has been well established and bears rehashing again. The calculator is just another tool and starting point. It serves the purpose of showing what things change when you alter your geometry. The hard/fast numbers simply don't exist because of the non-static nature of our Jeeps.



eace:



Joe :lecture:

 

[Goatman]

We really have two things here that are being referred to as "roll axis angle". In your explanation, which is correct for what you are talking about, it is the angle between the roll centers for the front and rear axles.

In the calculator, for the purpose of setting up a link suspension and determining the amount of roll steer, the roll axis angle is the angle of the axis around which the axle housing rotates. If roll axis angle is 0* then the axle rotates around a horizontal axis and there is no roll steer......the axle ends do not move forward and rearward as the suspension articulates, and so the tires keep pointing forward. In a rear suspension this is ideal, less roll steer is desirable. In a front suspension it hardly matters since the driver directs the direction of the wheels.

Now, in a very fast road car I'm sure roll steer is taken into account, but mostly in an independant suspension car. In a straight axle car there has to be quite a bit of axle articulation to induce roll steer, which wouldn't normally happen on a smooth surface road. And, again, we're talking about a rear suspension, not a front, and this discussion is about the front.

Wonder how many are still with us, or even care.


And I agree wholeheartedly that the best benefit of the calculator is to get a chance to understand the relationship between the links and mounting points and see how changes effect the geometry. The actual numbers are less meaningful, especially in a front.

 

[5spd_xj]

Wonder how many are still with us, or even care.

:cheers:

 

[vetteboy]

Wonder how many are still with us, or even care.

:wierd:

My front, on paper, has 'roll oversteer' at ride height of a little over 7 degrees. It's fine. I wouldn't change a damn thing. You will never be driving fast enough through curves on a road to roll the body far enough for it to matter, and if you do, you'll likely be cleaning the shit out of your pants before you're worrying about roll axis.

The values from that calculator are only accurate at static ride height with everything as shown on the screen. Flex it, sit in it, air down your tires, fill it up with gas, add a spare tire, climb a hill, and it's all out the window.

Put the links where they fit best and aren't rock magnets, get your upper link flat so your pinion angle and caster change stay reasonable, and go wheeling.