Design, geometry and functionality of tracbar-less suspension

Gojeep said:
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?
Click to expand...

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



standard.jpg
 
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......
 
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.
 
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
 
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???
 
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.
 
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
 
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
 
Beezil said:
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???
Click to expand...


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).
 
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....
 
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.
 
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.
 
Kaczman said:
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.

site1044.JPG

<snip>
-Jon
Click to expand...


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)?
 
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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
 
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