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Old 05-11-2011, 06:35 PM   #1
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Antirock Engineering Lesson

So, No one really responded to my original anti-rock post asking why currie recommends setting the arms level with the ground at mid suspension travel. Here is the link for reference.

http://www.wranglerforum.com/f5/anti...ing-92904.html

Since i am a junior in college working on my mechanical engineering degree i just decided to do some analysis on my own.

PAGE 1:

This shows how the antirock will be acting at full bumpstop and full droop. As an engineering background: The trorque created on the swaybar is caused by the suspension cycling. That torque(T) is opposed by the force of the suspension going down/up(F) multiplied by the perpendicular distance to the pivot point(X) {ie. the swaybar rod}. So as we can see the distance X is pretty important. If X is relatively big there will be a big opposing force. If X is small there will be a small opposing force.

Below you can see my representations and see that if you set the antirock level at mid travel the X distance will be the same at full flex and bumpstop. This will give the swaybar the same rate at both locations making it pretty stable.




PAGE 2:

Now if we look at a jeep with antirock set level at ride height. Also making the assumption that it has 3" of up travel and 7" of down travel. You will see that at full flex the X distance would be significantly smaller causing there to be less resisting force at full droop than at bumpstop. This causes an unstable setup becasue your swaybar rate will be different from side to side. If you are completely flexed out(full droop and bumpstop) your swaybar would theoretically be seeing different rates. See figure below





So this is my undergraduate engineering evaluation of the antirock swaybar. If anyone has something to add feel free! Just trying to answer my own questions since no one else responded.

Before anyone brings it up.... This is under the assumption of equilibrium:

ie. full flex with not much body roll that would cause a shift in weight that would affect some of the forces (F) shown above. That would make the problem very hard to solve without using some load cells and other instruments. My calculations just show the theory behind it.

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Old 05-11-2011, 07:00 PM   #2
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After about my third time of reading your analysis for full comprehension, I got it and it was eye opening to say the least. I'm going to be resetting my Antirock angle to be level at mid-travel thanks to your research. Mine is currently just above horizontal at rest but I'll dial it in more accurately asap.

Awesome work, it was actually a very cool analysis. You may even want to "pretty" it up and forward a copy over to John Currie who came up with the original Antirock design. You never know, you might just have a job at Currie awaiting you upon graduation if you're interested.

Thanks, what you did was very cool!

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Old 05-11-2011, 07:02 PM   #3
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Pretty wasn't my first thought. Most of this was thought of while at work. I have a Co-op as a mechanical engineer are TEREX materials processing right now.

P.S.
Thanks Jerry. I have actually read a ton of your posts and they have influenced me quite a bit on what i have chosen to do with my jeep. nice to know i could return the favor!!
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Old 05-11-2011, 07:29 PM   #4
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Quote:
Originally Posted by nelson08 View Post
This causes an unstable setup becasue your swaybar rate will be different from side to side. If you are completely flexed out(full droop and bumpstop) your swaybar would theoretically be seeing different rates.
In order to get one side compressed and one side drooped, you would have to disconnect the sway bar... The whole point of the bar is to equalize torsional load on the left and right suspension components.

It is set parallel at idle ride height because that's where your ride is for the majority of your street driving (where the sway bar is actively reducing body roll), you probably want to disconnect it if you plan on crawling over something that will compress one side of your suspension to the stops while the other side is swinging in the breeze.
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Old 05-11-2011, 07:33 PM   #5
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I recommend doing some research on the antirock swaybar. You are correct that it tries to equilize the loadings. This is to reduce body roll on the road. But the antirock has a much smaller torsion rate which allows the two sides to flex more than stock but it still provides some stability.
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Old 05-11-2011, 08:05 PM   #6
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In order to get one side compressed and one side drooped, you would have to disconnect the sway bar...
Not true, not even close. One side can easily compress while the other side is drooped with any of the following... Antiswaybar, factory antiswaybar, or simply disconnected.

Here's just one example of my previous (stolen) rig with an Antirock showing one side compressed (fully stuffed) and the other side fully drooped while on Sledgehammer in Johnson Valley.
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Old 05-11-2011, 08:15 PM   #7
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Post some pictures of your new rig jerry, im interested how it's looking so far.
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Old 05-11-2011, 08:31 PM   #8
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Here ya go, I didn't want to hijack this thread any further... Pics of Jerry's replacement rig...
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Old 05-11-2011, 10:38 PM   #9
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Originally Posted by C.L. View Post
It is set parallel at idle ride height because that's where your ride is for the majority of your street driving (where the sway bar is actively reducing body roll), you probably want to disconnect it if you plan on crawling over something that will compress one side of your suspension to the stops while the other side is swinging in the breeze.
I see you edited your original post. I still suggest you do some research on the "ANTIROCK" swaybar from Currie. It is not intended to be disconnected offroad. It is also a swaybar for offroad use. And thus is not set up for onroad use.

EDIT: Disconnecting the swaybar leaves a jeep uneasy and tippy in off camber situations. This is why they came up with the antirock. It has a low enough torsional rate to alow for flex but still counteracts a little body roll in off camber situations.


On a side note. If this were for on road use. Yes, you would want to set it to parallel to the ground at ride height. thus, giving equal resistance up and down for the small portion of that travel that is used on road.
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Old 05-12-2011, 08:57 AM   #10
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Why does the X distance have to remain horizontal? I would think that the force acting on the sway bar axis would remain a constant determined only by where you attach the lower links to the sway bar arm.
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Old 05-12-2011, 09:21 AM   #11
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Nelson, what school are you attending. Looks like some nice work with the math.
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Old 05-12-2011, 12:41 PM   #12
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I currently attend the University of Iowa. Thanks!

And as far as the X distance being horizontal. It has to do with the swaybar links. If you look at the links on the anitrock they have heim joints. This allows the joint to swivel. When the antirock is cycling the heims swivel keeping the swaybar link relatively vertical all the time. Thus all the force will remain in the vertical direction. For the engineers reading this the law of a two force member comes into play. As long as the link stays relatively vertical the forces will act in the vertical direction.


For everyone else here is an example of a two force member



The most common example of the a two force member is a structural brace where each end is pinned to other members as shown at the left. In the diagram, notice that member BD is pinned at only two locations and thus only two forces will be acting on the member (not considering components, just the total force at the pinned joint).

*this was taken from (first site i found with a good picture and explination)

Statics eBook: Two- and Three-Force Members


This shows that an antirock swaybar which has a link that is pinned on both ends(two heim joints) and only has two forces(downward force derived from the torsion in the sway bar and upward force from the suspension). Can be examined as a two force member. And I chose to neglect any movement away from verical in the swaybar links. I dont believe it moves enough to create much of a difference. Correct me if i am wrong though.


Thanks everyone for reading
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Old 05-12-2011, 12:44 PM   #13
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I currently attend the University of Iowa. Thanks!
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Old 05-12-2011, 06:27 PM   #14
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from a fellow engineer-in-training: looks good! statics class is coming back to me now.
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Old 05-12-2011, 06:53 PM   #15
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LOL! thanks. And its been a over a year since i took it too but for some reason using it on things i actually care about makes it all come back
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Old 05-12-2011, 08:11 PM   #16
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And as far as the X distance being horizontal. It has to do with the swaybar links. If you look at the links on the anitrock they have heim joints. This allows the joint to swivel. When the antirock is cycling the heims swivel keeping the swaybar link relatively vertical all the time. Thus all the force will remain in the vertical direction.
Makes sense. thanks.
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Old 05-12-2011, 08:18 PM   #17
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Great thread!
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Old 05-13-2011, 09:20 AM   #18
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Good work here...

As a quick comment, you might want to add to you assumptions list that the sway bar travel is linear. The anti-rock system moves radial about a center point, though I would imagine that this change would have little effect on your findings, it is still good to show that you thought of it.
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Old 05-13-2011, 10:49 AM   #19
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I didn't make that assumption. The fact that it moves radialy is why the X distance varies, thus why it is recommended you set it level at mid travel. The assumption i made is that the swaybar links stay relativly vertical. And i stated that assumption in one of my posts.
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Old 11-27-2011, 09:35 PM   #20
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Going to revive this one from the dead! I'll be setting my antirock next week. I've read over your calculations and it makes sense. Have you reported this to currie by any chance? So assuming the 3 up/7 down travel numbers, you'd want your ride height (non flexed) bars to be set aiming 2" downwards, correct?
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Old 11-27-2011, 10:15 PM   #21
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the reason no one could answer you in the other thread is you're smarter than everyone. it's easy for me to understand the way you explained it but there's no way i could've come up with that on my own.
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Old 11-27-2011, 11:01 PM   #22
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Those I have talked with at Currie like John Currie actually recommended the arm be just above horizontal.
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Old 11-27-2011, 11:40 PM   #23
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It's a good thing your in Iowa lol if I go buy an anti rock I'll come down there buy ya sum beers and you can help set it up haha
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Old 11-28-2011, 10:47 AM   #24
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Quote:
Originally Posted by Jerry Bransford
Those I have talked with at Currie like John Currie actually recommended the arm be just above horizontal.
Even with unbalanced travel?
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Old 11-28-2011, 02:10 PM   #25
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I actually thought about this a while back while going through some old statics notes...NICE! Well-done analysis, as well. I came to the same conclusion that at mid-travel you want the arms to be level. If you're running 12" travel shocks and are setup for 6/6 then at ride height, you'll be level. If you're setup for 5up/7 down, then you'll be just slightly above level at ride height. Currie makes a general assumption that most vehicles will be setup with a slight down travel bias and that translates to their recommendation.
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Old 02-26-2012, 03:56 PM   #26
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Quote:
Originally Posted by Jerry Bransford View Post
Those I have talked with at Currie like John Currie actually recommended the arm be just above horizontal.
Quote:
Originally Posted by MikalCarbine View Post
Even with unbalanced travel?
Quote:
Originally Posted by Imped View Post
I actually thought about this a while back while going through some old statics notes...NICE! Well-done analysis, as well. I came to the same conclusion that at mid-travel you want the arms to be level. If you're running 12" travel shocks and are setup for 6/6 then at ride height, you'll be level. If you're setup for 5up/7 down, then you'll be just slightly above level at ride height. Currie makes a general assumption that most vehicles will be setup with a slight down travel bias and that translates to their recommendation.
The key is to make sure that the arms can't swing backwards (Actually forwards in this case, but backwards from how they're designed to work) after one side experiences full droop. I've actually had mine buckle backwards on one side and lock my axle solid where it wouldn't flex upwards at all. (Picture your knee bending the wrong direction) I had to jack up the frame and pop the joint back to the right direction...
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Old 02-26-2012, 05:37 PM   #27
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I didn't make that assumption. The fact that it moves radialy is why the X distance varies, thus why it is recommended you set it level at mid travel. The assumption i made is that the swaybar links stay relativly vertical. And i stated that assumption in one of my posts.
The problem with that assumption is that for 98% of the TJ's out there, it's incorrect due to the forward and back swing of the front axle caused by the control arms. That effect is so pronounced that the anti-swaybar links are only vertical for a very short period of travel right above and below ride height. Couple that with the ends of the arms moving closer to centerline as they rise above and below the torsion bar and full droop has them at a severe angle.
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Old 10-22-2012, 10:21 PM   #28
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You wouldn't happen to have an illustration showing the travel of the front and rear suspension by chance?
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Old 10-23-2012, 09:29 AM   #29
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You wouldn't happen to have an illustration showing the travel of the front and rear suspension by chance?
I don't know of one but you could easily do this. Go out to the garage, set the frame and axles up on jack stands and pull of the tires and pull the springs. With the frame supported, cycle the axles from full bump to full droop. At full bump, the axles will be as far out as they're going to get and at full droop they'll be as far in as they'll get. The front axle will be shifted to the passenger side at full bump, shifted to the driver side at full droop and hopefully centered at ride height. The rear axle will be just the opposite. This isn't hard to visualize if you just go take a look at the control arms and track bars, the members that define how the axle moves.

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