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Unread 10-29-2012, 02:42 AM   #76
9703tj
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On an axle using a track bar the roll axis is going to be the height of the center of the track bar. Thus raising the track bar upwards will increase the roll stability of the vehicle.

On an axle using a triangulated 4 link set up and no track bar, the roll axis is going to be the center between the two upper links where they attach to the axle.

As far as a double triangulated system is concerned, the roll axis should be the same as a single triangulated 4 link set up.

A wishbone upper link will have the roll axis at the point that the wishbone link attaches to the top of the axle.

What I would like to learn is how does a sway bar, or lack there of effect the roll axis? If at all?

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Unread 10-29-2012, 03:48 AM   #77
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From my research the sway bar does not actually effect the roll axis since it is not part of the definition of a roll axis but it will effect the same things the roll axis does.
So from your definition the roll axis at the rear is the height of the axle mounting points of the uppers (so just over the pumpkin in most cases), and in the centre of the vehicle (in most cases)?
Im trying to set mine up for higher speeds and for use in forests and deserts mostly. So I want it to soak up bumps and washouts well without getting off balance around corners and hills or bouncy, but still be able to go slow in precarious situations. So my aim is about 110% anti squat (to keep the front end down on hill climbs but not enough to hop), 110% anti dive (same except descents), a low roll axis tilted forward (low roll axis absorbs bumps, no idea why tilting forward is better for street) and mid arm length. Will also run good swaybars not the stock ones to help keep it upright with the low roll axis. Anti squat and dive is easy for me to work out. But with the roll axis to achieve my aim I should mount the front track bar low and the rear upper axle end mounts should be a few inches above the centre height of the trackbar? Does all this seem like a good plan or am I off the mark for the numbers?
Thankyou.
Thanks.
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Unread 10-29-2012, 06:31 AM   #78
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The roll center height is defined by the track bar frame mount. If there's not a track bar present, then look at the next links that are laterally locating the axle. If the uppers are doing all of the work in that regard, then that's where the roll center is located on that end. Obviously the more work the lowers are doing (more triangulation), the lower the roll center will be.

The roll axis is just a line that goes through the separate front and rear roll centers and is the axis about which the vehicle rolls. The flatter you have the axis (the closer to 0 in the calc), the less unwanted steering input you'll get through the suspension cycle, regardless of arm length. Play with the separation, mainly the lower link separation at the frame and you'll see the roll axis change. If you look at the screen shots I posted early on in this thread, I'm sitting at 0 roll axis so I'll get minimal steering input when cycling.

FYI, just because the roll axis isn't low and tilting forward doesn't mean that your rig won't drive well at high speeds through rough stuff. Personally, I'd rather have a higher roll center and flatter roll axis than anything else. Stability and predictability are what I value the most. In my suspension's previous setup, it felt great through the high speed whoops and I was consistently the fastest one of the group through them. It should be considerably better this time around and the roll center and roll axis haven't changed.

When it comes to sway bars, don't let them even enter the equation. All they're doing is working with what they're given.

And as for your anti-squat, you don't want to shoot for >100% if you can help it, which you can. I know what you're thinking--if the rear end doesn't squat, the front end won't rise. Wrong. Focus on one end at a time. The last thing you want is high AS in the rear suspension when climbing steep ledges/walls. Hopping due to the high AS will cause a lot more trouble than you think. Shoot for 60-80% and when you do the rear, focus on keeping the anti-dive in a similar range. If you feel like it unloads on you too much, go with a center limit strap. I bet it ends up being a non-issue for you, though.
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Unread 10-29-2012, 03:05 PM   #79
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Yes that was exactly what I was thinking. I don't think hopping would be a huge issue though since we have such little traction. Now when I really think about it realize the front and rear are independent of each other and just because the rear raises it doesn't force the front to dive.
For the front the track bar should be parallel and equal length to the tie rod so would I be best making that correct and then making the 4 link have a flat roll axis with the midpoint of this?
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Unread 10-29-2012, 03:29 PM   #80
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Quote:
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Yes that was exactly what I was thinking. I don't think hopping would be a huge issue though since we have such little traction.
Little traction can cause more issues due to the vicious cycle of traction/no traction. That's exactly what causes hopping.
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Now when I really think about it realize the front and rear are independent of each other and just because the rear raises it doesn't force the front to dive.
Correct.
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Originally Posted by Motas View Post
For the front the track bar should be parallel and equal length to the tie rod so would I be best making that correct and then making the 4 link have a flat roll axis with the midpoint of this?
Thanks.
Tie rod? No. Drag link? Yes. The mounting points need to be parallel and as close to equal-length as you can manage. The roll axis in the front is defined by the height of the track bar frame mount. If you've got a flat roll axis, your front track bar frame mount and rear upper link mounts are at about the same height from the ground. Still, it's not something you really need to focus on. Work on the rear first and then worry about the front. If you do it right and focus on the right stuff, it'll all work out well when viewed from an aggregate point of view.
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Unread 10-29-2012, 03:44 PM   #81
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Ive been searching to no avail, so could someone one send me a link to the calc? I found one but it was in millimeters and I see Imped's is in inches.
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Unread 10-29-2012, 03:56 PM   #82
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Sorry I get confused between tie rod and drag link but I know what I mean. If I am trying to get them parallel and equal length wouldn't the mount be the same height as the steering box? So the roll height is decided for me.
So what should I be concentrating on? Obviously what fits where is a given, and strength is another. But what should I be concentrating on? I was planning on drawing it up in 3d in autocad to be made by a fabricator so can get the numbers pretty spot on and will draw in anything it could hit.
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Unread 10-29-2012, 05:25 PM   #83
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Quote:
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So what should I be concentrating on?
Build your rear suspension.
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Unread 10-29-2012, 05:26 PM   #84
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Imped

I know you used Poly brackets for the frame and I was wondering if what artec makes would work as well as those brackets have.



Only difference I see is having the weld the artec bracket together
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Unread 10-29-2012, 05:48 PM   #85
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Yep, I've dealt with those on another rig. Geometrically, they are very close to the Poly brackets. Very nice pieces that give you plenty of weld surface.
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Unread 10-29-2012, 06:21 PM   #86
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Those are some nice pieces of hardware. Wish they had another setup like that for non-framed vehicles...
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Unread 10-30-2012, 06:58 AM   #87
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Here's an old thread that just popped up in TJ Tech. It could be of some help.

http://www.jeepforum.com/forum/f9/wh...nking-1290791/
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Unread 10-30-2012, 05:57 PM   #88
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Quote:
Originally Posted by Treton View Post
Ive been searching to no avail, so could someone one send me a link to the calc? I found one but it was in millimeters and I see Imped's is in inches.
First post on this thread.

Quote:
Originally Posted by Motas View Post
I was planning on drawing it up in 3d in autocad to be made by a fabricator so can get the numbers pretty spot on and will draw in anything it could hit.
Been there, done that. End result: try rough fabrication, tacked-in-place mounts, and lots of cycling. That will take you much less time than a full CAD drawing and will eliminate any possible errors ahead of time.
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Unread 10-30-2012, 06:42 PM   #89
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Unread 10-30-2012, 07:49 PM   #90
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Is there something we should know about, Treton?

Still have'nt got under my rig to get some measurements. Life has a way of slowing down Jeep progression.
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