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Old 07-17-2013, 09:14 PM   #1
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How does articulation occur if track bar can't move?

When one corner of the axle drops low, it should theoretically pull the entire frame down with it because the track bar, with properly torqued bolts, cannot move or rotate inside its mounting brackets. Where the axle goes, side to side or up and down, is where the frame goes.

This clearly is not the case. We are all able to drop one corner of our axle at a time when installing coil spacers or rock crawling, etc.

This is really confusing me. Can someone completely explain it?

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Old 07-17-2013, 10:03 PM   #2
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Subscribed for a great question about understanding how stuff works.
I'd have to go look at the jeep to think about how it does work and it's raining right now.

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Old 07-17-2013, 10:12 PM   #3
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Quote:
Originally Posted by legitposter View Post
When one corner of the axle drops low, it should theoretically pull the entire frame down with it because the track bar, with properly torqued bolts, cannot move or rotate inside its mounting brackets. Where the axle goes, side to side or up and down, is where the frame goes.

This clearly is not the case. We are all able to drop one corner of our axle at a time when installing coil spacers or rock crawling, etc.

This is really confusing me. Can someone completely explain it?
It still only has one pivot point, if you notice it is much harder to get one side on than another due to where the attachment points on the axle actually are, also suspended in the air the axle will naturally droop a little more on the driver side in the front. Also, the steel sleeve and rubber bushing on the track bar slips in its mounts. It is a joint. If it were fixed and welded then the axle would not move at all and the frame would follow the axle until it all broke under the stress or your spine fell out.
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Old 07-17-2013, 10:17 PM   #4
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You misunderstand how your track bar works. Your track bar does in fact rotate where it mounts. There is a significant difference in the amount of force you can apply by hand and the amount of force your rig exerts on its own bits and pieces through the magic of gravity and Newtonian physics.

If you were to climb over an obstacle perfectly level on both sides of the Jeep that caused the suspension to compress or extend significantly, it is true that Axle and Body/Frame move laterally relative one another. This is, in fact, what happens when you install a lift on your Jeep and must then adjust the length of the track bar to re-center it underneath the Jeep.

Collectively, it is up to your control arms and chassis to account for the lateral displacement of the front and rear axle relative one another due to the restrictions imposed on the range of motion by the track bar. When torqued properly, track bars and control arms are secured at the bushing/joint and the bar/arm is free to rotate and articulate.


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Originally Posted by legitposter View Post
When one corner of the axle drops low, it should theoretically pull the entire frame down with it because the track bar, with properly torqued bolts, cannot move or rotate inside its mounting brackets. Where the axle goes, side to side or up and down, is where the frame goes.

This clearly is not the case. We are all able to drop one corner of our axle at a time when installing coil spacers or rock crawling, etc.

This is really confusing me. Can someone completely explain it?
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Old 07-17-2013, 10:50 PM   #5
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Sorry if I shorted the "complete" part - imagine my hypothetical example below.

Lets assume my track bar is 36 inches long - it then transcribes a circle with a diameter of 72 inches and a circumference of about 226 inches if it were to rotate fully. Assume the axle is about 60 inches accross and the frame and axle track bar mounts are ten inches apart vertically and approximately 34 inches apart horizontally (56% of the distance from the center to where the tire mounts). Imagine the frame as a fixed reference point (no twisting, as if it were on a lift) and consider that your stock suspension offers perhaps 9-10 inches of travel on one shock and is approximately center at normal ride height. One side compresses completely, one side extends completely - one side is five inches higher than normal, one five inches lower. How much has your axle rotated? Around 19 degrees or so. How much has your track bar mount displaced, vertically if we neglect horizontal travel? About 2.8 inches. How much angular displacement is that for your track bar? About 4.5 degrees.

So, if our track bar is naturally sitting about 16 degree below the horizontal and rotates another 4.5 degrees, what is the difference? Well, the horizontal difference between the mounts in now about 33.5 inches. And, as discussed, the vertical difference between mounts is about 12.8 inches (the original 10, plus 2.8 due to the axle's rotation).

We can consider the track bar and axle in different positions...perhaps the axles is level, but both shocks are extended downwards or upwards uniformly the full five inches and thus the track bar mounts would move the full five inches. It's not hard to see that's around twice the rotation at around 9 degrees from its static position. The point I'm making here is that track bar simply doesn't need to rotate very much in order to do its job. You can clearly see that suspension geometry can play a huge role and you can likely imagine designs that require significantly more or less rotation of the track bar and lateral movement of the axle.

Does that seem clear? That's not to say that the track bar doesnt restrict motion - it does, that's its entire job, in fact and it does have some impacts on articulation (why else would folks be so keen to eliminate them when running triangulated 3 and 4 link setups?). However, any who has installed a lift and/or watch the suspension travel as the shocks extend and compress can tell you with relative certainty that the track bar does in fact rotate.
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Old 07-18-2013, 10:04 AM   #6
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Very interesting. Thanks for that explanation.

But, if we read through all the death wobble threads, it suggests the track bar needs to be torqued to where it can't move horizontally at all. But if its torqued to where it can't move at all horizontally then it wouldn't be able to rotate either.

So it's all just confusing.
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Old 07-18-2013, 11:36 AM   #7
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No horizontal movement doesn't mean no rotation. You figure the bolt is stationary and the the TB sleeve rotates around the bolt. There is some side to side motion but it is so minimal it doesn't really matter. However that is also why a lot of guys upgrade to the 9/16" TB bolts so that even that play is minimized.

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