Death wobble

[mrblaine]

More precise language would say it was indirect causation. In reality I assumed it was directly caused by the MML. I also assumed Tim was being straightforward, probably a bad assumption. What I was missing was the rigidity issue. I was thinking this morning on the drive to work, how great your explanation was, and how helpful it is to me to have it at hand so I can help others who might run into this issue.

Still hammering out precession...According to the principle of the change of orientation of the rotational axis under torque, I think you are saying that for our front end components to hold the rotational axis stable is quite impressive, since naturally that axis would change and the wheel would go into a wild spin, possibly nutation. Am I on the right track???

Watch this and notice what happens when you try to change direction or add steering input. Notice that one side goes up and the other goes down. Now imagine what would happen if one side was out of balance. It isn't hard to see why we get DW and why any change in stiffness be it more or less can change how the structure can no longer control the natural tendency to precess.

 

[JMT]

You are. The tire is actually very similar to a gyroscope and wants to behave like one. Put two of those on the same axle and get them fighting each other and bad things happen.

So you'd still have two gyroscopes on an independent suspension, but they wouldn't be fighting one another like they do on our solid axle TJ's, right? Which is what makes for a generally rougher ride overall in a solid axle vehicle? My wife was just commenting on this yesterday and is a main reason I like to air down off-road. I still need to think about this more, and it would help if I had more background in physics and could do the math, but my questions are wandering in to how our steering and suspension components deal with this constant change of orientation of the rotational axis. What's happening to my upper and lower control arms, et al, which is far beyond my pay grade, but good to think about. I haven't been doing this long at all, but I have come to appreciate the Jeep engineers organization of all these components and the realization that one change has a whole series of effects, which makes building our TJ's almost an art, and if done well, good art, and if you Blaine, great art! So far I'm very happy with my art and looking forward to refining it into a, what was that acronym PSC build PEC (Performance something or other!)

 

[JMT]

Watch this and notice what happens when you try to change direction or add steering input. Notice that one side goes up and the other goes down. Now imagine what would happen if one side was out of balance. It isn't hard to see why we get DW and why any change in stiffness be it more or less can change how the structure can no longer control the natural tendency to precess.

Ha, elegant without spin, elegant with opposite spin, and Debussy elegant, of course, touchy when spinning in the same direction!

 

[bobthetj03]

So, what is the OP to do? Remove the MML? Strengthen other components to slow the process? I experienced seat staining DW way back when I installed my lift. ZJ steering, a JKS TB and Currie lower control arms cured it for me, so maybe we need to find his weak link and strengthen it to accommodate for the MML change. Change one thing, be prepared to change 3 or 4 more things to fix the one thing you changed. Welcome to Jeep life.

 

[tomtaylz]

So, what is the OP to do? Remove the MML? Strengthen other components to slow the process? I experienced seat staining DW way back when I installed my lift. ZJ steering, a JKS TB and Currie lower control arms cured it for me, so maybe we need to find his weak link and strengthen it to accommodate for the MML change. Change one thing, be prepared to change 3 or 4 more things to fix the one thing you changed. Welcome to Jeep life.

I just put on the JKS trackbar, going to try installing curries steering system too and I have new tires which were already ordered to rule that out. Hoping that does it.

I do need to use the Jeep next week though so hoping that the above fixes it. If not I guess I’ll get the control arms.

 

[Jerry Bransford]

One thing I would do is focus a little more on would be as close of a perfect tire balance as is possible. And the bigger the tires, the more important good tire balance becomes where DW is concerned.

To me, imperfectly balanced tires are a very common trigger for DW. Something else wrong elsewhere can let that trigger develop into DW. It isn't easy to get our TJ's tires balanced perfectly, it has taken three trips in the same day to my local Discount Tire store before they finally got them balanced perfectly enough for my needs.

Make sure your tie rod end nuts and track bar mounts are properly torqued. I hadn't checked mine in a while and a several months ago, I was getting a hint of DW. It proved to be a combination of my tie rod ends and track bar having loosened up since the last time I checked them, together with a front tire that was slightly out of balance. The steering system's tapered joints can seat together more and loosen up after their initial and even a subsequent tightening. I just tightened them again a tad yesterday while doing a little preventive front-end rust removal and paint touch-up. I had more rust than I could believe... all caused by scraping over rocks or running into stuff. Or as Blaine perfectly described it to me one day, driving by Braille.