Death wobble

[tomtaylz]

Last week I replaced the shocks and springs on my new Jeep and she drove fine, probably drove it two hours and no issues.

So today’s work included installing the body lift, and then MML and then new front brakes. While driving to break in the front breaks I got the death wobble (didn’t touch suspension today). Was able to reproduce it with my friend driving. Unsure what was going on and because I had the part we replaced the front trackbar with a JKS adjustable. Still was fairly easily able to reproduce death wobble. It occurred all times going over 40, once while decelerating.

The only thing I can think of is that I haven’t had an alignment yet but reading online that seemed more like something that could amplify the wobble over causing it.

Any other ideas? We were both a bit confused how the MML and BL could cause it to show up with it being so easy to reproduce.

(Also death wobble is terrifying)

 

[JMT]

Trackbar is out, assuming a good install. Trackbar is the culprit over 90% of the time. For those who are skeptical, I don’t mean it triggers DW. I mean it is the culprit, in the sense that it does not prohibit the rapid oscillation side to side once it is triggered. It is very often triggered by imperfect tire balance. So get your tires balanced, not just balanced, perfectly balanced. Make them do it right, or be willing to pay place after place until you find a tire shop that knows what they are doing. Tell them, I’ll be back in 10 minutes if they’re not right.

Do an alignment. 1/16” toe-in. There is really no reason with your ability to have a shop do it. In the How-To section on the forum there is a brief article describing how to align our TJ’s. It’s very simple because you are only adjusting the toe, not the caster or camber, which are set from the factory. Since this is so easy, especially for someone describing the work you’ve done on your TJ, it’s not worth spending $70-90 for that.

I’ve also wondered why DW begins after these same mods. I followed the same sequence you did last year. SL first, then a bit later a MML, and the day I put that MML on, DW! Oh Sh—-. Well, it wasn’t the MML and it wasn’t the SL. But somehow when you make these changes, suspension issues begin to show up. I imagine it has something to do with worn parts and changing the Jeep engineers technical specs. It’s just a speculation though, so take it for what its worth. I bought an MC front trackbar and it cured it for a little while. Tires still weren’t getting balanced correctly. I’d get a shimmy. There are only two tire shops in my town, and both suck turds. I apologize for the language, but it’s true. Then DW occurred again. Checked everything again; ball joints, hubs, pitman, upper and lower control arm bushings, tie rod ends. Tie rod ends seemed a bit loose. Since I already had the ZJ tie rod conversion on my mod list, I decided to do it early. Who can live with DW? It becomes all consuming. I’d always get it around 45mph. Talk about frustration. The ZJ Tie Rod conversion worked. That was last summer and I haven’t had DW since. I finally added my BL and stepped up from 31” to 33” from a tire shop NOT IN MY TOWN. Geez, did I say those two shops suck? It’s worth saying it again. Perfect balance, no shimmy, great tire shop and free lifetime balance. So, I hope this helps some and wish you well in your quest to solve the dreaded DW.

 

[tomtaylz]

Thanks that’s for sure a good starting point. And tell us how you really feel about your tire stores .

 

[tomtaylz]

Found some good articles and going to go through their checklists. Seems more common that I would have thought that the DW really occurs after a MML. I’ll beef up some parts in the process, it’s a problem I don’t want lingering around!

 

[bobthetj03]

Found some good articles and going to go through their checklists. Seems more common that I would have thought that the DW really occurs after a MML. I’ll beef up some parts in the process, it’s a problem I don’t want lingering around!

I remember reading some threads on JF about that. Blaine talked about bracing something, but can't remember what it was.

 

[mattyice2]

I'll never forget the first time I experienced the "death wobble". I was driving around Washington DC in my old Cherokee and I hit some pot holes. Scared me half to death. I pulled over thinking I had a flat tire, but the tire was fine. I was like what the?? It's a Jeep thing I guess.

 

[JMT]

MML does not cause DW. What’s happening is you are making changes and weak spots in your suspension show up.

 

[bobthetj03]

MML does not cause DW. What’s happening is you are making changes and weak spots in your suspension show up.

You sure about that?

 

[JMT]

You sure about that?

That’s what Tim at Brown Dog said. I got DW the day of or the day after I put on the MML. I think the day of. So I got on the phone with him. I mean, it was obviously the MML that caused my DW, right?! He worked with me on and off on the DW for three weeks or so. I never changed anything on my MML, just trackbar and tie rods solved it.

 

[bobthetj03]

So why did you get DW after the MML, with nothing else being changed?

 

[JMT]

So why did you get DW after the MML, with nothing else being changed?

Of course, that’s what I said to Tim. But he emphatically denied the MML had anything to do with it. So, if I never changed the MML and I did get an adjustable front trackbar to even out the axles, and then did the ZJ Tie Rod Conversion, and at that point the DW went away, what would you say was wrong?

 

[mrblaine]

MML does not cause DW. What’s happening is you are making changes and weak spots in your suspension show up.

Death wobble is an out of control oscillation due to the tire assembly's natural tendency to precess. The control of the rate and frequency of the precession is dictated by the stiffness of the structure and the components mounted to it that change or alter that stiffness. Same reason different tuning forks vibrate at different tones. More stiffness, different tone.

A motor mount lift raises the engine above the frame mount and gives the motor more leverage against the frame. That change can alter the stiffness of the structure. What's right near the motor mount bracket on the side of the frame? That would be the track bar mount. Moving the motor around can affect the frame's ability to keep the components under control. There are many many reported cases of folks doing no other mod except raising the motor and then getting DW. Yes, you are correct is that it is not the direct cause, but it can certainly be contributory to the ability of the structure to control the other components and allow little deficiencies to turn into big ones.

BTW- if you want to really open your eyes to just how great a job the front end components do at the tasks we ask of them, dig around on YouTube and look for videos on precession and then imagine something the size of a tire spinning at 70 mph and just how much force it takes to make it behave. I'm surprised we can drive something with 35" tires over 25 mph.

 

[mrblaine]

Of course, that’s what I said to Tim. But he emphatically denied the MML had anything to do with it. So, if I never changed the MML and I did get an adjustable front trackbar to even out the axles, and then did the ZJ Tie Rod Conversion, and at that point the DW went away, what would you say was wrong?

Rigidity changed.

 

[bobthetj03]

Of course, that’s what I said to Tim. But he emphatically denied the MML had anything to do with it. So, if I never changed the MML and I did get an adjustable front trackbar to even out the axles, and then did the ZJ Tie Rod Conversion, and at that point the DW went away, what would you say was wrong?

I don't know, but there is a correlation between a MML and DW. I wish I could find that thread on JF.

 

[bobthetj03]

Bam! There it is.

 

[JMT]

Death wobble is an out of control oscillation due to the tire assembly's natural tendency to precess. The control of the rate and frequency of the precession is dictated by the stiffness of the structure and the components mounted to it that change or alter that stiffness. Same reason different tuning forks vibrate at different tones. More stiffness, different tone.

A motor mount lift raises the engine above the frame mount and gives the motor more leverage against the frame. That change can alter the stiffness of the structure. What's right near the motor mount bracket on the side of the frame? That would be the track bar mount. Moving the motor around can affect the frame's ability to keep the components under control. There are many many reported cases of folks doing no other mod except raising the motor and then getting DW. Yes, you are correct is that it is not the direct cause, but it can certainly be contributory to the ability of the structure to control the other components and allow little deficiencies to turn into big ones.

BTW- if you want to really open your eyes to just how great a job the front end components do at the tasks we ask of them, dig around on YouTube and look for videos on precession and then imagine something the size of a tire spinning at 70 mph and just how much force it takes to make it behave. I'm surprised we can drive something with 35" tires over 25 mph.

Much better said than I could. Precession. Nice word...I’ll have to look into that

 

[JMT]

I don't know, but there is a correlation between a MML and DW. I wish I could find that thread on JF.

Yes, a correlation, but not a cause. My original post on the subject was, in all honesty, my theory after thinking about it for a long time. If it was directly caused by the MML, then I could have changed something there and corrected it. But the corrections were the trackbar and tie rod ends, and as Blaine mentioned, the relationship of the engine to the frame.

 

[mrblaine]

Yes, a correlation, but not a cause. My original post on the subject was, in all honesty, my theory after thinking about it for a long time. If it was directly caused by the MML, then I could have changed something there and corrected it. But the corrections were the trackbar and tie rod ends, and as Blaine mentioned, the relationship of the engine to the frame.

The problem is you didn't test it by removing the MML. Had you done the MML got DW, removed it and DW went away, is that causation?

 

[JMT]

The problem is you didn't test it by removing the MML. Had you done the MML got DW, removed it and DW went away, is that causation?

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???

 

[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???

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.