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The mechanics of the death wobble

Nuke Proof

NAXJA Forum User
All right geometry gurus,
What exactly is the physics behind the "death wobble," I know it is an uncontrollable wobble in the steering that often times will not go away until you stop or at least slow down. But what are the mechanics behind it? My XJ doesn't have it, at least not that I know of, but my 2005 F250 does. Sometimes if the planets are aligned and there is a series of consecutive bumps in the road at 52.7 mph with a light head wind it will get a wobble that doesn''t go away until I slow down.

Is it the worn parts, bad balanced and bald tires, poor alignment, too fat of a passenger, the Swedish bikini team having a nude volleyball tournament in the bed, come on guys I am running out of options here................................:wstupid:

Thanks guys
Darin
 
Why not try looking at all the steering components to see if there is excessive play in them? How about replacing the bald tires?

Could be anything.Worn out tie rods,ball joints,worn unbalanced tires,worn unit bearings,over torqued lug nuts causing the rotors to be warped. i mean really it could be anything.Until you start looking you are not going to know.
 
Well the old man's '01 chevy 1500 seems to have the same problem including only does it when the planets are aligned properly and when he's driving it not anyone else. So I talked to the lube tech at the good ole bowtie dealership I work at (since he used to be an alignment tech at a tire chain round here) and he was saying that sometimes the combination of toe, camber, and caster can make one of the tierods adjusted way out and the other way in to keep the toe in spec. According to him this will cause the dreaded death wobble. Although I doubt this is the main cause on our beloved XJ's. When I first started investigating this issue I checked all of the steering components and found only the pitman arm to have just a fuzz of play. So I'm gonna give it a good alignment and replace that pitman arm and see if "he" will drive it and see if it goes away.
 
having read other DW threads, i will say:

check alignment
check the balance of your tires
check the balance of driveshafts
check the condition of any suspension and steering linkage

i would venture to guess that the tires being bald is not the direct cause of the DW, but it could be playing into the equation by means of throwing the tires out of balance. you mentioned that it hits bump in your alignment of the planets case, but in all seriousness, does it depend on hitting bumps or does it always happen? if it always happens, i would be more inclined to say that the cause is bushing or TRE wear... HTH
 
I believe most cars are set up with some camber, which produces lateral forces on the tire. So any sort of missalignment or loose parts in the steering system could cause death wobble I suppose.

If I knew what fixed it, I could probably make a better guess.
 
I have alot of worn front steering components, and I have no DW at all, even at speeds of 85mph. I do however have a pulling to the right, like when my lower CA bushings were completely crushed on the passenger side on my old CAs, but now, with my less miles CA, I'm getting the same simptoms but no crushing, and also, I am getting a wandering feeling in the steering like I was getting a switching wind from right side to left and back, but no wobble. I've checked the track bar, pitman, and all CAs, but not sure what to tear apart first. How hard is it to remove the old bushings on the CAs anyway? I'm thinking on using poly bushings in the stock CAs for now, and that may help, as well help with death wobble others experience. Let me know if I'm wrong with that one, I'd like to know for the future problems I may have someday.
 
Some people have no sense of humor............hehehehehe

In a world of obvious choices I would hope that people are smart enough to figure that if they have worn parts and/or bald tires with this problem that that would be the first place to look. But like some stated earlier and in many other posts the death wobble tends to attack selectively, there are some who have replaced every steering and suspension component on their rig and still have the dreaded DW, but another guy that never maintains his rig and has welded drag links with jacked up steering geometry and a piece of rebar for a tie rod and he can enter the time trials with no ill effects.

My question is not necc. to answer my problem, that was pure sarcasm, however I was asking more or less exactly what I asked, "What is the mechanics of the DW? If in fact their is any actual mechanics to it or it is just a combo of factors."

Thanks
 
I'll take a shot at this LOL.
IMHO it's anything that effects toe. Many components that become worn create "play" that can effect the toe alignment.
Not enough caster causes the toe alignment to ride on a thinner line so to speak and causes it to be come more vulnerable to variables that can effect toe.
As for Bump steer, it is important to have the drag link and track bar parellel. When they're not, a bump forces the energy in two different directions. One way up the track bar and the other through the draglink throwing off the toe alignment. Like two shims sliding in and out in two different directions within each relative angle. On a parellel plane the force gets transmitted on the same angle and applies the same energy to each bar because they are at the same angle.

I don't know what I'm talking about but it sounds good lol.
 
Its a combination of things most times.A bald tire that is out of balance say combined with a worn out tie rod.The tire is going to have a bit of a wobble or shake.Now say you hit the magic bump.This makes the tire become irregular which makes it wobble/shake a bit more.Here is were the worn tie rod comes into play.Instead of holding the tire/knuckle/hub assembly firm and in place it allows it to start oscillating even more to a point it shakes violently.

Now that is not saying what causes it ever time but I'm sure you get the picture.I mean really it could be as simple as a belt in the tire is torn causing the tire to be out of round hence out of balance.A number of things can cause it.I have heard people say that over tightening the lug nuts can cause it .The unevenly torqued lug nuts cause the rotor to warp throwing it out of balance.Caster not being just right and you get what i call the shopping cart effect.I'm sure we have all had that cart at some point were the one caster wheel just wobbles around.What do you think causes that? Usually you look down and find the caster to be bent over or out of shape in some way.Since its not in the proper position it wobbles around,its that simple.Same thing with a vehicle.If something is not set at what it should be it will cause issues.


LR_2000_XJ said:
I'm thinking on using poly bushings in the stock CAs for now, and that may help, as well help with death wobble others experience. Let me know if I'm wrong with that one, I'd like to know for the future problems I may have someday.


Bad idea.Poly bushing are the worst thing this side of the dirt road.They are great were they belong,on cars namely sports cars.I tend not to like them on trucks were you want a certain amount of give and take.Putting a poly in the control arm is not the best thing .They are stiff and do not deflect or give like a rubber bushing would.Combine that with flexing the front end and you will soon be the proud owner of a ripped of control arm bracket.Stick to rubber is all I can say.The factory style bushing are cheap anyway.
 
Don't know how many Jeepers are also motorcyclists - for me there is a direct relationship between death wobble on a Jeep and a tankslapper on sportbike. Generally you'll find that on bikes with a stretched out caster angle like on a Harley, you'll never get a tankslapper and they are very stable, yet slow steering bikes (compared to a Jeep suspension setup with good caster, i.e. 7 degrees or more). On a sportbike the caster angle is reduced quite dramatically to make for a bike that has quick and nimble handling around a race track, but often when transversing bumpy backroads the steering could become a handfull as the bikes tend to tankslap when going over bumps in the road. The sole reason for this is because of the reduced amount of caster and trail these bikes run (compare this to a Jeep that is lifted with caster settings towards to lower limits of the spec in order to keep the pinion angles in check).

Naturally, like already mentioned, any slop in the steering will increase the effect and likelihood of death wobble, but in my mind caster plays a big role (provided toe and camber settings are within spec too).
 
Didn't see the actual mechanics of the death wobble addressed above, but it's basically this: One or more component mounting points are loose or worn, either tie rod ends, unit bearings, track bar ends, or occasionally really worn or out of balance tires. The alignment is set slightly "toe in", meaning the tire sidewalls are closer to each other in front than rear. This causes them to push in constantly, which is good, it's supposed to be set-up this way. Combined with one or more loose components though, when you hit a bump or sometimes at a certain speed/rotational force, the loose/out of balance components "break free" of the rut/neutral position they usually ride in, and the toe-in causes the axle assembly to cycle back and forth rapidly within the play of the loose/worn component(s). That is death wobble in a nutshell.


EDIT: Good point Gorman, too steep caster either way will accentuate the problem, and lead to a jumpy, darty feeling in the steering if everything else is tight.
 
Rotating parts introduce some sort of harmonic vibration, dampening devices isolate that vibration. The exact physics are inexact as they vary from case to case, suffice it to say that death wobble occurs at a specific speed where vibration introduced into the system exceeds the dampening devices ability to isolate it. Maybe you get it on a certain stretch of road, or when you introduce bump steer at a certain speed but, it is easily repeatable.

If it's only experienced by a single driver or "when the planets align" it's a shimmy, same physics but not "death wobble". With a shimmy, you can push through to a slightly higher speed and it goes away. Attempt to push through death wobble, and your sphincter tightens so hard it it introduces the thought, "I'm gonna die!" into your skull and causes an involuntary lifting of your foot from the skinny pedal. A second attempt to push through to a higher speed will confirm the initial diagnosis.

Eliminating the obvious (wheel alignment, poor geometry and visible steering slop), I've found "not obviously crappy" tires to be the culprit fairly often.
 
Thank you for the in depth answers. That is what I was looking for. If it was an oscilation (tire being out of round), a bounce (out of balance tire and/or a bad shock) or more of a wobble like the name implies (side to side shake) that could be caused by a multitude of things.

I wanted to ask this becuase it seems that when the few people who experince this phenomenon it appears the advice is always on the "is it this or that......?" and the actual "cause" of a death wobble is never addressed. I know that for me a problem is always easier to address if I know what causes the problem, not just its symtoms.

If that makes any sense at all good; if it doesn't, I was just saying thanks. Maybe this will come in useful and someone can use this to diagnose the entire problem instead of nickel and diming replacement parts until they get lucky......
 
Yes, caster reduction can help the Super Duties. I've found more often than not the track bar ball joint which is pressed into the axle will loosen up, causing the damper to work overtime. Then the damper fails and it wobbles. Replacing the damper will help but the new one will fail in short order unless the ball joint is replaced. Those trucks are definitely in the category of needing a damper.. call it a bandaid if you want, but they are undrivable without one.
 
This is my second SD both of which are lifted on 35's. It seems the DW comes into play with the 05, which, is a radius arm suspesion. It doesn't help that my tires are 13.5" wide and I have not installed the dual dampner either. I am fairly certain that the DW is a result of the alignment, being that I feel like I am driving a shopping cart that was hit by a train if I go around a corner that happens to have bumps on it.

I need to have the caster checked out, but that is only secondary to the reason behind my initial question on this thread. I was actually trying to get a "meeting of the minds" as to the physics and geometrical phenomena that causes the DW.

Thanks

edit: horrible punctuation
 
A mechanic buddy of mine described the caster problem like a shopping cart. If you get the wrong caster it "wobbles" just like a shopping cart. Mine was from the PO using the wrong bolt in the track bar. This allowed the axle to shift under the front. When the axle shifts, it turns the tires. When the tires turn, the weight of the vehicle shifts, which in turn moves the track bar the other direction, then the tires turn the other direction, and the action repeats itself until it feels like the jeep is shaking itself dry like Fido.
 
LR_2000_XJ said:
I have alot of worn front steering components, and I have no DW at all, even at speeds of 85mph. I do however have a pulling to the right, like when my lower CA bushings were completely crushed on the passenger side on my old CAs, but now, with my less miles CA, I'm getting the same simptoms but no crushing, and also, I am getting a wandering feeling in the steering like I was getting a switching wind from right side to left and back, but no wobble. I've checked the track bar, pitman, and all CAs, but not sure what to tear apart first. How hard is it to remove the old bushings on the CAs anyway? I'm thinking on using poly bushings in the stock CAs for now, and that may help, as well help with death wobble others experience. Let me know if I'm wrong with that one, I'd like to know for the future problems I may have someday.

if you have twisted up control arm brackets they bind the steering and eliminate any and all DW, but doesnt mean its fixed...
 
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