Apologies if this has been covered before, first post for me.....

1999 Cherokee, totally stock. Tires balanced, alignment fine, etc. Sometimes while traveling at highway speeds (i.e., more than 45 MPH) if I hit a bump or expansion strip in the road, even a relatively mild one, it sets up a shudder in the steering wheel that starts mild, and rapidly progresses to a point where I absolutely must slow down as it gets so violent I feel like I could lose control. Once the speed drops to about 35 MPH the shudder goes away. But it will come back if I hit another bump. I "assume" it's either a bad steering stabilizer or bad front shocks, and I plan to replace them promptly. FYI, I bought it used with 85,000 miles, and have only driven it about 5,000 miles since then. Anything else I should consider? Thanks in advance....

 

Also check your track bar bushing.

 

wheel bearings in good shape?

 

check all front end components (ie. tie rod ends, ball joints, yada, yada)

 

same as above , Check for worn ball joint , tie rod end , broken shock mount, bad stabilzer on the steering. Check also the steering box for play. and the pitman arm lastly make sure your steering box is securely ounted and your frame isnt cracked where it bolts up. Rare for it to go on a stock xj but a good curb licking in the snow could just break a bolt of cause a tear....

 

Death Wobble explained…
Here's an engineering description of DW. I get tired of seeing people guessing at what's causing their DW, so here goes. Hope it helps someone.

First, you've got to realize that the front suspensions on our vehicles were marginally stable, at best, from the factory. DW is a fundamental dynamic response mode of the entire front end...as a system. Lift and larger tires change (increase) the 'gain' associated with what becomes (or even starts out as) a marginally stable dynamic system. The damping factor (lambda) is also affected by larger tires...it decreases as a function of sidewall height/thickness ratio. Hysteresis in any control path (loose tie rod, steering box, track bar bushing) reduces the ultimate stability margin further. The fundamental frequency of DW is determined by the superposition principle where all springs involved are resolved (frame, tire resilience, hub bending, bushing deflection, etc, etc.) into one global spring constant, and all damping factors associated with friction, elastic elements, viscous damping (steering damper and shocks) are resolved into one damping factor. The natural frequency, damped natural frequency, and damping coefficient are then known. Now, if the system is overdamped and the gain is low...no problems...no oscillation. Increase the gain without increasing the damping and you go toward the critically damped, and beyond, specturm of responses. Critically damped means that DW would only 'hint' at being there, but would die out on its own without going totally unstable. This is also known as a decaying response.

Once the system goes beyond critically damped, any excitation, be it an unbalanced tire, a bent wheel, bumps in the road, etc. can set it off and the response will not decay...it will grow in amplitude, quite quickly in some cases, and may be limited only be physical non-linearities like hard stops...or breakage. That's classic Death Wobble.

A truck suspension is designed to stay in the overdamped to critically damped range. That is generally why a truck rides "rough". A Cadillac, on the other hand, is designed to stay in the undersprung range. It just "floats" down the road. Any change in the basic design parameters that affect the gain (e.g., lift, tire size, wheel backspacing, etc.), damping (tire size, steering damper, steering box condition), and hysteresis (any wear point that creates any slop) can push it over the edge and create DW. ANY ONE OR TWO of the factors discussed can do that...which is why everybody then thinks that whatever problem THEY found and fixed is the cause of all DW; it is not. It is plain and simply a marginally stable system in its original form that is easily made unstable by any of the myriad causes discussed already.

If your front end is loose (bushings, bearings, etc.) then you have a situation where your stiffness is removed and any jarring sensation (potholes, unbalanced tires, misaligned wheels, etc.) will cause the suspension to go crazy. It is no longer functioning where it is designed. On the other hand, your suspension could be very tight but an imbalanced tire would be spinning at just the right speed to throw the suspension into a unstable situation.

So unfortunately there isn't only one root cause to the problem of DW. The underlying problem is instability in the front suspension, the root causes can be a multitude of things ranging from bad/loose bushings, to loose bearings, to caster angles, to imbalanced tires, etc.

OKAY, HERE'S THE REALLY USEFUL INFO:

A steering damper only hides (maybe) the effect; it does nothing to fix the root cause.

There are two types of DW. The first typically is speed related. Whenever you reach a certain speed, bam, you get DW, no matter what. This is a vibration/oscillation issue. Look into tire balance, alignment, steering joints, missing bushings (totally shot), loose steering box (either loose bolts or worn internals), etc.

The second is an impact initiated DW. For example, hitting a pothole above a certain speed will start DW. This is more likely a bushings, loosening mounts, flexing components, etc. issue. Basically, something is tight enough that in general straight driving, it is ok, but give it an impact force, whatever is getting loose starts sliding, rebounds and starts going nuts.

Here is how you can tell if the issue is steering related or trackbar related. You are gonna need some ***** for this, but stick with me. Once you have played around with the DW awhile you find you can control it a bit by feathering the brakes. So go find a straight, deserted, bumpy road. Get the truck up to speed and get the DW going. You had it happen a few times, you have already been frantically avoiding potholes, so now go find one, quit whining. At this point, the truck is somewhat violently shaking, and you can keep enough control using the brakes to keep it on the road. Roll down the window and stick your head out and look at the front tire. What is it doing?

1. The front of the tire and the back of the tire are moving approximately the same amount side to side. In this case, the axle is stationary, and the wheel is pivoting on the ball joint during the oscillation. Therefore the problem is likely in the steering. Something in the steering has enough give to allow the movement.

2. The back of the tire is moving MORE than the front of the tire in the side-to-side movement. In this case, the knuckle is pivoting on the steering links, and allowing the axle to move back and forth under the vehicle. The problem here is most likely in the trackbar system.

This doesn't really answer a question about what's causing YOUR DW, but it should give you something to think about in your search for the root cause(s). I'd check the trac bar bushings, make sure your wheel bearings are in spec, make sure your tires are balanced, make sure your alignment is in spec - especially caster, make sure your ball joints & TREs are tight, see if you have play in your steering box, etc.

Everybody got all that? ;D
Steve
Here are a couple of Q&As I've received on another forum where I wrote about DW:

Can you explain why reducing caster helps on some vehicles? It doesn't seem like it should work, but it does.. and at other times more caster will cure it.
Basically, anything you do to get the front suspension back closer to original factory specs should help with DW. Remember, a lot of our vehicles were marginally stable from the factory, and when we lift them, change the steering, run huge tires, etc., we're making them even less stable. So, running the caster at whatever it was from the factory, along with making sure that the many other things affecting the front suspension are in good condition, will help greatly.

Unfortunately, there's no silver bullet for fixing DW, and what works for one person may not work for another. You have to consider the entire front suspension as a system and then make that system as stable as you can for the way you want to run it.

Pondering how too much caster can cause wobble, the more caster you have, the larger the vertical movement of the wheel will be, under steering input. More vertical movement = more influence that the weight of the rig can have on it, and particularly with large soft tires, that would be a fairly substantial, mostly undamped, weight hanging out there. Get it cycling, and it's not going to want to stop.

I really oughta crank down my caster a bit (front at 8 degrees) but no steering feedback = very difficult to get actual DW, so I've been lazy.
Scott, let's take it to the extreme to see why too much caster can make the suspension system unstable. Picture 90* of caster. The ball joints/kingpins would be horizontal, so when you turned the wheels they would turn top to bottom and not side to side. 90* is extreme, but the more + caster you have the more the wheels are turning top to bottom and less side to side. Besides getting pretty unstable (and VERY heavy steering feel) with much more than 10* or so, you also start scrubbing the tires pretty bad when turning with too much caster.


NOTE: For reference purposes, here is a diagram of what caster is. Positive caster is when the top of the tire "leans" toward the rear of the vehicle as depicted in the diagram.

http://www.familycar.com/classroom/I...ign_Caster.gif
gearida
I worked as an Alignment guru for several years. When asked to explain or help in remembering steering angle terms I found a simple way to remember: 1) toe in and toe out - simple right if not find something else to do in your spare time...
2) Caster, my analogy is like fishing and throwing the pole from over the shoulder. This is the angle in question called caster, so if your arm is in the strong muscle man position as if you are prior to casting the fishing pole your forearm is at positive caster (strong muscle=positive)
If you are at full throw of the fishing pole or weak muscle man pose then you forearm is at negitive caster.
3) Camber, now give me some here, again take your forearms and think of them as straight up in front of you. Lean them out (as if your dates legs where apart) that is a positive...camber.
If your forearms are again straight up in front of you and your fists are coming in close together (as if your dates legs are closing) this is negative...thus negative caster.
I hope you don't forget my little story!

 

nice post Mike.... Finally one that was longer then any of mine lol.. great explainations though should be a sticky...

 

I did not write it, I found it somewhere else.... But yes it is a good one. Very nicely written. It shows how there is no silver bullet, one size fits all cure for the dreaded DW.
How do we make it a sticky?
Mike

 

contact one of the admins and im sure they will point in the right direction if not do it themselves... Great info anyway...

 

[QUOTE=Mike1998XJ;233999]Mike

When you tip the axle back thus increasing the castor it puts forces on the wheels to steer them toward the inside. The forces are equalized because of the tie rod connecting them. We basically get death wobble because of play in the system and the ability through some type of slop in our connections for the wheels to move when they shouldn't. Thus by increasing castor and putting more equalized force into the system that tendency is reduced to the point that even with a little slop there is no death wobble even without the damper. In a practical sense, the more you increase the castor, the harder to steer the vehicle will be because it will always be trying to get back to center and equalize its forces. So a vehicle with a higher degree of castor will go back to center quicker when you let go of the wheel and want to stay there.

But if you tip the axle forward, the tires will want to dive to the outside. Thus if you have a little slop, it will want to start to make a turn and then the slop will happen and then the body will sway and move the drag link and then set up a harmonic resonance known as death wobble.

 

Sounds pretty convincing 4.3L. On the 3.5" lift like mine, RE designed a longer lower control arm to help offset this. I just drew it on paper and even though that's an approximation at best, I see how the geometry can be close to corrected with the longer lower control arm. Measurements would tell the actual tale. I can see where lengthening the lower CA's rotates the axel back into a decent arc range... I'm sure it's a compromise though!
Mike

 

Mike (and others), many thanks. Your detailed explanation makes emminently reasonable sense (I'm not an engineer, but know enough about engineering, vehicle dynamics, and other technical info to understand what you are saying), and will do the test as you described. OOH, I'm glad to know I'm not the first to experience the DW. OTOH, it's a bit unnerving to know that it happens to others too. I've experience some high speed vibration before in other cars (usually unbalanced wheel/tire), but never had a DW like this before, and it literally scared the cr*p outta me the first time. Had planned to replace the shocks anyway (and already bought a set) so that will be part of the project. Again, very much appreciate the detailed response.

 

After you take it for a spin and stick your head out the window... Why don't you inspect the ball joints and so on, then you could rotate the tires just for kicks... Maybe it's as simple as an out of balance tire... Are your fronts wearing even? Let us know the steps you are taking as you walk through. It may help others to troublshoot their DW.
Good Luck let us know what you find out.
Mike

 

This is an awesome thread guys, huge thanks to mike for finding that ridiculous amount of info. I too have had the death wobble, once on the I and once playing dukes of hazzard. Right now my jeeps pretty stock, but 142K is a lot of miles, when I do my lift I planned on getting a steering stabilizer, new bushings and ball joints for the front end, and maybe bigger/better brakes for bigger/better tires, just to do it all at the same time while I have it all apart. Shocks too, duh. I didn't read every last bit of info posted by mike or anyone else becuase im at work but I figured shocks, stabilizer, ball joints and bushings would be enough to eliminate DW.

 

I like to call it hospital wobble