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Death Wobble - Shimmy

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samcj2a View Drop Down
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    Posted: 17 July 2011 at 9:43am
Our old flat-fenders frequently wear to the point where a very bad shimmy develops where the only work around is to bring the Jeep to a stop and start up again.  This condition is often referred to as the "death wobble."  Once you experience it, you will understand why! 

There are a number of threads that discuss how various members have repaired the condition.  The bottom line is that any of the steering components that are worn or loose can cause the condition.  Here is a thread that mentions them.  Death Wobble

Items you need to look for are these:  loose king pins, loose tie rod ends, loose steering connecting rod joints, looseness in the steering box, loose wheel bearings, a loose steering column where it attaches to the tub, a loose connection of the steering box to the frame, worn or loose bell crank, under-inflated front tire.   Also, it can easily be a combination of these items.  Until you correct the last of them, the condition can persist.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Scott R Quote  Post ReplyReply Direct Link To This Post Posted: 17 July 2011 at 12:33pm
Good info Sam. I'd like to add this Service Bulletin posted by rocketeer in the following thread. http://www.thecj2apage.com/forums/front-shimmy_topic12860.html?KW=death+wobble




Edited by Scott R - 17 July 2011 at 12:56pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jwitt Quote  Post ReplyReply Direct Link To This Post Posted: 20 July 2011 at 11:28pm

The dreaded death wobble, what makes it happen?

There is a class of dynamic problems that are dealt with by nearly everyone, every day, mostly without consciously recognizing what they are and they have similarities that cut across many kinds of natural events.

These problems are, in mechanical terms, caused by vibrating systems.  A wobbling front wheel is a vibrating system and in fact is a fairly easy one to understand. These systems exist in all structures, electronics, optics and nearly everything in nature and operate under identical mathematics.

The simplest kind of vibrating system consists of a spring which is attached to a mass. If we jiggle the mass, it will bounce up and down.  In the real world various kinds of friction act on the vibrating mass and will eventually bring it to a stop.  This friction is called damping, and the whole system is called a spring-mass-damper system. 

There are some rules of thumb about such systems, the first being that the stiffer the spring is relative to the mass, the higher the frequency of vibration.  The action of stiffness is nonlinear, which means that if you double the stiffness, the frequency will increase by the square root of 2 or 1.4. A change in the mass has a similar effect. 

Without worrying too much about the numerical ratios of the three terms in the system we can say that increasing the stiffness makes it vibrate faster, increasing the mass makes it vibrate slower and increasing the damping causes the vibration to diminish faster.

So let’s look at a rolling pair of front wheels and see what might be going on.  If the wheels are rolling along on smooth pavement at a constant speed, the arrangement of toe-in, caster, tire stiffness and tire contact patch offset allows the jeep to cruise smoothly along.

If a disturbance occurs to one wheel, a bump or pothole, the wheel is moved vertically by some amount and things start to get interesting.  If all the parts are in good order, not much happens, the steering wheel moves a little and the nose of the Jeep darts to one side, the driver corrects and everything is fine.

However, suppose the shock absorber (it’s really a vibration damper) is totally worn out. This will allow the wheel to bounce up and down several times until the energy put in by the bump is dissipated.  Everybody has probably seen a car with this condition at some time, and the vertical bouncing of the wheel can be quite violent.  More on this in a moment.

In the case of a Jeep and many other cars, the wheel can’t really jump straight up and down. Because of the mechanical design, there is some left-right steering of the wheel (called bump steer by the race car guys) as it moves up and down.  The caster and tire stiffness now try to make the wheels center up again and so create a force acting around the spindle.  If we add in some looseness in the spindles, ball joints, etc., now the forces can wrap the whole thing up until all the slack is taken out.  The inherent springiness of the steel steering parts, the steering gear box and other components now have some energy stored up which is then used to snap the wheels back in the opposite direction.

If the wheels are rotating at something other than a certain critical rate, the wheels will wiggle some and the vibration dies out and we can continue until the next bump.  There is, however a critical rate which will vary depending on the way the individual system is designed, where the damping is completely overcome and the vibration continues with a great deal of force, slamming the wheels from stop to stop. At this point, you slam on the brakes and stop the Jeep (hopefully).

This critical point is called the resonant frequency of the system.  In this case, it is a destructive factor and we need to keep it from happening.  The remedy is to try to move the resonant point out of the operating range of the system, and as stated above we can improve on all three factors governing our suspension to do so.  We know it’s possible, because Jeeps don’t wobble when factory fresh.

Things to do:

Take the slack out of worn parts: springs and shackles, steering box, spindles, wheel bearings, bell crank, drag link, tie rods, etc.  This reduces the inertial forces which look like mass in the system and raises the resonant frequency.

Make sure the shocks are in good shape.  These are some of the cheapest parts in a Jeep, don’t skimp.

The alignment should be symmetrical, i.e., correct camber and caster on both sides. Correct toe in.

 Make sure the rear axle is parallel to the front axle.

Correct tire pressures, matching tires on the front – same age, design, tread depth

Add a steering damper to the front tie rod.  This is a patch to a system that has something else wrong, but might get you by for awhile.

This, admittedly a somewhat simplified explanation and there are a number of other factors affecting the way vehicle wheels vibrate.  These have been left out, mainly because I don’t have time or the expertise to write a dynamics textbook.

For those who want a bit more:

http://en.wikipedia.org/wiki/Speed_wobble

 
Hope this helps some,
 
John 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote m38mike Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2011 at 10:45am
WOW JOHN!!  Are you a philosipher in a 2A suit?  That was very eloquently stated. 
 
So to summarize, if you have worn parts, or loose parts, you could experience the "death wobble".  So keep your parts in-spec and tight.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jwitt Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2011 at 1:18pm
Yep, Sam and Scott both said it as well, though the factory assertion that the front seals provide enough damping to help seems a little overstated to me.  The loose spindle bearings would certainly be a contributor to the problem.
 
For what it's worth, as a general rule, tapered roller bearings should have some preload. There should be no detectable clearance in them.  Timken is very clear about this.
 
I have seen commercial built trailers delivered with the wheel bearings set up incorrectly, which leads to early failure.
 
John


Edited by jwitt - 23 July 2011 at 6:22pm
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