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Go Back   ClassicBroncos.com Forums > 66-77 Ford Bronco > Bronco FAQ > Steering FAQ

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Death Wobble Death Wobble
More than you ever wanted to know!
Steve
09/20/06
First, I apologize for how long it's taken me to post this. Been way busy this summer with work and other stuff. However, it's finally finished so here is way more info about Death Wobble (DW) than you probably ever wanted to know. If you have questions, post them here or send me a PM and I'll attempt to answer them.

VERY LONG POST WARNING!

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 balls 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?  

Last edited by Steve; 09/21/06 at 03:54 PM..
  #1  
Steve on 09/20/06, 08:29 PM
Here are a couple of Q&As I've received on another forum where I wrote about DW:

Quote:
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.

Quote:
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.

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  #2  
gearida on 09/10/07, 03:42 PM
Easy to remember

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!
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  #3  
mtfit on 09/10/07, 04:05 PM
thanks, great info, I have replaced, aligned, adjusted and balanced everything, DW still happenes occassionally with a certain bump in the road at certain speeds, but as you said 30 yr old truck, 37" tires?
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  #4  
is_wiz on 08/25/09, 06:59 AM
You enjoyed writing that. Well done.
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  #5  
4G5 on 10/09/09, 01:12 PM
Awesome info. guys. I have been having this problem for years. I'm glad that I finally understand it now.

Keep the good info. coming!
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  #6  
Hans on 11/16/09, 04:11 AM
I wish I remember where I saw the article, I believe it was an article from the late 1960's early 1970's (I only saw it about 10 years ago). It claimed that the shock location on Broncos, and the similar 2wd front end on the Econoline vans/pickups with the twin i-beam, was a big contributor to front end instability.

I don't remember the exact reasoning, but I want to say it had to do with the shorth throw length, high pressure, and the required valving for shocks that size. But I'm just going on memory here. The articles recommendation was the extended length shock mounts in place of the short factory location.

-Hans
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