Revisiting The “death Wobble”

I watched mine herky jerky yesterday, ongoing vehicle stopped and was freaking out more than I was. ha It looked like the front rim wanted to tear off, I checked torque on all my rims, to FSM spec.

 

The issue with Ross steering is that there are soo many points of movement that looseness and play allows everything to resonate with minimal mileage. The one pc system ties the wheels side to side with the wear points of:
Two sets of Wheel bearings, two sets of kingpin bearings, two TREs. So the opposite wheel dampens the oscillation before it involves the drag link and suspension.
With the individual short links one wheel can get very excited without affecting the opposite side.
A tire and wheel assembly is an excellent gyroscope. The questionable quality bellcranks and egg shape drag link ends don't help nothing.

 

I will have another shop location do so to check behind the others work.

 

A lot of F-250 work trucks ended up with DW. It seemed to happen to Fords when they went to the shortened Twin I-Beam. Some
of these trucks had 200- 400 thousand miles. Another cause is wheels bent from tire shops ripping the tires of the rims. I had this
happen and they denied it though I saw them do it. Ended up buying two rims and never going back.

 

I agree with Howard about the stabilizers, but I can't get the picture of Ken hanging over the front of a Jeep out of my mind.

 

Yeah Andy but truth is we were all
indestructible around age 20 or so.
My jeeps case of the wobble would come on at 45 mph when hitting the highway segments in a regular rythm.

 

I had DW in my 2009 F-250. The worst pucker moment was when I has my Lance camper on top and my kids in the back. The dealer went through several parts without a cure. To the best of my knowledge (well before I knew anything about cars), the fix was to replace with shock attached to the steering column (at about 50K), which I'm assuming is analagous to the Jeep steering stabilizer. It hasn't been a problem since then (currently at 170K).

 

I have a lot of thoughts about this. I struggled with it for a while and finally have it resolved. I put a lot of highway miles on my Jeep at around 65mph (twice now I have done road trips over 5k interstate miles) so getting smooth steering has been important to me.

The steering system is a mass - spring - damper system. The masses involved are the vehicle itself, the tire/wheels, and the axles. Springs include the tires (most important) the leaf springs, bushings, etc. and the linkage components, especially if worn. Damping is provided by the friction in the steering system and the damper shock, if present.

There are a couple of ways to avoid or reduce resonance (death wobble) in the steering. In order of priority (my opinion):
1. Reduce resonant excitation
2. Eliminate modes of vibration
2. Ensure that resonant frequencies are higher than the frequency of excitation
3. Add damping so that when resonant frequencies are excited, amplitude is reduced

Most important of all is to have well balanced, straight, and true wheels and tires. Even a perfect system will still vibrate to some extent if these conditions are not met. Worse yet, the tires will start to wear improperly and the shake will get worse over time even if you fix everything else.

Regarding #2, the main resonant modes for our Jeeps include both wheels moving in unison and wheels moving opposite each other. The second is the toe in - toe out cycling that Oldtime mentions, and can be totally eliminated with the one piece tie rod conversion. This does not prevent the wheels in unison mode so it is not a guaranteed cure, but I consider it a high priority upgrade. Its importance is even greater when the Jeep is lifted, because this increases the angle of the two piece rods, reducing the stiffness against toe change and increasing the likelihood of this mode of vibration. A loose bellcrank will also exacerbate this issue with two piece tie rods.

For #3, increasing caster is the main thing to do. Greater caster increases stability by increasing the return to center force and raising the resonant frequency. You want the resonant frequency to be significantly higher than the frequency of the tires at the highest speed you will drive. Of course this is a tradeoff for increased steering effort, especially with manual steering. You can also tighten up the suspension with stiffer bushings and shackles with a cross brace to prevent 'parallelogramming' which the stock ones are prone to. Tire stiffness is a major factor in resonant frequency as well, so tire choice and inflation are also important.

#4, adding damping, is the last resort. Damping doesn't change the resonance, but reduces its amplitude and can encourage it to die out. If your resonant frequency matches the excitation from a tire imbalance, you will still get vibration but it will be lessened. Where steering dampers shine is for sudden impulses, such as a pothole or railroad tracks - these excite nearly every frequency and the damper can encourage that to die out quickly and reduce stress on upstream steering components.

Some people say that to cure death wobble, it is critical to get all steering components tight. I disagree with this. Kingpin bearings, wheel bearings, tie rod ends, and caster are all you should need to eliminate vibration (steering play is another issue of course). If tightening up the steering box eliminates a wobble, it is only because the added friction (or your arms) are providing damping. It may work but it isn't the proper fix for the root problem. The vehicle should be able to go down the road straight and vibration free even if the drag link were totally removed.

Edit- just noticed @Fireball already covered most of these points

 

I agree with your other points but I think this is not the right conclusion. Of course you have a lot more first hand experience than most so I am wondering if there is some related factor for solid axle front suspensions - some other parameter that gets out of whack when the axle is turned for greater caster?

More caster should bring greater stability, and 7° still isn't that high in the scheme of things. Some production cars have 10+ and land speed cars can be in the ballpark of 20°. Also consider motorcycles that go to ridiculous angles.

Contrary to the shopping cart example - caster wheels flutter when the amount of trail is too small for the speed. In other words, they need more caster to fix the vibration. See here: https://www.bulldogcastors.co.uk/blog/castor-wheel-flutter/ "Use a longer 'trailing distance'"

 

More caster creates more self-aligning torque on the steering system, but it also means the tires have more leverage to input more torque into the steering system. This means a single tire hit can put a bigger input in the system to start resonance. As with all physical systems, it's a balancing act.

 

Fair point, and I would expect this to mean more stress on steering components and potentially a more jarring experience on bumps. But shouldn't this sort of brief impulse be what the damper excels in reducing? And if your resonant frequency is high enough, I would think that the vibration would be damped out fairly quickly and would not be sustained? Maybe this is not possible to achieve on modern 4x4s with their much larger and heavier tires?

 

I can't answer that, but there are cases where reducing caster has fixed the problem. There are also cases where increasing caster has fixed the problem. Depend on the vehicle and the components involved.

 

Another trigger point I thought of is the ackerman angle. When the tire scrubs going around a bend especially with body roll it can set it off. It seems there is not alot of scrub on the cjs compared to other vehicles i drive but still a possible contributing factor.

 

I had a short stretch of bad DW a couple years ago. I had bent my Tie rod causing a bunch of toe in. I twisted the rod out just guessing how much was needed. No DW since.

 

2021 kenworth t880 mind you $200,000 plus truck had a bad case of death wobble from the factory went to dealership multiple times no cure all new steering components springs tires wheels hundreds of hours of labor thousands of dollars lost revenue. It turned out to be a bad front shock absorber the factory warranty didn’t cover them so the dealership didn’t change them .our shop did as a last resort before the boss told kw to take it back and shove it 250,000 miles on it now and no further troubles.

 

Lots of very high expertise here.

However in my own experience DW very simply always came from loose king-pin bearings.

 

I think this is my issue, my adjusters were previously set for 7.50 tires, I am back to a 700, adjusted them all the way in, as the factory weld is long removed. So I need to adjust them back to factory setting, anyone know settings/inches etc?
I seem to encounter mine when backing out/wheels turned hard or turning hard left/right, and the ground is uneven from a dead stop, DW starts. I never encounter it driving down the road straight, only at low speeds when turning or backing out when turning.

So none of this was present until new tires and me backing off the turn radius adjustment bolts/nuts, so the ackerman angle seems most plausible as that seems to be my symptoms and when the DW occurs.