Death Wobble Re Visited

[jamesroney]

Re: Death Wobble Re Visited

OK, my turn.

I have some years of experience with the afore-mentioned death wobble. Let me assure you that it is NOT an Chrysler / Jeep issue. It is an issue with any live axle front suspension.

I have experienced death wobble in a 1946 Willys CJ-2A. Not a shimmy, or a shake. Death wobble, like tires moving 8-10 inches side to side. Ugly, scary, like the videos death wobble. Also had it on a 67 Bronco.

Death wobble does not have a specific "cause." Anyone that tells you otherwise is mis-informed. Death wobble is the convergence of four things that must come together to result in the wobble.

***Important to remeber. Correlation is NOT causation. This means that just because two things are correlated, it does not mean that one causes the other. Just like "speed kills." Accidents are correlated to speed. But you would not argue that accidents CAUSE speeding. It is equally INCORRECT to assume that speeding causes accidents. (because we know that you can successfully speed without having an accident...)

With that in mind, you need four key things to invoke death wobble.

1. Some compliance in the lateral location of the front axle. A loose trac-bar, a loose mount, bad shocks, soft leaf springs, etc.
2. Low centering force in the tires. (yes, I'm blaming the tires...)
3. Some kind of initiator to begin the oscillation. A bump, a shimmy, a tire out of balance, or out of round, something.
4. Low centering force in the steering geometry.

...and lateral drag links.

Death woble is the product of these four components. And this is why people will tell you that they can cure death wobble by changing item 1,2,3 or 4. ...and why people will tell you that they have exactly the same "thing" and do not have death wobble.

For example, lets start with Caster. That goes under item 4. Some people tell you that if you have 5-7 degrees of positive caster...you will not get death wobble. But if you have lots of problems with item 1,2, and 3...you can still get DW. On the other hand, there are examples of vehicles with 1 degree of caster that have never seen DW.

(aside...I have never heard of DW on cars with fore-aft drag links. That includes Toyota Hilux, and every GM, and Dodge 4x4 with leaf springs. This is an observation, but an important one. The ability to control DW is correlated to the tie rod / drag link geometry.)

With that, if you want to eliminate Death Wobble in your Bronco, ensure that the front steering / trac bar is not worn. Ensure that you have 7 degrees of caster. Ensure that you are using <33 inch round and balanced BFG radial mud terrain T/A's with <.006 inches of indicated runout.

Then tell me if you bronco still wobbles...Mine will run with 5 inches of lift, up to 80 mph on any road...and no hands on the steering wheel.

BTW, my 2007 JK Rubicon does not wobble. Every JK that I've seen with DW is running OEM Goodyears. Those tires love to wobble...but a tight front suspension keeps it in check. As everything wears, the Jeep starts to wander. Then wobble...

[TBS-POPS]

Re: Death Wobble Re Visited

My 05 f-550 at work has a slight case of "d" wabble when running empty at about 45 mph while turning left. The garage says they can't find anything wrong.

Marc, From all my research the DW in the Fords is from the Trac-bar. They have a bushing on one end and a ball joint on the other. There are aftermarket co.s already making replacements for their shitty set up.


Nice dissertation James, I like it. Good logical info.

[Broncrick]

Re: Death Wobble Re Visited

Nice straight info James!

[Lars]

Re: Death Wobble Re Visited

OK, my turn.

I have some years of experience with the afore-mentioned death wobble. Let me assure you that it is NOT an Chrysler / Jeep issue. It is an issue with any live axle front suspension.

I have experienced death wobble in a 1946 Willys CJ-2A. Not a shimmy, or a shake. Death wobble, like tires moving 8-10 inches side to side. Ugly, scary, like the videos death wobble. Also had it on a 67 Bronco.

Death wobble does not have a specific "cause." Anyone that tells you otherwise is mis-informed. Death wobble is the convergence of four things that must come together to result in the wobble.

***Important to remeber. Correlation is NOT causation. This means that just because two things are correlated, it does not mean that one causes the other. Just like "speed kills." Accidents are correlated to speed. But you would not argue that accidents CAUSE speeding. It is equally INCORRECT to assume that speeding causes accidents. (because we know that you can successfully speed without having an accident...)

With that in mind, you need four key things to invoke death wobble.

1. Some compliance in the lateral location of the front axle. A loose trac-bar, a loose mount, bad shocks, soft leaf springs, etc.
2. Low centering force in the tires. (yes, I'm blaming the tires...)
3. Some kind of initiator to begin the oscillation. A bump, a shimmy, a tire out of balance, or out of round, something.
4. Low centering force in the steering geometry.

...and lateral drag links.

Death woble is the product of these four components. And this is why people will tell you that they can cure death wobble by changing item 1,2,3 or 4. ...and why people will tell you that they have exactly the same "thing" and do not have death wobble.

For example, lets start with Caster. That goes under item 4. Some people tell you that if you have 5-7 degrees of positive caster...you will not get death wobble. But if you have lots of problems with item 1,2, and 3...you can still get DW. On the other hand, there are examples of vehicles with 1 degree of caster that have never seen DW.

(aside...I have never heard of DW on cars with fore-aft drag links. That includes Toyota Hilux, and every GM, and Dodge 4x4 with leaf springs. This is an observation, but an important one. The ability to control DW is correlated to the tie rod / drag link geometry.)

With that, if you want to eliminate Death Wobble in your Bronco, ensure that the front steering / trac bar is not worn. Ensure that you have 7 degrees of caster. Ensure that you are using <33 inch round and balanced BFG radial mud terrain T/A's with <.006 inches of indicated runout.

Then tell me if you bronco still wobbles...Mine will run with 5 inches of lift, up to 80 mph on any road...and no hands on the steering wheel.

BTW, my 2007 JK Rubicon does not wobble. Every JK that I've seen with DW is running OEM Goodyears. Those tires love to wobble...but a tight front suspension keeps it in check. As everything wears, the Jeep starts to wander. Then wobble...

James- I can vouch that it's possible to get death wobble with a fore/aft drag link. Vehicle in question was a 1948 Willys pickup, 2wd, typical-for-its-day solid drop axle with leaf springs. With play everywhere. Any bump (input) encountered above 30 mph would trigger an onset. Fortunately that front suspension wasn't on that truck for very long. Unfortunately (I think, looking back with rose-colored glasses) I sold it long ago. That truck had a fore/aft link from a steering box with horizontally oriented sector shaft, pitman arm rotating in a vertical plane, maybe 18" aft of the kingpin. Tie rod ends incorporated the adjuster that could take out play, retained with a cotter pin, seen often on vehicles of that vintage.

For what it's worth, I've never had death wobble with my Bronco, ever. Not when it was unlifted with some nasty Winston bias ply tires on stock rims, or now. But the first thing I did when I bought it was to replace every compliant bushing in the front end. At that time it only had 1.5 degrees of measured positive caster. In its current state with 4" of suspension lift, 6 degrees of caster, 35" (nominal) BFG A/T K/O tires, it'll go no-hands at a gps-verified 100 mph, assuming I'm willing to pay for the gas.

[earlybronco72]

Re: Death Wobble Re Visited

I still thing death wabble is caused by aliens. My bronco (that is now a pile of parts in the garage) had warn out bushings at both ends of the drag link sloppy tie rod ends, loose wheel bearings and unbalanced bias ply irocs on home built beadlocks and only had death wabble once. Figured out 1 1/4" of tow in was to much for it. Backed it off to 1/4" and ran it like hell. Passed the wife once on the freeway and she was doing 80. One hand on the steering wheel and one foot out the door.
Aliens I say.

[jamesroney]

Re: Death Wobble Re Visited

James- I can vouch that it's possible to get death wobble with a fore/aft drag link.

Thanks Lars,

I accept your experience, and will put it in my empirical knowledge database. Of course it kinda invalidates some of my assumptions...but I love to learn from real experiences.

As EarlyBronco72 correctly observes...it is possible to have a worn front suspension, and poor steering and still not exhibit death wobble.

My 1953 Willys pick up had incredibly poor maintenence, and had a full 1/2 turn of steering wheel play...and in fact had a loose steering arm / king pin cap but always drove generally where it was pointed. So I 100% agree that cheap, crappy tires, poor maintenence and sloppy steering do not guarantee death wobble.

thanks for the candid feedback.

[FLYINHIE]

Re: Death Wobble Re Visited

Thanks Lars,

I accept your experience, and will put it in my empirical knowledge database. Of course it kinda invalidates some of my assumptions...but I love to learn from real experiences.

As EarlyBronco72 correctly observes...it is possible to have a worn front suspension, and poor steering and still not exhibit death wobble.

My 1953 Willys pick up had incredibly poor maintenence, and had a full 1/2 turn of steering wheel play...and in fact had a loose steering arm / king pin cap but always drove generally where it was pointed. So I 100% agree that cheap, crappy tires, poor maintenence and sloppy steering do not guarantee death wobble.


thanks for the candid feedback.

And the opposite, where EVERYTHING is brand new (except tire) & I am fighting DW.

[jamesroney]

Re: Death Wobble Re Visited

And the opposite, where EVERYTHING is brand new (except tire) & I am fighting DW.

We should trade tires. My 73 drives straight as a string. Never a shake, bump, or wiggle. Any speed, any road surface.

If you are 5 lug...you are welcome to borrow my tires and wheels. I'm running fresh 35x12.50x15 inch BFG Mud Terrains.

My Bronco won't see much use for the next few weeks...

[Lars]

Re: Death Wobble Re Visited

Fun with resonance...

It can happen on motorcycles too, as I discovered once, on a 1989 Yamaha FZR1000. That bike came with radial tires, which were fairly new on the market. I allowed myself to be talked into a set of bias ply tires against my better judgement, at tire change time. They were on the bike for one day. At 70-ish mph any bump encountered would trigger a tank slapper (bars flailing wildly, seemingly lock to lock). Releasing the throttle and loosening my grip instantly damped it, but not before I nearly wet my pants- it happened on the freeway in rush hour traffic. Immediately went back to radials; it never happened again.

I observed a Bronco on the beach at SOB several years ago, with a fairly hot engine. 4wd was engaged, the driver pinned the throttle, and the front wheels started to flail back and forth. It actually appeared as if the wheels were toeing in/out, as if the tie rod was behaving like a jump rope. Though from my vantage point I couldn't be sure. It stopped when the front driveshaft snapped.

[jamesroney]

Re: Death Wobble Re Visited

Yep, anyone that has ever pushed a shopping cart too fast has experienced resonant steering wobble.

I'm not sure I agree with Bias-ply versus Radial as a root cause. There are many millions of miles travelled on motorcycles with bias ply tires long before radials were invented.

But I am increasingly convinced that tread design and tire carcass construction are a contributing factor.

Your Designed Experiment on the FZR demonstrates that with all other things held constant...tires are a factor.

[FLYINHIE]

Re: Death Wobble Re Visited

We should trade tires. My 73 drives straight as a string. Never a shake, bump, or wiggle. Any speed, any road surface.

If you are 5 lug...you are welcome to borrow my tires and wheels. I'm running fresh 35x12.50x15 inch BFG Mud Terrains.

My Bronco won't see much use for the next few weeks...

I have changed my steering geometry a little bit and if that doesn't do it I will swap tires. Your not too close so logistics of borrowing your tires won't work too well, but I DO appreciate the offer. A few others have offered to swap tires who are much closer and I will probably take them up on it if this doesn't work. Thanks

[Mike]

Re: Death Wobble Re Visited

Lars I'm glad you brought up motorcycles because I always thought Bronco DW was like laying a bike down. There are those have done it and those who are going to do it. I've had DW caused by something as simple as a loose c-bush cap bolt, as in one bolt.
I wish I had a paper I read long ago about how front end designs are inherently unstable due to all the conflicting needs. You guys could probably make more of it but it made sense to me. Since then I've given up on "getting rid" of DW and just accepted it a given, to be managed as best you can.
It really is amazing. I came home from Hollister once going 70+ the whole way, no problem. The next morning I could not make it down the street to the gas station. Good times.

[FLYINHIE]

Re: Death Wobble Re Visited

Lars I'm glad you brought up motorcycles because I always thought Bronco DW was like laying a bike down. There are those have done it and those who are going to do it. I've had DW caused by something as simple as a loose c-bush cap bolt, as in one bolt.
I wish I had a paper I read long ago about how front end designs are inherently unstable due to all the conflicting needs. You guys could probably make more of it but it made sense to me. Since then I've given up on "getting rid" of DW and just accepted it a given, to be managed as best you can.
It really is amazing. I came home from Hollister once going 70+ the whole way, no problem. The next morning I could not make it down the street to the gas station. Good times.

That's F*!cked up. That would piss me off just a little bit. I know with my problem it's something that is out of balance by my design, but for you. Good one day bad the next. That would be VERY frustrating!!!!!

[Lars]

Re: Death Wobble Re Visited

Yep, anyone that has ever pushed a shopping cart too fast has experienced resonant steering wobble.

I'm not sure I agree with Bias-ply versus Radial as a root cause. There are many millions of miles travelled on motorcycles with bias ply tires long before radials were invented.

But I am increasingly convinced that tread design and tire carcass construction are a contributing factor.

Your Designed Experiment on the FZR demonstrates that with all other things held constant...tires are a factor.

Ya, the "bias ply vs. radial" description is probably somewhat of a red herring. More likely something to do with the fact that the bias ply tires have stiffer side walls, and, at least in the case of those tires, a far rounder profile. In any case, the suspension and steering didn't like the change.

Designed Experiment... uh oh, the engineers are loose in our midst (it takes one to know one, etc).