Death wobble. Uncontrollable side to side movement of wheels.


by Les Lord
Tags: death, movement, uncontrollable, wheels, wobble
Les Lord
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#1
Aug19-09, 11:18 PM
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My question is what causes death wobble? Let me explain more...

The vehicle I've experienced this on is a Jeep with a solid front axle, but it can happen on other vehicles. While driving down the road at say 50mph, I hit a small bump, and the steering wheel and tires shake side to side, getting progressively worse in a matter of a second or two. The only way to stop it is to slow down.

I believe that it is inherent in the design of the steering and suspension and therefore all Jeeps have steering dampeners. I think that caster has something to do with it. Caster is the forward or rearward tilt of the steering axis. It is what cause the wheels to return to center. I have tried to tell people to make sure the steering dampener is not worn, and always get the response that replacing the dampener is just a band aide to the real problem.

Just wanted to hear other thoughts on this.

Thanks, Les
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Danger
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#2
Aug19-09, 11:31 PM
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Welcome to PF, Les.
I can't say that I've ever encountered, nor even heard of, this problem. I've driven a lot of different vehicles under a lot of different circumstances, and this is the first that I've heard of that. My pet vehicle, currently being stored pending restoration, is a '72 GTX Roadrunner which started to leave the ground at 160 mph. Some rough roads got in my way at that speed, without the problem that you refer to. My current drive is a '76 El Camino frame-stacked on a '74 Jimmy, with a 455 Olds Rocket motor. I've hit bumps at over 90 mph with it, and never had any trouble. Do you happen to have some really weird roads in your area?
Les Lord
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#3
Aug20-09, 07:59 AM
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It happens mostly on solid axle vehicles in my experience(Jeeps, Dodge trucks, 1 ton Ford trucks, etc...), but I've seen it on a vehicle with independent suspension before also. I work as an auto technician for a living. ASE Master tech. The roads aren't all that great around here in Utah, but even a small bump can set it off. It can happen on motorcycles too. They also call it speed wobble. Some motorcycles even have steering dampeners.

negitron
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#4
Aug20-09, 08:30 AM
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Death wobble. Uncontrollable side to side movement of wheels.


On motorcycles, this phenomenon is nicknamed a "tank slapper" for self-evident reasons.
Danger
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#5
Aug20-09, 12:47 PM
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Okay, Les. I'm going to bail out of this one. I have no experience with solid-axle vehicles (that I know of; I've driven a lot of borrowed machines that I didn't investigate the suspensions of), and I never encountered the problem on a bike. I was a very cautious rider, though, because I'm really bad at it.
I hope that someone here can help you.
mgb_phys
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#6
Aug20-09, 12:54 PM
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Quote Quote by negitron View Post
On motorcycles, this phenomenon is nicknamed a "tank slapper" for self-evident reasons.
Good explanation of it here http://www.triumphrat.net/riding-and...explained.html
turbo
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#7
Aug20-09, 12:56 PM
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What model Jeep? Also, are there any modifications, like lift kit, over-sized tires, etc?
Les Lord
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#8
Aug20-09, 07:03 PM
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Quote Quote by turbo-1 View Post
What model Jeep? Also, are there any modifications, like lift kit, over-sized tires, etc?
98 Jeep Cherokee Sport. 2" spacer lift. 30" tires, original size is 225 75 15(equal to about 28"). That's it. Although I've seen it on completely stock Jeep Cherokees.
turbo
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#9
Aug20-09, 07:24 PM
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Quote Quote by Les Lord View Post
98 Jeep Cherokee Sport. 2" spacer lift. 30" tires, original size is 225 75 15(equal to about 28"). That's it. Although I've seen it on completely stock Jeep Cherokees.
Kill the lift kit, and go back to normal tires. You'll be aces, though with the stress that set-up caused on steering-gear, you may need more than a normal maintenance call to repair the stressed elements. Vehicles are designed to work with geometry that is pretty constrained. Jeeps are pretty boxy, and it is easy to get them to behave badly (including instability and roll-overs) when you start pimping them out.
wmazz
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#10
Aug20-09, 07:49 PM
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I would suggest checking your ball joints, especially if you have wider and/or taller tires than normal stock. Then inspect your toe-in. Nothing like that has occured on my Cherokee
mg0stisha
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#11
Aug20-09, 10:33 PM
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When i used to skateboard when i was younger this used to happen also. I don't know if that helps in thinking what it could be, but its normally when the trucks were too loose, there for one little wrong movement would multiply very quickly and turn into the all to infamous 'speed wobbles'. Thats the only reason i could think for it, except bushing problems. Does this bring about anything that could be loose around the steering gear, or maybe a bushing thats too soft?
Danger
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#12
Aug20-09, 10:49 PM
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That skateboard thing reminds me of something that I never thought of the first time around.
When I was flying, and during some mechanical endeavours, we dealt with what, at the time, were referred to as 'overcontrolled oscillations'.
For instance, if you were cruising along and your left wing dipped, it was possible to feed in too much right stick, then too much left to compensate for that, and so on until you're tits-up in a field.
I wonder if that might have something to do with this steering problem. Thanks for the reminder, MgO.
mg0stisha
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#13
Aug20-09, 10:56 PM
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no problem, it sounds like it all starts with an over-correction. i'm wondering if there's a similar problem in the suspension. for example, you hit a pot hole, tire goes down. then, you come out of the pot hole and your tire lifts up too much. when that tire hits the ground again it causes the other to go up and then it intensifies quickly? this could also be 100% incorrect
Danger
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#14
Aug20-09, 11:05 PM
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Quote Quote by mg0stisha View Post
this could also be 100% incorrect
Could be; maybe not. Since I have no experience with solid axles, I can't comment upon that. It makes sense to me in basic terms, but someone who is more of a vehicular expert should probably weigh in. (Where's Stingray when you really need him?)
Les Lord
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#15
Aug21-09, 08:35 PM
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I really want to keep the lift because I actually do take it offroad here in Utah a lot. I really wanted to know if the steering dampener was put there by the manufacture to cure a design problem I guess.

I think mgOstisha and Danger are on to something though. I found on the net that even trains can experience this, called hunting. There wheels are coned shape so that when the train tries to veer right on the tracks, the diameter where the right wheel contacts the tracks increases and the left wheel decreases, cause the train to try and go back left. At certain speeds this can get out of control.

Here is what I came up with a while back...

I know that the wheels shake violently side to side, and it can start from a small bump and get worse in just a second. One reason it interests me is because I work on vehicle for a living and have seen it a number of times.

I've been trying to find info on the internet, but can only find what people say to check and do to fix the problem, but not the science behind it.

The following is my theory. I maybe partly wrong or totally wrong. First I want to explain a few things.

Camber is this tilt of the tire at the top either in towards the vehicle(negative), out(positive), or 0 deg.(straight up down). Caster is the forward or rearward tilt of the steering axis(imaginary line drawn through the upper and lower ball joints).

Caster is used for directional stability. Caster will normally be positive, upper ball joint behind the lower ball joint. If you turn the R.F. tire right it will be trying to lift the R.F. of the vehicle so it will try to turn back to the left. The L.F. tire will be doing the same. That is what gives directional stability and helps the steering wheel return to center after a turn. (This will also cause a pull if caster is higher on one side than the other. It will pull to the lower side.) You can demonstrate this w/ a coat hanger. Imagine the corners of the coat hanger are the ball joints and the hook part is the spindle. If you tilt it back and then pivot the coat hanger at the ball joints, turning one way, the the spindle will move down therefore trying to lift the vehicle. You can actually see this if you look closely at vehicle w/ a lot of caster that is stationary and turns the wheels all the way in one direction. Trucks, Jeeps, SUVs, etc have higher caster normally than cars. Another thing that happens because of caster is that camber changes when you turn. This is how caster is actually measured when alignments are done. The person doing the alignment turns the wheels a certain amount each way and the computer measures the camber change. It then calculates caster.

So what I think happens with death wobble is that first of you hit a small bump, and the tire turns right just a little. Then camber also increases in the R.F. tire, and decreases on the L.F. meaning both tires are leaning right. So now it is causing both tires to pull right and store more energy. Then the effect of the caster causes the tires to return to center and go past center to the left and the cycle continues. The only way to stop this cycle is to dampen the energy. One way is through the steering suspension. There is friction in the ball joints and steering linkage. Also the gear box, but not very effective because of the indirect connection(unlike rack and pinion), and even less effective in lifted vehicles because of the greater angle. Another is the steering dampener. It is just a shock. It takes the kinetic energy and changes it to heat energy. Also lowering caster. But lowering it too much will cause wonder. By the way Ford also has the same problem on their solid axle trucks and recommends lowering caster but still keeping it within spec and changing the steering dampener. I ran into this recently on a Jeep with only 12K miles. I still checked the suspension and everything was good. The only thing he had done was different wheels and tires and a 2" spacer lift. He decided to take it where the lift and tires were done though and have them look at it.
Les Lord
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#16
Aug21-09, 08:37 PM
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Quote Quote by wmazz View Post
I would suggest checking your ball joints, especially if you have wider and/or taller tires than normal stock. Then inspect your toe-in. Nothing like that has occured on my Cherokee
I checked all the ball-joints and bushings, and aligned it so that toe is in spec. Did I mention that since replacing the steering shock, I have not had a problem.
Danger
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#17
Aug21-09, 09:34 PM
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Quote Quote by Les Lord View Post
Did I mention that since replacing the steering shock, I have not had a problem.
I don't believe so, although I might have missed it. (Very long posts and over a dozen beers don't play nice together. )
It is certainly a very important clue toward a full diagnosis. I'm afraid that I'm in over my head now. I'll continue to monitor the thread, but I think that my usefulness has come to an end.
Les Lord
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#18
Aug22-09, 12:43 AM
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I started thinking, what if a whole bunch of people removed there steering dampeners from their Jeep Cherokees, I wonder how many would end up having death wobble issues? Hmmm.... BTW, just a thought, I'm not suggesting that anyone do this. :) I better get to sleep.


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