Intelligent Question About Death Wobble - Page 7 - JeepForum.com
Search  
Sign Up   Today's Posts
User: Pass: Remember?
Advertise Here
Jeep Home Jeep Forum Jeep Classifieds Jeep Registry JeepSpace Jeep Reviews Jeep Gallery Jeep Clubs Jeep Groups Jeep Videos Jeep Events Jeep Articles
Go Back JeepForum.com > Models > Jeep Wrangler Forums > TJ Wrangler Technical Forum > Intelligent Question About Death Wobble

Introducing MONSTALINER™ UV Permanent DIY Roll On Bed LineZone Offroad Body and Suspension Lift Kits, Brackets, DiscBilstein Shock Upgrade Kits, Jeep JK TJ XJ Wranglers and C

Reply
Unread 12-31-2013, 11:44 PM   #91
Necromancer_tat
Where'd all my money go?
 
Necromancer_tat's Avatar
2001 TJ Wrangler 
 
Join Date: Jul 2011
Location: Norfolk, Virginia
Posts: 5,092
Quote:
Originally Posted by SlackJaw View Post
Hmm...I think if you removed precession from the equation, death wobble would be an entirely different experience...maybe less "deathly" lol. It would certainly not sustain itself and could probably be turned off as quickly as it comes on say, with a quick jerk of the wheel, or perhaps hitting the next bump in the road.
That's like saying a guitar string would stop vibrating because you plucked it again.

__________________
TJ: http://www.jeepforum.com/forum/f22/p...0/index14.html

2006 LJ SOLD!
Necromancer_tat is offline   Reply With Quote
Unread 01-01-2014, 12:04 AM   #92
SlackJaw
baby jeep
 
SlackJaw's Avatar
2004 TJ Wrangler 
 
Join Date: Sep 2011
Location: Tulsa, OK
Posts: 5,178
Quote:
Originally Posted by Necromancer_tat View Post

That's like saying a guitar string would stop vibrating because you plucked it again.
No. Put an e-bow on it. That would be more analagous to precession as applied to DW. I would prefer to use a piano string and the term "percussed" and then you might see that it is possible. If we're going to maintain musicality as a reference point here, then I submit to you that the road and NVH are far too dissonant to maintain any single tone. It's more or less pink noise. And therefore the concept of cancelling a note with a single, well placed, strike becomes pointless.

The kind of "harmonics" you would find in a Jeep suspension would be at the imperceptible ends of the spectrum were it not for the resistance applied by precession of the rotating mass.

Without precession, death wobble would be "death clunk" or "death jerk"

Precession is required for full blown DW. The speed at which it is triggered might be a result of the "harmonics" you refer to, but the precession is what maintains the violence until you slow to such a speed that the stability of the system is greater than the force of the precession. Many times that is 10mph or less. If precession WERENT a factor, you should be able to kick off the wobbs dropping your kid off in the school parking lot.
SlackJaw is offline   Reply With Quote
Unread 01-01-2014, 06:53 AM   #93
mrblaine
Wizard of Brakes
 
mrblaine's Avatar
1999 TJ Wrangler 
 
Join Date: Dec 2004
Location: Quail Valley, California
Posts: 25,872
Quote:
Originally Posted by Necromancer_tat View Post
I still don't buy the precession thing... harmonics, resonance, un-dampened oscillation, make much more sense to me, and they also fall more in line with tightening and loosening various components allowing, or clearing up DW.
I'm not sure you have to buy it, but you can't say it isn't there. All tire shaped objects exhibit precession when they spin, it is physics.

Watch this and try to wrap your head around the angular momentum. In the video, to produce the same results in your axle that produces the up and down movement of each tire in opposite unison, you would be steering by rotating the axle around a center pivot like a toy wagon. Translate that into worn control arm bushings that hold the axle without letting it move to give you can idea of what forces we are dealing with.

Precession causes the angular momentum which causes one side of the tire to lift, the other to drop and that's what we are dealing with and controlling.

__________________
I am Savvy
blackmagicbrakes.com
mrblaine is online now   Reply With Quote
Unread 01-01-2014, 07:14 AM   #94
Necromancer_tat
Where'd all my money go?
 
Necromancer_tat's Avatar
2001 TJ Wrangler 
 
Join Date: Jul 2011
Location: Norfolk, Virginia
Posts: 5,092
Quote:
Originally Posted by mrblaine View Post
I'm not sure you have to buy it, but you can't say it isn't there. All tire shaped objects exhibit precession when they spin, it is physics.

Watch this and try to wrap your head around the angular momentum. In the video, to produce the same results in your axle that produces the up and down movement of each tire in opposite unison, you would be steering by rotating the axle around a center pivot like a toy wagon. Translate that into worn control arm bushings that hold the axle without letting it move to give you can idea of what forces we are dealing with.

Precession causes the angular momentum which causes one side of the tire to lift, the other to drop and that's what we are dealing with and controlling.

http://www.youtube.com/watch?v=vGun5athdfg
I'll agree that the tires and axle do experience precession, that's a given based on the fact that they're basically spinning gyroscopes. I don't think Precession is the leading culprit in DW though, mainly because precession becomes greater as the spinning gyroscope/top/bicycle tire etc, slows down. DW usually happens as the vehicle is accelerating, and sometimes at a constant speed, but I haven't heard of many cases where someone couldn't slow down or they'd get DW. If precession was the main cause of DW I would think that the wobble would get progressively worse as the vehicle slowed down.
__________________
TJ: http://www.jeepforum.com/forum/f22/p...0/index14.html

2006 LJ SOLD!
Necromancer_tat is offline   Reply With Quote
Unread 01-01-2014, 07:27 AM   #95
mrblaine
Wizard of Brakes
 
mrblaine's Avatar
1999 TJ Wrangler 
 
Join Date: Dec 2004
Location: Quail Valley, California
Posts: 25,872
Quote:
Originally Posted by Necromancer_tat View Post
I'll agree that the tires and axle do experience precession, that's a given based on the fact that they're basically spinning gyroscopes. I don't think Precession is the leading culprit in DW though, mainly because precession becomes greater as the spinning gyroscope/top/bicycle tire etc, slows down. DW usually happens as the vehicle is accelerating, and sometimes at a constant speed, but I haven't heard of many cases where someone couldn't slow down or they'd get DW. If precession was the main cause of DW I would think that the wobble would get progressively worse as the vehicle slowed down.
I have a buddy who won a very large settlement from Kawasaki many years ago. He was on the freeway with his bike and when he slowed down for an off ramp, the front fork started wobbling. He missed the ramp and sped up until it quit. Tried to slow again, rinse and repeat. He finally bailed off at 80mph and after a lengthy recovery, got in touch with a lawyer to help find out what happened. Turns out they installed the wrong size bushings in the pivot area.

I think you're hung up on semantics. The cause of DW is the inability to control precession. Without precession, there would be no DW.

I would never say that DW happens when accelerating. I've dealt with too many cases of it where a simple bump in a slight turn set it off with no other symptoms of it beforehand. Also look at the various speeds it happens at.

If you watch for it, most of the lower onset speed cases around 43-45 mph are typically trackbar related. Move up a bit higher in the range to around 50ish and those are usually upper control arm related.

I've had one case of my own where it would only start trying to go into DW at 60+ when going around slight turns on the freeway. It wouldn't exhibit any signs or hints accelerating through that speed unless you were in a slight turn.
__________________
I am Savvy
blackmagicbrakes.com
mrblaine is online now   Reply With Quote
Unread 01-01-2014, 07:28 AM   #96
SlackJaw
baby jeep
 
SlackJaw's Avatar
2004 TJ Wrangler 
 
Join Date: Sep 2011
Location: Tulsa, OK
Posts: 5,178
Quote:
Originally Posted by Necromancer_tat View Post

I'll agree that the tires and axle do experience precession, that's a given based on the fact that they're basically spinning gyroscopes. I don't think Precession is the leading culprit in DW though, mainly because precession becomes greater as the spinning gyroscope/top/bicycle tire etc, slows down. DW usually happens as the vehicle is accelerating, and sometimes at a constant speed, but I haven't heard of many cases where someone couldn't slow down or they'd get DW. If precession was the main cause of DW I would think that the wobble would get progressively worse as the vehicle slowed down.
You are looking at this wrong. The top is wobbling as precession decreases. The wobble in DW is not the same wobble that top exhibits. Rather spin that top as fast as you can, it's nice and stable, now give it a tap on one side. THAT'S the wobble in DW. The top (wheels) wants to right itself but the tap (instability in the system) throws it out of balance. It is the constant precessional force that keeps the wobbling rhythmic. If it weren't there, the top would just fall over the instant you tapped it.
SlackJaw is offline   Reply With Quote
Unread 01-01-2014, 08:01 AM   #97
Necromancer_tat
Where'd all my money go?
 
Necromancer_tat's Avatar
2001 TJ Wrangler 
 
Join Date: Jul 2011
Location: Norfolk, Virginia
Posts: 5,092
Quote:
Originally Posted by mrblaine View Post
I have a buddy who won a very large settlement from Kawasaki many years ago. He was on the freeway with his bike and when he slowed down for an off ramp, the front fork started wobbling. He missed the ramp and sped up until it quit. Tried to slow again, rinse and repeat. He finally bailed off at 80mph and after a lengthy recovery, got in touch with a lawyer to help find out what happened. Turns out they installed the wrong size bushings in the pivot area.

I think you're hung up on semantics. The cause of DW is the inability to control precession. Without precession, there would be no DW.

I would never say that DW happens when accelerating. I've dealt with too many cases of it where a simple bump in a slight turn set it off with no other symptoms of it beforehand. Also look at the various speeds it happens at.

If you watch for it, most of the lower onset speed cases around 43-45 mph are typically trackbar related. Move up a bit higher in the range to around 50ish and those are usually upper control arm related.

I've had one case of my own where it would only start trying to go into DW at 60+ when going around slight turns on the freeway. It wouldn't exhibit any signs or hints accelerating through that speed unless you were in a slight turn.
I agree that it's semantics. The process of dealing with the end result is the same regardless of the initial cause.

I've had DW occur at 75mph in a rig that had hydro assist, and everything in the steering and suspension was so tight that if you moved the steering wheel 1/8th of an inch, you could watch the tires move. It rode awesome and handled perfectly even at 75 mph, but there are a few sections of the highway between my house and work that have diagonal bumps where they change from asphalt to concrete and back again, and when I would hit those at 75 it felt like the front end was going to explode. The only thing I could trace it back to is either the frame itself was flexing, or the track bar was to skinny.

A local dealership nearby is really supportive of the off road community and has a nice off road park we can play on behind the dealership. I was talking to the service manager and one of the local off road club presidents about DW one day, and they were telling me about a case where everything was brand new, the tires were perfectly balanced, and they couldn't get rid of the DW except by putting different tires on the rig. So they took the perfectly balanced tires that came off, and just to see if they were the cause they installed them on a brand new show room Jeep and then took it for a test drive. Sure enough, the brand new Jeep got DW with the tires that were supposedly perfectly balanced. It turned out the was a defect in one of the tires, so even though it balanced out, it had some other issue with it's structural stability. I don't remember what they said it was but a bad belt is what pops in mind.

I think we all agree that DW is allowed to happen by some component being loose somehow. The tighter the system is the higher up the speed range DW will occur, and the looser the system is the lower the speed it will happen at down until everything is so loose that there isn't enough energy in the system to cause DW anymore.
__________________
TJ: http://www.jeepforum.com/forum/f22/p...0/index14.html

2006 LJ SOLD!
Necromancer_tat is offline   Reply With Quote
Unread 01-01-2014, 08:13 AM   #98
Necromancer_tat
Where'd all my money go?
 
Necromancer_tat's Avatar
2001 TJ Wrangler 
 
Join Date: Jul 2011
Location: Norfolk, Virginia
Posts: 5,092
Quote:
Originally Posted by SlackJaw View Post
You are looking at this wrong. The top is wobbling as precession decreases. The wobble in DW is not the same wobble that top exhibits. Rather spin that top as fast as you can, it's nice and stable, now give it a tap on one side. THAT'S the wobble in DW. The top (wheels) wants to right itself but the tap (instability in the system) throws it out of balance. It is the constant precessional force that keeps the wobbling rhythmic. If it weren't there, the top would just fall over the instant you tapped it.
Gyroscopic force is what keeps the top from falling over the instant you tap it. Your tap adds angular momentum into the system, and precession will occur. Precession would also occur if the top was left to spin all by itself as the top slows down it will start to spin in a cone shaped wobble that will grow larger and more pronounced as the top slows down. This is due to the top having less gyroscopic stability and the effect of gravity pulling down on it thus adding angular momentum to the system the same way as if you tapped it, but on a gentler scale.

Tires on an axle are spinning vertically not horizontally, so the forces on them are going to be turning one way or the other, which could be seen as gravity, or someone flicking a spinning top.

A spinning tire will see a huge angular momentum event every time you change directions. Any steering input into the system at all is a giant addition of outside influence into the natural gyroscopic forces of a spinning tire.

The Gyroscopic forces are what holds a bicycle and rider upright as they pedal down the road. If Precession was such a strong force it would be impossible to turn the handlebars without the whole front end wobbling out of control.
__________________
TJ: http://www.jeepforum.com/forum/f22/p...0/index14.html

2006 LJ SOLD!
Necromancer_tat is offline   Reply With Quote
Unread 01-01-2014, 08:15 AM   #99
SlackJaw
baby jeep
 
SlackJaw's Avatar
2004 TJ Wrangler 
 
Join Date: Sep 2011
Location: Tulsa, OK
Posts: 5,178
Noted, yes I was interchanging/wrong use the concept remains and yes this is entirely academic.
SlackJaw is offline   Reply With Quote
Unread 01-01-2014, 08:23 AM   #100
Necko2529
Registered User
2003 TJ Wrangler 
 
Join Date: May 2011
Location: Brookfield, Illinois
Posts: 2,547
This is actually turning into an intelligent and informative thread.

Slack, hows the swap going?

Happy Nee Year, all.
__________________
Rokmen TT, Rokmen control arms, Novak NV241 cable shifter, JKS 1.25" BL + 1" MML.

Anybody can take advice if it doesn't interfere with their plan.

[B]RIP "Prescott 19"[/B] Prescott Fire Department
Necko2529 is offline   Reply With Quote
Unread 01-01-2014, 09:37 AM   #101
mrblaine
Wizard of Brakes
 
mrblaine's Avatar
1999 TJ Wrangler 
 
Join Date: Dec 2004
Location: Quail Valley, California
Posts: 25,872
Quote:
Originally Posted by Necromancer_tat View Post
I agree that it's semantics. The process of dealing with the end result is the same regardless of the initial cause.

I've had DW occur at 75mph in a rig that had hydro assist, and everything in the steering and suspension was so tight that if you moved the steering wheel 1/8th of an inch, you could watch the tires move. It rode awesome and handled perfectly even at 75 mph, but there are a few sections of the highway between my house and work that have diagonal bumps where they change from asphalt to concrete and back again, and when I would hit those at 75 it felt like the front end was going to explode. The only thing I could trace it back to is either the frame itself was flexing, or the track bar was to skinny.

A local dealership nearby is really supportive of the off road community and has a nice off road park we can play on behind the dealership. I was talking to the service manager and one of the local off road club presidents about DW one day, and they were telling me about a case where everything was brand new, the tires were perfectly balanced, and they couldn't get rid of the DW except by putting different tires on the rig. So they took the perfectly balanced tires that came off, and just to see if they were the cause they installed them on a brand new show room Jeep and then took it for a test drive. Sure enough, the brand new Jeep got DW with the tires that were supposedly perfectly balanced. It turned out the was a defect in one of the tires, so even though it balanced out, it had some other issue with it's structural stability. I don't remember what they said it was but a bad belt is what pops in mind.

I think we all agree that DW is allowed to happen by some component being loose somehow. The tighter the system is the higher up the speed range DW will occur, and the looser the system is the lower the speed it will happen at down until everything is so loose that there isn't enough energy in the system to cause DW anymore.
I don't agree with that at all and your example clearly disproves that theory. Slightly semantical, but it occurs when the system lacks the ability to control the forces that are inherent to spinning tires. Remember, no one ever got DW from sitting still. Nothing has to be loose or worn for DW to occur.

I also strongly believe that you are way underestimating the angular momentum a spinning 35" tire has at 50 mph.

As to your 75mph example, what happened if you went through at 65? I suspect as the angular momentum decreased, the likelihood of your DW also decreased.
__________________
I am Savvy
blackmagicbrakes.com
mrblaine is online now   Reply With Quote
Unread 01-01-2014, 09:38 AM   #102
mrblaine
Wizard of Brakes
 
mrblaine's Avatar
1999 TJ Wrangler 
 
Join Date: Dec 2004
Location: Quail Valley, California
Posts: 25,872
Quote:
Originally Posted by Necko2529 View Post

Slack, hows the swap going?
A few more visits and her gender swap will be done. We've all got our fingers crossed.
__________________
I am Savvy
blackmagicbrakes.com
mrblaine is online now   Reply With Quote
Unread 01-01-2014, 09:48 AM   #103
Necko2529
Registered User
2003 TJ Wrangler 
 
Join Date: May 2011
Location: Brookfield, Illinois
Posts: 2,547
Quote:
Originally Posted by mrblaine View Post
A few more visits and her gender swap will be done. We've all got our fingers crossed.
Am i missing something? I've seen you crack a joke at it not too long ago. The thread went dead and i lost track...
__________________
Rokmen TT, Rokmen control arms, Novak NV241 cable shifter, JKS 1.25" BL + 1" MML.

Anybody can take advice if it doesn't interfere with their plan.

[B]RIP "Prescott 19"[/B] Prescott Fire Department
Necko2529 is offline   Reply With Quote
Unread 01-01-2014, 10:09 AM   #104
mrblaine
Wizard of Brakes
 
mrblaine's Avatar
1999 TJ Wrangler 
 
Join Date: Dec 2004
Location: Quail Valley, California
Posts: 25,872
Quote:
Originally Posted by Necko2529 View Post
Am i missing something? I've seen you crack a joke at it not too long ago. The thread went dead and i lost track...
You said that wrong. Will he be missing something?



















Just messing with him. Gotta start the new year off right ya know. Happy New Year to everyone!
__________________
I am Savvy
blackmagicbrakes.com
mrblaine is online now   Reply With Quote
Unread 01-01-2014, 10:23 AM   #105
Necromancer_tat
Where'd all my money go?
 
Necromancer_tat's Avatar
2001 TJ Wrangler 
 
Join Date: Jul 2011
Location: Norfolk, Virginia
Posts: 5,092
Quote:
Originally Posted by mrblaine View Post
I don't agree with that at all and your example clearly disproves that theory. Slightly semantical, but it occurs when the system lacks the ability to control the forces that are inherent to spinning tires. Remember, no one ever got DW from sitting still. Nothing has to be loose or worn for DW to occur.

I also strongly believe that you are way underestimating the angular momentum a spinning 35" tire has at 50 mph.

As to your 75mph example, what happened if you went through at 65? I suspect as the angular momentum decreased, the likelihood of your DW also decreased.
Yes, as my speed was lowered to 65 mph, I never again had any kind of wobble at all going over the same section of bumps on the highway. I agree that with greater speed the angular momentum increases when hitting something that makes the tires deflect. That's why in other motorsports circles it's called "speed wobble" instead of "death wobble" but Precession by itself doesn't sit well with me when I picture the entire ovrall event that is DW, I'm sure it does play a part, but I don't think its as significant as you hold it to be.

Picture a man on a unicycle, he's riding down the road and hits a bump that bounces him one way or the other... if he's skilled at riding he'll just cope with the bounce, lean whichever direction he needs to correct things, and keep pedaling down the road. If precession had such a violent effect as you're hypothesizing, the unicycle rider would go into violent shakes every time he hit an uneven surface.
__________________
TJ: http://www.jeepforum.com/forum/f22/p...0/index14.html

2006 LJ SOLD!
Necromancer_tat is offline   Reply With Quote




Jeep, Wrangler, Cherokee, Grand Cherokee, and other models are copyrighted and trademarked to Jeep/Chrysler Corporation. JeepForum.com is not in any way associated with Jeep or the Chrysler Corp.