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Unread 06-25-2013, 03:05 PM   #16
matteo92065
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Quote:
Originally Posted by SlackJaw View Post
Push it past 70 and you should be able to find another magic speed for a even wilder ride
Funny!
But the Jeep and I both lack the 'eggs'.

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Unread 06-25-2013, 03:07 PM   #17
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I've thought about it but I think it's pointless......at least I don't think I'll ever be able to come up with a perfect formula, or even close to it. I'll continue to do what I do, which has worked so far.....that's find the problem through a methodical, systematic approach, fix that problem, and use good components to keep the axle constrained. It doesn't matter what speed I go or how balanced or unbalanced my tires are. Things feel good.
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Unread 06-25-2013, 06:17 PM   #18
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Originally Posted by JEK3 View Post
I noticed a temperature dependence with mine also. I believe it is because the CA bushings are stiffer when they're colder.
Remember when I said "you know nothing"? This is precisely why. What I have not seen mentioned is what happens to tire inflation pressures at different temperatures and how that affects the spring rate of the tire. I have also not seen any mention of the resonant frequency change in the front suspension as the weight transfer happens going up or down hills.

When I say "you know nothing", it also means I know nothing. The more I learn about DW, the less I am convinced I know.

For every single point, there is an equally valid counterpoint which refutes the point making both null and void.
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Unread 06-25-2013, 06:24 PM   #19
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Originally Posted by mrblaine View Post

Remember when I said "you know nothing"? This is precisely why. What I have not seen mentioned is what happens to tire inflation pressures at different temperatures and how that affects the spring rate of the tire. I have also not seen any mention of the resonant frequency change in the front suspension as the weight transfer happens going up or down hills.

When I say "you know nothing", it also means I know nothing. The more I learn about DW, the less I am convinced I know.

For every single point, there is an equally valid counterpoint which refutes the point making both null and void.
Interesting comment. I have often thought I felt a difference in how things behaved as my tires warned up. I've always run radials though and thought that was more of a bias ply issue.
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Unread 06-25-2013, 07:30 PM   #20
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Originally Posted by mrblaine View Post
The more I learn about DW, the less I am convinced I know.
In that case, perhaps all he needs is a $20 steering stabilizer.
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Unread 06-25-2013, 07:47 PM   #21
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Originally Posted by biffgnar View Post
Interesting comment. I have often thought I felt a difference in how things behaved as my tires warned up. I've always run radials though and thought that was more of a bias ply issue.
It's a complex concept that you have to ponder a bit. As the tires warm up, the inflation pressures rise, but the rubber becomes more compliant. Both in the tire and in any joints that have it.

As to why I've considered that has to do with a buddy's rig. It's a very nice Unlimited, I've built most of it with new Currie and Savvy all around with the exception of the stock fuel tank and factory belly skids.

It currently wears toy status and isn't daily driven. I swapped out the beadlock bolts in a set of Racelines when it was cooler several months back and that simple act rendered it nearly undrivable so he parked it. He wanted to start driving it again, so on his way to the tire store several weeks later to have the tire balance checked, there was zero wobble, zero shimmy.


The only changes between then and now are direction and temperature. On his way home, he climbed a large grade for several miles in cooler temps, on his way to the tire store it is downhill and much warmer.

We know nothing.

A set of problematic 37" GY was removed from one set of rims and put on another set of rims in two different cities, one was balanced and nearly undriveable, they are just fine in their unbalanced state on the other rig. Both are Unlimiteds.
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Unread 06-25-2013, 07:58 PM   #22
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I had a 97. I sold it because of the DW. Everything in the front end was changed. Some things twice. It would only happen on right turns going down a hill. I've had many Jeeps but this one had me and a few others stumped. I got tired of dumping money into it and sold it. Everytime I put on new parts I thought I had it fixed. maybe 2-3 weeks would go by with no DW. Then when you wasnt ready it would do it's dance.
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Unread 06-25-2013, 08:01 PM   #23
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Originally Posted by mrblaine View Post
Remember when I said "you know nothing"? This is precisely why. What I have not seen mentioned is what happens to tire inflation pressures at different temperatures and how that affects the spring rate of the tire. I have also not seen any mention of the resonant frequency change in the front suspension as the weight transfer happens going up or down hills.

When I say "you know nothing", it also means I know nothing. The more I learn about DW, the less I am convinced I know.

For every single point, there is an equally valid counterpoint which refutes the point making both null and void.
Mr Blaine,

I was just going to mention the relation between frequency, stiffness and mass...
The specific relation is:
Critical Frequency = sqrt(stiffness/mass)...(sorry I can't use proper engineering notation here)

While the mass may not vary while driving a specific vehicle, (well, you could have mud accumulated, or throw a wheel weight), the mass can and does vary from vehicle to vehicle, and with variations in wheels and tires.
And while it is true that the critical frequency is dependent on the mass of the moving component, the moving componet (axle, wheels, tires, steering linkage) is all tied to the vehicle...the mass of which varies from vehicle to vehicle (think hard top/soft top, winch, passengers, engine, etc).

Now, let's look at the stiffness term. As you pointed out, variations are present with respect to tire pressure, loaded or unloaded suspension, tire stiffness (sidewall height, number of belts, tire design).
But wait, there is the variations in stiffness of the control arms, their bushings, the all important track bar, ball joints...even the knuckles themselves.

With all these variables, some of which are non repeatable, it's no wonder that DW (under damped or unstable, or inadequate log decrement, in my world) is so difficult to predict.

If I had the time, I would like to take FFT measurements of the axle motions with an accelerometer, from a wide range of vehicles, with their various tire sizes, lift springs, worn bushings, etc.
Maybe then, a near universal solution can be created.

From my professional perspective, all of this DW discussion is fascinating.
As a Jeep owner who had DW at 70 mph, I also find this terrifying...

And finally, one more point and I'll stop blathering:

A wise man knows what he'll never know,
A fool knows everything.
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Unread 06-25-2013, 08:08 PM   #24
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Originally Posted by stripperguy View Post
And finally, one more point and I'll stop blathering:

A wise man knows what he'll never know,
A fool knows everything.
Where is Slackjaw to read and live this?
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Unread 06-25-2013, 08:16 PM   #25
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Originally Posted by biffgnar View Post

Where is Slackjaw to read and live this?
Hey I never said I know everything, just that there are a finite number of variables. Shimmy more mysterious, D/W not so much. If Blaine (we) "knows nothing" then why do the JFTJ parrots keep yapping the old "steering stabilizer only masks the real problem" mantra? Seemed a good enough bulletin for Chrysler engineers who designed our plastic radiators with mechanical fans.

If my shimmy persists after JJ rebuild and new rotors on all 4 corners, I'm putting a SS on. EVERYTHING else is up to snuff. Care to wager the SS solves it?

Side note I am working on hypothesis that after a certain point of lift, that mid/long arm setup may be necessary to insure that possibility for D/W decreases.
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Unread 06-25-2013, 09:32 PM   #26
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Hey I never said I know everything, just that there are a finite number of variables.
That's a cute cop-out. Last I check, 100, 1000, 10,000 were all finite numbers and if the factors that influence DW are any where near even the smaller of them, it's still fairly mind boggling.

Quote:
Shimmy more mysterious, D/W not so much. If Blaine (we) "knows nothing" then why do the JFTJ parrots keep yapping the old "steering stabilizer only masks the real problem" mantra?
If we're going to continue down this path of amicable intercourse, you're gonna have to hop down off that high horse of yours about JFTJ. I dare you to post the same DW question on 50 TJ specific forums without several responders mentioning something about DW being fixed by their steering stabilizer replacement, alignment settings, and tire balance.

If we parrot in that vein, then I fixed my buddies rig by manipulating the seasons.


Quote:
Seemed a good enough bulletin for Chrysler engineers who designed our plastic radiators with mechanical fans.
Do you ever don't doublespeak?

Quote:
If my shimmy persists after JJ rebuild and new rotors on all 4 corners, I'm putting a SS on. EVERYTHING else is up to snuff. Care to wager the SS solves it?
Everyone should run a stabilizer. If it prevents one instance of DW and the resultant damage, it's worth it.

Quote:
Side note I am working on hypothesis that after a certain point of lift, that mid/long arm setup may be necessary to insure that possibility for D/W decreases.

That number would have to be in a realm that would be ridiculous to run anyway. I have experience with far too many rigs over the years on 4" of lift and short arms to believe otherwise.
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Unread 06-25-2013, 09:41 PM   #27
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You know full well the finite number of components in play here is closer to 10 rather than 100.

Not double speak - we give godly credit to Chrysler for the plastic radiator and mechanical fan, but we poo poo them on the D/W bulletin, even though for very many the bulletin solved their problem.

If you say "we know nothing" then that should include our thinking of the SS as well shouldnt it? If not, then "we know something".

As stated my long arm idea is still a hypothesis, I have not evaluated the angles, but it shouldn't be difficult to do a little math and then perhaps estimate a compromisational length for theoretical purposes. Jmo. You may be correct, I have not done the observational diligence on it yet.
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Unread 06-25-2013, 10:27 PM   #28
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You know full well the finite number of components in play here is closer to 10 rather than 100.
Actually I don't know that. I've been around far too many cases of it where the cause and effect were too tenuous to readily define.

I know what the root cause is but from there the causation and exhibited symptoms are too varied. I also strongly believe there is a correlation between damping from the shocks and instances of DW that hasn't been explored nearly enough.

Quote:
Not double speak - we give godly credit to Chrysler for the plastic radiator and mechanical fan, but we poo poo them on the D/W bulletin, even though for very many the bulletin solved their problem.
Whatever.

Quote:
If you say "we know nothing" then that should include our thinking of the SS as well shouldnt it? If not, then "we know something".
I say we know nothing because what works on one rig doesn't work on another even though both exhibit the same symptoms. It doesn't matter how acutely you try and dig into the nuance of the equipment either because that information is typically filtered by the mechanical aptitude of the owner.

Quote:
As stated my long arm idea is still a hypothesis, I have not evaluated the angles, but it shouldn't be difficult to do a little math and then perhaps estimate a compromisational length for theoretical purposes. Jmo. You may be correct, I have not done the observational diligence on it yet.
When faced with trying to distill information down into a hypothetical, I run at it from the other direction and I've done that many times to prove or disprove DW theories.

It's touted a bunch that toe settings affect DW. I've taken several rigs and set the toe on them to many angles and attempted to induce DW. If you can induce something and then make it go away, it is very easy at that point to make statements of certainty. Until then, it's all speculation.
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Unread 06-25-2013, 11:06 PM   #29
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Speaking of math. I took two control systems classes and one of the typical problems was a simple screen door with a spring and dashpot to control closure. It took at least a page of probably the highest level of math to optimize just that small simple system. I started thinking how cool it would be to analyze the front end of the TJ, but instantly decided I'd need some serious free time to relearn about half of the math and then probably about 3 months of evenings to probably make make any headway or realize I was in over my head on the devotion to get to the end. Being that you'd need to do the "math" for every connection that has potential to flex, I'd guess it would take about 50 pages of math I haven't done in 15 years. The guy who taught the classes was a Nuclear Physicist and could probably solve it in about 1/3 the pages, he had all kinds of ways to simplify stuff. I only wish I would have paid more attention and/or gotten a job out of college to actually use the stuff.

Also, wouldn't know any of the constants for the spring/dampeners anywhere in the system, and that would be probably double the work if I even had a way to try to measure it all myself, likely not. Not saying it couldn't be done, but not even a mediocre task for someone who does it every day like the Engineers who designed the system and obviously missed something or chose to ignore it.
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Unread 06-25-2013, 11:12 PM   #30
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Speaking of math. I took two control systems classes and one of the typical problems was a simple screen door with a spring and dashpot to control closure. It took at least a page of probably the highest level of math to optimize just that small simple system. I started thinking how cool it would be to analyze the front end of the TJ, but instantly decided I'd need some serious free time to relearn about half of the math and then probably about 3 months of evenings to probably make make any headway or realize I was in over my head on the devotion to get to the end. Being that you'd need to do the "math" for every connection that has potential to flex, I'd guess it would take about 50 pages of math I haven't done in 15 years. The guy who taught the classes was a Nuclear Physicist and could probably solve it in about 1/3 the pages, he had all kinds of ways to simplify stuff. I only wish I would have paid more attention and/or gotten a job out of college to actually use the stuff.

Also, wouldn't know any of the constants for the spring/dampeners anywhere in the system, and that would be probably double the work if I even had a way to try to measure it all myself, likely not. Not saying it couldn't be done, but not even a mediocre task for someone who does it every day like the Engineers who designed the system and obviously missed something or chose to ignore it.
I've chatted with Jim about this and they have some serious computer programs than can't even come up with a way to make sure it won't happen right out of the box. The best they can do is a close approximation and then go test it.
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