Death Wobble continuation

[Nik]

Hey fellas I'm starting to getting to understand how this all works. I can tell you it is definantly DW when I hit 35 and a slight dip or bump the jeep bounces around so much that I almost want to sell the damn thing. I have been working on this for over 3 weeks and cant get it straight. I just had the tires all rebalanced. I had the toe and alignment set perfect at a shop specializing in aoffroad alignment in front of my eyes. I have a buddy with a CJ 5 running full width 60's and 40 inch swampers and he can cruise 80 with no vibes. I cant hit 35 without shitting my pants. The shop said the tires seemed to be a little outa round, not much though, so I am getting them replaced under warranty. They are the 39.5x13.50 Irok's. I have 11 inches of lift with major trimming. I have built this rig for over 9 months and cant even get it to roll down the road. It wheels nice but its my only vehicle and wont street ride. I know the tires will go fast on pavement so I dont wanna haer it:). I run DB with adj. LCA'S and UCA'S. Every shop I have trailered it to said it is one of the most well done XJ's they have ever seen. I'm not as extreme as you all are into crawling yet, but I am 16 trying to bild a sick rig to crawl with in the future. Does anyone know anybody I can take this thing to in colorado springs and fix this damn problem, I am getting sick of this problem and have tried a lot to find out what it is. Any other suggestions?

Thanks again

NikNAXJA

[yellowxj]

any pictures of the suspension set up. might give us somewhere to look for problems. $0.02

[RCP Phx]

Not even drop brackets will work at that lift,what are you thinking!Youve got geometry problems everywhere.

[juicexj24]

I would look into a long arm set up. You didn't say if you did or didn't have one. I'm guessing not because of the Adj control arm thing. Using the short arm design on such a tall lift is a major problem. Also the trac bar is another problem. It needs to be dropped if you haven't done that already. Also the the angle of the drag link is something to look at. A dropped pitman arm could help. Also the pinion angle in relation to the steering knuckles. If the knuckles are to far forward your in big trouble as far as getting everything to work right. CJ's and XJs are two different animals when it comes to suspension. So you really can compair with that. Now someone else can chim in. Juice

[Duane]

With that much lift (even with the drop brackets) the angle of the control arms will cause a forward and aft movement of the tire during suspension travel. This creates a condition known as unstable tire patch contact. Positive castor produces what is called trail (tire patch is behind the steering axis) this is what gives you a good directional stability or return to center feeling in the steering,(imagine dragging something behind a boat---it wants to straighten the direction of travel-- this is called the anchor effect) but when the tire patch is not stable, moving forward and back, castor actually creates more instability. this is because as the wheel moves towards the back (extended suspension) the tire patch is on the wrong side of the steering axis and wants to get to the other side. So now when you hit a bump and the tire moves forward and aft this will trigger the harmonic vibe DW (not shimmy). Out of balance tire will cause a shimmy that will cause the tires to rotate left and right this movement will cause the tires to move up and down also because of the steering angle and the castor. This will set off DW. Out of balance tires are a given with large tires and the abuse we put them through. So what do we do. We compromise--we back off the castor setting so that it breaks one of the components in the mechanical system so the massive harmonic vibration of DW does not happen. Back your castor angle off (longer uppper control arms or shorter lower control arms) until the truck is stable. You may not have a good return to center feel, but it is better than DW.

[Eagle]

You still have not put the alignment numbers up for us to look at. Don't mean to pick on you, but you simply could not have "... had the toe and alignment set perfect at a shop specializing in aoffroad alignment in front of my eyes." Toe, maybe. But with 11" of lift, if they set the caster angle where it's supposed to be your u-joint angle at the pinion would be so big that you'd be eating u-joints on a daily basis. Therefore, I suspect they set the caster (which is part of the alignment) to keep the u-joint angle tolerable, and the result is not enough caster.

[rockwerks]

pinion would be so big that you'd be eating u-joints on a daily basis. Hey Eagle maybe that is where the vib iscoming from....I've seen it on CJ's with massive lifts....a nasty vibration in the front and it was actually the U joints binding...Hey NIK is the vibration mainly in your hands or distrubuted evenly over your whole body?
.
Any other suggestions

Drop it down to 7.5 to 8" of lift......10" is so far past the tollernces of this set up.....What I see is happening is the inverted y steering is stretched to its max adjustment and flex is happening at the adjustment locations

What axles are under your XJ with the 39.5x13.5 tires on em?

thats alot of unsprung weight for a dana 30 to try to control...I would bet jsut going down to 37" would really help...how many steering stabalizers do you have on the system?

[Eagle]

Good points, Brian. I was close-focused on just control arm, pinion, and drive shaft angles. You're quite right that in the other plane that much lift is going to do a real number on the steering geometry and components.

[rockwerks]

I feel the only fix might be to replace the entire steering setup to a more standard system........and over knuckle...tie rod that goes from knuckle to knuckle and draglink that sits on top of that...with a misalignment spacer...and a drop pitman arm....over size all the parts 1 1/4" 120 wall tube and use 5/8" heims...that shoud do the trick!

Nik you are gonna have to stop the flex in the steering system itself, and to do that replace it with a custom job!

doesnt someone here have a pic of their setup simular to what IM saying
.
.
here you go this is your fix!
.
.bullet proof steering setup (http://bulletproofsteering.com/steeringsystems.html)

[Eagle]

Don't forget the track bar angle. With 11" of lift, the bloody track bar is sitting almost vertical, so any wobble force from one of those big tires is going to be vectored into a huge vertical component, causing the chassis to bounce up and down while wobbling side-to-side. Gotta do something with the track bar whilst playing around with the drag link and pitman arm.

[MaXJohnson]

Originally posted by Duane
With that much lift (even with the drop brackets) the angle of the control arms will cause a forward and aft movement of the tire during suspension travel. This creates a condition known as unstable tire patch contact. Positive castor produces what is called trail (tire patch is behind the steering axis) this is what gives you a good directional stability or return to center feeling in the steering,(imagine dragging something behind a boat---it wants to straighten the direction of travel-- this is called the anchor effect) but when the tire patch is not stable, moving forward and back, castor actually creates more instability. this is because as the wheel moves towards the back (extended suspension) the tire patch is on the wrong side of the steering axis and wants to get to the other side. So now when you hit a bump and the tire moves forward and aft this will trigger the harmonic vibe DW (not shimmy). Out of balance tire will cause a shimmy that will cause the tires to rotate left and right this movement will cause the tires to move up and down also because of the steering angle and the castor. This will set off DW. Out of balance tires are a given with large tires and the abuse we put them through. So what do we do. We compromise--we back off the castor setting so that it breaks one of the components in the mechanical system so the massive harmonic vibration of DW does not happen. Back your castor angle off (longer uppper control arms or shorter lower control arms) until the truck is stable. You may not have a good return to center feel, but it is better than DW.

Nice to see some fresh thinking around here on this topic. :)

But, you'll have to do a better sell job to get me to visualize my contact patch moving backwards.

If my front tires hit a bump, the suspension is going to compress, moving ever so slightly forward in reaction to the bump, not rearward. And if my front tires hit a dip or pot hole, the contact patch will move "less forward", not backward as you describe. I agree with you that castor is both friend and foe, but not for the reasons you describe.

[Duane]

MaxJ and the XJ team

My description of the wheel moving forward and aft is in relationship to the vehicle. If the vehicle is moving forward, this is really an acceleration and deceleration of the wheels in relationship to constant forward speed during suspension up and down travel. In true DW the wheels alternately move up and down (the bow tie of the axle). This up and down movement coincides with a forward and aft motion because of the angled forward facing CAs. The more angle of the CAs (at static vehicle height), the more forward aft motion per given amount of suspension travel. At stock vehicle height, and horizontal CAs, there is very little forward and aft movement of the wheel during the (mathematical sine function). Castor, and the trail it produces, requires a stable contact patch to function properly and produce directional stability.

Castor and the steering angle combine to cause the tire to move up and down as it is turned. The combination of trail and the wheel moving fore and aft cause a wipping of the wheel back and forth that in turn causes the wheels to alternately rise and fall (bow tie) alternately compressing the front springs.

Every harmonic system has four factors: controlled motion or linkage, Mass, Spring rate, and some damping. The linkage and motion is as I have described. The Mass is tires, axle etc. The Spring is the front springs and to some extent tire pressure, and the damping are the front shocks and steering damper.

If the mass of the system is increased the frequency is lower. For example: Nik’s 40s produce the DW at 34 MPH –a low frequency. Lighter tires and axels will produce higher frequencies in the 50MPH range.

Higher spring rate springs will move the frequency higher. This is the reason that those that use real stiff springs don’t have as much problem with DW.

Stiff shock will also help damp the system—real stiff shocks help too

And finally there is the mechanical system: changing leverages etc can stop the trigger, and this is where reducing castor enters the picture.

More later—got to get back to work

[Eagle]

Well, that's an interesting theory, which just happens to be contrary to everyone else. The consensus of a great number of threads and opinions on these forums has been that more caster helps defeat death wobble, and that reducing caster below about 5 degrees may in fact be a cause of death wobble.

And now you want us to reduce caster to effectively zero. Gotta think about that some more ... my brain hurts. :(

[rockwerks]

And now you want us to reduce caster to effectively zero. Gotta think about that some more ... my brain hurts.

On teh 95 I currenty have It as after market LCa's that area about 1/4" longer than stock..there is no lift...this combination helps produce DW....I would on get DW while turning on certain banked corners....because the axle is laid back several degrees (more Caster), when you turn the steering wheel the leading tire rolls onto its outside edge quicker than it would with less caster.....causing the tire to scuff on the pavement. causing the DW....putting stock arms back on made the problem go away...so more caster than stock can also cause DW.

Ya gotta love our jeeps dont cha....the biggest improvement we all can do to get rid of it, is to replace the inverted Y steering system with a standard type system

[96XJ]

This poses a question to ponder. Wouldn't you put a longer upper control arm on at the same time of the longer lower control arm? If they were both 1/4" longer, then the DW would not be a factor but because the lower is a 1/4" longer than the upper, the pinion on the front diff is tilted downward, causing a bind on the u joint. Just something to think about.

[Nik]

I have heard you all out and have come to a conclusion. I dont even have the front shaft hooked up at the moment so PA is not it. I have seen 4 Xj's at a almost Identicle set up to mine with drop brackets and they have none of this. Owners of 4 wheel stores out here have the same set up at rock crawls in the springs. A fella in Castle rock is running drop brackets with 10 inches of lift, A guy from predator has 12 and runs with 38.5x14.50's with no problem and has a stock super 30 up front, locket, shafted, and geared. I have a Dana 60 full width like the rear in my shop now and I am starting to work on it. I'm tired of hearing that the Drop brackets will not work because I have seen them work at 70 mph with no shimmy on 3 other jeeps. I know If I'm not running long arm or radius arms I will not have the correct angles according to you all, I have another friend running skycrappers 8 inch lng arm and I have almost similar or better steering angles. I'm not really coming off as not believing or mad I just have seen and had a chance to drive these other jeep with the same set up's. When I get the money it will definantly be a long arm set up. Two other factors, I havn't put shocks or a stabilizer on it yet, would this be a major contribution? Its so bad at a low speed that I wouldn't think this is it. I dont have the Dana 30's pinion angle set up at all, I will with the 60 when it is done, I just want to get it rolling down the road.

Any more suggestions and obvious flaws you can see

[Nik]

I do know the shocks and stabilizer will contribute to the prob.

Anyone willing to take a look and fix in Colo Springs?

[xjaddiction]

Duane, I'm with you on this theory!! See my previous post about my DW and a witnessed account of exactly what you describe. I talked to Tereflex almost 2 years ago when I had DW. They told me to lower caster to 2 to 2.5 degrees pos. Thats well below the 5.3 to 8.0 degrees stock, Essentially the lower caster is pushing the axle forward, or holding it forward with the D-shaft and CA's. That stopped my DW, but still gave me that spooky pre-DW feel on certain bumps.

I experimented with this on one of my DW's. I pulled it into 4H and floored it. Bye DW, because the axle was being pulled forward by the acceleration.

You obviously understand this better than myself, but I'm with you on this theory. Why else would RE come out with drop brackets after offering their soft rate 4.5" springs. I have Tereflex which are also a soft rated spring. I asked RE this, they would only say that they saw a need for drop brackets.

This is great, only 3 years in the waiting for me to finally meet someone with a similar theory.

Greddy

[Nik]

Hey!! After all of this confusion I got my spare today and slapped it on and it turns out this entire problem was the damn out of round swamper after 4 shops said it was fine. I can hit 65 with a slight shimmy and I think shock and a stabalizer will solve that a little till I get the 60 in. I wont go much faster than 60 for a while anyways.

Nik

[MaXJohnson]

Duane wrote: "My description of the wheel moving forward and aft is in relationship to the vehicle."

The centering effects of castor and/or mechanical trail react at the tire contact patch. The amount is proportional to steering axis offset and speed of the tire relative to the ground, not to the vehicle.

"the wheels alternately move up and down (the bowtie of the axle)."

I have used this term to help describe DW many times in the past. Constrained by the control arms, the "bow tie motion" moves predominantly up and down, but somewhat fore and aft dependent on the amount of lift induced angle on the control arms.

You are neglecting the shimmy motion of the tires. This is one of the major components of DW. The fluttering "bow-tie" motion of the axle is an oscillating system, dependent on mass and spring rate. The steering "shimmy" motion of the front wheels constitute a separate oscillationg system with additional variables for mass and spring rate. The closer the natural frequency of these two systems coincide, the more likely they will work together to produce DW. The spring rates of these systems are based on many more factors than you mentioned, such as sway bar rate, bushing compliance, sidewall thickness, tread design, depth and compound, frame flex at the steering box and more. Wheel backspacing plays a part as well.

Changes to the mass or spring rate of either system should affect the speed at which DW is likely to occur and the magnitude of the event required to trigger it.

Contrary to some opinions, replacing the "inverted Y" steering with a cross-over design has a negative affect on the problem. DW is dependent on a coupling of the left and right wheels for each of the mentioned oscillating systems. The solid axle provides the couple for the "up, down, bowtie" motion and the tie-rod for the "steering, shimmy" motion. A more rigid tie-rod provides an efficient means of transmitting the motions from one wheel to the other. Any improvement seen from this modification is due to a replacement of old components with new, not the change in design.

Castor dampens the small steering motions that can evolve into DW. As the magnitude increases, castor becomes a contributing factor that increases the instability of the system.

[Duane]

Maxjohnson

Please, before you try to correct or disagree with my statements -- read all my posts in the thread. For example: your point on relative speed or movement of tire to vehicle and it relationship to ground speed was covered in the very next sentence. Second I did not neglect to mention the shimmy. Please see first post.

Do you have a suggested solution to this problem or are you just arguing to argue?

I do disagree that the shimmy components and the flutter/”bowtie” are separate systems. They can be analyzed separately, but they are linked by the rise and fall caused by SAI and Castor, and the moment load induced on the steering axis from the fore and aft movement of the axle. You do mention that they “work together”, and this is how they do it.

Reducing the Castor helps break this link and does create separate systems.

To explain more on the fore and aft movement of the suspension travel: At a moderate 4 inch lift, the static control arm angle is approx. 15 degrees. Fore and aft movement through the full range of travel, assuming a 9” shock, is approx 6 inches. Fore and aft movement for a travel of plus and minus 2 inches is a little over one inch. This movement will cause an Acceleration/deceleration of the Steering axis causing a push pull around itself. This “somewhat” and not “predominant” motion is the main reason for the lack of stability in the forward facing control arm system. You don’t hear of this problem in the SFA flat leaf sprung systems because there is very little fore and aft movement in comparison to the forward facing CA system. Some leaf systems don’t even have a trac bar, and they are more stable.

The bottom line is this forum should give suggestions and not just theorize. So, Maxjohnson are you ready to jump on board with the reducing castor method to break the link that creates the DREADED DEATH WOBBLE? --- I think we are almost in violent agreement. Especially about steering linkage.

[Duane]

EAGLE

I agree that when you reduce castor you will feel more of the out of balance shimmy condition, but it will not grow into DW----- the link has been broken. I have adjustable JKS CAs and have experimented with the Castor adjustment. At 6 to 7 degrees I would have terrible DW. Yes my tires are old and out of balance but a lot of trucks and cars would be tolerant of tires in this condition. When I go to 3 degrees, no more DW. I can feel the shimmy at 55 and drive there all day and it never goes into DW. I think true death wobble is more prevalent in those XJs that are using stock length CAs and are lifted in the 3.5 inch range. At this lift, most don’t pay much attention to their pinion angle and the castor will be on the high side of stock settings. As we lift higher we start rotating the axle to align the pinion with the drive shaft and the castor is reduced.

My next test is to increase scrub radius. The one- inch wheel spacers are in the mail. I will install the spacers with the exiting rims and old tires to see if I can feel a stability increase. My theory is that at stock back space, the tire patch is close to the center of the steering axis. Because the steering axis is close to the CG of the tire, it can more freely oscillate. Decreasing back space (increasing positive scrub) will move the tire out and away form the steering axis and make it more difficult to shimmy. The added scrub will also add some damping---after this test I will install the new 32 BFG ATs the real reason for the alloy spacers.

I vote for a Death wobble forum!!!!

[Eagle]

Originally posted by Nik
Hey!! After all of this confusion I got my spare today and slapped it on and it turns out this entire problem was the damn out of round swamper after 4 shops said it was fine. I can hit 65 with a slight shimmy and I think shock and a stabalizer will solve that a little till I get the 60 in. I wont go much faster than 60 for a while anyways.

Nik

Hmmmm ...

I'm not usually one to hollar "I told you so" but in this case, having started out by suggesting that your problem was in the tires and getting my assets handed to me ...

I TOLD YOU SO!

:D I'm glad you found the problem. The moral of this story (which I think I also alluded to) is that most of the guys who run those machines don't have a clue what they're doing. You are not the first person to find that a tire declared "balanced" by one (or even two or three) shops was in fact not balanced or even round. Find a GOOD shop and stick with them.

[MaXJohnson]

Welcome back, Duane ;)
Your cart pushing the horse concept of castor only seems to work in suspension droop. Using your arguements, suspension compression would increase stability. Would this create the anti-DW???

"I did not neglect to mention the shimmy."
You didn't neglect to mention shimmy, you mentioned it once. Wheel shimmy is half of the DW problem and you mentioned it once.

"Do you have a suggested solution to this problem or are you just arguing to argue?"
I'm just arguing. :)

"I do disagree that the shimmy components and the flutter/”bowtie” are separate systems."
Separate oscillating systems with their own mass, spring rate and dimensional components, coupled by more than castor/SAI.

"Fore and aft movement through the full range of travel, assuming a 9” shock, is approx 6 inches"
Giving you a generous ratio of 3" of up travel and 6" of down travel, I come up with approximately 2 1/2" of fore/aft motion. Less than half of your figure.

"causing a push pull around itself"
Is this like a "reach around"???
Moving forward, the steering axis is pulling the contact patch, never pushing.

"You don’t hear of this problem in the SFA flat leaf sprung systems"
I do. My first experience with DW was in 1971 in a Chevy 1/2 ton 4x4. That's with SFA flat leaf springs. I've heard or read of countless examples since then.

"lot of trucks and cars would be tolerant of tires in this condition."
Lots of cars and trucks are set up and maintained properly. Lots more have independent front suspension.

"...in the 3.5 inch range. At this lift, most don’t pay much attention to their pinion angle and the castor will be on the high side of stock settings."
A 3" lift decreases positive castor. This would move it towards the low side of stock specs.

"at stock back space, the tire patch is close to the center of the steering axis."
Increased tire height can offset the effects of less back spacing.

[Duane]

Yes,Max I do agree the the caster will decrease slightly at 3 inch lift, but will still be suficient to link the two systems. At three inches of lift, without changing anything, my caster was 7 degrees. I also agree there is more than one couple of two component of the whole system such as the angle of the SFA putting tires on edge.

Again,the bottom line--- are you going to recommend a reduction of caster to help regain stability, or are we going to continue to just mental masurbate on the topic---no I don't need a "reach around".

analysis does nothing without a final conclusion and recomendation!!!!!

[Eagle]

Originally posted by Duane
Again,the bottom line--- are you going to recommend a reduction of caster to help regain stability, or are we going to continue to just mental masurbate on the topic---no I don't need a "reach around".

analysis does nothing without a final conclusion and recomendation!!!!!

I can't -- and would not attempt to -- speak for MaXJohnson, but personally I remain unconvinced by your arguments because they don't square up with what I have seen and experienced in over 40 years and about 1.3 million miles of driving. Basically, EVERYONE else says positive caster reduces/controls death wobble, and now on the basis of one thread you want us to "recommend" the exact opposite.

I'm sorry, but I cannot do that. MaX will have to speak for himself.

[MaXJohnson]

Originally posted by Duane
...are you going to recommend a reduction of caster to help regain stability, or are we going to continue to just mental masurbate on the topic..."

Neither. I'm satisfied with the status quo.

Your blanket recommendation of reducing castor to almost zero is based on false assumptions and anecdotal evidence.

[Duane]

Eagle,

Sorry to inform you that "EVERYONE" is NOT saying the same thing. Reduction of caster has been mentioned by others on this forum.

Here two major after market manufacturers recommend reduced caster settings.

From R.E.installation manual:

http://www.4x4groupbuy.com/installInstructions/RE-INSTR-XJ3-4.5.doc

"HIGH SPEED WOBBLE: IS A CONDITION WHERE FRONT TIRES WILL SHIMMY AFTER HITTING A BUMP. AVOID BIAS PLY TIRES AND WHEELS WITH EXCESSIVE OFFSET. CHECK FOR WORN OR LOOSE PARTS. IN MOST CASES A REDUCTION OF POSITIVE CASTOR WILL ELIMINATE THIS CONDITION."

From Teraflex:
http://www.trailquest.com/teraflex/wj/s4w.shtml

"For best results, Teraflex suggests using a 4 to 5 degree castor and a toe-in of .25 inches."


From RE forum:
http://www.rubiconusa.com/forums/threads.asp?threadid=3729&fsort=&flastret=&numitems=5&forumid=1010&sort=&lastret=0

[Duane]

MaXJohnson

I think you need to to call these aforementioned vendors and inform them that their recommendations are based on "false assumptions and anecdotal evidence". I am not suggesting this as a blanket cure but as measure to increase the stability of the system---all the typical recommendations of balance, proper maintenece etc. still apply

One other thing --Sparkys should leave the dynamic analysis to the mechanical guys--just kidding

This is it for me on this subject unless other "experts" want to chime in

It's always fun.

D

[MaXJohnson]

Originally posted by Duane
MaXJohnson

I think you need to to call these aforementioned vendors and inform them that their recommendations are based on "false assumptions and anecdotal evidence".

I was referring to your recommendations. I don't know what basis RE and Teraflex used for their recommendations, but would guess it was experience.

RE's intallation instructions specify setting castor to factory minimum which would be around 5° or better. Makes sense, I'm guessing they are trying to compromise pinion angle while staying within factory specs.

Teraflex also recommends 4° to 5° castor, but trades off tire wear for stability by recommending 1/4" toe-in. Castor is in the same ball park I play in, but I wouldn't agree with their toe-in setting; hiding a problem at the expense of tire scrub and wear.

RE's forum mentions adjusting your castor to avoid bump steer. You go ahead and figure that one out. :rolleyes: