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  1. #1
    Captain Radon Steve's Avatar
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    Arrow Death Wobble - More than you ever wanted to know!

    Long post warning!!!

    Here's an engineering description of DW. I get tired of seeing people guessing at what's causing their DW, so here goes. Hope it helps someone.

    First, you've got to realize that the front suspensions on many of the vehicles we're working on were marginally stable, at best, from the factory. DW is a fundamental dynamic response mode of the entire front end...as a system. Lift and larger tires change (increase) the 'gain' associated with what becomes (or even starts out as) a marginally stable dynamic system. The damping factor (lambda) is also affected by larger tires...it decreases as a function of sidewall height/thickness ratio. Hysteresis in any control path (loose tie rod, steering box, track bar bushing) reduces the ultimate stability margin further. The fundamental frequency of DW is determined by the superposition principle where all springs involved are resolved (frame, tire resilience, hub bending, bushing deflection, etc, etc.) into one global spring constant, and all damping factors associated with friction, elastic elements, viscous damping (steering damper and shocks) are resolved into one damping factor. The natural frequency, damped natural frequency, and damping coefficient are then known. Now, if the system is overdamped and the gain is low...no problems...no oscillation. Increase the gain without increasing the damping and you go toward the critically damped, and beyond, specturm of responses. Critically damped means that DW would only 'hint' at being there, but would die out on its own without going totally unstable. This is also known as a decaying response.

    Once the system goes beyond critically damped, any excitation, be it unbalanced tire, bent wheel, bumps in the road, etc. can set it off and the response will not decay...it will grow in amplitude, quite quickly in some cases, and may be limited only be physical non-linearities like hard stops...or breakage. That's classic Death Wobble.

    A truck suspension is designed to stay in the overdamped to critically damped range. That is generally why a truck rides "rough". A Cadillac on the other hand is designed to stay in the undersprung range. It just "floats" down the road. Any change in the basic design parameters that affect the gain (lift, tire size), damping (tire size, steering damper, steering box condition), and hysteresis ( any wear point that creates any slop) can push it over the edge and create DW. ANY ONE OR TWO of the factors discussed can do that...which is why everybody then thinks that whatever problem THEY found and fixed is the cause of all DW; it is not. It is plain and simply a marginally stable system in its original form that is easily made unstable by any of the myriad causes discussed already.

    If your front end is loose (bushings, bearings, etc.) then you have a situation where your stiffness is removed and any jarring sensation (potholes, unbalanced tires, misaligned wheels, etc.) will cause the suspension to go crazy. It is no longer functioning where it is designed.
    On the other hand, your suspension could be very tight but an imbalanced tire would be spinning at just the right speed to throw the suspension into a unstable situation.

    So unfortunately there isn't only one root cause to the problem of DW. The underlying problem is instability in the front suspension, the root causes can be a multitude of things ranging from bad/loose bushings, to loose bearings, to caster angles, to imbalanced tires, etc.

    A steering damper only hides (maybe) the effect, it does nothing to fix the root cause.

    drueg, this doesn't really answer your question about what's causing YOUR DW, but it should give you something to think about in your search for the root cause(s). I'd check the trac bar bushings, make sure your wheel bearings are in spec, make sure your tires are balanced, make sure your alignment is in spec - especially caster, make sure your ball joints & TREs are tight, see if you have play in your steering box, etc.

    I could go into the calculus needed to model the entire front end as a system, but I've probably lost most of you already so I'll stop now. Someone needs to do a full modal analysis of all of the factors involved; just not me.

    Everybody got all that?
    Last edited by Steve; December 7th, 2005 at 05:30 PM.

  2. #2
    do i need to do a book report now?


    good little article. i learned a couple things. thanks!
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  3. #3
    Wild Hare's Avatar
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    WOW!, that was long. I love it when you brainiacs put stuff in writing. Makes for good reading.
    Thanx Steve

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    Can you explain why reducing caster helps on some vehicles? It doesn't seem like it should work, but it does.. and at other times more caster will cure it.

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    Captain Radon Steve's Avatar
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    Quote Originally Posted by Norco
    Can you explain why reducing caster helps on some vehicles? It doesn't seem like it should work, but it does.. and at other times more caster will cure it.
    Basically, anything you do to get the front suspension back closer to original factory specs should help with DW. Remember, a lot of our vehicles were marginally stable from the factory, and when we lift them, change the steering, run huge tires, etc., we're making them even less stable. So, running the caster at whatever it was from the factory, along with making sure that the many other things affecting the front suspension are in good condition, will help greatly.

    Unfortunately, there's no silver bullet for fixing DW, and what works for one person may not work for another. You have to consider the entire front suspension as a system and then make that system as stable as you can for the way you want to run it.

    Quote Originally Posted by GrandWheeler
    Sounds like someones just took a final in a Mod-Sim class of mechatronic systems. Mines next week
    That was actually a long time ago for me. Have fun!

  6. #6
    Quote Originally Posted by Steve
    If your front end is loose (bushings, bearings, etc.) then you have a situation where your stiffness is removed and any jarring sensation (potholes, unbalanced tires, misaligned wheels, etc.) will cause the suspension to go crazy. It is no longer functioning where it is designed.
    On the other hand, your suspension could be very tight but an imbalanced tire would be spinning at just the right speed to throw the suspension into a unstable situation.
    That is a bunch of bla, bla, bla.

    All that is a description of a spring-mass system applied to the suspension and steering system. Fancy words for something we all know. If your nuts and bolts are not tight, you are going to have problems. If a bushing is bad, you should replace it. If you tie rod end has some miles on it, you should replace it.

    I have a hard time believing the author really could do a full dynamic model of the force being applied to a truck go thru a pothole.

  7. #7
    Captain Radon Steve's Avatar
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    Quote Originally Posted by EEJEEP
    That is a bunch of bla, bla, bla.

    All that is a description of a spring-mass system applied to the suspension and steering system. Fancy words for something we all know. If your nuts and bolts are not tight, you are going to have problems. If a bushing is bad, you should replace it. If you tie rod end has some miles on it, you should replace it.

    I have a hard time believing the author really could do a full dynamic model of the force being applied to a truck go thru a pothole.
    I don't disagree with you. And I never said I was going to do a full dynamic model of the entire front suspension, going through a pothole or not. It could be done, but it would be extremely complicated, time consuming and expensive, and it would be specific to the vehicle being modeled.

    I simply tried to give a big picture explanation of DW and its very many possible causes and interrelationships. Don't like it? Don't read it!
    Last edited by Steve; December 6th, 2005 at 01:01 PM.

  8. #8
    Thanks Steve. My education is a couple years short of being able to quantify what is going on, but I completely know what you are saying. Everyone "knows" the cause of DW, and everyone has a different explanation that has nothing to do with resonance.

    DW is even a problem that Jeep engineers have trouble overcoming. It may not be possible to fully model all the factors accurately. Again, I don't know enough to say for sure though.

    Wouldn't it be possible to position the panhard and drag link in a way that would oppose the oscillation?

  9. #9
    Steve you didn't have to get a Slide Ruhl out to figure all that out did you?
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  10. #10
    Captain Radon Steve's Avatar
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    Quote Originally Posted by dalec
    Wouldn't it be possible to position the panhard and drag link in a way that would oppose the oscillation?
    I don't believe so; it would have to be some kind of active system, not passive. The oscillation, when it occurs, is in two opposed directions. You would have to have some system to sense that and and then actively move the panhard or trac bar in the exact opposite two directions of the oscillations to counteract it.

    Slide Ruhl Bob? (Good pun BTW ) I haven't used one since graduating from Navy Nuclear Power School, and I doubt I could do anything with one nowadays - that's what calculators and computers are for.


  11. #11
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    Good writeup Steve. Another reason people fighting DW have such a hard time, is the DW is often resulting from more than one source. Typically it is a combination of items that, when put together in one vehicle, tend to make the suspension more prone to DW. So no one clear answer is clear to solve the problem.

    For example, bushings are sorta old and loose, a tie rod is beginning to wear and ball joint is on the way out. Just sounds like an older truck, right? Where likely no single one would be a problem, when you put them together, you are open to DW. But when you try to diagnose it, nothing really jumps out as the culprit.

    One more area to include is suspension members of insufficient rigidity. A prime example is Rusty's adjustable trackbar. Maybe he has improved it lately, but I doubt it. The stock trackbar is beefier. Anyway, this trackbar is spongy enough to start flexing following a pothole strike, leading to DW. Not for everyone, but for some. Sometimes you have to look beyond the joints and bushings to find the source.

    Caster is an issue often beat about. One guy decreases caster and solves teh problem, one guy increases caster and solves the problem. For the why, see my first point, everyone's truck is a little different. Like Steve said, some changes will alter teh system enough to get it out of the critical zone and find some stability. It is best to set the caster at the factory setting for the vehicle, or if the axle was swapped in, the factory settings for the vehicle the axle came from.

    Generally, axles are set with negative caster to allow the tire to track straighter. Too little caster and the tires are too easy to turn, allowing them to oscillate. Positive caster make the wheels prone to turn rather than roll straight, very bad. To much negative caster and you are back in DW zone due to stability of the rolling shape.
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    Factory XJ caster spec is as high as 7 or so degrees, but keeping that number when lifted is asking for wobble, 3 degrees works well.

    There's a TSB out for 05 Super Dutys with wobble. Among other things including torquing all the bolts and inspecting the damper, it tells you to reduce caster by .5 degrees. Fixes 'em.

    I had a Crown Vic with vicious wobble once. On that one, I maxed the caster and used the minimum toe spec. Fixed it.

    So.. what is it about caster that contributes to this problem? Understanding that caster uses the weight of the vehicle to straighten the wheels, you'd think that more is always better. Not the case.

    edit: must have been typing at the same time as Clodhopper. You're a little off on your definition of caster. Almost everything has positive caster. Zero is no return to center force.
    Last edited by Norco; December 6th, 2005 at 02:24 PM.

  13. #13
    Clod Hopper's Avatar
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    Quote Originally Posted by Norco
    edit: must have been typing at the same time as Clodhopper. You're a little off on your definition of caster. Almost everything has positive caster. Zero is no return to center force.
    Huh. every diagram I have seen (to my memory anyway, but that is worth less and less every day ) indicates caster leaning toward the rear of the vehicle as negative, leaning forward is positive. zero is straight up, wiggle zone. I seem to remember the XJ stock is somewhere at 5.5 to 6, but the allowable zone in the FSM could be up to 7 deg. dunno on that.

    I dunno, I still get east and west backward (grew up in New England, spent six years on west coast of FLA, the ocean is which way?). Why would it be any different with caster?

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    other way round, you were close

  15. #15
    Captain Radon Steve's Avatar
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    Norco's correct, positive caster is when the top adjustment "leans" toward the rear of the vehicle, like so:


    Typically, 3-5 degrees positive is good and you can usually get by with a degree or so on either side of that. If the caster is different from side to side, the vehicle will pull to the side with the less positive caster. If the caster is equal but too negative, the steering will be light and the vehicle will wander and be difficult to keep in a straight line. If the caster is equal but too positive, the steering will be heavy and the steering wheel may kick when you hit a bump.

    Chip, you're right about there usually being more than one cause of DW; that's the point I was trying to make with this thread. What works for one vehicle may not work on another. In my experience with lifted vehicles, it's almost always more than one thing that causes DW, although fixing the worst offender will usually quiet it down for a while.

  16. #16
    Clod Hopper's Avatar
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    Quote Originally Posted by Steve
    Yep, ME=Dumbass. I can conceptually understand a problem, then go and ruin it all by mis-remembering the generally accepted coordinate system.

    For everyone's benefit, I will no longer post any tech information as it will likely be laced with inaccuracies.


    Quote Originally Posted by Steve
    Chip, you're right about there usually being more than one cause of DW; that's the point I was trying to make with this thread. What works for one vehicle may not work on another. In my experience with lifted vehicles, it's almost always more than one thing that causes DW, although fixing the worst offender will usually quiet it down for a while.
    Yepper on that. No one "cures" DW, just a matter of "managing" it.

    Oh yeah, there are alot of people who profess to have DW, even "real bad" DW, but in reality, they are just dealing with the shimmies.

    If you tires don't leave the ground, if you don't need to change your pants the first couple times it happens, I submit to you, YOU DON'T HAVE DW!

  17. #17
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    Quote Originally Posted by Clod Hopper
    One more area to include is suspension members of insufficient rigidity. A prime example is Rusty's adjustable trackbar. Maybe he has improved it lately, but I doubt it. The stock trackbar is beefier. Anyway, this trackbar is spongy enough to start flexing following a pothole strike, leading to DW.
    I just put a Rusty's front OTK steering setup with an adjustable trackbar on the Dirty 30 - but couldn't work out the anti-sway bar mounts before Moab in October. The thing did fine on the way into town from camp, but on the way back, a pretty bad DW . Sounds like it could be the Rusty's trackbar? I'm hoping getting the sway bar hooked up and the maybe the stabilizer back attached will help.

    Clod Hopper - was the DW experience you had with a Rusty's on a TJ or an XJ?

  18. #18
    Clod Hopper's Avatar
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    Quote Originally Posted by Dirty 30
    I just put a Rusty's front OTK steering setup with an adjustable trackbar on the Dirty 30 - but couldn't work out the anti-sway bar mounts before Moab in October. The thing did fine on the way into town from camp, but on the way back, a pretty bad DW . Sounds like it could be the Rusty's trackbar? I'm hoping getting the sway bar hooked up and the maybe the stabilizer back attached will help.

    Clod Hopper - was the DW experience you had with a Rusty's on a TJ or an XJ?
    It was on an XJ, but IIRC, the same trackbar is on the TJ. Or same construction, so essentially the same. Really, I don't have a huge issue with the trackbar itself (you get what you pay for and his is cheap), but while trying to sort out the problem Rusty was less than helpful (to be nice about it). His stuff is inexpensive compared to others kits, and the part quality difference shows also. The trackbar is the worst, real junk.

    Again, the stabilizer may add enough damping to the system to cover up the problem and make the truck driveable. I had to run 2 stabilizers for a while to make it streetable enough to drive over 45 mph until I got it sorted out.

    There are two types of DW. My examples are not all inclusive or iron clad. Just suggestions to help diagnose problems.

    The first typically is speed related. Whenever you reach a certain speed, bam, you get deathwobble, no matter what. This is a vibration/oscillation issue. Look into tire balance, alignment, steering joints, missing bushings (totally shot), loose steering box (either loose bolts or worn internals), etc.

    The second is an impact initiated wobble. For example, hitting a pothole above a certain speed will start DW. This is more likely a bushings, loosening mounts, flexing components, etc. Basically, something is tight enough that in general straight driving, it is ok, but give it an impact force, whatever is getting loose starts sliding, rebounds and starts going nuts.

    Personally, it was the second one. So I could rule out the tires, alignment and that kind of stuff. My problem was finding what was moving.

    Here is how you can tell if the issue is steering related or trackbar related. You are gonna need some balls for this, but stick with me. Once you have played around with the DW awhile you find you can control it a bit by feathering the brakes. So go find a straight, deserted, bumpy road. Get the truck up to speed and get the DW going. You had it happen a few times, you have already been frantically avoiding potholes, so now go find one, quit whining. At this point, the truck is somewhat violently shaking, and you can keep enough control using the brakes to keep er on the road. Roll down the window and stick yer head out and look at the front tire. What is it doing?

    1. The front of the tire and the back of the tire are moving approximately the same amount side to side. In this case, the axle is stationary, the wheel is pivoting on the ball joint during the oscillation. Therefore the problem is likely in the steering. Something in the steering has enough give to allow the movement.

    2. The back of the tire is moving MORE than the front of the tire in the side to side movement. In this case, the knuckle is pivoting on the steering links, and allowing the axle to move back and forth under the vehicle. The problem here is most likely in the trackbar system.

    Personally, I had situation No 2. Since I had a new Moog TRE and a harder oem bushing on the trackbar, the only thing left was the damn thing was flexing to allow the movement. Picked up a used Rubicon Express trackbar (those things are really nice for the price) and what do you know? Problem solved.

    Ya know, Rusty's kits are cheaper out of the box, which was the reason I bought it originally years ago. But when I added in the money spent to get the bugs worked out, I would have been better off buying a RE kit, and been happier to boot. As I have continued with the truck, at this point, the only thing of that kit I still have on the truck are the main leaves of the rear springs. Everything else has been swapped out for one reason or another. That is your best indication right there of the quality of the product.

    Hope it helps.

  19. #19
    Captain Radon Steve's Avatar
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    Great additional info Chip!


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    So you're saying it's the steering stabilizer that causes DW???




















    Hehehe...Just kiddin.
    Good post
    I only have one eye! ;)

  21. #21
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    Quote Originally Posted by nomo4x4
    So you're saying it's the steering stabilizer that causes DW???
    Yep, you got it!

  22. #22
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    Pondering how too much caster can cause wobble, the more caster you have, the larger the vertical movement of the wheel will be, under steering input. More vertical movement = more influence that the weight of the rig can have on it, and particularly with large soft tires, that would be a fairly substantial, mostly undamped, weight hanging out there. Get it cycling, and it's not going to want to stop.

    I really oughta crank down my caster a bit (front at 8 degrees) but no steering feedback = very difficult to get actual DW, so I've been lazy.
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  23. #23
    Clod Hopper's Avatar
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    Quote Originally Posted by Scott@Rockstomper
    Pondering how too much caster can cause wobble, the more caster you have, the larger the vertical movement of the wheel will be, under steering input. More vertical movement = more influence that the weight of the rig can have on it, and particularly with large soft tires, that would be a fairly substantial, mostly undamped, weight hanging out there. Get it cycling, and it's not going to want to stop.

    I really oughta crank down my caster a bit (front at 8 degrees) but no steering feedback = very difficult to get actual DW, so I've been lazy.
    I don't know the exact mechanics of it, but I have experienced it. This has been backed up by others on NAXJA (national XJ board). I couldn't wrap my brain around the whyst, so I gave up and just accepted it.

    Big tires can accentuate DW, but low pressure or soft tires can fight it with the added dampening, until you get so low that the tire won't spin true and the inbalance can start it up again.... DW is a wicked little imp....

    Yah know.... one more thing that affects DW is temperature. Your truck will be more prone to DW on a very hot day, than on a cold morning. WHY you ask? warm bushings provide less resistance to movement, or less dampening, than cold ones. This explains why you can drive to work with fewer problems than when you drive home in the afternoon.

  24. #24
    Captain Radon Steve's Avatar
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    Quote Originally Posted by Scott@Rockstomper
    Pondering how too much caster can cause wobble, the more caster you have, the larger the vertical movement of the wheel will be, under steering input. More vertical movement = more influence that the weight of the rig can have on it, and particularly with large soft tires, that would be a fairly substantial, mostly undamped, weight hanging out there. Get it cycling, and it's not going to want to stop.

    I really oughta crank down my caster a bit (front at 8 degrees) but no steering feedback = very difficult to get actual DW, so I've been lazy.
    Scott, let's take it to the extreme to see why too much caster can make the suspension system unstable. Picture 90* of caster. The ball joints/kingpins would be horizontal, so when you turned the wheels they would turn top to bottom and not side to side. 90* is extreme, but the more + caster you have the more the wheels are turning top to bottom and less side to side. Besides getting pretty unstable (and VERY heavy steering feel) with much more than 10* or so, you also start scrubbing the tires pretty bad when turning with too much caster.

    You rarely drive yours on the street, and with 40+ inch tires aired way down, you probably don't need to worry much about DW.

  25. #25
    Tube Monkey Scott@Rockstomper's Avatar
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    Quote Originally Posted by Steve
    You rarely drive yours on the street, and with 40+ inch tires aired way down, you probably don't need to worry much about DW.
    Agreed on the first (chopped) paragraph, you put it into words better than I did.
    DW would however, be really really really scary at 70mph in the desert (yep, I've driven mine that fast). I actually should crank down my caster a bit because it's tied to my pinion angle, which could benefit from a couple degrees more (it's currently at zero). But that's a whole separate discussion.

    Back (sorta) on track... whaddaya think would be an ideal caster setting for the rear? And which direction is positive or negative on the rear?

  26. #26
    Quote Originally Posted by Steve
    Scott, let's take it to the extreme to see why too much caster can make the suspension system unstable. Picture 90* of caster. The ball joints/kingpins would be horizontal, so when you turned the wheels they would turn top to bottom and not side to side. 90* is extreme, but the more + caster you have the more the wheels are turning top to bottom and less side to side. Besides getting pretty unstable (and VERY heavy steering feel) with much more than 10* or so, you also start scrubbing the tires pretty bad when turning with too much caster.

    You rarely drive yours on the street, and with 40+ inch tires aired way down, you probably don't need to worry much about DW.
    I always thought that positive caster affected the SAI, kinda rounding out the axis causing the tire to constantly "push" into and out of the wheel well aka DW.
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  27. #27
    Quote Originally Posted by Scott@Rockstomper
    Agreed on the first (chopped) paragraph, you put it into words better than I did.
    DW would however, be really really really scary at 70mph in the desert (yep, I've driven mine that fast). I actually should crank down my caster a bit because it's tied to my pinion angle, which could benefit from a couple degrees more (it's currently at zero). But that's a whole separate discussion.

    Back (sorta) on track... whaddaya think would be an ideal caster setting for the rear? And which direction is positive or negative on the rear?
    In the rear there are no ball joints to measure caster. Solid axle. In any case the axle isn't turning left and right so caster shouldn't be an issue. So caster in the rear is called pinion angle. I would be curiours to know if there is a need for rear caster on a 4 wheel steering car? I imangine that the turning angle in most 4 wheel steering isn't enough to make a difference.

  28. #28
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    Quote Originally Posted by D-MASTER
    I would be curiours to know if there is a need for rear caster on a 4 wheel steering car? I imangine that the turning angle in most 4 wheel steering isn't enough to make a difference.
    In my case (special, I'll admit) my rear axle will steer as sharply as my front, being that it's a more-or-less normal kingpin 60. Measuring the caster is a relatively simple exercise (incidentally, most A-arm independent rear suspension car would also have rear caster, measurable and in some cases adjustable), but I'm curious as to if there's a reason to set it at something particular. Mine's at zero, and AFAIK, the Quadrasteer GM rear axle also is normally set either at or very close to zero caster as well, but I'm pondering the ramifications of changing it. Back to the book...

  29. #29
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    Quote Originally Posted by Scott@Rockstomper
    Agreed on the first (chopped) paragraph, you put it into words better than I did.
    DW would however, be really really really scary at 70mph in the desert (yep, I've driven mine that fast). I actually should crank down my caster a bit because it's tied to my pinion angle, which could benefit from a couple degrees more (it's currently at zero). But that's a whole separate discussion.

    Back (sorta) on track... whaddaya think would be an ideal caster setting for the rear? And which direction is positive or negative on the rear?
    rear steer. I don't know how much DW is likely to affect a rearsteer axle. The suspension on a front axle pushes the axle forward, so the axle tends to be unstable (wants to move aft of the suspension mounts), where a rear axle is being pulled, so already in a more stable position. Really, I have no idea how much what I just said impacts DW....

    My guess would be to set it at the standard 5 or 6 deg POSITIVE (see, I am learning ) caster. I would shy away from negative caster, or even zero.

    But I gots no maths to back it up. Just gut feelings and intuition.

  30. #30
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    Quote Originally Posted by Scott@Rockstomper
    ......Mine's at zero, and AFAIK, the Quadrasteer GM rear axle also is normally set either at or very close to zero caster as well, but I'm pondering the ramifications of changing it. Back to the book...
    Why would you run caster in the back?

    The purpose of running caster is to alleviate driveshaft angles. A rear driveshaft is so long, there aren't really any angle problems.

  31. #31
    Clod Hopper's Avatar
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    Quote Originally Posted by JeffW
    The purpose of running caster is to alleviate driveshaft angles.
    Not true. Why would there be any caster in a stock vehicle if this were true? the axle is built for the vehicle and when assembled could be set for zero caster.

    Caster is needed for steering stability and return to center. Probably some other stuff that I am forgetting. Caster is based on suspension, the chunk rotation is then chosen for the driveshaft being used.

    Now when those in our offroad bent start swapping axles, yeah some will sacrifice caster for improved driveline angle to save rotating the C's. What is best is up to the builder/driver and what they want. I prefer to let my caster be correct and solve driveline issues in other ways.
    Last edited by Clod Hopper; December 7th, 2005 at 01:28 PM.

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    Captain Radon Steve's Avatar
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    Quote Originally Posted by JeffW
    Why would you run caster in the back?

    The purpose of running caster is to alleviate driveshaft angles. A rear driveshaft is so long, there aren't really any angle problems.
    Because he has rear steer. And no, the purpose of caster on the front axle has nothing to do with alleviating driveshaft angles. Rear wheel drive cars still have the exact same caster issues without a front DS. You could have a perfect pinion angle with negative caster, but I don't want to ride in that vehicle.

    Good question Scott, never thought about caster angles for rear steer. I suspect you'd still want some positive caster to help them return to center easier, but since you probably have full hydro rear steer, that's not an issue. Since the rear axle is "trailing" instead of "pushing," my gut feeling is that anything a few degrees either side of zero would probably work fine. That's just a guess tho...

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    Tube Monkey Scott@Rockstomper's Avatar
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    Quote Originally Posted by JeffW
    Why would you run caster in the back?
    Any steering axle has caster... I just have zero rear caster currently. Many independent suspensions that technically don't have "steering" are nonetheless set up like front suspensions that do have steering--the steering is simply anchored to a fixed chassis location rather than a movable member that allows for steering control.

    The purpose of running caster is to alleviate driveshaft angles. A rear driveshaft is so long, there aren't really any angle problems.
    I wish; if that were the case, I wouldn't be running a high pinion rear axle at all, but a stronger, cheaper, and more readily available low pinion. Unfortunately for me, my shaft is rather short, and the angles ugly, so I have to run a high pinion. Fortunately for me, I can set the pinion angle independently of the caster (once) to optimize both--I currently have ~14 degrees of rear pinion angle, but if I go to a low pinion, I have to run ~26 degrees (sub-24" shaft).

    But further, caster is for steering constraints; if the only reason for having caster at all were to alleviate driveshaft problems, there'd be no discussion ever of having to cut and turn a front axle--that's done to keep caster but change pinion angle.

    Since my rear steer is entirely hydraulic, and without allowance for return to center, the only thing caster can do is reduce the effect to which a loose tierod end or kingpin bushing/bearing is felt--I have to return the rear to center manually, and there is not actually a defined center--it's just "turn around and look at it, and stop when it looks straight".

    Any bets on how long it takes for Steve and Clodhopper to send me back to Pirate?

    Actually, caster angles for a rear-wheel-drive car are usually different than they would be for the same car front-wheel-drive. I'll have to look into that more to find examples and explanation of why, which I'll have to do in between real work, but I'll poke at it a bit.
    Last edited by Scott@Rockstomper; December 7th, 2005 at 01:30 PM.

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    Quote Originally Posted by Scott@Rockstomper
    Any bets on how long it takes for Steve and Clodhopper to send me back to Pirate?
    Never. You should send them here to learn something.

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    Quote Originally Posted by Scott@Rockstomper
    Any bets on how long it takes for Steve and Clodhopper to send me back to Pirate?
    Hell no! I like have big tech discussions over here too! I just wish I had more experience with rear steer. (I will in the future, just not right now....)

    I wasn't aware that the quadrasteer was set at 0 deg. I guess there is some indication from that that caster is less important in the rear... or that they were trying to minimize steering effort to help make the steering equipment cheaper. I tend to believe that the automakers put some effort and engineering into designs and it is a matter of us to figure out what the design focus was (ie. D30 or D44 debate).

    Your rear is full hydro. Do you have any kind of return-to-center system on it or is that all manual? I am thinking that since it is full hydro that the little bit of return-to-center benefit of caster would be lost or at least not enough to affect the hydro system. If so, then stability is the only issue.

    Course I imagine this is all speculative as you already have the axle completed. Are you thinking about cutting and turning the C's to change it?

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    Quote Originally Posted by Steve
    Never. You should send them here to learn something.
    (two thumbs up!)

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    Tube Monkey Scott@Rockstomper's Avatar
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    Quote Originally Posted by Clod Hopper
    I wasn't aware that the quadrasteer was set at 0 deg. I guess there is some indication from that that caster is less important in the rear... or that they were trying to minimize steering effort to help make the steering equipment cheaper. I tend to believe that the automakers put some effort and engineering into designs and it is a matter of us to figure out what the design focus was (ie. D30 or D44 debate).

    Your rear is full hydro. Do you have any kind of return-to-center system on it or is that all manual? I am thinking that since it is full hydro that the little bit of return-to-center benefit of caster would be lost or at least not enough to affect the hydro system. If so, then stability is the only issue.

    Course I imagine this is all speculative as you already have the axle completed. Are you thinking about cutting and turning the C's to change it?
    I'm not sure Quadrasteer is actaully zero. I know it's set at very little caster, may actually be zero, but I've only eyeballed it, not put an angle finder to it.

    I have no return on the rear steer at all; if I roll the rig with the rear at full lock, regardless of impact on the rear wheels, it stays at lock (assuming parts don't break). Similarly, if it's straight, there it stays.

    It's both speculative, and intentionally trolling a bit, but also intended to be informative--I was actually on the phone this morning discussing pinion and caster angle relationships for a set of axles to be built shortly, that I plan on swapping to for next year. I have the opportunity to change angles now; if I don't capitalize on that opportunity, it's fairly difficult to change them later. I'm semi-deliberately not posting the numbers because I'd like to see what this discussion brings, but I'm pretty sure I have the numbers I want to run next year, laid out.

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    Keep throwing out tech info here Scott. This board has needed a real tech post like this for a long time.

    Thanks for initiating this Steve. Your explanation of "critically dampened" fits the DW I experienced to a "T". I had driven my Jeep in it's then new suspension setup (with no shocks yet) for several weeks with no problems whatsoever at any speed. I let it sit for a couple of weeks, during which time the tires flat spotted to some extent and the outside temperature dropped which would have slowed the time it took for the tires to warm and eliminate these flat spots. I took it out for a spin and upon entering a slight left turn at 45mph, my front left tire began to bounce to the point where it was off the ground 6-8 inches (so I was told). Apparently I was on the edge of dampening without the shocks and the flat spots in the tires were just enough to initiate the DW. I was close to needing a fresh pair of underwear when it happened too as it was enough to dislodge my clutch linkage. The installation of shocks put me back into the favorable side of critically dampened and I haven't experienced DW yet since then.

    Excellent post, especially the part about one man's fix not working for another man's problem.

    Slide Ruhl

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    Captain Radon Steve's Avatar
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    So Scott, what caster angle does your axle builder recommend for the rear axle?

  40. #40
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    My current axles are slated to be going to another (relatively quiet) board member (and have been so for a while, I just need to get them out and deliver them, fortunately, he's in no hurry).

    My axle builder recommends that I tell him what angles to use, and what shaft lengths to build to, as he doesn't particularly know or care what ideal caster on a rear steer axle is. He concurs with several on here that the 10 degrees of front caster I currently have (I just measured it) is too much. Honest truth is, he's just building the housings, the rest is, as they say, on me. He only cares about the angles because it's easier for him to put an axle together by putting the C's on the tubes, then putting the tubes in the center (Diamond Axle 9" centers, 60 knuckles) than it is for him to put the tubes in and then the C's on. He's wanting to hang the C's first as much as a courtesy to me, as for any real fabrication reason.

    Anyway, back to the DW, this calls for more info on kingpin inclination too--changing wheel backspacing can impact DW, because the contact patch is (ideally) relative to kingpin inclination (which is a generic term that applies to all steer axles, regardless of the actual pivot joints used). I'm curious as to what the kingpin inclination is on a newer D30 (or a balljoint 60) relative to some of the older stuff, and all of that could be relevant to why some axles DW under certain rigs, but not under others.

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