Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

Some engineering perspective on the Death Wobble

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blazer1970, Mar 26, 2002.

  1. Blazer1970

    Blazer1970 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    1,600
    Likes Received:
    0
    Location:
    Old Mission, MI
    Here are some thoughts from an engineering perspective that may help some of you guys get to the bottom of your DW problems.

    Resonant Frequency
    Every mechanical system has a resonant frequency. This is the frequency at which the system will go into oscillation, which is a condition of uncontrolled or increasing vibration. This is what is happening with the DW, your front suspension is being excited by a bump, etc., at its resonant frequency, which causes it to go into oscillation.

    The heavier something is, the lower the resonant frequency. Automotive designers spend all of their time trying to raise the resonant frequency of their subsystems. A higher resonant frequency is not as likely to be induced by the forces placed on the vehicle from external sources such as road imperfections.

    Unsprung Weight
    If you have spent any time paying attention to modern automotive engineering publications, you have heard the term "unsprung weight". This is the weight of the suspension components that are below the springs. One of the major pushes in modern suspension design is to reduce unsprung weight. This increases the resonant frequency of the unsprung components, and decreases the likelyhood of oscillation. It also means that smaller, lighter components (shock absorbers) can be used to control the oscillation. This is the main reason why everyone has gone to IFS - it reduces the unsprung weight, therefore reducing the weight of the entire system (along with improved steering geometry).

    Spring geometry
    The stock front springs, with their negative camber are about as good as it gets with a leaf spring suspension to keep the axle firmly located side to side. Since the axle is pretty much in line with the shackles, there is very little leverage on the spring bushings when the axle is forced to the side, such as when you are turning a sharp corner. As your lift springs get taller, there is increasingly more leverage on the spring bushings, shackles, and the frame. This allows for a lot more side to side motion of the axle assembly with a given amount of input force.

    Now for the DW.
    You have added a great deal of unsprung weight to your front suspension between the Dana 60 (+200 to 300lb?), and the big tires (100 - 200lb?). This weight lowers the resonant frequency of the system, making it more susceptible to oscillation induced by external factors. Most also have lift springs, which add leverage to the side to side motion problem. The front shock absorbers do a good job of damping the up and down motion of the front axle. They do basically nothing for side to side motion. So what does damp the side to side motion? The only side to side connection is through the steering system. Even a steering stabilizer only acts between the axle housing and the tie rod - the connection to the chassis is still through the steering box.

    Why do so many different things seem to make a difference?
    Anything that affects the resonant frequency of the entire system could change or eliminate the DW. If you change the steering stabilizer, maybe the DW will go away, maybe it will happen on a different stretch of road.

    Why would changing the PS pump make a difference?
    The power steering system is a closed loop system that attempts to keep the angle of the output shaft of the steering box in synch with the input shaft. When you induce a large external load onto the output shaft, the PS system (steering box and pump) will attempt to counteract this and keep the output in synch with the input. Because of this, the particular pressure that your pump is developing will effect the resonant frequency of the system. Higher pump pressures will increase the resonant frequency of the system - lower pressures (read - worn out pump) will decrease the resonant frequency. Under these large external loads, the PS system is doing some steering that you are not involved with. Once the oscillation begins, the loads that are being transmitted through the steering box are likely orders of magnitude larger than what the engineers at Saginaw Division ever expected this stuff to encounter.

    Stiffness
    Stiffness is a good thing (ask the ladies). Any part of your front suspension or steering system that lacks stiffness is going to lower your resonant frequency. The little short drag link that GM used on these trucks is really stiff. Does anyone use a crossover drag link that is not straight? That is not as stiff as a straight one. Is your tie rod bent? A bent tie rod is not as stiff as a straight one. Do you use soft spring bushings to increase flex? They are not as stiff as hard ones.

    But why do some almost stock trucks have the DW?
    I would suspect that the PS pump or box is bad, the spring bushings are worn out, bad tie rod ends, or a combination of any of this stuff. DW is a huge problem with D/C vehicles that have coil spring front suspensions. Some of them do it at a very low mileage - replacing the steering box or pump has been known to cure it. I would guess that if you could compare side to side, you would see that the trucks with Dana 60s and big tires resonate at a much lower frequency than the trucks with stock stuff.

    OK, what can I do to stop it?
    I would like to see someone try running a steering stabilizer from the frame to the axle housing. You would want to mount it as close to horizontal as possible. Maybe make a bracket that mounts to several of the diff cover bolts so that you can attach the stabilizer to the top of the diff housing, and then as straight as possible to the frame from there. This should not really limit your suspension flex if it is horizontally mounted. Somebody try this and let me know.
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

    Joined:
    Jul 23, 2000
    Posts:
    36,167
    Likes Received:
    1,365
    Location:
    E-town baby!
    I don't know about anyone else...but that was very informative, clear and easy to read and understand. Thanks!

    This is getting earmarked as a favorite post immediately!/forums/images/icons/cool.gif

    Rene
     
  3. Blazer1970

    Blazer1970 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    1,600
    Likes Received:
    0
    Location:
    Old Mission, MI
    Not to have a discussion with myself on this, but the more I think about it, the more convinced I am that it is the power steering system that is the main cause of this problem. Someone who has it bad should try removing their power steering pump belt, and see if the problem is there without any power assist.
     
  4. Blazer1970

    Blazer1970 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    1,600
    Likes Received:
    0
    Location:
    Old Mission, MI
    I am an engineer, and this has been on my mind lately. Just took some time to get the energy to type it all out. Every problem has a cause and solution.
     
  5. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    779
    Likes Received:
    0
    Location:
    Ontario, Canada
    I agree with Rene! /forums/images/icons/smile.gif

    Definitely informative. Some good info in there Tim! /forums/images/icons/smile.gif

    This is the kind of info I like to see on CK5! It's what made it my favorite site back when it first got started. The tech section was actually technical!! /forums/images/icons/wink.gif
     
  6. fortcollinsram

    fortcollinsram 1/2 ton status

    Joined:
    Nov 8, 2001
    Posts:
    2,261
    Likes Received:
    0
    Location:
    Fort Collins, CO
    Tim...YOU DA MAN...I have been waiting for someomne to throw up a post like this...I have wanted to know what is at the heart of the problem, not what "seems" to fix it...

    Chris
     
  7. BadDog

    BadDog SOL Staff Member Super Moderator Author

    Joined:
    Feb 24, 2001
    Posts:
    7,777
    Likes Received:
    11
    Location:
    Phoenix, AZ
    All I can say is, Nice job!
     
  8. ramjet gmc

    ramjet gmc CK5 Staff Staff Member Moderator GMOTM Winner

    Joined:
    Dec 19, 2001
    Posts:
    3,370
    Likes Received:
    0
    Location:
    Milltown NJ 08850 NJ sucks
    Would a rock ram with a cross over steering kit help stop this?
     
  9. Sparky

    Sparky 1/2 ton status

    Joined:
    Aug 20, 2000
    Posts:
    423
    Likes Received:
    0
    Location:
    San Antonio(summer) Texas A and M (school year)
    I'm a mechanical engineering student (junior) and what he says is 100% correct. I'm taking a vibrations class right now so I know a little bit about it. Funny, i have been thinking the whole shock from the axle to the frame thing for a while. I dont have any problems with DW at any speed so i dont worry about it much, but seems like it should work. I just dont know if the damping factors for any steering stabilizers will be anywhere near what they need to be to cure this problem.
     
  10. Blazer1970

    Blazer1970 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    1,600
    Likes Received:
    0
    Location:
    Old Mission, MI
    <blockquote><font class="small">In reply to:</font><hr>

    Would a rock ram with a cross over steering kit help stop this?

    <hr></blockquote>

    I am not sure exactly how this setup installs, but if the only connection to the frame is still through the steering gear, it will not likely solve the problem. It will change the resonant frequency though so you might not experience the same problems. The Rock Ram attaches to the axle housing, correct?
     
  11. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

    Joined:
    Mar 5, 2001
    Posts:
    15,680
    Likes Received:
    1,391
    Location:
    642 Days to BB2018
    Tim makes the rest of us in the "1st Gen Community" very proud.....! /forums/images/icons/smile.gif

    As much as I wanted to suggest something along these lines, I lacked the "vocabulary" and Mechanical Engineering background to explain it as clearly and succinctly as you did. I hope that this thread becomes a part of the "Technical Archives" along with any supplimental "Test Results" that people can add to this.....

    I think you've got the answer 100% right on this one.
     
  12. Blazer1970

    Blazer1970 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    1,600
    Likes Received:
    0
    Location:
    Old Mission, MI
    Let's see if we can fix it before we get too excited!
     
  13. René

    René 1/2 ton status

    Joined:
    Mar 14, 2002
    Posts:
    208
    Likes Received:
    0
    Location:
    Michelbach, Germany
    Hello colleague. That was appropriate like on the university, but very good. I concerned myself also with this chassis problem and my solution is a reaction triangle. Two corners are fastened swivelling to the left and right framerail, the third corner with a strong tie rod end centrically at the axle. Thus to move of the axle are prevented from it sideways and the steering becomes many more accurate. By a little bending I achieve an anti dive effect when
    braking. Ideal would be the whole system with coil springs.

    You should consider modern chassis of serious offroaders (like Mercedes G
    or Landrover Discovery). One comes on quite nice ideas.
     
  14. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    779
    Likes Received:
    0
    Location:
    Ontario, Canada
    Tim

    In terms of an axle to frame link; would a rigid one do as opposed to a damper? If so; that theory may need to be examined too cause Stephen Watson said that he used to/still does run a panhard rod with his cross-over set-up. I guess that shoudl have either taken care of the problem of the steering box taking the vibration or I guess changed the frequency; adding the frame to the mix.

    Your thoughts??
     
  15. Blazer1970

    Blazer1970 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    1,600
    Likes Received:
    0
    Location:
    Old Mission, MI
    I am torn at this point as to whether the entire axle assembly is moving from side to side, which could be cured with the frame link, or if it is all contained within the steering system. Someone had posted another thread where they said that they saw a TJ do it, and the guy stopped it by sawing the steering back and forth. This kind of points to the oscillation being caused or worsened by the attempt of the power steering system to "return to center". It is very common for a closed loop hydraulic system to get into an oscillation like this when the external conditions are outside the original design parameters. I really think it is a combination of all of the above, and different causes on different trucks. I would like to see someone try it without the power assist though, as that would really help narrow down the possible solutions.

    A worn or misadjusted steering gear could also contribute to the problem.
     
  16. Blazer1970

    Blazer1970 1/2 ton status

    Joined:
    Feb 17, 2000
    Posts:
    1,600
    Likes Received:
    0
    Location:
    Old Mission, MI
    Also, when you say "rigid", is it really a rigid connection, or are there rubber or poly bushings on the ends of the link?
     
  17. StonerK5

    StonerK5 1/2 ton status

    Joined:
    Feb 13, 2002
    Posts:
    183
    Likes Received:
    0
    Location:
    Phoenix Arizona
    That should get an Honorary CK5 PHD! You da man..thanks for that explanation. Wow!

    Chris
     
  18. 1BAD88

    1BAD88 Registered Member

    Joined:
    Feb 23, 2002
    Posts:
    89
    Likes Received:
    0
    Location:
    bristol ct
    Man now this is what it's all about .I think that now we all can find a sure fix for this problem with the valuble info donated by our brother k-5er .I fixed my DW by installing a new stab and steering pump and wheel bearings and I am glad to say it has'nt come back .I too am sure that somehow the ps pump had a major roll in this problem because after I put in a new one I could tell right away that the steering felt more precise and tight .( I am running 33s on a stock lift and 10 bolt axles with a new autozone stab and ps pump with timken wheel bearings )and I am sure I found the cure for my truck .It was well worth the $200 to eliminate the DW .Hopefully all of us can find the right combo of parts for there application to get rid of this forever.......Thanks again for the helpfull info and professional courtesy.........................
     
  19. BadDog

    BadDog SOL Staff Member Super Moderator Author

    Joined:
    Feb 24, 2001
    Posts:
    7,777
    Likes Received:
    11
    Location:
    Phoenix, AZ
    I can see it now. We are well on our way back to the '80s with 4 stacked steering stabilizers on each side! /forums/images/icons/laugh.gif /forums/images/icons/wink.gif
     
  20. yeild2me

    yeild2me 1/2 ton status

    Joined:
    Jan 7, 2001
    Posts:
    1,309
    Likes Received:
    0
    Location:
    fayetteville, nc
    lol...maybe the ole schoolers knew something we dont!!

    anyways...i agree, esp, the part about the steering. my steering has been increasingly worse lately, and i am thinking steering box. the pump was replaced last year because of leaks, so i will mess with the box first.

    to tell yeah the truth, there is nothing i hate messing with more than steering!!

    BTW-very informative tim, like was said before, this is why i love this site..

    heres a bump for the west coasters getting off of work/forums/images/icons/cool.gif
     

Share This Page