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Adjusting Roll axis with 4 link

Discussion in 'Center Of Gravity' started by yeild2me, Nov 6, 2003.

  1. yeild2me

    yeild2me 1/2 ton status

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    Since my 4 link is mostly done except for a few small details, I have been driving it alot on the street. When I am in a Parking lot (i.e Mall...lol), with the wheel to full lock and I give it gas, I have a lot of body roll. I have since mounted my shocks, and that seemed to help tons, but now I am thinking that I should move my links to make it a little more stable. With out a bunch of cool programs to calculate everything, I can tell you that when the truck is put in to gear, the ass-end will raise slightly, so maybe I have too much anti-squat. I just dont want it to flop the truck in off-camber situations when a bit of throttle is applied.


    Now that I am thinking about it, I need to go measure everything out (like I meant to on my other 4-link post), and let guys calculate it (like Jason hehehehe).

    I think I can fine tune it abit more. But other than that, I am very happy with the way it rides, drives, flexes etc.

    I hope this forum is the proper place for this post....there does seem to be quite a bit of discussion in here lately.


    Thanks....Rob
     
  2. Greg72

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

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    Rob,

    You've come to the right place.....


    The characteristic of the rear suspension "standing up" under throttle is anti-squat.....and in your case, it might be a higher value than you actually want.

    There are a number of places you can make adjustments to reduce the anti-squat value.....link lengths, postion of each of the link ends, ride height, etc.

    From some of the preliminary number crunching I did on my own suspension design, I found that the lower links are a good place to attack first. Take a look at that new "Karnivore"? or whatever it's called.....the BFG Demo vehicle. The lower links are really nice and flat.... That reduces anti-squat (or anti-dive) if we're talking front suspensions. My guess is that your lower links have a pretty steep angle to them? By either raising the mounting point on the axletube end (or) lowering the mounts on the frame end, you should be able to get some of that angle removed and that should calm down the anti-squat.

    If you can provide some basic measurements, I can plug them into my Excel spreadsheet and tell you where you're at now (anti-squat wise) and maybe give you some options....

    If you're interested let me know, and I'll tell you what specs I'll need. /forums/images/graemlins/thumb.gif



    .....this could get juicy!
     
  3. yeild2me

    yeild2me 1/2 ton status

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    Greg...Thanks man, I just got called into work for a little while, but when I get back, I will measure everything out and post up. Like i was saying above, its not bad, but now that its mostly done, I dont mind "Toying" with it to get the most out of it.


    Rob
     
  4. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    I agree with Greg. Your gonna have to run the numbers and figure out exactly what your A.S. number is along with your roll axis. It sounds like A.S. is pretty high jacking like that at idle. What does it do when you accelerate hard? Tried a climb yet?

    For a stable roll axis, you need it as high as possible. But there are trade offs involved and how you get it there depends on the link arrangement. For instance, depending on link style, improving roll axis may come with increased A.S. I don't remember what link arrangement you are running, but some pics and/or diagrams along with the measurements will be necessary for discussion. I know you may already know this stuff, but not everybody does so I’m throwing out the things that occur to me (still not completely awake though)..

    This is the problem I have with all the guys who say, “Just do it and don’t worry about all the calculations.” Sometimes they get lucky, or they don’t realize how much better it could have been. Those are the guys who make that statement everywhere they go. Then people follow their “experienced” advice and wind up scratching their heads, then doing things twice. I hope yours can be corrected without too much surgery…

    [edit]
    Just remembered your pics in the sig link. I couldn't find a really good pic of the rear links, but based on what I saw, it does not look like it should be too terribly far off on A.S. or Roll Axis. Hard to say from just those pics...
     
  5. Greg72

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

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    Rob,

    OK when you're ready, post up the following:

    Wheelbase (inches)
    Tire Diameter
    Estimate of your vehicles CG (center of gravity) in inches.
    Length of upper links (inches)
    Length of lower links (inches)

    The distance from the ground of the upper link on the axle side
    The distance from the ground of the upper link on the frame side
    The distance from the ground of the lower link on the axle side
    The distance from the ground of the lower link on the frame side

    If the attachment points of the links on the axletube side do NOT run through the vertical centerline on the axle, please provide the distance from the centerline (in inches)



    It sounds like a lot, but it's really just some basic info to help me get your suspension plotted graphically and so that I can run the calcs. I'll post up the image and data once I have your numbers.....

    that way EVERYONE will be learning from this! /forums/images/graemlins/thumb.gif
     
  6. yeild2me

    yeild2me 1/2 ton status

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    Ok Guys, just got home and gonna get the measurements and a few pics.

    Rob


    BTW-Baddog, I guess all it really does is unload the springs when I put it in gear (Aerostar coils). From a standing start, accelerating quickly, the truck stays almost perfectly level.
     
  7. Sandman

    Sandman 3/4 ton status Author

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    Thank you! I'm toying with doing this also or just going with the Ford springs. /forums/images/graemlins/deal.gif
     
  8. yeild2me

    yeild2me 1/2 ton status

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    [ QUOTE ]
    Rob,

    OK when you're ready, post up the following:

    [ QUOTE ]
    Wheelbase (inches)
    Tire Diameter
    Estimate of your vehicles CG (center of gravity) in inches.
    Length of upper links (inches)
    Length of lower links (inches)

    The distance from the ground of the upper link on the axle side
    The distance from the ground of the upper link on the frame side
    The distance from the ground of the lower link on the axle side
    The distance from the ground of the lower link on the frame side

    If the attachment points of the links on the axletube side do NOT run through the vertical centerline on the axle, please provide the distance from the centerline (in inches)



    [/ QUOTE ]

    Weelbase- 120"
    Tire Diameter- 37.5" now, but I designed it with my 44's (42.5") in mind
    COG-Top trans bolt--41"
    upper link length-33"
    lower link length-51 3/4"
    Lower link---ground to frame-28 5/8"
    ground to axle-16 13/16"
    Upper link---ground to frame-31 5/16"
    ground to axle-26"


    Attachment points on axle---the lowers are 3.5" forward of center, and 2" lower than horizontal center
    uppers are .5" behind center (Because I have 0 pinion angle with cv shaft)

    Side notes-----My lowers are par. with the d shaft and the frame attachment points end a little behind the output shaft. I mounted them outboard as far as I could on the axle for a little more added triangulation. I wanted to keep them lower on the axle to keep leverage forces down.

    Uppers-Triangulated at about 43* at axle.

    I think that is about all.

    Thanks alot guys.

    Rob
     
  9. yeild2me

    yeild2me 1/2 ton status

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    [​IMG]

    [​IMG]
    [​IMG]


    Dang....Those are some tiny pics.....for bigger ones, do a search for "yeild2me" in member rides

    Rob
     
  10. yeild2me

    yeild2me 1/2 ton status

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    LOL....I have been meaning to post the specs for all who are interested. Its not perfect, but doesnt bind, has 16" of travel (6 up and 10-down----this was the most I could measure by just backing up my trailer) and rides awesome down the road. My shocks are Heckethorne's for a chevy with 12" of lift. These are the longest I could get (13" of travel). They are nitrogen charged and although may be too stiff on most trucks, complement the Aerostars quite nicely. I have them mounted kinda weird for right now. I need to take it to the shop and put it on the lift to see if there is any other places to better mount them.

    Rob
     
  11. Greg72

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

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    Rob,

    Here you go:

    The orange horizontal line represents the CG height.
    The orange dotted line (angled) represents a 100% anti-squat value
    The yellow dotted line represents the ACTUAL anti-squat value.
    The green dotted lines represent the converging points of the links which will determine IC (instant centers) for both the X and Y axis.

    The 8-sided circles are bias-ply Swampers!!!

    [​IMG]



    It gets a little fuzzy because my gallery squishes everything down to 640x480, but you should still be able to make out most of it.

    Your current setup (including current tire size) gives you 107% Anti-Squat (ignore that my sheet says Anti-Dive, it was being used for my front end work)

    That's not as bad as I'd thought when you described the symptoms.... /forums/images/graemlins/thumb.gif


    Here are a few possible mods you can try:

    #1 - Relocate the frame-side mount of the lower link from it's current 28-5/8" height down to 25". This will reduce your anti-squat to 54%....that should be a fairly simple mod, and the resulting change should be easily noticeable.


    (or)

    #2. Move the lower link on the axle side UP from 16-13/16" to 20.5" which will get you an anti-squat of 57%.


    Initially it would seem like the second option is the best choice because it "costs" you nothing ground clearance-wise. HOWEVER, there is a big downside. The vertical separation between the upper and lower link ends determines the LOAD on the links (and therefore the sizing of the rod ends to prevent catastrophic failure). Right now your design has slightly over 9" of vertical separation between the uppers and lowers....that keeps the loads reasonable. If you do option #2, the vertical separation drops to only 5.5" and your loads go up SUBSTANTIALLY.

    I can't calculate the actual loads without knowing a precise vehicle weight and F/R distribution of that weight. But suffice it to say, since your in the "experimental phase" of this....I'd say give up a few inches of "breakover clearance" and drop the mount on the frame side......

    If you like the handling at ~50% antiquat, then you can strategize on a way to resdesign the suspension to get you that number AND maximize clearance.




    ....I told you this was going to get JUICY!!! /forums/images/graemlins/thumb.gif /forums/images/graemlins/thinking.gif
     
  12. CK5

    CK5 In my underwear Administrator Premium Member GMOTM Winner Author

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    [ QUOTE ]
    Dang....Those are some tiny pics.....for bigger ones, do a search for "yeild2me" in member rides

    [/ QUOTE ]
    Fixed them for you, just needed to link to the full size pics instead of the thumbs. /forums/images/graemlins/thumb.gif
     
  13. DPI

    DPI 1/2 ton status

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    Rob said his currently tires are 37.5" and that is what you used in the calculation. But he also said he designed it to run 42.5" tall tires. How will this effect the anti squat and roll axis? Also, I have read a bunch of stuff on rear 4 link. What if anything do you want to do differently for a front 4 link setup? SORRY FOR THE POST HI-JACK.
     
  14. Greg72

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

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    [ QUOTE ]
    Rob said his currently tires are 37.5" and that is what you used in the calculation. But he also said he designed it to run 42.5" tall tires. How will this effect the anti squat and roll axis? Also, I have read a bunch of stuff on rear 4 link. What if anything do you want to do differently for a front 4 link setup? SORRY FOR THE POST HI-JACK.

    [/ QUOTE ]


    The tires will change the CG height, and all the measurements he provided from the ground to the suspension links become invalid. ( I could do some simple math to get new ones....)

    I figured that it would make more sense to solve the "existing" problem...then take a look at a re-design of the setup (including new tire size, adjustable bracketry, etc) if the 50% anti-squat is a change in the right direction......


    We must do hard math, even before we can crawl...."
     
  15. Greg72

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

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    Oh.....a couple quick clarifications on the diagram:

    The orange horizontal line represents the CG height.
    The orange dotted line (angled) represents a 100% anti-squat value
    The yellow dotted line represents the ACTUAL anti-squat value.
    The green dotted lines represent the converging points of the links which will determine IC (instant centers) for both the X and Y axis.

    The 8-sided circles are bias-ply Swampers!!! /forums/images/graemlins/rotfl.gif
     
  16. DPI

    DPI 1/2 ton status

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    Where can I find the formulas used to determine anti-squat, roll-Axis, and the other outputs in your pic? That is something I can build into MS Excel or an Access form - including the graph.
     
  17. Triaged

    Triaged 1/2 ton status

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    That is a program that Greg made called ExcelCADv1.1.xls /forums/images/graemlins/thumb.gif

    He did a real good job combining all the data and displaying it all. I had been working on about the same thing for a while and Greg through that together in like 2 days /forums/images/graemlins/thinking.gif

    At the moment it doesn't calculate roll axis (that will be in v1.2) and just has an approximate for the link loads (I'm still working on teaching Greg 3-D vector algebra /forums/images/graemlins/eek.gif) but what it does calculate it does correctly and quickly!

    BTW:
    I love the bias-ply swamper comment...maybe v1.2 could have some radial tires /forums/images/graemlins/rotfl.gif

    Edit:
    I think one of the posts that Steve moved in here to start this forum has some info in it for calculating all that stuff. Anit-Squat is almost the same as Anti-Dive except AD has a added factor for % front breaking.
     
  18. DPI

    DPI 1/2 ton status

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    Cool, thanks for the info. I thought that looked like Excel. You think he might be willing to share the hard work?
     
  19. Triaged

    Triaged 1/2 ton status

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    I don't think he would mind at all!

    In fact I think he would love it!

    We need get a spot online to put all these spreadsheets in one place /forums/images/graemlins/thinking.gif
     
  20. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    I can host them on my site if you guys like.
     

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