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4 Link Gurus

Discussion in 'The Garage' started by 1977k5, Jan 5, 2007.

  1. 1977k5

    1977k5 3/4 ton status Vendor

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    I have been thinking about 4 linking the rear of my truck for some time. This is my first attempt at designing a 4 link so please help me out with what I need to change. I have lots of reading but that only takes you so far.

    Sorry about the way I had to attach the screenshots of the 4 link calculator, I couldn't figure out how to do it any other way :doah:

    The COG number I put in seems really high (almost twice as high as the base number in the calculator), does that sound right? I got that measurement from the top bolt on the bellhousing.

    I've heard 100% antisquat is too high but it seems I might have a little more wheelbase than most which wants more AS, right?

    I chose frame mounting points that seemed reasonable to me and I am capable of making whatever mounts I need to. The bottom of the frame will be at 25.5"

    Let me know what you think
     

    Attached Files:

  2. ntsqd

    ntsqd 1/2 ton status

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    W/o an actual measured number set, I usually assume the CG w/ a V8 to be at the intersection of the cam centerline and the rear face of the flywheel. Sounds like yours might be a bit high. Getting the fore-to aft CG location is relatively easy. It's getting the height that can be problematic.
     
  3. 1977k5

    1977k5 3/4 ton status Vendor

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    Aside from that, anyone else have any feedback? Its snowy and I don't feel like re-measuring the COG.
     
  4. Cricket

    Cricket 3/4 ton status

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  5. BLZN4FN

    BLZN4FN 1/2 ton status

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    Chris,
    I have been thinking about doing 4 link the rear of my k5. If you need a hand let me know maybe we could do it on both of our rigs
     
  6. 1977k5

    1977k5 3/4 ton status Vendor

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    The attachments I have in my first post are screenshots of the calculator :D

    You can't just plug stuff into the calculator and come up with a suspension design. Plus I don't know a ton about 4 links which is why I am asking for help.

    Thanks though :D
     
  7. 1977k5

    1977k5 3/4 ton status Vendor

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    I will definitely let you know, if I do mine first you can probably learn from my mistakes :doah:.

    After I get the design laid out I will buy the parts and hopefully be ready to wheel in springtime :D
     
  8. Cricket

    Cricket 3/4 ton status

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    If you pm Marv Springer he might be able to help or point you in the right direction. Or there's always Pirate. :dunno:
     
  9. ntsqd

    ntsqd 1/2 ton status

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    oh gawd, not there!

    What is it you're looking for other than the CG location?
     
  10. 1977k5

    1977k5 3/4 ton status Vendor

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    Does the anti-squat number seem reasonable with my wheelbase? What about the roll center and roll axis angle? The instant center? Are the links at the right proportion to each other (I had heard to aim for the uppers to be 70% as long as the lowers)? Are the links too long or too short?

    It goes on, any help is appreciated :D
     
  11. Brians89K5

    Brians89K5 1/2 ton status

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    Paxx is doing this to his Blazer, Im sure he has tons of info to help you with.

    ~Brian
     
  12. Greg72

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

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    I know a few things about ExcelCAD :wink1: so maybe I can help you....

    100% AS isn't a good target. It only gets worse (higher) when your truck starts climbing, and that's going to cause a lot of hopping. If you MUST have a target, try for something a lot lower like 40-50%.

    I can't see your numbers because the screenshots are too fuzzy, but another thing to endeavor to do is get the roll center and instant centers as close together as reasonable. Everyone seems to build a "bridge" on the axle and converges the links there, but if you converge them at the frame end instead, your numbers will be better.

    Roll Oversteer and Understeer can be reduced by making at least one set of links completely parallel to each other. I am using a simple design with two parallel upper links and a set of converging lower links (frame side) that looks quite nice

    Watch your FS values, and link lengths! (Material Selection Tab) You will be tempted to use longer links to get "nice" values, but the links will become SOOO long that they will have very poor resistance to bending loads.

    If you want to post up your values from the "blue cells" (user defined specs) other people could plug them into ExcelCAD and see your design....mess with it, and potentially post up suggested changes. [/hint hint]


    :usaflag:
     
  13. 1977k5

    1977k5 3/4 ton status Vendor

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    100% anti-squat wasn't my target, I just chose some link lengths that seemed reasonable and took measurements on the truck and threw them in the calculator.

    Here's what I have for the excel sheet:

    Wheelbase: 116
    Tire Diameter: 42
    Tire Rolling Radius: 21
    Vehicle CG height: 41.5 (measured at top bellhousing bolt)
    Vehicle Mass: 5200
    Front Unsprung: 850 (these are guesses)
    Rear Unsprung: 750


    Upper Links

    Frame End

    x: 28.00
    y: 13.00
    z: 31.50

    Axle End

    x: 0.00
    y: 2.50
    z: 31.50

    Lower Links

    Frame End

    x: 39.70
    y: 13.00
    z: 25.87

    Axle End

    x: 3.00
    y: 26.00
    z: 21.00



    That yields this Geometry Summary:

    Anti-Squat: 99%
    Roll Center Height: 31 in.
    Roll Axis Angle: -1 deg
    Instant Center x-axis: 82 in.
    Instant Center z-axis: 32 in.


    There ya go guys, thanks for the help!
     
  14. 1977k5

    1977k5 3/4 ton status Vendor

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    Is this what you mean Greg?


    Wheelbase: 116
    Tire Diameter: 42
    Tire Rolling Radius: 21
    Vehicle CG height: 41.5 (measured at top bellhousing bolt)
    Vehicle Mass: 5200
    Front Unsprung: 850 (these are guesses)
    Rear Unsprung: 750


    Upper Links

    Frame End

    x: 28.00
    y: 13.00
    z: 31.50

    Axle End

    x: 0.00
    y: 2.50
    z: 31.50

    Lower Links

    Frame End

    x: 39.70
    y: 22.00
    z: 25.50



    Axle End

    x: 3.00
    y: 22.00
    z: 24.00



    That yields this Geometry Summary:

    Anti-Squat: 52%
    Roll Center Height: 32 in.
    Roll Axis Angle: 2 deg
    Instant Center x-axis: 187 in.
    Instant Center z-axis: 32 in.
     
  15. 1977k5

    1977k5 3/4 ton status Vendor

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  16. Greg72

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

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    Chris,

    You might try a design where the parallel links are your UPPER ones, instead of your lower.....also, I'd suggest converging the links (what would then be your lower links) up at the frame end, instead of over the pumpkin. Out back, it's a lot easier to do since you don't have an oilpan in the way. You will still need to be aware of the driveshaft position though.

    The angle of your converging links appears to be rather small, so as-designed it's unlikely to hold the axle in position. You'd probably need a panhard bar wtih what you have, or work on getting a stronger angle on those bars.

    Your link separation is only about 7 inches....that's going to put a lot of stress on the heims. You can buy the "beef" for those to compensate, or try to get a little more separation.....even small gains will help a LOT to reduce the forces on those ends.

    :usaflag:
     
  17. 1977k5

    1977k5 3/4 ton status Vendor

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    Like this? The roll center height and instant center seem pretty different, aren't they supposed to be close? Thanks for the help

    Wheelbase: 116
    Tire Diameter: 42
    Tire Rolling Radius: 21
    Vehicle CG height: 41.5 (measured at top bellhousing bolt)
    Vehicle Mass: 5200
    Front Unsprung: 850 (these are guesses)
    Rear Unsprung: 750


    Upper Links

    Frame End

    x: 28.00
    y: 11.00
    z: 31.50

    Axle End

    x: 0.00
    y: 11.00
    z: 31.50

    Lower Links

    Frame End

    x: 39.70
    y: 5.00
    z: 25.50



    Axle End

    x: 3.00
    y: 25.00
    z: 24.00



    That yields this Geometry Summary:

    Anti-Squat: 52%
    Roll Center Height: 26 in.
    Roll Axis Angle: 0 deg
    Instant Center x-axis: 187 in.
    Instant Center z-axis: 32 in.
     

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