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4 Link 101

Discussion in 'Center Of Gravity' started by zcarczar, Jan 15, 2004.

  1. zcarczar

    zcarczar 1/2 ton status

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    Ok, Ive decided I want to do a 4-link rear end on my rock crawler. I also have noticed that a few of the members have designed and are building linked setups on there trucks. So how about lets have a thread with the basics of a standard triangulated 4 link like Willyswanter did on his K3500.

    Ok, so what are some of the basics for all of us 4-link "newbies?" Also what are some good books to read to further understand the design theory's that go into making a 4 link truly work as performed? Also what are some ways to find COG, Roll Center and Roll Axis and other various terms I have seen in the 4-Link discussions?
     
  2. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    One of the best books is generally regarded to be Race Car Vehicle Dynamics by Milikin. This is more track car oriented, but the concepts and math are generally applicable once you actually understand it. Unfortunately most of the book has no relevance to wheeling and it's quite expensive. /forums/images/graemlins/frown.gif

    Another is Fundamentals of Vehicle Dynamics by Gillespie. I've heard references to it, but not nearly the positive input as that of the Miliken book.
     
  3. Triaged

    Triaged 1/2 ton status

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    I really liked Carroll Smith's (R.I.P.) books. They don't have any equations (I think there are 2 total in his 5 books) but give you alot of good info and rules of thumb on what is a good design and what will get you killed. A good one to start with would be "Tune to Win". If you remind me I could bring my stack of books (it is about 2' at the moment and I just bought 2 more /forums/images/graemlins/eek.gif) to some meeting and you could take a look at them and see what you want to buy. As for the "RCVD" book there is only 1 chapter that most people would want to read. It is not a book you could just sit down and read straight through where as Carroll Smith books are good reading.

    You also need to go over to Pirate4x4 and read the "god of suspension" and "link suspension's for dummies".
     
  4. willyswanter

    willyswanter 1/2 ton status GMOTM Winner

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    Yep, I read for months on pirate before doing anything... It's free and probably better than any book...

    Next I read RCVD by Millekin and it is a huge book and rather spendy as mentioned... But some good info.

    Then I read Chassis Engineering by Adams and Engineer to Win by Carrol Smith. All good stuff but it's all car related but still works for us... Just harder when your frame is 2 feet higher off the axle than a race car is... /forums/images/graemlins/rotfl.gif

    You can see my autocad drawings in my 4 link thread that shows all the variables and what my measurements came out to.
     
  5. Bubba Ray Boudreaux

    Bubba Ray Boudreaux 1 ton status

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    How "awn" was that write-up that appeared in 4WOR a few months ago? Was it just enough info to get a newbie in trouble, or was it a good primer for coming close to getting a 4 link setup right?
     
  6. willyswanter

    willyswanter 1/2 ton status GMOTM Winner

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    It was actually pretty good according to me... For me it basically helped show me how to relate what I was reading in the books about "car" setups to the truck more... But nothing helped me more that pbb...
     
  7. Bubba Ray Boudreaux

    Bubba Ray Boudreaux 1 ton status

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    [ QUOTE ]
    It was actually pretty good according to me... For me it basically helped show me how to relate what I was reading in the books about "car" setups to the truck more... But nothing helped me more that pbb...

    [/ QUOTE ]

    For the next project, shall we see what it takes to 4 link a Honda Accord?
     
  8. willyswanter

    willyswanter 1/2 ton status GMOTM Winner

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    Bwahahahahaha, that would be kinda cool... /forums/images/graemlins/rotfl.gif
     
  9. Bubba Ray Boudreaux

    Bubba Ray Boudreaux 1 ton status

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    [ QUOTE ]
    Bwahahahahaha, that would be kinda cool... /forums/images/graemlins/rotfl.gif

    [/ QUOTE ]

    Some super A-Arms also. Keep it the same ground clearance, and hide some shocks up in the body, kind of a closet prerunner. Something along the lines of these new fangled Trophy Trucks coming out that are sitting close to the ground like a short course truck, but has some serious wheel travel.

    Seriously without wrecking the thread too much, I was just wondering about that 4WOR article. I don't pay attention to the threads on PBB, but I read that article and actually comprehended the science into the calculations and measuring and thought "well heck, if it's somewhat in the neighborhood, even my dumbass could probably whup something up."
     
  10. Greg72

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

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    [ QUOTE ]
    ...Also what are some ways to find COG, Roll Center and Roll Axis and other various terms I have seen in the 4-Link discussions?

    [/ QUOTE ]


    Jason,

    Have you spent any time looking at ExcelCAD v2.0?

    That whole project started when I was asking these same questions about 6 months ago. I didn't understand how all the terms related to each other. Once Triaged starting answering my questions in the forums here (and through massive amounts of PMs!) I started plugging in formulas and generating some very basic spreadsheets....

    As we collaborated, Dan started introducing me to more sophisticated ways to calculate things (using more real math, and fewer assumptions) and the chart continued to evolve and grow. Once it had a "graphical" component, my learning curve seemed to REALLY move quickly. I tend to be VERY visually-oriented, so when I could make changes and see the resulting changes in a suspension diagram, the concepts really started to make sense.

    If I were you, I'd spend some time with ExcelCAD (it's FREE!) The hardest thing is getting used to using a coordinate (x,y,z) system to take measurements off your existing truck/buggy so that you can plug the data into the sheet in the correct format.

    Some quick comments on what you can learn:

    1. AntiSquat/AntiDive - Depending on your link design, these values can be all over the place. Making some seemingly minor changes to the design can "reel in" numbers that are way out of line. They are tied to the CG height (and CG is not that easy to figure out) so you still have to make SOME assumptions...

    2. Loads - The stuff that Dan did for calculating loads was some of the more fascinating stuff to me. I never had a good understanding how "how much is enough" with regards to tubing thickness, rod end sizing, etc. It was always a "well THAT one looks like it won't break" kind of guess. Now with the safety factor values that are calculated, you will KNOW how well your parts will work.

    BTW -> When you start playing around with the position of your end links, you will be amazed at how different the FS values can be. A well designed suspension, can get away with smaller rod ends and be just as safe as lesser designs with massive rod ends. It all comes down to vertical seperation of the links....it's not always easy to do, but if you can make the packaging work in "real life" you can save a small fortune by using the smaller rod ends!


    The other thing that really helps (me at least) is mocking this stuff up in real life. (I'm visual, remember?). That experience of building a 3-link front suspension, and then building a 4-link front was priceless. There is nothing like doing something in full-scale.....being able to "cycle" the suspension....realize it has some sort of massive interference problem or geometry problem, then go back to ExcelCAD and try a new setup. Those repeated attempts are another great way to learn in a hurry..... experience is worth a lot.

    BTW -> A rear multilink will be a LOT easier than a front suspension design. There's no steering to worry about and the tires won't be trying to turn and rub against your links and do all sorts of wacky stuff... I started at the front because I knew it would be the hardest, but once I get it nailed....designing the rear will be a cakewalk.

    I remember talking to Dr. Watson, and he said that his rear suspension came together VERY fast once he had the experience of doing the front. I recall him saying that he designed it, built the first mock-up, cycled the suspension ONCE to confirm that everything was cool.....then he welded it up and went wheeling! /forums/images/graemlins/cool.gif (Your experience may vary.... /forums/images/graemlins/wink.gif)


    Anyway, try out the program. Save a copy with your data in it and post it up....we can give you feedback and hints about what looks good, and where the concerns might be.

    /forums/images/graemlins/thumb.gif
     
  11. zcarczar

    zcarczar 1/2 ton status

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    Awesome, thanks for the help. Now for a Quick Question on COG, The vehicle that is getting the 4 link is already been beat to hell body wise(read:Rolled a couple times). So Can I use a Hi-Lift or a Forklift to find the COG before it rolls to give me a more accurate measurement of COG?
     
  12. Greg72

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

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    You'll have to go back and look at the threads about taking those measurements.

    There is some disagreement about what constitutes a "valid" measurement. I recall something about whether the springs should be allowed to compress, or if they should somehow be "locked out" so that they don't flex during the measurements.

    I have never measured my CG....(you can use the camshaft centerline, or top tranny bellhousing bolt measurement as a rough estimate)....but I plan to design my suspension bracketry such that I can change the link positions to account for a CG that might end up a few inches off in either direction.

    If you're building something basically "from scratch" it's hard to know where the CG is going to end up anyway. You kind of need to build based on a "best guess" and then measure and validate it later on.
     
  13. Triaged

    Triaged 1/2 ton status

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    Balancing the truck right before it rolls is a very good way to find the overall CG. That is how we did our little race car at school. We took the shocks out and made solid bars with with holes drilled in them so it would always stay at ride height to replace the coilovers. We than tilted it forward till it balanced then backwards till it balanced and used the average of the 2 values. As Greg said there is a bit more that goes into it than that...In order to get the AS value you need the sprung mass CG. All you would need to know is how much un-sprung mass you have and it could be calculated (un-sprung mass should be light enough to find on a set of bathroom scales).

    Besides trying to get your truck as close to rolling as you can without actually rolling it sounds like fun /forums/images/graemlins/thumb.gif
     

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