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4-link debate

Discussion in 'The Hokey Ass Message Board' started by sideweeler, Apr 4, 2013.

  1. VoodooTwin
    Joined: Jul 13, 2011
    Posts: 3,455

    VoodooTwin
    Member
    from Noo Yawk

    Back to the original question and topic: whether the mounts are installed parallel to the axle or not, there will be some bind in the link ends. But with street driven cars, the amount of expected articulation and resulting bind should be able to be accommodated by the elastic bushings in the link ends. Placing the upper link mounts as close to the pumpkin on the axle housing will minimize the bind forces in those links. In this setup, the lower links will feel the brunt of the bind, so make sure those bushings are compliant, to absorb the bind forces.
     
  2. Absolutely. We have been focused (at least Kerry and I) on the upper links. There is a WHOLE lot more going on under acceleration. The more torque that is applied through the tire, the more reaction we get at the tires and through the chassis.
     
  3. Wow, Dan, that is a killer video!

    And coincidentally enough, the De Dion tube rear suspension acts exactly the way Kerry has been proposing. All 4 links that locate the De Dion beam front to rear are in compression under acceleration... BECAUSE the Halibrand quick change rear diff is mounted rigidly to the chassis. The reaction force at the pinion is fed directly into the chassis, rather than into the suspension links as it would be on a conventional axle.
     
  4. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,284

    Kerrynzl
    Member


    I learned a valuable lesson at school many years ago. You can either go there to get an education............... or you can go there just to pass the exams.
    Sadly because of the grading systems, most of us choose the latter [ the odds are stacked in your favour ]

    Now here on the H.A.M.B I joined to make friends and to educate myself and to help educate others.
    I am not here to win an argument ,But If that is what you want to do I will give you the victory now.

    I am not here to get into arguments and bitching sessions [ I could have stayed with my ex-wife and attended “dog shows” to achieve this :D ]






    Now thanks for posting up this picture , this is now a giant step in the right direction.

    Now I am going to ask you a big favour [ or anybody else with better computer skills than I care to have ]

    On the bottom of your picture you have a Yellow Arrow pointing left towards the rear [ indicating tyre rotation ]
    Please note this arrow doesn’t represent thrust as is quoted it actually represents TORQUE [ the wheel centreline is what moves to the right in this picture ]

    So could you add another Yellow Arrow at the centreline of the axle and pointing towards the right [ this represent axle thrust ]
    And then could you add another 2 arrows to the upper and lower pivots ,also pointing towards the right [ to represent the axle thrust being transferred into the car via the links ]

    Don’t remove any of your existing arrows they are all correct. [ just add some arrows to represent thrust which also represents acceleration ]
    If you don’t understand what I mean, go back to the picture I posted on # 138 of this thread [ the lower drawing ]

    You will notice on the upper link 2 arrows going in opposite directions [ cancelling each other out ]
    You will also notice on the lower link 2 arrows going in the same direction [ increasing the load ]

    After you’ve done this, really have a good look at the result and think about it [ it is exactly the same forces found in a 5-link , except they are now combined together in a 4-link ]

    All these force need to be represented here , if there is no thrust the car remains stationary
     
  5. pitman
    Joined: May 14, 2006
    Posts: 4,918

    pitman

    Kerry, you raise questions that help me to think this one through, appreciated!
    If you take latest force diagram, and pin the top mount (a tool used in free-body diagram analysis) then consider the tire traction force 'balanced' against the lower mount's force, it started to gel for me. As the tire friction generated force varies, so too the upper and lower mount forces then will vary. The axle-to-bearing centerline force is intermediary, and what it does help to visualise, are where the applied load (hypoid gear C-L) and the 'reaction' loads end up. Tire friction "causes" increases in mount reactions.
     
  6. Done.
    [​IMG]

    Never denied those forces exist, ever... My point, this whole time, has been that the reverse torque reaction from "pinion climb" greatly, (and visibly), exceeds the forward thrust force on the top links...putting them in tension. (By the way, the arrows lengths do NOT represent magnitudes of vector forces. I never intended to get that technical with it...)

    And I don't need you to concede victory... What I don't want to see is someone ELSE taking the information and incorrectly designing a suspension system that doesn't work properly, or worse.

    Once again, I am only looking at the "worst case scenario" of hard acceleration; which is exactly the situation where everything needs to be right.
     
    Last edited: May 13, 2013
  7. I'm going to ask a simple question, anyone feel free to answer...

    IF the thrust force always exceeds pinion climb (top of the housing is pushing forward), why would we ever need a pinion snubber or traction bars to stop "axle wrap"?

    It doesn't matter what suspension we have attaching the housing to the chassis, the net force at the axle centerline is always the same...
     
  8. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    CutawayAl
    Member
    from MI

    I wasn't going to post any more about this, but apparently I'm not smart enough to take my own advice.

    This statement; "The thrust only becomes thrust because the torque of the tire is anchored to the ground through the tire contact patch." is the essence of the misunderstanding. The tires are not anchored to the pavement. It is essentially a hinge. If the tires were fixed to the pavement then yes, the pinion could climb the ring gear and lift the front of the car. Since the tires are not clamped to the pavement the only force available to make this happen is the inertia of the axles/wheels/tires.

    Visualize a giant pair of hands holding the wheels so the pinion climb the ring gear, lifting the front of the car. The moment the giant hands let go the front of the car will pivot on the tire/road "hinge" causing the front of the car to fall. As has already been posted, wheelstands happen when the car's CG, reat wheel location, and forward acceleration, all compliment each other to make it happen. Saying that axle wind-up(torque reaction) can lift the front of the car is like saying one can lift themself off the ground by pulling on their shoe laces.

    As to experts agreeing; I have heard true experts in their field say things that are completely wrong. No one knows everything. Just because a mob all agrees doesn't mean they are right. Look back through automotive history and racing. There are a lot of things that were once common practice that aren't done anymore. That happens when eventually someone not brainwashed by current practice figured out it was a bad idea. Some of the claims and observations posted are not invalid. The error is in the interpretation of what is being observed.
     
  9. The tires ARE fixed to the pavement at any instantaneous point in their rotation; that is, if the tire contact patch moves one foot, we'd like the ground to move the same one foot relative to the chassis. Just because the tire is rolling across the ground doesn't mean the contact patch isn't anchoring the torque from the axle; that is traction. If it weren't we'd have wheelspin, which is exactly what happens when your "giant hands" let go.

    And sorry, but your statement about "lifting yourself off the ground by your own shoelaces" is simply idiotic. Nobody said anything about reversing the law of gravity...! With the amount of torque produced by a racing engine, gear multiplication, and sticky tires; it is MORE than possible to lift the front of a car off the ground by pinion reaction.

    Try to explain your point to any driver who has experienced a wheelstand in a rear engine dragster. They have a VERY low C/G and NO weight behind the rear axle C/L... If the pinion reaction were so insignificant, and the "giant hands" let go, WHAT force is rotating the front of the car up?

    Racing is a very empirical pursuit; in most cases the technology is based on YEARS of trial and error. Those top pull bars are used BECAUSE they work (they make the cars faster), not because one person who was "wrong" convinced everyone else (manufacturers) to take the wrong approach.

    Believe it or not, I do understand the fundamentals of physics. I also understand that there is a LOT going on with the rear suspension as a car accelerates. You can show me pages of calculations, but when I SEE a thing happen, and have dealt with tuning that reaction, and with breakage caused BY that reaction; I have to think that something was left out of the equation if the result states that the OPPOSITE is happening....

    The question still remains: If the top of the housing truly is pushing forward under acceleration, then WHY do we need a pinion snubber or slapper bars? WHY do the top links on ALL drag race 4 links run downhill to generate anti-squat? WHY do the pull bars in the videos visibly get longer? WHY does a truck arm suspension with ONLY lower links lift a 3400 lb NASCAR stock car several inches above ride height when accelerating?

    I've been enduring you 2 inferring that I'm some kind of idiot (as are pretty much the entire racing industry) and I will gladly take my lumps and agree with you as soon as you provide the answer (all I have heard is "if you don't get it by now, you never will").
     
    Last edited: May 13, 2013
  10. blowby
    Joined: Dec 27, 2012
    Posts: 6,592

    blowby
    Member
    from Nicasio Ca

    Did any of those pull bar cars not have floaters? As has been noted, with the axle pushing the car through those the rear is free to rotate, not good evidence. You have stretched heims and pulled threads from upper links, that should be proof right there since it didn't likely happen under braking.

    Bending leaf springs on launch is also proof of lift created, or at least attempting to be. But I'm not sure it's proof that an upper bar would be in tension at that point, it could still possibly be pushing the car forward.

    If anyone is inferring that you are an idiot it's not working on those of us that are worlds behind in understanding this. I'm in the midst of a rear suspension design so it's of particular interest to me.
     
  11. Paul,

    Even if the rear axle is on floaters, it makes no difference. The bottom line is the top link itself is visibly getting longer, against spring pressure. That's why I picked those videos. Sure, the rear axle is free to rotate, and the point is it wants to rotate...to the rear! If that's not in tension, I guess I need a new definition....

    If the top link were always in compression (of any amount), the axle could still drive forward under it; but the link itself would never try to stretch, which it very obviously does. If I clamp the floaters, or weld the brackets solid to the housing tube; that basic force does NOT change.
     
  12. DICK SPADARO
    Joined: Jun 6, 2005
    Posts: 1,887

    DICK SPADARO
    Member Emeritus

    Boy you guys are having a good time with this thread. exwest this is my answer to your last questions #252 post.

    (The question still remains: If the top of the housing truly is pushing forward under acceleration, then WHY do we need a pinion snubber or slapper bars?)

    The pinion snubber is used to limit U joint alignment angle or in the case of leaf spring package control spring wrap..


    (WHY do the top links on ALL drag race 4 links run downhill to generate anti-squat?)

    Top links run down hill to produce lift and weight transfer rearward. Top link angle down ward to front produce lift weight transfer rearward. Angled up produces chassis squat but less weight transfer.

    (WHY do the pull bars in the videos visibly get longer?)

    If the pull bar located atop on center it has to get longer to compensate for the gear torque rotation of the rear axle, it's a slip joint. The spring is used to control the rate and distance of rotation so the torque doesn't spin the tires.

    (WHY does a truck arm suspension with ONLY lower links lift a 3400 lb NASCAR stock car several inches above ride height when accelerating?)

    Because the arms point upward and the gear torque rotates the axle housing thus producing a big lever lifting the chassis.


    Didnt this tread start out to determine the compliance of a 4 bar unit with triangulated upper links during bump or roll. Where the tri upper link rotated to a central axis point forming a locating link lessening the torsional twist while the lower link longitudinal link twisted thru the materials composition and the locating end design. In both upper and lower bars with rubber bushings the compression composition of the bushing determines the degree of twist of the rods. If rod ends are used the degree of lateral rotation of the rod end before the twisting force is applied to the rods. .

    NNNNNEXT
     
  13. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    CutawayAl
    Member
    from MI

    Really?.... You are saying if the trans is in park, the front of the car is jacked up, it will stay there? That's what would need to happen for your theory to be correct.



    I used the example because it is idiotic. Unfortunately it is also analogous to what you are claiming. Per my comment above, no matter how much pinion reaction, sticky tires can't hold the front of the car off the ground.



    Geez, this has been explained to you at least three times. Once again; It is the relationship between the car's CG, the location of the rear tires, and forward acceleration. The same effect could be duplicated by putting the car on an angled surface with the rear wheels locked. In that case the angle of the surface would be equivalent to acceleration. Although the cars do have a low CG, the CG is close to the rear wheels. The harder the car is capable of accelerating, the further forward(or lower) the CG needs to be to avoid a wheelstand.



    I am niot saying you are clueless. I am saying you just happen to be mistaken about this. If you asked the participants at a drag race about this most would agree with you. It seems intuitive, but it's wrong. As I already pointed out, you are misinterpreting what you have seen and what you do know.



    I have in no way insinuated you are an idiot. I have said you are wrong. I'm sorry you don't like that, but things are what they are. Contrary to your claim, information has been provided. Apparently you are either unwilling or unable to understand it. I believe you are so committed to your misconception that nothing anyone could say would change your mind. When a person already has their mind made up they are not open to alternate paths of thought. It happens to everyone.

    I am already repeating points made previously. I don't see an epiphany shaping up here, so good luck to you.
     
  14. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,139

    gimpyshotrods
    ALLIANCE MEMBER

    Don't make me turn this board around, you two.
     
  15. blowby
    Joined: Dec 27, 2012
    Posts: 6,592

    blowby
    Member
    from Nicasio Ca

  16. Ok, you have GOT to be screwing with me now, right? Right??? If the contact patch of the tire is NOT anchoring torque output to the ground at any point in time, it is spinning, period. I didn't think you could top "pulling myself of the ground with my shoe laces". I was wrong...


    Funny, I was thinking EXACTLY the same thing.

    I have learned 3 important lessons from all this:

    1) I cannot trust what is right before my eyes.

    2) An entire industry has been making progress for 30+ years along a path that is completely wrong.

    3) Isaac Newton was an idiot.

    Paul, don't bother... I'm going out now to lift up my car, put the trans in park, and sleep under it, because I KNOW it will just hang there. I love spending nights under the stars.
     
  17. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,284

    Kerrynzl
    Member

    Hey Guy’s please keep the sarcasm and insults out of this thread. And no ganging up on people please.
    Some of the greatest creative minds in history went against conventional thinking.

    If you don’t agree with anything here [ including mine ] Question it !

    What we have going on here is a “black swan problem” or a Deductive falsification

    I’ll give ALL you guys are question here to think about [ please look at all the arrows that exwestracer drew on his drawing ].[ this is not an answer, but purely to make you think about it ]

    The question.........
    If a car accelerates forward 6 inches from a given point , but the top of the rear end only accelerates forward 5-1/2 inches from the same given point [ due to pinion torque reaction ],
    Is the top moving backwards 1/2 inch or the bottom moving forward an extra 1/2 inch ??

    With deductive reasoning it is all dependent on where you look at it from [ if you look at it from inside the car, the top appears to pull back ]
    If you look at it from the outside of the car [ from the side of the track ] the bottom appears to be pushing forward.


    Here’s another question [ a chicken or the egg type ]
    On a dirt modified with a “floater” 4-link and a torque arm , If you got 2 strong men to lift up the rear of the car the diff will rotate upwards.
    Does the torque arm rotation cause lift , or does the lift cause torque arm rotation??

    Understand a 5 coil modified usually has a 250-300 lb spring on the torque arm, yet they can accelerate a 2400lb modified at approx 1G.
    Modified racers also alter the angle of the links to the floaters to increase “forward bite” [ There is a picture on posting # 138 for you all to think about ]

    So does the majority of the lift come from the outer links or the torque arm??

    So sorry to throw a spanner in the works here :D






    "If we knew what it was we were doing, it would not be called research, would it? Albert Einstein
     
  18. blowby
    Joined: Dec 27, 2012
    Posts: 6,592

    blowby
    Member
    from Nicasio Ca

    Hi Kerry,

    My guess at your first question is: Whichever one had the extra slop!

    Don't know enough about the dirt cars to even guess at the second.
     
  19. DICK SPADARO
    Joined: Jun 6, 2005
    Posts: 1,887

    DICK SPADARO
    Member Emeritus

    Great first thing in the morning a test! Hi Kerry. Here are my answers to your questions.

    The question.........
    If a car accelerates forward 6 inches from a given point , but the top of the rear end only accelerates forward 5-1/2 inches from the same given point [ due to pinion torque reaction ],
    Is the top moving backwards 1/2 inch or the bottom moving forward an extra 1/2 inch ??

    With deductive reasoning it is all dependent on where you look at it from [ if you look at it from inside the car, the top appears to pull back ]
    If you look at it from the outside of the car [ from the side of the track ] the bottom appears to be pushing forward.

    If the bracket holes are perpendicular mounted to the rear, the top rotates rearward and the bottoms rotates forward equal distance however the central axis of the axle will move a distance of the operation radius of the arc created by the rods based upon the position of the front mount.

    Here’s another question [ a chicken or the egg type ]
    On a dirt modified with a “floater” 4-link and a torque arm , If you got 2 strong men to lift up the rear of the car the diff will rotate upwards.
    Does the torque arm rotation cause lift , or does the lift cause torque arm rotation??

    Not quite. If you just lift the rear of the car the axle falls and the fixed torque arm rotates the axle in the arc of the arm front mount position. Points up slightly. Under acceleration the gear torque of the axle produces the force and the torque arm is a lever fixed at the rear mount producing a lifting force at the front mount. Points up slightly again based upon torque arm radius.
    Understand a 5 coil modified usually has a 250-300 lb spring on the torque arm, yet they can accelerate a 2400lb modified at approx 1G.
    Modified racers also alter the angle of the links to the floaters to increase “forward bite” [ There is a picture on posting # 138 for you all to think about ]

    So does the majority of the lift come from the outer links or the torque arm??

    This adjustment changes the center of gravity pickup point, arm generates the lift.

    Did I pass or fail???


    So sorry to throw a spanner in the works here :D






    "If we knew what it was we were doing, it would not be called research, would it? Albert Einstein[/QUOTE]
     
  20. Well, it seems we are returning to some productive discussion... I'll take a swing at this one.

    First off, let's make sure we are all on the same page. I am looking at this question pertaining to a drag race 4 link with steel heim joints. Lower links level to the ground (as we agreed they typically are). No bushing slop, no "floaters", no nothing. Let's purify the case as much as possible, ok? My answer:

    It does...not...matter. We can say that the bottom (and the chassis) is "out accelerating" the top of the housing by that 1/2". Fine. Since the top and bottom chassis brackets are rigidly connected (in most cases the SAME piece of steel), then the top link chassis bracket must also be "out accelerating" the top of the housing by that same 1/2". Which would mean that a link connecting the chassis bracket to the top of the housing would be pulling the housing along after it; in other words, in tension.

    So my return question is, what force is causing the top of the housing to "lag behind" that 1/2"? Something must be restraining it from keeping up. If not pinion climb, then what?

    (SEPARATE POINT) As I mentioned to Paul in #254, I like looking at the reaction on an axle that does have floaters, because the top of the housing is free to do whatever it wants. It is not constrained by any forces going through the outer tubes and links. In every video I've posted, the top of the housing CHOSE to stretch the top link under acceleration, regardless of what the rest of the housing had in mind. If we want to look at it as the bottom of the housing was "out accelerating" the top; as long as the top of the housing was still "pushing", it would NOT have compressed a SPRING built into the link. Even if we agree that the top was pushing (at some reduced amount), the link still visibly extended (tension), did it not?

    The difference (and I believe a flaw in the logic of this question) is that the 5th coil will not compress if the chassis is physically lifted at the rear. You admit here that the 5th coil does compress under acceleration, correct? And that 250lb spring will compress 2-3 inches at the end of a 3 foot long torque arm.

    Let's keep the math simple. 250lb spring compressed 2" takes 500lb at the END of a 36" arm. Since torque is divided by the length of the bar (your math in post #202), to convert to lb/ft we multiply by the 3 feet and find 1500 ft/lb of reverse rotation torque at the axle centerline. Maybe my maths are a bit off here but I would hardly call that insignificant!

    Oh, and since the axle in question IS on floaters, exactly NONE of that force against the 5th coil is coming from thrust against the lower links...

     
    Last edited: May 14, 2013
  21. For those of you to whom this "5th coil" stuff sounds like gibberish, this might help:

    http://www.youtube.com/watch?v=1ixkibG2hOQ

    The 5th coil is the spring and shock mounted at the front of the torque arm that connects to the rear axle center section. Looks like it might even be moving further than my 2" example...;)
     
  22. DICK SPADARO
    Joined: Jun 6, 2005
    Posts: 1,887

    DICK SPADARO
    Member Emeritus

    return question is, what force is causing the top of the housing to "lag behind" that 1/2"? Something must be restraining it from keeping up. If not pinion climb, then what?

    It doesnt lag behind the axle housing is rotating under the gear torque to the rear tires.

    (



    The use of a spring rod or spring-shock combination in a torque arm is mainly used to control the rate of acceleration torque force and prevent the tires from spinning when the power is applied to quickly.
     
  23. Right, Dick. Just trying to put it in the terms as Kerry presented it. You kind of have to read the WHOLE thread to see how we got (back) here...
     
  24. Pops1532
    Joined: Jun 19, 2011
    Posts: 544

    Pops1532
    Member
    from Illinois

    [/QUOTE]

    From someone that's been around dirt racing for 50 years I'd say you passed with flying colors. Rayburn, Masters, Bloomer,Richards,Pierce and the rest of the guys that make a very good living building dirt late models would agree.
     
  25. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,284

    Kerrynzl
    Member

    You all passed !
    It is not important whether get it correct . The purpose of those questions is to get you all the “Step outside the circle” and look back in.
    In other words try to think about it from a different perspective.

    I asked my brother today the following question except I also scribbled out a diagram



    His 10 year old daughter was listening in. She said “if the top bar moved backwards ,then it would have to be behind the starting point”
    Ya gotta love kids, the simplicity of their thinking.

    Here’s another one for you dirt car guy’s
    If you have a “pull bar” [ not a torque arm ] off the top of a floating diff housing , does it create any lift during torque rotation [ without any bottom link ]
     
  26. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 8,042

    theHIGHLANDER
    Member

    Royce made mention of how something simple will always become complicated. I also see the mixing of usage in this theoretical discussion. I like simple, easy, basic. They never lie. 4 links are long "imaginary" ladder bars. All of them, ladder bars, 4 links, 3 links and truck arms, are all LEVERS. It's foolish to ignore the forces that make these levers work. exwest, good call intorducing the massive multiplication of TQ that imparts leverage to the rear suspension arms. I know this all started about bind within the arm's design, but the bind is insignificant in all but rock crawlers.

    The car moves before the axle does when instant power is added. Instant power being the engine's TQ multiplied from say 600lbft to as much as 4500lbft. Does the axle move or "roll" or "climb the ring gear"? Not really but it's a good way to understand the forces that do the WORK. Bars pointing down, when measured from the axle or wheel centerline, GET LONGER when the leverage designed into the system is applied. I went through this with the ladder bars on my racer. "You gotta have em point up so it leaves more violent. Your 60 foots will lower by a bunch!" No, they won't and didn't. Bars pointing up want to move the car rearward the moment the trans brake is released. Using the "anchor" analogy, for a very brief moment in time when power is applied the tires "gain weight" due to the leverage used for the work. The car, the point where the bars are anchored, moves forward by as much a 1/2" giving it a quicker and more consistent reaction time. Understand that this was in MY case, not all cars doing battle in the 1/4 mile, but several did improve when looking at what I'd reasoned out for bar placement. We see a lot of discussion about instant centers, CG, pinion angle, and most of that complicated sypherin is needed when the packaging demands a 4 link due to limited space.

    In the topic's 1st post, while it may be a bit better to have the links all on the same axis, there's not enough travel to make it an issue. The ride would be more compliant as it moved through it's travel when all of that "swing" is happening in harmony. Frankly, it looks better too. Looks more "OEM" having them all in sync. But still, imagine those big heavy "X-Frame" GMs with tht simple 3 link. How could it possibly work?
     
  27. Well, I'm glad we can all be friends again. Call it what you want, I'm trying to point out to others who DON'T understand all of this that running the top links of a 4 link suspension uphill will not generate anti-squat; as generically saying they are in compression would have us believe... I may have over-simplified my version of the explanation (the "winch cable theory", for example) to emphasize the NET effect that placement of the links will have on the system. This:
    [​IMG]

    And this:
    [​IMG]

    cannot BOTH be correct, can they? (As I mentioned earlier, the top image IS of a "birdcage" or housing floater)

    It "creates" lift by anchoring the top of the housing to allow the floaters to drive under the chassis, pushing up on the outer links. I believe most of us understand that principle.
    http://www.youtube.com/watch?v=BqAoMRnNnoU&list=PLE2ABA35D0392CD3D
    Now, I'd still like an explanation of how that is possible if the top link (as you have said MANY times) is under a net compressive force.

    I've asked this question a number of times, but here it is again (for benefit of those showing up late I guess)...

    To keep this somewhat HAMB relevant, let's go back to a non-floating type 4-link (as in the lower drawing above). The SAME reaction that "pulls" on the pull bar (again, visibly present in the video) is now connected to the upper links of the 4 link (unlike the case of a floating housing). If, as you say, they are under a net compressive force, would that not cause them to push down on the rear of the chassis, causing the chassis to squat under acceleration?
     
  28. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,284

    Kerrynzl
    Member



    You have to realize that gear reduction is torque multiplication [ this the very reason that we “change down” a gear on hills or to accelerate more ]
    There is no way that driveshaft torque can be greater than axle torque in a typical rear end.

    Pinion torque reaction is a product of driveshaft torque [ it tries to climb the crownwheel ] creating lift.
    Axle torque reaction is a product of driveshaft torque multiplied by gear reduction [ it tries to propel the axle forward against traction ]

    Altering the rear gear ratio doesn’t alter pinion torque reaction [ this comes from the engine end of the vehicle ], but altering the rear gear ratio certainly alters the axle torque reaction

    So this is the reason why axle thrust is always greater than pinion lift [ it is purely because of mechanical gear reduction ]

    The ONLY reason you SEE the diff rotate backwards in a typical 5-link [ birdcages, torque arm etc ] is because the pinion torque reaction is disconnected from a greater load of axle thrust [ which would basically cancel it out any tension the top link ,if combined ] Lift at the front pivot caused by weight transfer can also alter the pinion angle

    All you have to do is understand that it is combining greater compression loads with lesser tension loads on the same link [ refer to drawing on posting.# 250 ]


    Now if we look at a typical 5-link which is where the debate here seems to differ from 4-links, you can clearly see the instant centre for the 4 outer links is very low and behind the rear axle.

    http://www.youtube.com/watch?v=m_uYfLs0WD0&list=PLE2ABA35D0392CD3D Have a good look at the angle of the links from about 55 secs on
    .
    Having the instant centre behind the rear axle puts the IC above the anti squat line, and pointing the links upward also increases the forward bite and rear lift during acceleration [ it also shortens the wheelbase during acceleration, which would cause “pull bar” rotation ]

    These cars in the videos have 60%+ rear weight bias, a high CGH, and a short wheelbase. When they accelerate hard they can easily “carry the front” due to weight transfer.
    Now the accelerating forces through the links cause the rear to lift [ which is evident in the videos ] a pull bar cannot do this
    The combined forces of thrust and weight transfer cause the whole car to “Leap up” out of the corners.

    To the untrained eye, it looks like this was caused by the torque arm / pull bar at the centre of the car. Many people have been fooled by this

    There is always torque reaction in all these situations but it is certainly a lot less than the axle thrust [ or axle torque reaction ]
     
  29. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 8,042

    theHIGHLANDER
    Member

    Just a personal peeve of mine, it's the load that transfers rather than the weight. The weight remains the same, the load becomes greater when the CG is "relocated" by raising the front of the vehicle in question. In a static state the weight is on all 4 (or 2 or 3) tires in various percentages, but using 50/50 and 3000lbs you have a 1500lb LOAD on the front and rear pairs. If upon launch the CG is now higher and further back, AND the front tires are no longer LOADED, the load is now placed on the rears. Like the TQ being multiplied by gearing, the load also becomes multiplied by LEVERAGE. I know we colloquially use the term "weight transfer" an it's a generic way of mentioning the dynamics, but I was able to get a better grasp on things by understanding those dynamics and the actual forces applied by separating weight and load. This came about through discussions with a friend (now deceased) who was an industrial designer/manufacturer in the hydraulics industry that took an interest in my adjustable shocks. Just sayin, carry on...
     

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