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  • Anti Squat Modeling



    Before we talk about the consequences of antisquat tuning I want to figure out where the 100% antisquat (aka no force) line and instant center (IC) exist on the S14. I tried to get some discussion going on zilvia, but I don't think there are enough engineer minded folk finding the thread.

    It's well accepted that the antisquat line always go through the IC. But there seems to be some disagreement as to where that line should start? Can anybody confirm that a car with a independent rear suspension calculates anti squat with a line through the instant center and the center of the wheel, rather than through the rear tires contact patch? And if so why? I found this textbook page, but I've seen a lot of contradicting information stating that IRS cars' anti squat is calculated the same as a sprung 4-link.



    The difference is dramatic. Using the contact patch, as is standard when calculating antisquat in sprung 4 link drag cars you get about 200% [BLUE LINE] antisquat for an s14 and a completely rediculous >300% in an s13. But Using the textbooks wheel center method for an s14 gives a makes-no-sense 0% antisquat [ORANGE LINE]. 120-150% [GREEN LINES] is standard for a road race car, and of course it's possible that my pic is a bit off, but this isn't making a lot of sense for a performance car.



    Originally I stated that I've been getting conflicting reports as to whether the anti-squat line should originate at the contact patch or the center of the rear wheel. The more I read, the more certain I am that the contact patch is only used to create the antisquat line in solid axle designs, and that the orange line is indeed the correct line to use (rear wheel CENTER to IC) to calculate the antisquat line in IRS designs.

    Here's a link to Bill Shope's web page. He is a retired Chrysler Engineer (one of the original ram chargers) and Professor of race car dynamics at Florida State agreeing

    http://www.racetec.cc/shope/

    and a quote from that site:

    "It is apparent, then, that...with an IRS or DeDion...the IC will be much higher for a given percent anti-squat [compared to a solid beam axle]. This can create excessive roll oversteer. The launch squat common with production IRS cars indicates that the designers have chosen to sacrifice some anti-squat to avoid the roll oversteer"


    Clarification: I refer to 100% antisquat line (no induced force) as the pink line (a lot of discussions will refer to the blue line as 100%. I call the blue line 200% antisquat.



    Further evidence that the s14 has ~0% anitsquat: I remember the one time I drag raced my s14 at the track and at launch the rear would squat so much that I was a fraction of an inch from draging my stock tow hook! Any car with 200% anti-squat (as the blue line starting at the contact patch would indicate) will actually lift the rear end of the car on launch like a properly set up drag car. It's obvious that the 240 doesn't lift the rear on launch.

    Here's a s14 running real QA1 drag shocks, with noticeable squat

    http://www.youtube.com/watch?v=wMgVy3yxi8c

    Here's a mustang. While the guy struggles to stage you can see how softly sprung the car is, it wobbles all over the place. But whatch his launch around 1:12, a couple times. Even though he has enough power and traction to lift the front wheels completely off the ground THE GAP AT HIS REAR TIRE NEVER SHRINKS... no squat... That's how a car with 200% antisquat should behave on acceleration.

    http://www.youtube.com/watch?v=Lq4FzEWdDNM

    Finally here's a paraphrase from Herb Adams book "Chassis Engineering"

    http://books.google.com/books?id=rY2...ing%22&f=false

    "Only real downfall of a IRS is the anti-squat, practical limit is about 25%. While a drag/live axle car can get 100% or more"
    Last edited by Gigapunk; 11-05-2010, 01:01 PM.

  • #2
    Found this site:
    http://www.racetec.cc/shope/

    The correct point for IRS is the center of the wheel.

    To tired to think clearly now. Will have a closer look at this tomorrow.

    Edit: Looked a little closer at your picture. Isn't the esstimation of the IC a bit low?

    What is the correct way of finding or esstimating the IC of our complex rear suspension?
    My guess is to use the BJ, RCA-front pivot point and the traction rod.

    Going by gutt feal I would probably have placed the CG a little higher also.

    Anyway, you might be right that the anti-squat of the S14 is close to zero. Would that mean the S13 has PRO-squat?

    Does anyone have pictures of there cars accelerating in a straight line? Preferably in low gear (1'st or 2'nd)

    Edit2: Ride hight of the car is going to effect the squat on our cars alot. Another good reason to get drop spindels for the rear.
    Last edited by BeerBringer; 11-07-2010, 01:36 AM.

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    • #3
      I noticed that you stated that a lot of anti-squat in an IRS system would cause the rear to actually lift on launch, this is not true. On a SOLID AXLE car the rear lifts because the torque response on a solid axle with a LOT of anti-squat will actually drive the axle into the ground. With an independant suspension this is not the case because the axles are not an integral suspension member.
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      • #4
        The rear weight transfer causes the back of the car to squat DOWN. The equal and oposite reaction (inertial force) to that is tire force pointing UP. That's bad for traction of course. <100% (where I believe the S14 is set up) will enhance this "tire lifting". 100% will not effect it at all. 101% to 199% will resist the "tire lifiting" by transfering loads through the suspension arms instead of through the damper/springs. At 200% you effectively have locked the suspension and the rear suspension will not compress. None of the above will change between IRS and a live axle. But it will be harder to achieve the higher percentages with an IRS though, because the 100% line will have to travel throught he rear wheel center rather than through the rear wheel contact patch. Regardless of the axles being suspension members there is a rotational force created about the CG by the interaction of wheel torque at the ground and inertial forces at the CG (the forces that cause wheelies/blowover). With the axles as integral suspension members this force is greater though, and this is accounted for by having the 100% line start at the ground and not the wheel center.

        Some cool stuff I found. A viper rear end being grafted to a PT cruiser. The viper is said to have about 130% anti-squat. About where I'd like to get my s14, that RUCA gunna need to come up!

        Driver side:

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        • #5
          So... in theory, S13 rear subframe with uber anti-squat is good?
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          • #6
            I havn't plotted it yet, but that's where I think we're headed. Right now I don't think that the s13 even has 100% anti-squat. I don't think it's as uber as everyone thinks.

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            • #7
              Seat of the pants feel definitely give the nod to the S14 rear subframe for traction powering out of a corner, as well as more controllable weight transfer with the throttle.

              I'd have to think Nissan analyzed things once computing power of this level became more prevalent(early 90s), and that's why the S14 and all the later RWD cars had reduced antisquat.
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              • #8
                I'm sure Nissan did analyze things. But the conclusions I think they came to were:

                1) This is a comuter car with less than 210 ft lbs of torque, being shipped out with decent 205 series rubber. Aka, it's not a traction limited 500 ft lb monster looking for corner exit speed.

                2) As you increase anti-squat you increase the RATE of rearward weight transfer. Not a good thing for an inexperienced RWD driver in the rain.

                3) Also as you increase anti-squat, you start to get more weight transfer forward on breaking, another bad thing for soccer moms jumping on the brakes without ABS in the rain.

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                • #9
                  Wait, why did I swap my rear subframe then? Oh yeah because its just better then what ever the maths says.

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                  • #10
                    What if it's better because of less camber change on squat? Or because of less stiction, or less flex? And what if even the mighty s14 subframe can be improved upon with a slight relocation of pick-up points?

                    I know that there is some serious S14>>S13 subframe mantra out there in the community. And the benefits are blindly attributed to the obvious differences in antisquat geometry. All I'm saying is that I think there's a good chance that the benefits of the S14 subframe may come from something other than the antisquat difference.

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                    • #11
                      I get confused!

                      The S14 has LESS anti-squat than the S13?
                      It will squat MORE giving more weight over the rear wheels -> and as Ff = N x mu we will get more traction and acceleration?

                      And if the S13 subframe is the better one for traction then why have we reports of the S14 subframe giving better traction? (Logr, Bill DSG 240RS maxi)

                      Where

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                      • #12
                        The confusion comes from the the counter-intuitive physics. With more squat, the rear will squat more. And while that looks like more weight transfer is happening, there is actually LESS! The reason for this is that with a lot of squat the force at the tire is upwards.

                        MORE SQUAT = LESS FENDER GAP

                        LESS FENDER GAP = TIRE FORCE AWAY FROM THE GROUND

                        TIRE FORCE AWAY FROM THE GROUND = LESS TRACTION

                        ie

                        SQUAT = LESS TRACTION

                        The confusing part is that with high squat situations it empirically "looks" like there is more weight transfer, when in reality, there is less.

                        go back and watch that properly set up drag mustang video link in my first post... no squat.

                        The other confusing part is what everyone is so eager to point out. If more antisquat is beneficial, then why is the s14 (with its reduced antisquat) an upgrade? If the only difference between the s14 and s13 subframes was the antisquat, then the s14 should theoretically give less traction with identical set ups.

                        As antisquat decreases (?upgrading? from an s13->s14 subframe) the spring/damper takes more of the weight transfer force and the suspension arms take less. This will make the same spring/damper act softer in the s14 subframe.

                        Unless you increased your rear spring rates when you swapped from an s13 to s14 subframe you really aren't comparing apples to apples. This is just ONE possible explination as to why the s14 subframes seem to be prefered, they make your car handle as though you had put softer springs in the rear.

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                        • #13
                          I'm attempting to learn what I can from you guys but sometimes a question is needed for me to fully absorb things so...

                          I'm just RLCA's away from setting my coupe on a built s14 subframe with VLSD, would I be better off switching to softer springs and swapping everything on to my S13 subframe or going the route I initially intended? I've been holding on to this sub for a few months trying to decide. LOL
                          Originally posted by Matt93SE
                          in engrish, it's all about the length of your shaft... :P

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                          • #14
                            Originally posted by Gigapunk View Post
                            I'm sure Nissan did analyze things. But the conclusions I think they came to were:

                            1) This is a comuter car with less than 210 ft lbs of torque, being shipped out with decent 205 series rubber. Aka, it's not a traction limited 500 ft lb monster looking for corner exit speed.

                            2) As you increase anti-squat you increase the RATE of rearward weight transfer. Not a good thing for an inexperienced RWD driver in the rain.

                            3) Also as you increase anti-squat, you start to get more weight transfer forward on breaking, another bad thing for soccer moms jumping on the brakes without ABS in the rain.
                            So why do you think increasing the rate of rear weight transfer is a good thing in a traction limited, high powered RWD car? Sounds like the exact opposite of what you want.

                            Seat of the pants feel is that the S14 allows you to get on the throttle much harder, faster, and put down more power on corner entry/exit(depending on the corner) than the S13 rear subframe. The math backs this up(i.e. "softens" what the rear contact patch sees). I also don't want more forward weight shift under braking, if anything I want more in the rear so I can make those brakes do more.

                            Are you approaching this from a drag racing perspective or something, because you seem to be getting a bit "backwards" in your analysis as to what helps you get around a track quicker.


                            I want every advantage of making the car transfer weight smoother and be more receptive to power inputs in "traction limited" situations like cornering.
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                            • #15
                              Originally posted by Gigapunk View Post
                              What if it's better because of less camber change on squat? Or because of less stiction, or less flex? And what if even the mighty s14 subframe can be improved upon with a slight relocation of pick-up points?

                              I know that there is some serious S14>>S13 subframe mantra out there in the community. And the benefits are blindly attributed to the obvious differences in antisquat geometry. All I'm saying is that I think there's a good chance that the benefits of the S14 subframe may come from something other than the antisquat difference.
                              Actually, the upper geometry doesn't change between the S13 and S14. It would actually induce more camber change in the same time just because for the same amount of force the suspension movement is bigger. Another reason, is that you don't want to "shock" tires. Like steering inputs, you want the force applied on the tire to be gradual.

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