instant center, anti squat and percentage rise - 460 Ford Forum
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post #1 of 3 (permalink) Old 07-24-2011, 03:43 PM Thread Starter
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instant center, anti squat and percentage rise


I have a few question about instant center, anti squat and percentage rise of a drag race car.

I understand that if the instant center is above the neutral line (100%) anti squat that the rear of the car will rise and the tires are hitting the grond harder. And if the instant center is under the neutral line the car will squat and hitting the tires less.

The thing what's i don't understand is what the position of the instant center (IC) affects this all if it's in front of the center of Gravity (CoG) or behind it.

Let's say that i adjust the four link at 100% anti squat with the IC in front of the (CoG). What will be the difference with a adjustment of the four link with 100% anti squat with the IC behind (toward the rear) the CoG???

What best the settings for drag car with a auto trans (1000hp and 2700lbs)??

What's the purpose of calculating the percentage rise?? Is 100% anti squat the same as 50% rise ??

Already thanks

Freek (Holland)
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post #2 of 3 (permalink) Old 07-25-2011, 06:11 AM
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Some might agree, and some will disagree & get all pissy.......but.....

To be honest the problem I have with the whole "squat/anti-squat/neutral line theory" thing is it doesn't take into account the effect engine torque & shock/strut valving also has on the total suspension picture. Suspension leverage (I/C placement) doesn't "hit/plant" the tire all on it's own, it's only part of what's going on. Engine torque is another force that also "hits/plants" the tire into the track surface too. And without engine torque the suspension isn't going to move anyway (regardless of where the I/C is placed) without engine torque/power first causing housing rotation to happen first.

I have seen first hand where the squat/anti-squat's theory of having the I/C above the "magical" neutral line always causes suspension separation, and below always causes suspension squat just is not true with every car/combo/situation out there. With enough power hitting the suspension at the launch a so-called "squat" suspension setting could actually separate the suspension instead at the initial hit. And a car way down on power might actually squat even though it's I/C placement says it should separate the suspension at the initial hit. And shock valving adjustment can also throw the squat/anti-squat/neutral line theory a curve-ball making a car/suspension not always do what the "theory" says it should.

I do like the old Alston "percentage of rise" theory a little better (but only a little) since it doesn't claim a suspension action (squat/separation) must happen regardless of all the other car/combo factors. It instead just gives you an intersecting reference line in relation to the C/G to work from. But in the end truth is neither "theory" can tell you 100% what the suspension will do, only actual track testing can.

Years ago Pro Mod racer Bill Kuhlman in a magazine article basically said when ironing out a new car/combo/suspension, I/C placement is less important than shock valving. He basically said just choose an I/C setting/placement (as in "flip a coin") then go to the track & test different shock valving settings. In effect the direction the shock valving goes during testing (producing better & better times) also helps point the direction the car want's the I/C to go. I have seen cars where his opinion is 100% right, especially on some big power cars that use/need real tight shock settings & wheelie bars.

So regardless of what suspension "theory" someone might want to go with, or if they decide to instead go with a generic setting that "everyone at their track uses", only track testing will show for sure what any given suspension setting might actually do vs what the car really needs.

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post #3 of 3 (permalink) Old 07-25-2011, 08:26 AM
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Dave, I agree with you 100%. Finding the IC location that a car "wants" is dependent on what you throw at it power wise, shock valving, spring rate and suspension travel. Knowing where it approximately wants to be is the thing that I try to get people to understand, and in understanding, the whole theory about how a cars suspension works..... and that puts them on the path to understanding how to go about sorting it out. There's a lot of science that a person needs to understand, as a baseline, before they can start applying the math in order to sort it all out. I personally don't know everything.... I learn more each day that I draw breath, and learning is one of the things that keeps me going, that and the smell of tire smoke and nitro methane lol. Any way.....yes, knowing approximately where the IC that a certain chassis might want is only a part of the whole formula. There is no one magic # that makes it all perform flawlessly. That's one of the reasons that I personally prefer a 4 link set up.....the adjustability.....'cause you're going to be changing lots of things before you make that grocery getter run the magic 3 second quarter mile .

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