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 Post subject: Instant center
 Post Posted: Fri May 05, 2006 12:13 pm 
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Location: Western Illinois
The simpelest way I can explain instant center is, if your finger was the center of the mounting points on the rear axel and where the upper and lower bars converged out front some where was the fat end of a bat. Then you tried to balance the bat on your finger, like we all did as kids. Straight up would be neutral center line (balance point). If bat end stayed on neutral center line your finger would not have to move to keep it balanced. Now put your finger at the instant center, because the axel should not move up or down. The finger moving could be looked at as squat or rise in the rear of chassis, depending on instant center hight. Now the car could care less about balancing the bat, It will just react to where the instant center is. If higher than nutral center line, the rear will rise. If lower than the nutral line, the rear will squat. Any rise or squat is wasted time and power, but some benifits can come from it at times, A bit of rise can increase presure on the contact patch of tire(too much can make car hop like a bunny from loading tire and tire rebounding) been there, no fun. A little squat can cushion the hard hit of a high H/P car with a clutch, too much will lose traction. That pretty well explains instant center hight. Now how about how far out it is? Now put a short pencil on finger alot harder to control that the bat because of less levrage. That levrage is one of the contributing factors in creating wheel stands. I just hope I haven't confused anyone more. This is just a part of what tunes a good launch.

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 Post Posted: Fri May 05, 2006 9:01 pm 
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Hey ,shucks, I get it .......Thats a good thing......me understanding is no easy task.. #-o Thanks

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 Post Posted: Sun May 07, 2006 3:55 pm 
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Now that we have an Idea of what Squat and antisquat does, we can move on because a launch is a changing thing. We have torque forces to control. There are 2 basic forces. Remember Newtons law of motion. For every action there is an oppisite and equal reaction. First force is torque applyed to the ring gear, (this wants to press the drivers side tire into the ground and lift the pasengers side. the oppisite reaction of this is to twist the chassis, lifting left front corner. If there was no control of this, it would raise the left rear and squat the right rear. The good thing is weight transfers cross corner and we have the weight from this to help plant right rear. To control this we need either more anti squat in right rear or more spring rate in right rear or a stiffer rebound setting on left rear shock or a stiffer compression setting on right rear shock, or a comination of all. Most common used is a preload on right upper bar. shortening this bar actually raises the front mouting points, wich gives it more antisquat. (raising instant center on that side). Raising the ride hieght on right rear would do similar but to a lesser degree. With this in mind, you could lower the left rear ride heigth and lose some antisquat. (lowering instant center. All of these adjustments are tools you can use to fine tune. Consider the antisquat settings as a control over initial hit, then regulated to applying a steady presure to it's side. The other force is weight transfer from front to back. Second force is front rises reaction is more weight applyed to the rear contact patches. We need to control this also, because if front comes up too fast it can cause wheelies and put enought presure on rear tires to distort tire and ruining contact patch. comming up too slow and just not getting the presure back there at all. First control we have is wheel base(longer car lifts harder), the next is center of gravity hieght. the lower it is the less travel to achieve weight transfer(remember the bat it applys here too) A side advantage is that a lower frontal area is less wind resitance, because space under car is added to frontal area. The other controls we have over the weight transfer to the rear is spring and shock rates. Springs are funny things, they don't nessaesarly work like you would think. a heavy spring will asorb and give up as much weight as a light spring but the travel will change. If we have a 400lbs spring and a 200lbs spring and you put 800lbs on them, the 400 would compress 2 inches, the 200 would compress 4 inches, unload them and they extend the same 2 or 4 inches. The heavier spring will give it up a shade faster and compress a shade slower. The shock rates have controll over the quickness and temporaryly add or subtract from the rate. Now to put this into practical use, lets say You have a long board in your hands and your feet are the rear tires. you have a buddy on the other end if he was a heavy spring he would help you lift it a short ways fast or if he was a light spring, he would help you lift it farther, but slower. either way you end up with the weight. his shock rate speed or slows this. Now the rear spring/shock recive the weight. the heavy spring will transfer the weight to the tire faster with less compresion, lighter spring the opposite. Remember you have help in the rear also in the form of antisquat. I wouldn't recomend different rate springs on the rear of a drag car, even though it's common practice on circle track cars, but changing the shock settings can direct weight temorarly to your advantage, by softening left rear and stiffening right rear compression. Then evening out rebound so you don't pich when stopping. usually a fairly stiff rebound setting because you want it stiff on left side to control the body rise of left rear. One other thing. Anti roll bar is a spring. You could say it is the Robbin Hood of springs. It steals from the rich and gives to the poor. The body roll creates weight on the bar which is transferred to the opposite side, keeping the car level side to side. A hard roll will cause rear ride hight to lower. It's a good tool but I prefer to tune with bars and shocks then put it on for insurance pollicy. If you have patience and under stand it you can tune one with an open rear end. Back in the 60s it was illeagal to have welded or locked rear ends, no spools. And even black marks for as long as tires churned.

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 Post Posted: Tue May 09, 2006 6:48 am 
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What can you do with a ladder bar suspension, since the bars don't have all the adjustment a 4-link has? I know it would be nice to have a set of wheel scales too but I don't. :-k

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 Post Posted: Tue May 09, 2006 2:12 pm 
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us7race wrote:
What can you do with a ladder bar suspension, since the bars don't have all the adjustment a 4-link has? I know it would be nice to have a set of wheel scales too but I don't. :-k
On a ladder bar car you use a line drawn from the contact patch threw a line coming straight down from the center of gravity, intersecting at half way from the ground. this is called percentage of rise, and where it crosses the Ctr of gravity line is 50% percentage of rise. on a ladderbar the instant center is the front mounting point. Generaly an automatic trans likes a % rise higher than 50% and manual trans cars like lower. harder it hits the less%. Extend the instant center line from the center of 2 rear mounting points threw the centerof gravity line to find yours. You can also preload if you have ajustable ends on your ladders. you can also change instant center heigth by raising or lowering the front mounting point or the height of the rear of car. you can fine tune with double ajustable shocks What you can't do is change how far out the instant center is. I'm a bit rusty on ladderbars. Scales are not nessary. Just set chassis level side to side to start. If you want to see what wieght change an ajustment made you can use 2 bath scales and a piece of 2 by 12 and 3 blocks the same hight under the 3 other tires. just add weight from both scales, they may be off a bit in real lbs but will give a good % of increase or decrease.

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