*** I took this from a website that KEVINS put together. I think it is a really good start to understanding all the terminology and ways to manipulte suspension setups. ****
Here is how I learned to think about launching a car. Imagine pushing on a refrigerator on a tile floor. Your feet are representing the tires on your car. The Center of Gravity (CG) of the refrigerator is imagined to be in the very MIDDLE on one of the shelves. If you were to push on the refrigerator somewhere above the CG then you will probably push it over (the refrigerator won't move) and you won't feel much pressure on your feet. Now, if you push on the refrigerator 2" from the ground, you will slip on the ground (not getting traction) and the refrigerator won't move. If you push on one side of it then it will turn and not go straight.
But somewhere in between the CG and the ground will be the "sweet spot" where you will get the most traction and the refrigerator will move the easiest. When changing the suspension, what you are looking for is the Least amount of power to move the car the quickest. This is Efficiency.
Definitions:
Center of Gravity (CG):
Imagine the weight of your car concentrated in an area the size of your fist and located, for example, on top of your shifter handle. The actual position will need to be calculated but this is just for a visual aid.
Instant Center (IC):
Ladder Bar- The IC is the front ladder bar mounting hole.
Four-Link- The upper and lower bars are angled toward each other. The IC is the imaginary point of intersection if you were to draw a line along the length of the lower and upper control arms forward.
Percentage of Rise (PR):
Percentage of Rise (PR) is best used for ladder bar cars but a few do use it for 4-link cars. Imagine a line drawn down to the ground from the Center of Gravity (CG) like your shifter handle in the example above. Now draw a line forward from the contact patch of the rear tire through the line from the CG (the line should be drawn so it is below the CG). This intersection is the PR.
The goal in setting up a suspension is to apply just enough force to the tires to keep them from spinning and let the rest of the force push the car forward.
Draw an imaginary line through the lower control arm forward. Now draw an imaginary line through the upper control arm forward until it intersects the lower line. This intersection is called the Instant Center (IC).
Now imagine the Center of Gravity (CG) of your car concentrated at the shifter handle. Where the IC (Instant Center) is located compared to the CG (Center of Gravity) is what determines how the force of the suspension acts on the car to get it moving. If the IC is too high then there will be too much energy wasted pushing the car skyward. If the IC is too low then there won't be enough force applied to the rear tires and the tires will spin. There are also variables if the IC is in front of the CG or behind the CG. Somewhere there is going to be a position (or more than one) that will apply just enough force to the tires to keep them from spinning and the rest of the force will push the car forward.
While the car is sitting still, the tires have 100% traction. During the launch you don't want to change this. Properly adjusting the IC will maintain the existing traction during the initial launch or Tire Shock. The IC starts working during the first .001 second of the launch and continues for the next 30 feet or so. However, after the first .001 seconds the shocks are playing a very important roll in "maintaining" the traction and allowing the IC to apply the desired force to the tires. For the most part, the IC is what is adjusted to get the initial hook and the shocks should be adjusted to maintain the traction throughout the 60'.
The four control arms on a four link type suspension, when adjusted correctly for a particular car, can help launch a car quicker and use less horsepower doing it. Compared to a ladder bar suspension a 4 link suspension has the added benifit of changing the length of the Instant Center to accomodate more or less front end lift without adding any more Anti Squat.
Definitions:
Anti Squat (AS):
This is a term used to describe how much the rear of the car will Rise or Squat during launch. If a chassis has 160% of AS then the rear will rise drastically, planting the tires violently. By comparison if a chassis has 100% AS then the rear of the car will not rise or squat and if a chassis has less than 100% the rear of the car will squat.
~A common problem with high horsepower cars running Too Much AS is that the tires will plant the tires very hard for the first several feet but then start to unload the tires and spin when the chassis starts to settle. If your car does this, then look into the value of the AS. Much too often racers will detune the engine to "calm down" the launch instead of look into the suspension geometry.
~A common problem with high horsepower cars running Too Little AS is that the suspension will cause the rear of the car to squat which tries to unload the tires. Again racers detune the engine to maintain traction instead of looking into the suspension geometry.
Normal/Neutral Line:
1. Draw a line up through the center of your front wheel.
2. Draw a Horizontal line through the CG until it crosses the line drawn through your front wheel and mark this intersection point. The Normal Line (or Neutral Line) is the line drawn from the Rear Tire contact patch through this intersection. Neutral Line
The location of the IC relative to the CG and the Normal Line (or Neutral line) is what determines what the chassis is going to do. One term used to describe the chassis movement for a 4 link suspension is Anti Squat. An IC that is located ABOVE the Normal Line is said to have more than 100% Anti-Squat. An IC located BELOW the Normal Line is said to have less than 100% Anti Squat.
~4-link settings of 100% Anti Squat should accelerate the car w/o any raising or squatting of the rear of the car.
~4-link settings with MORE than 100% Anti Squat will Raise the rear end and hit the tires HARDER.
~4-link settings with LESS than 100% Anti Squat will cause the rear end to Squat and hit the tires SOFTER.
~IC's that are located IN FRONT of the Center of Gravity (CG) will tend to lift the NOSE of the car.
~IC's that are located BEHIND the Center of Gravity (CG) will tend to lift the REAR of the car.
