Roll center is the point around which the chassis will roll when side loads are encountered whether it be by cornering or a side hill angle. AKA Body Roll.
Roll center on independant suspension is the imaginary point at which the A arms meet in the middle of the chassis then splitting the difference between left and right. On triangulated four link it is where the two upper links converge. With a track bar it is located along the axis of the bar between the upper and lower attach points.
Now back to body roll. Body roll is not necessarily a bad thing, think of it as weight transfer. If you have no body roll, then as the chassis is loaded laterally (cornering forces or side hill forces), the outside or down hill tire will be loaded and the inside or up hill tire will be unloaded. Once the max traction threshold on the tire with all the force is reached, it will slide. As the chassis rolls, it is distributing part of the force to the up inside or uphill tire so more total force can be applied.
Roll moment is the distance between the CG and the roll center. The greater the roll moment, the greater the chassis roll will be. Think of it as the length of the torque wrench: the longer the wrench, the more the bolt will turn. In this case with a high CG and low roll center, you will get more chassis roll. There is another phenomenon associated with roll center and CG known as "Jacking". This is where the chassis rises, jacking, as lateral forces are applied, and at some point it will go over. I have seen this on IFS Chevys with lots of lift, high roll center, and hard cornering. Although not as common with four link suspension, I have seen it there also. On four link systems it is caused by steep angles from the upper link attach point on the chassis, angling down to the axle. On independent suspension it's the angle of the A arms that effect this.
Roll axis is the line drawn between the front and rear roll center. When setting up a chassis for fast cornering, you want a lower roll center in the front and a higher roll center in back. By doing this you can reduce roll moment, and not suffer from jacking. By having a lower roll center in the front than the rear, the rear tires will break loose first, allowing you to maintain control with the front tires. Several stock car chassis have adjustable rear roll centers to make them tighter or looser in a corner (Higher roll center=more loose, lower roll center=tighter)
In addition to the items listed below when dealing with independent suspension, you also have to worry about camber gain and anti-dive among other things.
Camber gain is the amount of camber gained as the suspension cycles through its movement. If you have no camber gain, then as the chassis rolls, the tire will remain parallel to the chassis and not to the ground.
Anti-dive is the resistance to diving while breaking. On IFS it can be changed with the angle of the upper control arm attach point on the chassis (the higher the front attach point in relation to the rear=more anti-dive), and on 4-link it can be changed by raising or lowering the upper link attach point on the chassis (Higher attach point=less anti-dive)
Once again this will change the weight transfer of the front suspension. Too much anti-dive can cause wheel hop under braking, and wheel hop under power in 4WD applications.
Definitions from Baseline Suspensions:
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.
Getting back to the CG. Let's say the CG is 26" above the ground, for example, sitting on top of your shifter handle. This 26" is represented as 100% and 13" (halfway up from the floor) is represented as 50%.
When you found the PR (when you drew the imaginary line from the contact patch through the vertical line from the CG) it intersected it at, let's say 18" from the ground. Then the suspension is said to have a PR of 69% (18" / 26" = 69 or 69%). Generally, cars equipped with automatic transmissions need a PR greater than 50%. Cars equipped with manual transmissions need a PR less than 50%.
The HIGHER the PR the HARDER the suspension Hits the tires. The LOWER the PR the SOFTER the suspension Hits the tires. Manual transmission cars already hit the tires plenty hard when the clutch is dropped so need very little PR compared to an automatic equipped car. By comparison, automatic cars (no T-brake) don't hit the tires hard at all.
*See Anti Squat below in the 4-Link section for another term used for describing how to set up a 4-Link chassis.
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 benefit of changing the length of the Instant Center to accommodate 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.
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.
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