To counter this problem there are aftermarket control arms that lower the outer pivot to restore a proper camber curve and roll center on an aggressively lowered, strut-equipped car. SPL sells control arms for the front suspension of the S13 and S14. Several suppliers sell block spacers to place between the lower control arm and ball joint to correct the roll center and camber curve on MacPherson strut-equipped cars.
Corner balancing
Corner balancing is the adjustment of weight distribution at each wheel. Ideally, the cross-weight percentage is the same diagonally between the car's corners. This is done so a car's understeer/oversteer balance is the same in a right- or left-hand corner. Corner weight can be adjusted on any car with a height-adjustable suspension. There are applications for nearly all popular performance cars nowadays. For older or non-mainstream cars, companies like Ground Control sell parts to make any car's coil-over suspension height-adjustable.
Corner weights are set by adjusting the suspension ride height at each corner while the car is on four linked electronic scales. The scales display the weight supported by each wheel. With the driver in the car, the spring perches are adjusted to achieve the desired cross-balance. Raising the perch increases the weight at that corner; lowering decreases it.
Weight also tends to be transferred diagonally across the car when changing perch height. Through trial and error, the weights should be adjusted to be as close to equal as possible from side to side and diagonally.
The complexities of modern suspension design, from anti-lift geometry to camber curves, are fundamentals that must be understood before making effective changes to a vehicle's suspension. For more information on suspension design and tuning, look for the "Making It Stick" series on the Web at www.sportcompactcarweb.com
Next, we'll dive into the mystery of dampers, discussing design, operation and valving strategies that work together to make or break a car's handling.
Calculate anti-dive, anti-lift and anti-squat
1) Find the center of gravityThe calcuated center of gravity (C.G.) usually ends up 15 to 20 inches above ground-a few inches higher than the plane of the crankshaft-in a typical sedan. The horizontal location is approximately a foot or so forward of the middle of the wheelbase on a front-engine, rear-wheel-drive car. On a rear- or mid-engine car, it's a foot or so aft of the middle of the wheelbase. On a front-wheel-drive car it is about even with the driver's seat. Draw a vertical line from the C.G. to the ground.
2) Find the instant center
Calculating anti-lift and anti-dive requires finding the side-view instant center for the front suspension. Anti-squat calculations require finding the side-view instant center for the rear suspension. To find either instant center, draw lines through the suspension pivots of the upper and lower control arms at their attachment points to the chassis. On MacPherson strut cars, draw the upper line from the upper strut mount perpendicular to the axis of the strut. These lines should converge somewhere in the middle of the car between the wheels. This intersection is the side instant center.
3) Find the percentage
Anti-dive, anti-lift and anti-squat are expressed as percentages of the C.G. height. Draw a line from the center of the tire's contact patch up to the instant center intersecting the line from the C.G. to the ground. This line represents the force vector where the acceleration or deceleration force acts on the mass of the car. To calculate the percentage of anti-lift, anti-dive, or anti-squat, compare the overall height of the C.G. to the distance between this intersection point and the ground. For example, if the force vector intersects the line between the ground and C.G. at one-quarter of its height, the suspension has 25 percent anti-dive.
Other Installments:
Making It Stick Part 1: Four basic steps to better handling
Making It Stick Part 2: Four more steps to better handling
Making It Stick Part 3: It's all in the geometry
Making It Stick Part 4: More lessons in suspension geometry
Making It Stick Part 5: Damper fundamentals
Making It Stick Part 6: More advanced dampers
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SPL Parts
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Global Performance Parts, North American distributors for Whiteline
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Ground Control Inc.
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By Ti Tong
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