I'm researching ways to improve the handling of my street driven 4th Gen Camaro. Its a daunting task since most successful autocrossers and road racers are somewhat secretive about their setups to maintain their competitive advantage.
DESIGN CONSIDERATIONS
Some design considerations include: center of gravity height; roll centers; roll rates; roll couple distribution; springs rates; anti-roll bar rates; shock absorber rebound/compression rates; bushing durometer ratings; tire size, construction, compounds, tread design, and tire pressure; suspension alignment specs .... the list of interacting variables seems endless.
WEIGHT TRANSFER
When cornering, we want to reduce the weight transfer from the inside to the outside wheels. This weight transfer has to be taken up by the suspension system, or else it will be expended at the tires with loss of traction. This weight transfer is handled through body roll.
BODY ROLL
The amount of body roll is affected by the vehicle's center of gravity (cg) height, the suspension's roll centers, spring roll rates, and anti-roll bar rates. The greater the distance between the center of gravity height and the roll center height, the more amount of body roll is attained. We can't change the roll center height too much except for changing tire widths, but we can lower the cg by using lowering springs.
For a fixed cg height and a fixed roll center height, we control the amount of body roll with our spring rates and our anti-roll bar rates. I've found the general consensus is that we want the front end roll rate to be almost evenly divided between the front springs and the front anti-roll bar.
HOW THE ANTI-ROLL BAR WORKS
The function of the anti-roll bar (or anti-sway bar) is implicit in its name - that is to resist the roll of the vehicle when cornering. The front anti-roll bars on our F-bodies are connected to the front lower control arms by end links and the center section of the bar is attached to the frame rails with bushings. The suspension doesn't know the difference between one wheel hitting a bump or the vehicle leaning. When either case occurs, the anti-roll bar twists and resists this motion. For example, during right cornering, left front wheel moves up into the wheel well and the right wheel moves down, twisting the bar. This twisting motion eliminates some of the body roll by making the entire front end squat.
ANTI-ROLL BAR RATES
The only job of the anti-roll bar is to handle roll resistance. We can determine the roll rate of an anti-roll bar through the mathematical formula:
R = [(pi)(G)(d^4)] / [(16)(b^2)(L)
where R is the roll rate in pounds per inch, pi is 3.14, G is the modulus of the material; d is the diameter of the bar; b is the length of of the roll bar arm measured from center to center; and L is the length of the anti-roll bar.
As we can see, the diameter of the bar has the greatest effect. Small changes in bar diameter result in large changes in anti-roll bar rate since it varies as the fourth power.
Our F-bodies have a "formed" bar. So we can't exactly determine the length (L) of the bar, and measuring the arm length (b) is difficult. Also, we usually don't know the modulus or twisting strength (G) of the bar since many manufacturers don't specify the exact material used. Also hardening of the bar's material by means such as heat treating will affect the modulus.
SPRINGS
The main purpose of the springs is to keep the wheels/tires in contact with the ground. They are also used to control body roll. There are basically two types of coil springs used on our F-bodies. Linear springs have a constant rate and non-linear springs which have a progressive rate. Spring rate is measured by how many pounds of weight it takes to compress the springs a constant number of inches. For example G2 front springs are rated at 550 lb/in. So, it will take 1100 pounds of force to compress this spring 2 inches. By comparison Hotchkis front springs are variable from an initial rate of 285 lb/in to 525 lb/in at full compression.
SPRING SELECTION
Selecting a spring rate is a compromise between ride height and roll rate. Basically we want to use as soft a spring as possible so the wheel can follow road surface irregularities and still keep the tire in contact with the road. If the suspension is too stiff the car will not give proper feedback. If it is too soft it will handle unpredictably with excessive roll.
Remember that weight transfer is formulated by cg height and roll centers, and roll rates. If we use lowering springs, we reduce the center of gravity height and reduce roll. This allows us to use "softer" springs. But we don't want to lower too much or the suspension geometry gets screwed up and also might encounter coil bind. Also remember that roll rate is a combination of both spring and anti-roll bar rates.
ROLL COUPLE DISTRIBUTION
Here's a basic principle: The stiffest end of a car (in terms of roll rate) will slide outward in a turn first. The front and rear suspensions each separately handle body roll passing through their roll centers. Roll couple distribution determines how much body roll is divided between the front and rear suspensions. Usually the front roll couple is between 77% and 93%. The higher the front roll couple percentage, the more the car has a tendency to understeer or push. The less front roll couple, the more oversteer. We want a roll couple distribution that allows us a fairly neutral balance in the corners - with perhaps some understeer for safety on the street, and maybe a touch of oversteer on the track.
So we're seeking that perfect balance between spring heights, spring rates, anti-roll bar rates, and roll couple distribution.
ANTI-ROLL BAR COMPARISION (93-02):
Since the manufacturers don't spec out their anti-roll bar rates, I've listed the specs I could find on a few bars.
1LE: 32 mm front, 21 mm rear
BMR: 32 mm front solid, 21 mm rear solid - 4140 chrome moly steel
(BMR claims front same roll rate as G2 and 41% more rigid than 1LE)
Spohn: 32 mm front solid, 22 mm rear solid - 4140 chrome moly heat treated
Eibach: 32 mm front, 21 mm rear
Intrax: 32 mm front, 25 mm rear
LGM G2: 32.5 mm front, 21.5 mm rear - chrome moly
Suspension Techniques: 35 mm front, 25.4 mm rear
Addco: 35 mm front, 22.5 mm rear - high carbon steel
Strano: 35 mm front hollow, 22 mm rear hollow - chrome moly
SLP: 35 mm front, 21 mm rear
Hotchkis: 36.5 mm front hollow, 25.4 mm rear hollow - high carbon steel
SPRING COMPARISON (93-97):
Does not include coil overs from Ground Control or Global West
1LE: 360 lb/in front, 130-170 lbs/in rear
Global West: 1" drop front
BMR: 1" drop 310-550 lbs/in front , 1" drop 145-170 lbs/in rear
Hotchkis: 1" drop 285-525 lbs/in front, 1" drop 100-140 lbs/in rear
Eibach Pro: 1.3" drop front, 1.3" drop rear
Eibach Sportline: 1.8" drop front, 1.8" drop rear
B+G: 1.4" drop front, 1.4" drop rear
H&R: 1.5" drop front, 1.5" drop rear
G2(HyperCo): 1.75" drop 550 lbs/in front, 1.75" drop 180-220 lbs/in rear
Intrax: 2" drop front, 1.8" drop rear
Origionally Posted on Z28.com by CheapChevy
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