SCGT Car Setups by Mad Aussie (My own experience mixed with excerpts from GTVE4 info)
I'm not the fastest guy on the block, but if you are having trouble getting your car setup
maybe this will help :)


Tire Pressure:
High Pressure results in more grip and more tire wear.  
Low pressure results in less grip and less tire wear unless you go too low and cause uneven grip.
I use as high as possible, as long as my tires wear out as my fuel loads ends.

Tire Compound:
Soft compounds mean better grip, less durability.  
Hard compounds mean less grip, more durability. 
I use 'soft' tyres as my qualifying tyres and for short races. 

Brake Balance:
This is the percentage of braking power that is applied to the front wheels.  
Increased braking power to the front wheels means more stablity under heavy braking, but does result
in less steering under braking.
Lower braking percentage results in more application to the rear wheels and therefore less stability
under heavy braking, but gives more control to the steering.
I usually run braking to the front, a lift of the foot to regain steering is better than oversteer for me.

Front AirDam: Rear Spoiler Angle:
The higher the value, the more downforce and control during cornering (and more drag, less top speed).  
The lower the value the less downforce and more understeer (and less drag, more top speed).
I always set my Front Air Dam to less than my Rear Spoiler to lessen oversteer.
Too much front ... and you get great turn in, but Oversteer. 
Too much rear .... and you get and stable rear but 'push' or understeer going in and out of corners.

Ride Height:
Adjusting the vehicle body height also effects the center-of-gravity.  
Usually lower is better, unless on a particularly bumpy circuit.

Spring Tension:
Determines the 'stiffness' of the front/rear springs.  
Higher values will make the vehicle 'stiff' (good for cornering, but not very forgiving on bumps).  
Lower values give the vehicle a 'softer' ride (less cornering response but more forgiving over bumps).
I always set my suspension as hard as possible, but lower rear spring tension if I'm suffering rear
wheel spin I can't fix another way.

Shock Bump:
Reflects shock 'stiffness' when hitting a bump.  This determines how hard/easy the shock compresses.
Generally more front bump can cause understeer and less can unsettle the rear.

Shock Rebound:
This is somewhat the opposite of shock bump.  It determines how fast (in stiffness) the shock rebounds back after 
hitting a bump.
Front Bump and rebound need to be of similar values so as not to cause bouncing off bumps or not rebounding off a bump at all.
This means that a Front Bump value of say 4150 should be in conjunction with a Front Rebound value of 3800 - 4500 or so.
The same applies to the rear.
I keep my bumps as high as possible but must lower them on bumpy circuits.

Camber:
Negative camber measures the degree at which the top of the wheels lean towards the center of the vehicle.  
Adjust this to keep the tires flat on the ground thru turns. 
More negative Front camber causes stability under braking but lessens 'turn in', less cause the car to wiggle under
heavy braking but 'turns in' better. Too much 'turn in can cause oversteer.
More negative rear camber has little effect in a straight but will slow up oversteer although can cause wheelspin.
I use a low front neg camber and a high rear neg camber. 

Anti-Roll:
Increases the vehicle's resistance to lateral rolls, resulting in better handling.  
Too thick a setting will cause the vehicle to be too bouncy over bumps.
I keep mine thick (High)

Final Gear:
Diff ratio and Therefor, changing this gear ratio effects ALL gear ratio's indirectly.  
Your normal gears still retain their range relative to each other, but all gears are increased/decreased by 
the same amount by changing the Final Gear. Higher values increase acceleration. 
I try to keep my gear ratio high.

Weight Distribution:
Percentage of weight towards the front of the vehicle.  Add more weight to the rear for better traction.
I keep my weight toward the rear a little usually ... but this will vary on driving styles. As balanced as possible, avoiding
oversteer or understeer usually results in a higher overall corner speed.

Summary:
Basically Harder Suspension for smooth tracks and softer for bumpy circuits. Its not unreasionable to have a hard front setup 
to steer in with and a softer rear setup to ensure no tyre spin. Adjustments are made according to how you take corners....fast in..slow out??
Slow in ... Fast out?? Even in Even Out?? Go faster down the straights and sacrifice cornering, or go slower down the straights and
get round corners better. Weigh up where the most time can be won and lost on each circuit.

