Basic GT5 suspension trouble shooting

Understeer and oversteer
Car and motor sport enthusiasts often use the terminology to describe vehicle cornering behavior. A vehicle that is less responsive (going straight thru corners) to steering might be said to understeer or be "tight" or "plowing"; a vehicle that is more responsive to steering (tail wagging or drifting thru corners) is sometimes called oversteer or "loose" or "free".

And how to tame these behavior in Gran Turismo 5 
Problem: Understeer 
INTO corner
Possible solutions:
*soften front comp. dampers
*soften rear rebound dampers
*softer front anti-roll bar
*harder rear anti-roll bar

MID corner
Possible solutions:
*softer front anti-roll bar
*harder rear anti-roll bar
*increase front downforce
*decrease rear downforce
*increase camber

OUT of corner.
Possible solutions:
*harder front rebound dampers
*harder rear comp. dampers
*softer front anti-roll bar
*harder rear anti-roll bar

Problem:Oversteer
INTO Corner....
Possible solutions:
*harder front comp. dampers
*harder rear rebound dampers
*harder front anti-roll bar
*softer rear anti-roll bar

MID Corner
Possible solutions:
*harder front anti-roll bar
*softer rear anti-roll bar
*decrease front downforce
*increase rear downforce

OUT of Corner
Possible solutions:
*softer front rebound dampers
*softer rear comp. dampers
*harder front anti-roll bar
*softer rear anti-roll bar
*raise rear ride height
*soften rear suspension stiffness

Helpful suspension tips:

If you increase overall stiffness, vehicle will become responsive, grip reduced over bumps and rough surfaces, and resistance to body roll increased

If you increase front stiffness
Induces understeer
Reduces oversteer
Front resistence to body roll increases

If you increase rear stiffness
Induces oversteer
Reduces understeer
Rear resistence to body roll increases

If you increase front anti-roll
Induces understeer
Reduces oversteer
Reduces camber change in corners

If you increase rear anti-roll
Induces oversteer
Reduces understeer
Reduces camber change in corners

For discussion on these principles and how they apply to GT5 http://www.gtplanet.net/forum/showthread.php?t=149604