From my own experience:
Oversteer( car turning faster + cars losing grip = spinning out)
Fix this by lowering height and increasing spring rate for compensation.(more grip).
Alter the camber and toe angle to increase grip(according to the descriptions that are included in-game)
Now all that remains is to alter the LSD settings. This is a very important factor to whether a car will understeer or oversteer.
Understeer{ Car being pulled out of a corner(difficulty/inability to turn)}
The key to this distributing the weight in a specific way ideal to the car you're tuning.
Once again, LSD is very important, espcially acceleration sensitivty.
Brakes will also need a tweak.
To sum up:
To fix oversteer:
Tweak the suspension(all the settings)
LSD
Braking
Aerodynamics
To fix understeer:
LSD
Brakes
Suspensin
Weight
Tires are important as well; if you can always buy the soft tires(only really good for a few laps) buy them(36,000 cr).
Anyways, focus on those settings(depending on what car does)
If you're planning on tuning a car, then alter all the settings that affect handling. The lists I mentioned is for any tweaking to be done after a tuning.
It would also be useful to mention what car you are trying to tune/tweak but you already probably went over to the tuning forum. Anyways gl.
