Openpilot steering tuning guide

• Kp too high = the car overshoots and undershoots center
• Kp too low = the car doesn't turn enough
• Ki - dampens the overshoot / undershoot of Kp and allows the car to be centered faster, allowing for a higher Kp value
• Ki too high = it gets to center without oscillations, but it takes too long to center. If you hit a bump or give the wheel a quick nudge, it should oscillate 3 - 5 times before coming to steady-state. If the wheel oscillates forever (critically damped), then your Kp or Ki or both are too high.
• SteerRatio - too high, and the car ping pongs on straights and turns, too low, and the car doesn't turn enough on curves. If you're on a turn and the wheel is oversteering and then correcting, steerRatio is too high, and it's fighting with Kp and Ki (which you don't want) - although in the past I've been able to have an oscillating oversteering tune which could do tighter turns, but the turns werent pleasant.
• Kf - lower this if your car oscillates and you've done everything else. It can be lowered to 0

All of these parameters interact with each other so finding the balance is a bit experimental

Source: https://github.com/kegman/openpilot/blob/kegman-0.7.5/README.md

See also: https://github.com/commaai/openpilot/wiki/Tuning