The general rule is, nothing below the scrub line, usually it's steering components but chassis and suspension pieces count too. The easiest way to describe scrub line without a diagram is to say nothing below the bottom most point of either front wheel, the assumption being that a flat tire will allow the car to drop to that point. If anything else hits the ground before the wheel (other than minor pieces of weak sheetmetal or plastic that tear away), you gonna have problems controling the car.