"Pill inserts" or "suspension mount inserts" are pill-shaped plastic inserts that are used within suspension mounts that allow precise, discrete adjustments to be made to the position of the pin that holds the suspension arm.
In the front of a car, they are used to set the roll center, track width, and kick-up (and caster). They also affect sweep, but that is a rarely-used setting.
In the rear of a car, pill inserts are used to set the roll center, track width, anti-squat, and toe.
** Need help figuring out your suspension geometry when changing pill inserts? Try our Pill Insert Calculator
For more information on tuning with pill inserts, see our article on suspension mounts.