KPI= Equals King Pin Inclination. Basically the difference between the angle of an imanginery line drawn thru the center of the King Pin and the true vertical axis at the center of the king pin.
beam axles , not supported correctly can bend slightly to cause an increase or decrease in that inclination. A LOT of rat rods, street rods and even factory model Ts and As can develop this problem. Especially if one hits a big enough bump and have a weight drifting over the front beam. IE, a big block that weighs twice or more what the axle was designed to support.
The modern term for it, sin
the 37-48 ford front I beam axle had a problem with this as the king pin angles were set at what many say is the max for a beam axle. Not a lot of room for flexing. Add in the fact the 37-48, some were cast iron vs forged (most ended up being cast due to costs) you have an axle that potentially is prone to KPI movement.