K.P.I – kingpin inclination,
S.A.I – swivel arm inclination,
B.J.I – ball joint inclination,
The kpi is the angle formed between the true vertical, and the centre line through the ball joints or struts, the term kpi is no longer relevant since it is reliant on older forms of construction, in fact, as far back as the horse and cart, the evolution of new technology forming the cars geometry, commanded new terminology such as S.A.I and B.J.I, my preference is K.P.I as this term is recognized globally,
The kpi is designed into the suspension system and is there to aid keeping the wheels straight, and to aid self centring of the steering, to understand how this is achieved, it is necessary to imagine a system with 0 degree kpi.