KPI (King Pin Inclination) AKA SJI (Steering Joint Inclination) SAI (Steering Axis Inclination) owns one of the most important areas Geometrically on your car but is often dismissed due to it's complexity.... With luck this post will develop an understanding and questions are invited.
The Kingpin position is perpendicular to the vertical viewed from the front of the car, the inclination in measurement decreases figuratively as the inclination is increased, so a position of 12 degrees holds less than a figure of 10 degrees this means the inclination is more inclined mechanically but lower figuratively, it's important that you can image this!
So what?: The Kingpin deserves respect since it controls every area within the steering axis it truly is the (Kingpin)... Geometrically!
Purpose: The KPI forms the lower pivotal axis positioning the Castor trail (Mechanical) also inviting the activation of the TOOT (Toe Out On Turns) during a turn, but we have some dynamics to consider... Since the Castor trail is established then the KPI is left to react during compression of the suspension, this is known as the Camber curve, calculating the curve is dependant of the length of the suspension compression and the length of the wishbone this realises the curve positions (one for the mechanical engineers)
Example: Front:- Camber compression on (loaded) -1 degree 10' Camber compression off (unloaded) +10', this is the Camber curve.
So how come with independent suspension can the car still drive straight if one wheel hits a bump loading the Camber curve since the conical force generated by the Camber position would insist that the car will violently pull toward the negative direction?
Solution: All cars follow the same principle by design... Imagine the car like a Sandwich upper and lower decks... The upper deck rests it weight on the Kingpins and since the positions are linked through the steering rack then the inclination commands the opportunity to be at rest and equal, a natural equilibrium.
Deviation by the driver in straight line travel off-sets the equilibrium by declining/inclining the KPI, in essence the inner KPI is lifted holding more weight than the outer KPI, again since they are both linked through the steering rack the drivers deviation is replied by the KPI returning the steering position retaining the natural balance resting on the inclination.
To the bump: The production of independent suspension suggests that the Camber will indeed 'Curve' attenuating it's force, so how is this belayed?..... The hero is the KPI..
Positions: Assume the static front camber position of -30' and a KPI position of 9 degrees, during a compressive load the curve could position the camber at -1 degree adding conical force but the KPI due to the pivotal position will incline suspending an independent reaction involving the vehicles weight pushing back down on the Kingpin belaying the conical force offered by the camber curve, the reactive position of the KPI would be 8 degrees 30' this is more inclined.
You try it?: You can witness the KPI in action... simply turn the front wheels from side to side, you will see the car rise as you go from lock to lock, this is solely due to the Kingpin inclining.
Chassis dynamics is complicated but not impossible to understand or indeed develop, grasp the reasons why one particular angle is there then you are well on your way to develop your own theory.... My biggest criticism with technicians is that they only realize the positions not the forces... in reality most desires or complaints are born through the recognition of the forces!
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Topic: Geometry and the KPI/SJI/ SAI interesting read (Read 2423 times)