Macpherson SAI or KPI
It is a common misconception that adjusting the inclination of the strut within a Macpherson Strut assembly will alter the king pin inclination (also referred to as the steering axis inclination, SAI or KPI).
What is Kingpin Inclination
King pin inclination (also referred to as the steering axis inclination, SAI or KPI) is quite simply the axis about which the wheels steer. Generally, the greater the KPI angle, the more straight line stability the vehicle will have, KPI is also a factor when considering bump steer and scrub radius.
Strut Offset
Within a Macpherson Strut assembly the KPI is the line between the top mount of the strut (where it meets the chassis) and the lower control arm outer joint. If you want to reduce the amount of KPI at design time you will probably find the strut will clash with the wheel and tire as you can see below on the left.
Adding an offset makes no difference to the KPI, but does allow for clearance between the strut and the wheel and tire as you can see below on the right.
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Comments (7)
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Racing Aspirations
March 17, 2026 at 4:09 pm
Yes, you are correct. The KPI is based on the LBJ and the top strut mount, which are the two points around which the steering rotates. |
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F70NR
March 17, 2026 at 12:36 pm
When measuring the angle for instant centers. Am I measuring the phyiscal angle of the strut or the straight line between the LBJ and top strut mount rotation point? My head says LBJ to top mount as I can’t see how th actual inclination of the strut has any bearing when it rotates about the LBJ and top mount. The actual strut inclination would only effect (in a very minor way) the Motion Ratio? |
