Macpherson SAI or KPI

by Racing Aspirations on September 5, 2017

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 (5)
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Racing Aspirations December 31, 2025 at 5:23 pm

If I’m wrong and the line is right, where to the bottom of the coilover do I measure to, I’m assuming it’s either one of the two clevis bolts or the lower locking collar

Correct, measure from the bottom of the coilover to the lower ball joint. You can pick any point of reference, as long as it is on the shock body and accessible.

Huw
stephenryan2525 December 31, 2025 at 10:24 am

I’m struggling with KP/spindle and KP/hub offset. My coilover is offset like the picture above and the line that is displayed for these two measurements go from the LCA ball joint to the bottom of the shock, shouldn’t it be to the ball joint top of the strut?
If I’m wrong and the line is right, where to the bottom of the coilover do I measure to, I’m assuming it’s either one of the two clevis bolts or the lower locking collar
defjux23 December 2, 2022 at 5:41 am

Does the change of strut angle in comparison with KPI have any negative implications? Thanks!
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Racing Aspirations September 6, 2017 at 7:40 am

Yes, that’s correct and the KPI change would also affect the scrub radius.
mathieub September 5, 2017 at 8:16 pm

So with an adjustable top hat on a McPherson strut that adjusts camber, would also increase the KPI /SAI if pulled to the centre of the vehicle?
And if the adjustment is fore and aft it would adjust the caster angle but not the KPI?