Ron, one more question on KPI/SAI: is the angle relative to ground plane? In other words, does the KPI change when static camber is changed? Or is it a hard angle directly from the manufacturing geometry of the spindle? So with a particular spindle, the KPI angle would always be the same for that spindle? Just wondering as I am trying to visualize all this. I like your analogy of the wheel turning 90 degrees, it's easier to see whats happening.
Ron, one more question on KPI/SAI: is the angle relative to ground plane? In other words, does the KPI change when static camber is changed? Or is it a hard angle directly from the manufacturing geometry of the spindle? So with a particular spindle, the KPI angle would always be the same for that spindle? Just wondering as I am trying to visualize all this. I like your analogy of the wheel turning 90 degrees, it's easier to see whats happening.
Ron, one more question on KPI/SAI: is the angle relative to ground plane? In other words, does the KPI change when static camber is changed?
I "believe" front end alignment machines show it relative to the ground, which would change with camber. But that's not how we do it, nor how we see it in racing.
Or is it a hard angle directly from the manufacturing geometry of the spindle?
Correct, it is a hard angle that does not change, regardless of camber. It is the angle difference from the wheel hub axis compared to the centerline of both ball joints.
So with a particular spindle, the KPI angle would always be the same for that spindle?
Yes, the KPI stays the same on a spindle, unless you physically modify it.
Modifying the KPI of a spindle is not common for street cars. But it can & has been done with welding up the ball joint holes & re-milling them in a different location. Some racers "cheat" this way in classes where "stock spindles" are required by rules.
Also, there are racing spindles with a removable slug in the top, for the upper ball joint. Different slugs move the upper ball joint location in towards the wheel or out away from the wheel, effectively changing the KPI. They also have removable steering arms where the lower ball joint mounts. To keep the ball joint alignment true to each other, this lower arm is changed too.
I have run this type of adjustable KPI spindle, but don't prefer it. It's fine for testing stuff. But I know what I'm looking for when I order custom spindles & have designed the whole package to work together. There is no need to change it later.
Just wondering as I am trying to visualize all this. I like your analogy of the wheel turning 90 degrees, it's easier to see whats happening.
Cool. I am very visual. So I need to see or at least visualize things working to understand them. I also found using BIG examples is an easier way to learn. Once you're clear on a concept ... you can always imagine the concept to a lesser degree.
What an awesome discussion, thank you.
You're welcome.
Oh, more Torino pics Rob!
I don't think you're jacking the thread. Rob indicated he wants us to have these discussions. (I asked him)
If someone wants to ask specific questions about a different car, then we should start a new thread.
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
again for such a detailed answer on KPI. Can't wait for roll center lessons 
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