Alignment question, KPI vs Caster...

[pitts64]

When a car is going straight down the road and the static alignment is set so there is more positive caster then KPI, shouldn't the car drift toward the side with slightly more +caster?



Take the GM A body for example, the KPI is 8 and the Caster is set at left +9/ right +9.5.. Shouldn't the car drift toward the right?


I was wondering about this today when I had my 69 el Camino out with SC&C Stage 3. I have the caster set pretty high with slightly more on the passenger side. I noticed how it favors the right side of the road.. It doesn't pull or anything like that but most times I'm correcting to the left..


Thank you....

[Marcus SC&C]

Cars pull to the side with less + caster. If you drive on a lot of high crown roads and want to compensate for the natural tendency to drift slightly to the right put a little less in the drivers side. You can get a good overview of alignment tech, cause and effect from specprod.com Click on their Knowledge Center.
KPI vs. Caster is more of a hypothetical chassis design issue than a simple street car alignment issue. Adding caster (like so many things) is an exercise in diminishing returns. More isn`t always better, just right is always better and what the *just right* combination of performance, road manners, tire clearance etc. is has a myriad of variables that vary from car to car. What works great on one car may work poorly on another. In your case setting the caster very high for the platform at +9.5 is really messing up your bump steer because it has pivoted the steering arms and tie rod ends up to a much higher location than they were designed to be with your suspension package. Back it off to +5.5 and you`ll find that drive ability and performance will both improve. Tire clearance will also improve.

[pitts64]

Thank Marc! I actually have my caster set at 5.25 drivers and 6ish passengers side.. That was just an example..


I had the alignment rack set it to your street specs and tuned it slightly. Glad I bought the adjusters with the flats, I have every move charted.. The SPC adjustable uppers are a suspension tuners dream come true.. It would make life so much easier if SPC found a way to lock the locking bolt into place.

[Marcus SC&C]

I agree, the SPC arms make accurate tuning super easy. Not sure what you mean about that bolt, it`s necessary that it be loose when doing adjustments to let the geometry of the arms change shape. Once tightened down the arms are for all intents and purposes solid arms. Those caster numbers sound much better for your combination!

[Chad-1stGen]

Make sure you put anti seize on those SPC arms. Myself and many others have had the "race" version with the aluminum sleeves gall up.

[pitts64]

Thanks Chad. I keep my cars underbody and chassis well lubed since I drive it in the rain a lot.. I use Lubi-plate spray grease everywhere that's not in normal reach.. So far it is the best rust prevention I ever used, only issue is getting greasy when working on it but its worth not having any rust..