Guys,
I'm getting conflicting information on the C6 spindle KPI angle. So take that into account when reading #1234 & #1235.
If the KPI #'s I have been given are incorrect, I will correct the KPI # & the equation in these posts and let everyone know.
-----------------------------------------------------------------------------------------------------------------
Part 3 of 3 ... answering Carbuff's question about caster & KPI.
Camber gain & chassis roll angle are next. Chassis roll angle hurts the contact patch of both tires. Camber gain (towards negative) helps the contact patch for your outside tire & hurts the contact patch for the inside tire.
If you worked out your camber gain to be 1.5 degrees negative “in dive” on the outside tire … and add that to the combination of your caster & KPI/SAI angle of zero … and factor in the car has a chassis/body roll angle of 2.0 degrees … you would still end up with 0.5 degrees of positive dynamic camber (Bad).
Assuming you have a modern low roll angle suspension … to achieve this chassis/body roll angle of 2.0 degrees … the inside tire, of this car in the same corner, is compressed, but not as far, so it doesn’t have as much camber gain towards negative (reminder: camber gain towards negative is bad on the inside tire).
Let’s say we end up with 2/3 the travel & end up with 1.0 degrees negative camber gain (the bad direction for the inside tire) … and add in the 2.0 degrees of roll angle … makes the inside tire with 3.0 degrees of negative dynamic camber (Bad). Then we factor in the 3.05 degrees of positive dynamic camber (Good) … provided by the combination of caster & KPI/SAI … and we end up with the inside tire at 0.05 degrees of positive dynamic camber (Good).
So … at this point ... dynamically we have:
IF Tire +0.05 degrees (Good)
OF Tire +0.5 degrees (Bad)
Not optimum yet, but we’re going the right direction & we’re not done yet. 
The next layer of the onion is static camber. You need SOME static camber … to help with initial steering turn-in responsiveness. Just don’t get greedy. In road racing or AutoX where you’re turning left & right, static camber is like camber gain. It helps the contact patch on the outside tire & hurts on the inside tire. For this example, let’s add 1.0 degrees of static camber.
Now with static camber added … with your car hard in the corner … suspension in dive, wheel turned 15 degrees for a tight corner … we have:
IF Tire -0.95 degrees (Bad)
OF Tire -0.5 degrees (Good)
Not optimum yet, but we’re going the right direction & we’re not done yet.
Now, here is another part I love. You simply add caster until the contact patches of both tires are flat & happy. And from this point the math is easy.
Add 1.0 degree of caster and …
IF Tire +0.05 degrees (Good)
OF Tire -1.5 degrees (Good)
Add 1.25 degree of caster and …
IF Tire +0.30 degrees (Good)
OF Tire -1.75 degrees (Good)
Add 1.5 degree of caster and …
IF Tire +0.55 degrees (Good)
OF Tire -2.0 degrees (Good)
There are many factors that will define your optimum set-up, but this creates a baseline that is darn close.
**P.S. I like to end up “around” 1-2 degrees more dynamic camber on the outside tire, since it is loaded so much more.
---------------------------------------------------------------------------------------------------------------------------
There is a little more involved in this, when you involve exact steering angles for specific corners. The good news is … when you run tighter corners requiring more steering angle than 15 degrees … the caster increases the dynamic camber to help the tires maintain flat contact patches. I use a spread sheet I developed to plug in all the info & know exactly what dynamic camber I have at different steering angles & different camber gain & different suspension travels.
The best way I have found to work out a front end setting is to start with KPI/SAI & caster … then bring in camber gain … and finally static camber … to achieve the optimum dynamic camber for BOTH tires. Regardless of how you get there ... all of these geometry pieces need to work together in harmony to achieve full, optimum contact patches for both front tires in hard cornering situations … for optimum cornering grip & speed.
It probably is clearer now why getting advice on one setting that worked for a buddy’s car … without knowing the whole picture … can be misleading. As a tuner, I couldn’t imagine setting the caster without knowing the spindle KPI & the car’s camber gain … and then of course testing on track with tire crayon on the edges every run (plus taking tire temps).
It’s been said a zillion times. It’s the whole package, not one part or one setting.
-------------------------------------------------------------------------------
Remember the KPI/Caster Split concept ... if the caster is slightly greater than the KPI, the outside wheel is going to gain camber as the steering is turned, creating a flatter, better tire contact patch. The inside wheel also gets cambered the correct direction (for the inside wheel) and both front tires have more grip, better turning & higher corner speeds.
When I'm designing a front suspension for a specific class with rules on what spindle we can run, I pick the best spindle available under the rules and design everything else to either fix or compliment that spindle. Factory spindles usually have a lot of KPI/SAI ranging from 7-10 degrees. When I have to run a factory spindle ... I know I'm going to end up with 1-2 degrees of caster more than the KPI.
I designed & raced NASCAR Modifieds with factory GM #2 spindles with 8.75 KPI. 10-10.25 degrees of caster produced awesome results. We had a crew chief go off the range with set-ups & try 6-7 degrees of caster, but the cars always pushed in mid corner ... and snapped loose on exit. He was used to running less caster, but didn't take into account the KPI of the spindles we had to run.
The whole combination of KPI/SAI, caster, caster gain, camber, camber gain, Ackerman, toe, steering ratio, etc. ... ALL have to be designed together for optimum cornering performance. All of them are important, but the KPI/Caster Split is critical & often not fully understood.
When I'm designing a front suspension with no rules on what spindle we can run, I design the spindles & have them built. Then I'm not trying to fix anything ... and everything else in the front suspension can be designed to compliment that spindle. For a road racing car, I designed the spindle with 3 degrees of KPI/SAI ... and designed the rest of the the front suspension around what is called a "zero scrub" set-up ... & we ended up with 4.0 degrees of caster for optimum handling. This car did NOT require a high caster number, because the spindle KPI was lower. What is optimum for tight cornering is having the KPI/Caster Split slightly favoring the Caster.
Another successful car I designed with 5 degrees of KPI/SAI ... ended up with optimum handling with 6.5-7.0 degrees of caster ... depending on the track. Again, the key was the KPI/Caster Split slightly favoring the Caster.
This higher amount of caster seems odd to most mechanics & street car guys, but when you look at the newer Corvettes you'll see they run a lot more caster than what most think of as "typical" in the 1.5-3 degree range. The Factory GM specs for the C6 ZR1 is 7.7+ degrees of caster, with a 9.15 degree KPI spindle. The KPI/Caster Split is LOW, but still slightly favoring the KPI by 1.45 degrees. But owners that compete & win in the C6's increase the caster to 9.5-10.5 degrees of caster ... with the KPI/Caster Split favoring the Caster.
The common denominator is we are almost always running more caster than KPI/SAI ... if we can. We run spindles with lowerr KPI where we can too ... but it requires running wheels with DEEP back spacing to get the scrub radius low (or sometimes zero). So simply ordering a spindle with smaller KPI is NOT a bolt-on solution.
Make sense?
Got questions? ... chime in.
...