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.
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Part 2 of 3 ... answering Carbuff's question about caster & KPI.
First … think of caster as “dynamic camber” … since caster has no affect on angle of the tires & wheels … until you turn the steering. Then caster is tipping the top of BOTH tires towards the inside of the corner you’re turning into (Good).
KPI angle is important to keep the scrub radius lower. You can look at the illustration in the previous post & imagine how big the scrub radius would be if the KPI was straight up & down through the ball joints.
Think of KPI as “dynamic camber” … since it also has no affect on the angle of the tires & wheels … until you turn the steering. But unlike caster, it is not tipping both tires towards the inside of the corner you’re turning into. KPI is tipping the top of the outside tire out towards the outside of the corner you’re turning into (BAD) and tipping the top of the inside tire in towards the inside of the corner (Good).
When the KPI/Caster Split favors the KPI … the tire & wheel, on the outside of corners, goes into a state of positive camber (BAD) … rolling over on the outside part of the tread and sidewall of the tire … with the inside part of the tread becoming unloaded. Basically, at this point, the actual tread making contact with the pavement (contact patch) gets narrower, making it incapable of maintaining the speed it was capable of an instant earlier, when it had a full contact patch.
Now let’s talk about the tire on the inside of the corner. Some cars roll so much the inside suspension goes into a “droop” or state of extension … and if that car has negative camber gain built in … the droop actually helps the inside tire stand straighter. For cars don’t roll as much … and that compress the suspension on the inside tire & wheel when cornering, the negative camber gain on the tire on the inside of the corner is tilting that inside tire the wrong way. It is rolling over on the inside part of the tread and sidewall of the tire … with the outside part of the tread becoming unloaded. Also making the contact patch narrower, making it incapable of maintaining the speed it was capable of an instant earlier, when it had more contact patch.
So your front tires that were already at their limit of grip … just lost a significant amount of contact patch & essentially got narrower … and lost even more front traction … creating a push or understeer condition.
The amount of dynamic camber loss is minimal with slight amounts of steering input on large sweeping corners, but grows exponentially worse with higher rates of steering input (front wheel steering angle) on tighter corners. More caster would help both situations … creating more dynamic camber the correct way for both tires … keeping the tire contact patches flatter on the track surface. But how much is enough? Read on.
Let’s start with understanding how spindle KPI/SAI works. Let’s use a C6 based spindle with 9.15 degrees of KPI/SAI. If you were to set both the caster & camber at zero … and rotated the spindle 90 degrees each direction … the difference would be 2x the KPI/SAI angle … so in this case 18.3 degrees.
We know the wheels don’t turn anywhere near 90 degrees, but this example makes everything more clear. Please humor me & follow along closely, because I’m about to share something that is one of the most overlooked keys to proper cornering set-up. We will account for the ACTUAL steering turning radius later.
If you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 9.15 degrees of camber loss (goes into positive camber). Bad … very bad for the outside tire of a corner. 
If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel also experience 9.15 degrees of camber loss (goes into positive camber). But this good for the inside tire of a corner.
Reminder, we not turning the wheel 90 degrees in the real world, so don’t lock in on the numbers “too much” … just the concept.
Caster is different. If we set caster at 9.15 degrees positive (top to the rear) & leave KPI/SAI out of the equation, as if we had a spindle with zero KPI/SAI … and you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 9.15 degrees of camber gain (goes into negative camber). The right direction for the outside tire in a corner.
If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel experience 9.15 degrees of camber loss (goes into positive camber). And this is the right direction for the inside tire of a corner.
So … caster helps both the inside & outside wheel & tire.
Here’s the most important piece of info to know at this point. It is the first & most important key to getting the front tires to use their full contract patch when cornering … increasing front end grip & turning speed. Drum roll please …
Caster offsets KPI/SAI on the wheel & tire on the outside corner … and compounds (adds to) KPI/SAI on the wheel & tire on the inside corner.
Read that again. It’s very important.
This is called KPI/Caster Split. When the Caster & KPI are equal … the caster offsets the negative effects of the spindle KPI on the outside wheel ... and compound the advantages of the KPI on the inside wheel. When the KPI is greater than the caster (unless the car has a TON of Camber) the outside wheel is going to lose camber as the steering is turned & roll over on the outside front tire. Ugly.
The greater the split favoring the KPI, the worse the problem. On the other hand if the KPI/Caster split favors the caster … meaning 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 stay flatter to the road, have more grip, better turning & higher corner speeds.
Sooo … if we set the car up using spindles with 9.15 degrees of KPI/SAI and 9.15 degrees of caster … and you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 0 degrees of camber gain or loss.
Frankly it is zero, no matter what degree you rotate it to the front, because 9.15 degrees of caster counteracts … or neutralizes … the 9.15 degrees of KPI/SAI.
If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel experience 18.3 degrees of camber loss (goes into positive camber). This is the right direction for the inside tire of a corner … way too much ... but we’re not turning 90 degrees. We’re turning somewhere from 0 to 25 degrees.
What if the wheels were turning 15 degrees? … that’s 1/6 of 90 degrees … times 18.3 … equals 3.05 degrees … the right direction.
So … at this point ... we have:
IF Tire +3.05 degrees (Good)
OF Tire +0.0 degrees (OK)
You’re probably going “Hmmmm” … but we don’t have the whole picture yet.
We have a lot of other geometry to factor in. Remember, we’re peeling this onion a layer at a time, so we’ll get to camber gain, chassis/body roll angle & static camber in steps.
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