Spindle Axis Inclination-how much

[High Plains Mopars]

...is the best all around number? A little bit more is probably not a bad thing. If the opportunity arises to add more than stock, is that a good thing or not? Obviously it impacts some dynamics changes as the suspension cycles. What is a good number for our dual purpose cars adn is that different than a dedicated race car?

[David Pozzi]

later models seem to have more SAI.
The way I see it, the extra angle helps move the center of pivot outward, which may help with wider tires. It should move the center of pivot point outward closer to the tire center.
A first gen Camaro has 8.75 deg, Second gen has around 10 or so. When you turn the wheel, the tire will camber out (neg) more with higher SAI. You would need to increase the static caster setting.

It would probably help increase self-return of the steering after a turn.

If your center of turn is not way out from wheel center, then I'd not increase SAI, but that's just off the top of my head without thinking a lot on it.
David

[High Plains Mopars]

I know there are alot of guys out there with wiz-bang suspension software. What have you seen with SAI changes?

[High Plains Mopars]

Could also be a chance to alter adjustment for caster and camber too.

I know someone out there has plugged figures into their program for this...

[gt1guy]

There is no clear cut best number. And it depends on what your doing. Ground up, clean sheet design, or or maybe designing a spindle for a car who's suspension pickup points are already set.

If you know the height of the spindle and where the tire will sit, you can use KPI to set the scrub radius, as Dave stated. Scrub radius, spindle length, and KPI are interrelated.

Also, KPI and castor work hand in hand to determine camber gain(+ or -) when steering. A KPI of X and no + castor will make the outside tire gain +camber, not what we want. So if your limited on the amount of +castor that can be dialed in, you can make up for it by reducing the KPI. But, then you take a hit in the scrub radius.

KPI also has a self centering affect. But this is done by lifting the front end when the wheels are turned. The weight of the front end actually straightens the wheels out through the KPI. The amount the front end is lifted is related to spindle length, the taller the spindle, the more it lifts.

So, everything is related to everything else. And in the end you have a compromise. Where you make the compromises is determined by what your doing with the car, which is another compromise. Fun stuff isn't it?

I will say this. I don't know of many (if any) cars running more than 10* KPI. The trend as I see it is to minimize KPI, and make up for the increased scrub radius with wheel back spacing. Looking at the front rims of most new perf production cars (even race cars) will give you a clue.

Kevin