Rodger, I was reading all my books again and trying to understand the rear "caster". Among them are, Alan Staniforth-Competition car suspension, Carroll Smith's books, and Len Terry and Alan Baker-Racing car design and development. Nobody uses the term "rear caster" But somebody used the term "rear upright incident angle" or something like that. So I will now not use "rear caster" anymore because I don't think it exists except as a description of misadjustment.
So I think the Corvette "0" spec is meant to eliminate misadjustment errors and establish a preliminary basis for further adjustment of toe. The more I think of it, the more I believe you hit the nail on the head. If the "ball joints" can only move in a perfectly vertical plane then the only thing that affects the rear toe is the toe link, its adjustment and it's geometry. Given that, it seems to me that "caster" on a double a-arm rear suspension really has no effect. The term really does not have any meaning in this application.
Another way to think of this is the two A-Arms on the rear are fixed at all three pivot points. Two inboard and one on the upright each. None of them swing on an arc and only move vertically. Therefore they can't CHANGE the toe from its at-rest setting. ONLY the toe link geometry can do that. There is no DESIRED "rear caster".
Mounting the toe link on the A-arm at one end and the upright at the other prevents the toe link from moving in an arc as well. Thus resulting in a stable toe setting throughout the suspension travel. Every race car pic I found does it this way including the green RCR SLC. Pretty much done and dusted on this question for a race car.
I think I will try and set the rear toe as close as possible to 0. The above texts do warn that too much neg rear toe will cause oversteer so it should be kept to a minimum with a preference on preventing ANY positive toe. That will also produce oversteer but much more dramatically.
Dave, since my SLC is a track-only car and I am running R7 Hoosiers I need to run a lot more camber. The Hoosier guy I talked to wanted me to run at least -2 and as much as -3.5 depending on tread section temps and tread wear. It is currently set to -2.5 F and -2.0R. I ran -.75 on my GT40 when it was a dual use car and then went to -.5 as I returned it to street only use. Runing a lot of camber on the street will really effect tire life. I view about -.5 as a street car maximum.
Here's a C8 specs.
Claimed stock C8 alignment specs
10-19-2019, 12:38 AM
So they claim :
The stock curb alignment specs (a full tank of fuel with no passengers) alignment specs :
FRONT
Caster: 7.4 +/-0.6 deg
Camber: -0.5 +/-0.6 deg
Sum toe (left + right): 0.10 +/- 0.20 deg
REAR
Caster: 0 +/-0.8 deg
Camber: -0.5 +/-0.6 deg
Sum toe (left + right): 0 +/- 0.20 deg
Track Alignment Specs
FRONT
Caster: 8.0 deg
Camber: -3.0 deg
Sum toe (left + right): 0.1 deg
REAR
Caster: 0
Camber: -2.5 deg
Sum toe (left + right): 0.1 deg"
Of particular note is that the rear caster specification is tightened from 0-(-.8) to absolute 0. This would confirm the information Dave posted above. Also, you can see that GM thinks -.5 degrees of camber is correct for a street car. I suspect this is to preserve tire life.
I love this forum. Always a good thoughtful discussion. I couldn't get by without it.