Here is what I think.
Toe first. Ideally, a tire trace precisely true only when the car is going dead straight with toe=0. In most other cases, some lateral slip happens inevitably. Toe in at the rear makes the tail stable because the loaded outside tire is pointing inward to push the tail back in the line. So far so good, but what if the [additional slip angle] by the [toe in] is too much?
A tire's max lateral grip happens at a certain amount of slip angle. This optimal working point is varied by the tire. (Also, different tires have different characteristic curves -- a curve describes relation between lateral grip & slip angle.) Anyway, when the slip angle is larger than the optimal point, grip drops. That's when the car slides too much to be saved. (If the slip is more or less the same at both front and rear, the car would do four wheel drift. Drifting too much or the line is not good, it's going out of track. Unequal slips make it wash wide or tail out etc.)
When a car is driving hard along a curve, there're some slips at all four tires. And the conditions of loaded outside tires pretty much dominate the stance and trace of the car.
With toe in at rear, the steering angle in front must be larger to keep the car tracing the required line. Otherwise, the tail tucks in, the front would be pointing outward relatively. Meantimes, the partially loaded inside rear is pointing outward. Although its grip is less than the much more loaded outside tire, it still contributes to the total lateral grip. Now it's not helping, it's doing the opposite. And the more inward angle of outside tire could be approaching or even going over the optimal slip angle in some violent changes of direction.
So, a toe in rear setting is actually making the 2 rear tires fighting with each other, and also need more help from the front to maintain the line. It could use up the usable slip angle sooner. That's the price we pay for a little more inherent straight line stability.
What about toe out? The much loaded outside tire is pointing outward, it seems the car would be going sideway. It might, but if the grip is still enough to track true, this just alters the stance of car (related to the trace). Similar to 4-wheel steering with opposite phase on f/r. Meantimes, the lightly loaded inside tire is pointing inward, is giving help to the overall lateral grip.
At this stage, I can have a (preliminary) conclusion: toe out is good for cornering (and also the stability in corner), with a condition of ample rear grip to keep the tail in check.
I examined this thought on some tail happy cars -- BTR, F40, Diablo GT/GT2. I tried -0.2 ~ -0.36 on them. Guess what? Spinning like tops? No, they are very drivable with toe out! I adjusted their suspensions as below:
Spring rate --- front: stiffest / rear: softened to be the same as front
Damping--- follow the spring rate proportionally
ARB---- front: 3~5 / rear: 1
camber--- front: 1.0~1.2 / rear: 0
Yes, lift off oversteer is still on them, but not snappy and easy to control. Actually, I like progressive lift off oversteer very much. When in a series of corners, steering by throttle is a real joy. With such setting, these cars are rock stable in corners when you're on gas, tucking in when you lift. Line is freely adjustable by the throttle. Some are more tolerable than others on the degree of lift. Anyway, they are all drivable, enjoyable and not requiring full concentration all time to keep them on track. I love them yet again, by this newly found aspect.
In my distant memory of GT3, those F1 cars have large toe out at the rear. I was surprised at first, but they all drive very well with the help of massive grip of tire and the DF.
I should have thought of it and tried. Thanks again
ULTRAVIOLENZZ, if it wasn't you, I won't rediscover this wonder.
And yet, the setting is not completed -- the crucial LSD. That will be in the next post.
Again, any comments are welcome.
Edit: it's BTR, I typed CTR previously. They are similar, though.