I don't understand why less locking factor results in more lift throttle oversteer. Take the extreme case of an open differential. During engine braking combined with cornering, it seems that the inside rear tire would lose grip and slow down while the outside tire would not, giving more warning, and less prone to oversteer, because only one tire would be sliding. However I gather than an open diff creates more oversteer reaction to lift throttle than a mostly closed (LSD type) diff.
Can someone here (maybe Todd Wasson) explain why this is so?
it's very simple. when a car turns, the outside wheels must travel a greater distance (and consequently turn faster than the inside wheels). an open diff permits this to happen freely, thus allowing the car to turn without any resistance.
an lsd does resist the wheels turning at different speeds, and consequently causes the car to tend to drive straight.
so it's actually more correct to say that an lsd causes understeer than that an open diff causes oversteer, but of course everthing is relative.
The first that you seem to be referring to is caused by the lateral tire forces. As you said, the capability for this would decrease when you go to a tighter diff while coasting into a turn because there is a longitudinal (braking) component that tends to get larger as you stiffen the coast side of the diff. That component by itself indeed does tend to increase oversteer as you envision because there's less lateral force available on the outside rear tire. A fully open diff gives the most possible lateral force, so it does seem silly to suggest that it causes oversteer indeed.
However, there is a second yaw torque that comes from the longitudinal tire force (the braking component on the outside rear which is what evilgeek is referring to). A bigger rearward force on the outside tire because the diff is tight would tend to straighten the car; an understeer effect by itself. Two competing torques exist. The question is, which one generally wins?
In my simulations so far (LFS and all others too that I've tried) the longitudinal effect has won in every case by a pretty considerable margin. However, you do have a valid point in that an open diff gives you the most possible lateral force in any given situation, which is why I recommend people run the loosest coast lock they can get away with, then adjust the rest of the setup accordingly. That in itself would suggest more open = understeer, but the opposite usually winds up being true because of that longitudinal force that counteracts it.
That's showing longitudinal components of tire force under acceleration. Flip all the arrows to point down and you have the situation under coasting/braking while cornering.
Thanks. I was also thinking that if the inside rear tire loses grip, then the rear end of the car is getting less overall grip than the front end, and this would result in an oversteer reaction, but this would depend on the suspension setup and the difference in roll stiffness between the front and rear end of the car.