EVOldtimer said:
Donald, thanks for the input. Understeer vs oversteer is not even an issue here. The car departed the roadway straight ahead. Besides the stability system can only work if the ABS is working. That is, if the brakes are applied and the SAS engages, you will hear the chattering of the ABS.
Well, it's your car and your thread, but it is not at all correct that the stability control only runs with the ABS, if that is what you are saying. Yes, if the ABS has
failed then the ESC will also be non-functional, but the ESC should operate whether you are braking or not. It might already have cut in if you made a sudden, albeit small and/or sub-conscious, steering input in these circumstances that you might not have realised you were making. The whole point of the system is that it responds to cues from your inputs and the attitude of the car that you do not even realise is happening (else you'd do something about it for yourself and there's no point making a car that does that!). Clearly, the NCAP test video shows the ESC working overtime to try to keep the car straight ahead, and no braking was going on...
The issue of oversteer is not simply about steering. Even in a straight line your tyres still need to 'steer', albeit in a straight line. The issue is that the rear tyres become unloaded when the car is performing dynamic manoeuvres. So a car that will tend to oversteer will also need to have a very light bias on the rear brakes, because the whole point of oversteer [in a FWD - i.e. not due to loss of traction due to driven wheels spinning] is that as the weight shifts around on the tyres that the rear tyres in an oversteering car become more easily unstuck than a comparative car with little oversteer tendency. A car with an oversteering tendency will lose stability in a heavy straight-line braking manoeuvre more readily that a car will less oversteer tendency.
If a car with ESC is tending to yaw under heavy braking (all cars will tend to yaw under heavy braking - but the less a car's oversteer tendency, the less it will tend to do it), the ESC system will compensate and increase the braking force on one of the rear tyres. In a straight line yaw under braking, this, unfortunately, happens to be the tyre on the 'inside' of the yaw motion, which is already experiencing a shift of vehicle weight
off it, thus elevating the risk of locking that wheel up. You might well have thought you were braking in a straight line, but the car might have been struggling to maintain a straight line at all and the locked tyre you mentioned was a rear unloaded tyre that the ESC was trying to actuate to keep you on that straight line. If it had been struggling in that manner then it would seem plausible the system maintains an 'elevated' level of braking force above your own input and that is what you might have experienced. I am not defending that as a good algorithm, clearly if this is the case then it is too easy to 'confuse' the ESC system and to put the car in a compromised attitude, which one way or another is a safety issue. The question is whether this is the sequence of events leading, or if not what is?
The test to reproduce this possible effect is very simple to try, if you can find some safe space: Just drive across a nice open space and give the steering wheel a small but sudden jerk (not enough to actually change direction but just enough as if you were shifting over within you running lane suddenly, like if you were dodging a nail or pothole you spot in the road at the last second) and then stamp on the brakes almost simultaneously but just slightly after the jerk of the wheel. See what happens.