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I asked one of the stability guys. It's the physics of ABS braking on *very* low traction surfaces or *very* high yaw/side slip events that can cause issues. The wheel deceleration and acceleration between each brake pulse is more difficult to calculate because the inertia/torque at the wheel is lower, hence harder to calculate the brake pressure needed (it's easier to work out and apply/release pressures when the system is heavier). So when you finally get traction the wheel speed *could* be anywhere (even completely stopped). Modern systems almost completely fix the issue as they can still read the wheel speed at very high resolution, even though it's effectively a gibberish number that's unrelated to vehicle speed.
I don't full understand it myself either...
👍
But pointing this out, and saying that my choice is to change around that point makes me a doom and gloom fanny apparently
For the record that comment wasn't aimed at you.
But the comment about getting out wrong side of the bed was.
(even completely stopped
Or turning backwards even if the car is moving forwards if you haven't dipped the clutch. If one wheel has grip and the other doesn't and engine speed is below half of road speed the differential will spin the wheel with no grip in reverse. I have no idea how ESP/ABS copes with that. "Clutch down and steer"
Whilst "clutch down and steer" still works one other 'when all seems to be lost' method doesn't work anymore. In pre-ABS cars if you'd lost it but the car was heading in a safe direction you could jump on the footbrake and grab the handbrake thus locking all four wheels. The car would then slide to a stop in whichever direction it was moving regardless of where it was pointing - sometimes you could even let the brakes off if the car ended up pointing where you wanted to go again. Modern cars don't have a handbrake and jumping on the brake doesn't lock wheels.
However ESP/traction control is so good you're much less likely to end up in those situations. The ESP on our little Zoe is probably pretty basic but it requires serious efforts at provocation to unsettle it even on ice and snow. Sure the laws of physics still apply but ESP and Cross Climates mean you've got to be making special efforts to get out of line. Early ABS was pretty crap on snow but I no longer feel the need to spend time tinkering under the bonnet to disable it (even if that's possible) on snowy days now. It's great.
You know you’ve got grip issues due to the tread depth when it’s pissing down with rain on the uphill stretch of the M4 to the Bath, Tormarton junction in a BMW, and the revs start to go up under gentle acceleration, because the rear wheels are actually spinning in the water! Mainly because the water is running along troughs caused by the weight of trucks in the inside lane. At that point I chose to leave the motorway and use the A-roads.
The Beemer had 1.6mm of tread on the back tyres, the legal limit, and it was deemed safe for me to drive it. But the heavy rain at that point hadn’t been anticipated.
If it had had between 2 and 3mm on the outside edge of one tyre, there wouldn’t have been any problems at all, I certainly wouldn’t have noticed any loss of performance.
Less than a millimetre difference across the width of any given tyre, especially when it’s well outside the legal minimum limit of 1.6mm is completely irrelevant, it’s perfectly safe and legal to drive, and I defy any regular driver to notice any difference to the actual performance of the car and/or tyres.
If, like my rear tyre, it had a foreign object stuck in it, in a position that was irreparable, I’d be changing it before any significant journey, which is exactly what I did, because of the very real likelihood of the tyre going down at the worst possible moment, but a slightly lower tread depth on the outer edge? Please, give me a break! *rolls eyes*.