The threshold argument might not convince you that a car driver can tell if a wheel has locked, or is getting close to locking, but my experience of looking at data tells me that good drivers don't lock wheels very often. They very frequently get into the zone of slowing a wheel (occasionally two, almost always inside front on corner approach) to 80% of road speed, sometimes lower. The sound and look of the car tells me they've momentarily locked a wheel, but the data tells me they haven't.
Threshold braking isn't smooth, drivers need to go past the optimum slip point into the "unstable" slip region to know where the optimum is, so they tend to modulate either side of it. Weirdly enough I only figured out how they do this when reading the excellent Lotus 72 book by Michael Oliver. Once Chapman had put 100% anti-dive into the front geometry, the drivers could no longer judge when the front wheels were going to lock. They were actually using the dive characteristics of the front suspension to judge how hard they were braking, as going past the point of optimum braking grip causes the nose to rise a very small amount.
I've never tried to prove it, but I think in race cars the drivers modulate the brake pressure around this, as if they are either side of optimum grip the nose will ride higher than just at the point of maximum braking force being available. I don't think they know that's what they're doing, but many don't even know what gear they use for a particular corner...
Liam
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