Hard braking at high speed (>250km/h) makes the car slide

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You are testing with ABS disabled, zero camber and toe?

Have you tried to compare the values? Maybe a small app where you feed a set of slip, slide values maybe with fixed/zero camber, fixed load and output the forces with different combing approaches.

From what I remember VDrift has additionally the problem that the velocity at contact is calculated in wheel space and not in contact space. As soon as you correct it the handling deteriorates. You'll get the car drifting to a certain side(right?). But that was more a camber, toe in issue I think.

TBH I'd love to replace the Beckman stuff or even the Pacjeka(won't be possible I guess) with something more simple/predictable as long as one gets somewhat plausible handling.

If you find out more regarding your issue, please share.