Q: “Specifically, on the entry phase, how does one determine whether it is steering (amount of steering or rate of input) or braking input (or more generally the rate of deceleration in a linear sense) that is the cause of over/understeer while turning in? More briefly, how does one know if they have applied too much linear deceleration or tried to induce too much rotation?”

A: Well, the easiest – and best – way of answering that is to suggest experimenting with both your braking and steering inputs. As you know, there is no better way to learn something than from experience, as the lessons will stick with you better and longer than if I, or anyone else, tell you what to do. Plus, every car responds slightly differently, so what works in one car – or even on one corner – may not on another.

Here’s how I’d approach this:

• Drive some “baseline” laps, getting consistent with your BoB (Begin-of-Braking) and turn-in points.
• Next, deliberately start releasing the brakes earlier in the brake zone, but much slower than you ever have before, and see how your car responds. Entering a corner with more trail braking (which is what you’ll be doing), one of two things are going to happen: (1) The additional load/weight on the nose of the car will give the front tires more grip, resulting in either less understeer or some amount of oversteer/rotation. (2) The additional load/weight on the nose of the car will overload the front tires and cause them to give up grip, resulting in understeer.
• Now, enter the same corners, but release the brakes quickly, and see what happens. Again, it’ll either result in more understeer, or less understeer.
• Once you’ve learned more about what changing the timing and rate of release of the brakes does to your car’s handling, it’s time to experiment with the steering. Use the same strategy of experimenting with both a slow and gentle turn-in, and a quick and crisp turn-in.
• Finally, combine what you’ve learned from the changes in brake release and turn-in, and see how your car responds.

Ultimately, what I’m suggesting is that you drive the car proactively, rather than reactively. Instead of waiting to see what the car does, make it do something. When you do that, you’ll answer your questions yourself… and be a much better driver.

The best drivers are the best because they’ve figured things out for themselves. They’ve experimented and learned what doing different things does to the car. If they simply followed what others have told them, they would only learn part of what they need to become the best. We can all learn from how they learn.

Finally, I answered a similar question a little while ago, so also check out https://speedsecrets.com/ask-ross/q-how-do-i-know-if-its-me-or-the-car-thats-causing-a-handling-problem/. I think you’d also get something out of this question about End-of-Braking points: https://speedsecrets.com/ask-ross/q-what-is-the-end-of-braking-point-what-squeal-do-tires-make-at-their-peak/.