Q: “At an HPDE, how do I practice trail braking without being a hazard to the other participants by accidentally going sideways or spinning out of control in the corner?”
A: How’s this for an answer: “Carefully.” 🙂
Seriously, I don’t think you need to worry as much about spinning when practicing trail braking. It shouldn’t be that extreme. First, pick a corner where trail braking will be a benefit, such as a relatively tight radius corner that requires changing direction of the car a fair amount. If the corner is not lined with concrete walls, that will help only from the comfort level perspective (again, you shouldn’t go spinning off while working on this).
Next, start by doing some laps solely focused on where your EoB (End-of-Braking) point currently is. If it’s right at the point you turn into the corner, or 10 feet past there, halfway to the apex, or whatever, it doesn’t really matter – other than you wanting to become aware of it. You might find that you’re already trail braking more than you thought. Just do enough laps to the point you’re able to be fairly certain as to where you currently finally release the brake pedal (think of it as the point where I’d notice your brake lights going off if I was following you).
Now, do some laps where all you focus on in that corner is being slower with the release of your brake pedal. In doing so, you’ll notice that your EoB is now a bit later, and you’re trail braking more. If you find that the car is now rotating or changing direction a little more easily (turning in quicker), then the trail braking is helping. If you find that it isn’t rotating, then the slower release is probably just over-slowing the car, and you don’t have enough corner entry speed for the trail braking to really have much effect on the rotation. In that case, you need to think about moving the whole brake zone in. In other words, make your BoB (Begin-of-Braking) and EoB later – shift the entire brake zone in. By doing so, you’ll enter with a little more speed, trail brake a little longer, and the car will rotate more. When I say “move your brake zone in,” I don’t mean by 100 feet! Start off with one car length later, and see what that does. If there still isn’t much rotation, move it in another car length; then another, if needed. At all times, be aware of your EoB. In fact, as you approach the brake zone, look to the EoB, and only notice your BoB with your peripheral vision.
This is kind of a step-by-step process, right? First, be aware of your EoB, then slow your brake release, then move the brake zone in (if necessary) – but in “car length” steps.
Check out “How should I practice and get better at trail braking and my brake release?” for more info, and links to a couple of videos that might help, too.