However, threshold braking is hard to achieve. The point of threshold is not static, and the pressure on the pedal has to be modulated to keep the car braking at the threshold. A common mistake is to brake far too lightly (about 60%).Therefore, the suggested approach in a road car, is to stomp on the brakes and let locked-up wheels or ABS stop you. However, in a race, there are multiple ways of achieving threshold braking:
1. Cadence braking: a variety of method that involve pumping the brake, and therefore theortically reaching through the threshold for small periods of time. This method is almost never advised due to a dramatically increased braking distance and a not nessecarily improved control or steerability.
2. Progressive braking: A method of progressively squizzing the brake to the point of the threshold, and than keeping it there. While it is true that "stomping" on the brakes will not allow to exploit the brake potential efficiently, modern brakes can still be pressed quite quickly in high efficiency, and therefore, squizzing it down will have you put in low pressure for quite some time.
3. Regressive braking: This is the suggested approach. It is better to start off hard and release, rather than go easy and increase. Yes, even if it costs a momentary lock-up or activating the ABS. That's because the hardest pressure is needed at the begining of the process, when the car is still going very fast. Once you have reached the point of threshold, you will have to constantly release the brake pressure to keep it there. Ideally, you should start to brake hard, and than go through the entire braking zone graduatly easing off, untill you reach the turn-in with minimal or zero pressure. There are three methods of regressive braking:
When caught under surprise: Brake as hard as possible, at once, and than release the locked wheels and keep them roughly at the threshold by regressive braking.
Before a typical racing corner, for beginners: Brake regressively and smoothly, but not nessecarily at the exact threshold.
Before sharp corners, advanced technique: This is the classic method of regressive or threshold braking that racing drivers should adopt. Sadly, it's also the hardest skill to adopt. Start braking hard, at the threshold, and than constantly release to keep it at the threshold. As you are about to corner, release more pressure, and enter the corner with slight brake pressure. You should combine this with Heel and Toe and possibly with trail braking. I repeat, begin by applying the brakes powerfully and quickely (but not instantly!) and begin easing-off progressively.
Regressive braking
This is the most efficient way to use threshold braking. You start off applying the brakes rapidly, almost stomping upon them (though not quite stabbing it) while simultanously applying pressure with the left foot against the footrest, and than progressively release pressure. Ideally, this should be very smooth. The marking of success are that the car keeps on slowing down in unison, while the driver puts almost full brakes on the begining of the braking zone, and spends the whole braking zone releasing pressure untill reaching the turn-in with a minimal amount of brake pressure. In most Formula cars, as an example, it is possible to apply the brakes almost fully but with sympathy. As you are about to reach the turn-in (which might be later than you think!) you need to release even more pressure in order to reach the turn-in with only light brake pressure ("Brake-turning"). Than, declutch and Heel and Toe to downshift into the most appropriate gear to get you through the corner (I.E. Accelerate you out of it).
Don't be afraid if the tires do squeal slightly during threshold braking, it does not nessecarily mean that the wheels are locked. It means that a certain precentage of slip is existent, and that is a natural part of threshold braking. In fact, a constant, faint howle should be emitted from the front tires during this procedure. The pedal should be vibrating and the wheel of the car might jerk a bit in the begining. Make sure that the wheel still feels responsive. However, one of the best signs for a good braking during a race, is that the seatbelt remains equally pressed against your body throughout the entire procedure.
In order to get the feel for threshold braking, the starting racing driver should use several techniques: At first, simply stomp the brakes at once, see how it feels. Do not hesitate and do not keep the heel of the foot on the floor, but rather lift the foot in the air and stomp the pedal at once. Now, try and squizzing down the pedal untill you reach the threshold point (the wheel should jerk slightly as you reach it). If it seems to really lock-up, release it althougether and re-apply slightly less hard. Repeat as nessecary. Later on, you will be able to brake, release a little bit and than reapply just once. After that, you could curle up your toes to release enough pressure to release locked wheels. Now, you are ready for the real thing: It's quickly brake and slowely release (like steering).
Indications of threshold braking:
Tire squeal: In theory, a minimal amount of tire squeal, just a feint howle, should indicate good threshold braking. However, if you have a tire with silent elements, you are not likely to get as much as audible feedback.
Steering feel: As the threshold is reached and maintained, the wheel should jerk. If the wheels lock-up, it should feel very light.
Pedal feel: The pedal should feel more firm and maybe a bit vibrating, the constant release and adjustments are made with the anckle. The pedal should actually squizze back and try to release by itself.
Seatbelt: This is a sign of smooth regressive braking which can indicate a sustained threshold braking. If the seatbelt remains similarly pressed against your body through the whole procedure, it's a good sign.
The key at first is to apply smooth regressive braking, without locking up the wheels, or locking them up for a moment in the begining. When you start to use it, don't worry about staying right on the threshold through it. At first, you simply need to get the braking hard at first, and than concentrate on easing up smoothly.
http://www.trackpedia.com/wiki/Threshold_braking