I've heard the term "Threshold braking", and just wondering what exactly it means? is it braking to make fine adjustments to speed during corners that tighten? or what? Thanks!

 

It's braking at the absolute limit of the tires' grip.

You can also "threshold brake" in an ABS car by treating the kicking in of the ABS as your tires locking. By doing this, you can actually outbrake your ABS.

Braking is one of the hardest techniques to learn though.

 

It's difficult partly because when the tire locks up and you have to release some pressure off the pedal, and then once the tire's spinning again you can add a little more, it's not a linear thing, there's some hysterysis there. It's a balancing act and is hard to do really well.

 

Effective braking IE threshold braking is the key to winning or loosing in any sort of roadracing.

It's the most difficult thing a road racer will have to perform because it's a decision that's made at the highest speeds with the most important end result. At very high speeds you have to determine how much space is required to enter at the propper turn in point with the proper speed and balance required to rotate the car allowing you to get into the gas asap. Too fast and you over rotate or worse you just cook it through the corner into a tire wall, too slow and you aren't able to turn in soon enough or get through the corner at maximum speed.

If you get into the brakes too soon the back end will get light and you'll have to ease off to settle the car. Get into the brakes to soft and you end up safety checking at the end and leaving all the weight on the nose so you over steer going in and loose speed correcting for that or worse yet are not able to make the corner. Ideally you go in smooth and once the car has taken a good brake posture or set you really lay into into it either nibbling at ABS or nibbling at lock up if you don't have ABS. At the last second as the corner approaches you release the brake pressure smoothly to balance the car for turn in. If done properly it's a thing of beauty if not you can really get yourself into serious trouble. With that said it's the last thing an amateur driver should concentrate on because you'll need excellent car control to save any kind of problem that may arise from missing the mark.

 

Fritz is correct.

It is breaking as hard as physically possibly, on the THREASHOLD of locking up the tires.

The fastest way to stop a car is not lockup. It is also not breaking without skidding.

The fastest breaking is done when the tires are rotating at a percentage of the vehicle speed. I cant tell you what this is, I simply dont remember. BUT. It isnt lockup, and it isnt no skid.

The danger of threshold breaking as Fritz mentioned, is the razors edge you are on at that limit.
You are NOT going to have equal traction on all 4 wheels. You are also NOT going to have a perfectly even road surface.
Your brake calipers also are NOT going to brake perfectly equal.
There will be slight variations in tire tmperature, brake pad temp, rotor temp, suspension geometry changes as you have weight transfer, etc etc.

All these things come into play rapidly when braking as hard as possible.
You have to adjust your braking constantly, you are downshifting, you are picking the right gear, you are keeping the car pointed the right direction, you are modulating the brake pedal constantly ever soo slightly to keep it RIGHT on that edge of lockup.

Depending on where its at.. its quite a eye opener. If you lock em for 1/4 of a second, you just went 20 or 50 feet. And the corner you are coming to may be only 30 feet wide. Missing your corner by 20 feet equals Wall or grass.

To get an idea. Get someplace where its SAFE. Nothing to hit off the sides of the road, and its FLAT. Go 80, and apply the braked FIRMLY. Not stomp. APPLY. And push hard and fast enough so that you are on the verge of locking up. ANd DONT LET OFF. Stay on that down to 20 or 30 MPH. Its difficult the first time you do it, to truly get the most of your braking. You can feel when you are breaking too hard because the g-forces lessen on your body. The same for not hard enough.

Unless you have a car with competition pads, and or race fluid, you will only be able to do that 3 or 4 times tops. Before you have to let it cool down.

NOW.

Picture doing that at 160. To a corner with a wall. In heavy traffic. On a persons BUTT. And someone next to you. And someone On YOUR butt.

Thats where you need to be able to do it right.. every time.

 

have the car lined up straight (and suspension side to side neutral) for thresholding, helps to keep the back behind the front

 

the thing i noticed and im sure others will chime in here, the more you slow down the less pedal effort you will need to stay at the threshold so you have to actually lift accordingly as vehicle speed decreases

this technique is pure ban kai once you learn it

 

thanks you guys for an excellent explanation.

 

This is true. As the speed drops the inertia of the spinning wheels falls so it takes less pedal pressure to lock the wheel. If the car has any downforce that will also lessen as the car slows which reduces tire grip and available braking force.

Jackie Stewart has said it's how he came off the brakes that made him quicker than everyone else. Not something totally related to threshold braking but something to ponder none the less.