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      08-09-2012, 03:33 AM   #1
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Threshold braking/emergency braking

Hiya,

I need some help...

I have a question about braking at "high" speeds - say around 160 km/h. I try to do regressive braking, i.e. get on the brake as hard as possible at the start, and slowly release towards the corner/obstacle.

My question is, if I stomp on the brakes, I reach maximum braking force as soon as possible, however the car shivers and starts to slide a bit - it feels unsettled. However, if apply the brakes more smoothly, I do not reach maximum braking force as soon as before, however the car tracks straight.

Which method will stop me faster?

Second, with the "stomp" method, I feel like ABS activates way too soon. With the "smooth" method, the ABS seems to activate later.

Also, the faster you go, the less downforce you have (our cars create lift, v.s. M3 which creates downforce). Does this come into play? Does it mean that the faster I go, the less I can apply the brakes at the start?

Thanks
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      08-09-2012, 05:27 AM   #2
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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
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      08-09-2012, 06:53 AM   #3
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ABS has been shown in scientific testings to best pro pilots in braking distance in all road conditions, except on dry, flat, perfect asphalt. On the wet, bumpy road, the ABS could stop 20% faster than pros, while the human pro-pilots could stop slightly faster on the dry, flat patch (and average experienced drivers were performing marginally worse than ABS in this simple situation, unexperienced drivers would get as much as 20% increase again).

So, the conclusion is that you should trust your ABS on open roads, it will outperform you in almost all situations, because surprise, fear and unknown pavement condition makes it improbable that you'll reach perfect threshold braking.

On sand or gravel (and sometimes wet leaves), ABS doesn't work well though, so stay alert and prepare to release if you don't feel the ABS pumping in roads with these hazards.
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      08-09-2012, 06:38 PM   #4
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Quote:
Originally Posted by turugara View Post
Good read. 1 question: why heel off the floor while braking? There was a debate about this but while clutching; the verdict was depending on what you get used to, and it was about 50/50 touch/not-touch the floor.

I hear many beginners actually overheat their brakes by not utilizing their full braking torque and by dragging the brakes at lighter pressure for longer distance and time.
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      08-10-2012, 08:43 AM   #5
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Sorted this out for myself, rear tires were overinflated, triggering ABS prematurely. I'm running 2.3/2.5 Bar now on 18s (vs 2.3/2.7).
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      08-10-2012, 08:56 AM   #6
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This thread is awesomely useful.
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      08-10-2012, 12:03 PM   #7
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Sigh...

I have mentioned this before, and I will mention it again. Trying to sort this stuff out yourself is damn near impossible, because a lot of the concepts that goes into driving fast is counter-intuitive. It is best practiced with a coach or instructor on the passenger side. That's how *I* learned.

I can type until blisters form on my fingers and it will still not make any sense to some of you. But I'm going to try.

Threshold braking is a misnomer. There's no such thing as a pre-defined "threshold" that a braking system is capable of generating. The actual "threshold" to a braking system is the dynamic grip level of the tires. Simply put, your car can ONLY slow down as quickly as your tires can provide maximum grip.

"Well, tires are tires, right? It's rubber meets road surface. How can grip level on a tire be dynamic?" Well, I'm about to launch into some basic physics principles so if you don't have at least high school level AP physics in you, skip this discussion entirely.

Friction, or grip of tires, is defined by a coefficient (a ratio) between two surfaces. Tire (rubber) against road surface (concrete or aggregate). That coefficient is NOT CONSTANT. It varies between "static," meaning the two surfaces remain constant relative to each other, or "kinetic," when the two surfaces are moving PAST each other. The static coefficient of friction is almost always larger than the kinetic coefficient of friction. And in the case of tires vs road, the difference is dramatic.

"Well, tires are always moving on the road, so that means the friction coefficient is always kinetic. So what's your point?" The tire surface, when it's not sliding on the road, stays relatively static against the road surface as the wheel/tires rotate. When the wheels LOCK UP, and the tire stop spinning against the road but continue to move forward, that's kinetic friction acting.

Find a piece of eraser, and slide it across a desk surface. You'll probably notice that it takes a little bit of force, but once the eraser start sliding on the surface of the desk it becomes increasingly easy to slide it. But once it stops sliding, you'll need to increase the force applied to "break" the static friction then it'll be easier to move it again. Try another trick. Put some static weight on the eraser (like, I dunno, a stapler). With weight on the eraser it becomes increasingly HARD to move the eraser.

THIS is where tire's dynamic grip comes from. A tire is like a huge balloon. If you've ever played with balloons before, you'll notice, that if you press the balloon up against a window, there's a flat patch on the window where the balloon contacts the window. The more weight or force you place upon the balloon, the larger the patch. Well, tires are like that. The more weight you place upon the tire, the larger the CONTACT PATCH grows against the ground. In addition, additional weight also contributes to increase in grip, or available static friction.

Since a typical vehicle rides on 4 sets of springs and dampers, when you BRAKE, the front springs compresses and shifts weight to the front of the car, in essence increasing static load on the tires and thus increasing the dynamic grip a tire is available. Unfortunately, this formula is not linear. The harder you brake, the more weight shifts to the front, and the more grip you have on the front, right? Wrong. There'll come a point that the clamping force of the brake pads on rotor will overcome the static grip and turn it into kinetic grip on the tires.

So the "secret" to threshold braking is to find that maximum amount of weight transfer to the front tires, with the maximum amount of pedal pressure WITHOUT resulting in the tires locking up. The following diagrams will demonstrate. The area under the tire grip chart equals the amount of deceleration:

1: Soft pedal pressure increase:



If you slowly and progressively increase your brake pressure as you enter the braking zone, you will eventually feed more grip up front, but as you continue to press the brake harder, it will eventually transition from static grip to kinetic grip. This is the first technique we teach beginning HPDE drivers, because it's easy to learn and as you can see, a tremendous amount of braking can be done over a long period while still retaining plenty of grip in the tires to slow down the car. Where the brake pressure crosses over the tire grip line, is where "threshold" of the brake occurs.

2: Stomping on the brake pad:



This is what the typical users do when they get sick and tired of the slow, progressive braking. "Well, if adding brake pressure results in increase in front grip to slow down the car, why don't I stomp the crap out of the pedal and I should slow down faster, no?"

No. As you can see, the tire's grip almost always drops to the kinetic grip and therefore there's far less braking or slowing down going on here. Stomping on the brakes is NEVER a faster way to slow down*.

3: Rapid ramp up:



There are drawbacks to the smooth, progressive braking. You spend too much time ramping up the brakes, and not enough time slowing down. Plus the weight is not transitioning to the front of the car fast enough to add more grip. So the logical way to improve braking distance and slow down the car faster, is to ramp up the brakes rapidly. Engage the brake pedal in a nice and smooth fashion, but get to maximum brake pressure quick (not fast, but quick. As the great John R. Wooden is fond of saying, "be quick, but don't hurry."). This will allow the front tires to increase grip very quickly. But as we'll quickly find out, the front tire's grip will also get overwhelmed quickly too...

4: Now we're talking. Threshold braking.



IF you can manage to hold the brake at even pressure near the threshold limit of breaking the tire's static grip, while maintaining the maximum weight transfer to the front of the car to increase the tire contact patch AND weight to increase frictional force, you will be able to slow the car down MUCH faster than any of the techniques described above. However, there's a problem here with threshold braking. I added the theoretical "brake temperature" line. As you continue to hold on to the brake at the threshold point, heat will build up exponentially. You will soon overwhelm the brake's capability to convert speed into heat, since the system can only evacuate so much heat at a time. On a high speed roval like Auto Club Speedway, where you're braking from 130MPH down to 40MPH entering from the oval to the infield, you are on the brakes for literally 5-8 seconds. If the brakes were to fail to evacuate heat half way into your braking, you'll find yourself taking the long trip around the whole oval to get back on course.

5. The ultimate braking technique...



Hence regressive braking is taught as the technique to use once you've mastered threshold braking. The area beneath the tire grip line is only marginally smaller than threshold braking, but tire temperature is easily managed within the normal operating range. Less brake heat build-up means you can consistently brake later, deeper and harder into each corner than if you were to threshold brake WITHOUT tapering off for each turn. In addition, rolling off of the brake pedal also allow for a smooth transition from braking in a straight line to turning into the corner, otherwise if you continue to threshold brake into the turn, you'll find that the tire's grip is used up all for braking, none for turning and soon you'll be into kinetic grip again.

6: The ABS myth



Some of you will say, "just brake as hard as you can and let ABS take care of it." While for street driving, this is certainly a preferred way to slow the car down quickly, but as you can see, the oscillating properties of tire grip means you're only really doing better than the absolute STOMPING of the brake sans ABS. The area below the tire grip is an average of static grip and kinetic grip. It's certainly better than just kinetic grip, but it's not ideal for High Performance Driving applications.

There you have it folks. If you're still reading it at this point, no there's no reward for glutton for punishment. Honestly, if you read through that whole thing? You must not have a lot of stuff to do. But if you DO read through the whole thing and were able to apply it the next time at the track, and shave a few seconds off per lap...Well then my time typing it up and drawing up these lame diagrams is time well spent.
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      08-10-2012, 01:20 PM   #8
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Correction, now its awesome
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      08-10-2012, 02:00 PM   #9
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The Hack, thank you for the great post!
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      08-10-2012, 02:21 PM   #10
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Very good summary Hack!

If I may suggest a comment, not only ABS is better than locked wheels, but it is also better than "shy driver afraid of the brakes". It appears from accident statistics that, on open roads, many accidents could have been avoided if the driver had used all the ABS available braking power, instead of pressing mildly the pedal and using only 60% of possible static friction braking. Said otherwise, the performance of on-off ABS is inferior but close (in the general case) to perfect static friction, and road conditions being unknown, vastly superior to human typical underestimation of perfect braking.

This is why it is good advice to do "rapid ramp up" with ABS on open roads, because it limits the typical pitfall of under-soliciting the brake. Now it is clear that on a racetrack, a competent driver that has made several runs already will do much better than ABS by applying the proper technique you described (btw this is what I had been taught as threshold braking - the constant pressure thing, I've never heard of this before as a proper technique).
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      08-10-2012, 02:34 PM   #11
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Oh, and reading again the original message, stomping is always a bad idea. You want to wait until weight transfer has started before pushing brakes to max. As Hack has explained, weight transfer is what helps tires get grip, if you don't wait for it to happen, your total grip is lower, hence ABS will kick almost immediately which will prevent proper weight transfer, repeat, and you result in longer stopping distance. For street emergency braking, you really want what he described as rapid ramp-up, you press for hard braking that should not lock the wheels, as soon as you feel the dive, reach for ABS to kick in. You may do better if you are talented, but how talented do you feel when you are panicked?
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      08-10-2012, 04:48 PM   #12
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Quote:
Originally Posted by Meeni View Post
Very good summary Hack!

If I may suggest a comment, not only ABS is better than locked wheels, but it is also better than "shy driver afraid of the brakes". It appears from accident statistics that, on open roads, many accidents could have been avoided if the driver had used all the ABS available braking power, instead of pressing mildly the pedal and using only 60% of possible static friction braking. Said otherwise, the performance of on-off ABS is inferior but close (in the general case) to perfect static friction, and road conditions being unknown, vastly superior to human typical underestimation of perfect braking.

This is why it is good advice to do "rapid ramp up" with ABS on open roads, because it limits the typical pitfall of under-soliciting the brake. Now it is clear that on a racetrack, a competent driver that has made several runs already will do much better than ABS by applying the proper technique you described (btw this is what I had been taught as threshold braking - the constant pressure thing, I've never heard of this before as a proper technique).
Correct, the graphs assume that you are driving sans ABS, which most race cars are not equipped with. With the inclusion of ABS, then it is preferable for most average drivers to quickly ramp up the braking and hold past threshold for the quickest way to slow down.

Of course, if you actually ramp up the brakes correctly, your ABS won't kick in until much later in the pedal travel, allowing you to actually brake harder...
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      08-10-2012, 10:07 PM   #13
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Quote:
Originally Posted by The HACK View Post
Correct, the graphs assume that you are driving sans ABS, which most race cars are not equipped with. With the inclusion of ABS, then it is preferable for most average drivers to quickly ramp up the braking and hold past threshold for the quickest way to slow down.

Of course, if you actually ramp up the brakes correctly, your ABS won't kick in until much later in the pedal travel, allowing you to actually brake harder...
Great information!

I just have to say: I LOVE your sig!
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