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      08-10-2012, 12:03 PM   #7
The HACK
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Drives: 2006 MZ4C, 2013 Veloster Turbo
Join Date: Oct 2007
Location: Jerry Brown's little tree-huggin' state

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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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