jakermc

Car is a GT4CS with Motec 125 installed and braking system is the dual master, non-boosted set-up. I only have 2 days in the car thus far and trying to reprogram my muscle memory for the non-boosted brakes. Thus far my brake pressure is WELL BELOW what a pro can do in the car and I am trying to understand what to correct, other than just push harder.

Is the max PSI generated at the beginning of the pedal travel, i.e. the initial application? Or is it generated at the bottom of the pedal travel when the pedal feels the hardest and you are getting into the ABS? I am getting into ABS so pushing harder there would not seem to generate benefits, so is the max psi data telling me I am too slow and weak on initial application? (Not that I needed data to tell me that, LOL)

Matt Romanowski

This shows long G, but brake pressure is the same shape (brake pressure and long G are very similar in braking zones). You want to build peak pressure quickly and then slowly trail off. I'm sure there will be plenty of other posts saying the same idea.

Frank 993 C4S

It's not about max PSI but in general you want a pretty quick initial ramp up in terms of PSI and then a gradual bleed off as you trail into corner. Couple of other considerations are throttle to brake timing. With a CS you minimize that through left foot braking. Also, not all corner are threshold braking zones where you should feel ABS intrusion. Most of the braking is about brake release and transferring weight.

The only way to find out what you should be doing at a particular track at a particular corner is by hiring a pro and compare brake pressure traces. Everything else is just theory.

ProCoach

I agree with Frank and Matt that you want a relatively quick rise time ("brake attack") to pMax, then trailing off immediately as the inertia of the rotating assembly is overcome to a lesser pressure, then trailing off relative to steering input. Agree it's about brake release more than anything, but your question was about initial brake application.

I'll disagree with Frank on "hiring a pro and compare brake pressure traces" as being the best way to improve. In reality, you're just comparing one variable against another, and there ARE wide variations to what different pros do with their feet. Sure, you can look at their absolute number, but unless you TEST it yourself, you don't know. Most top teams know the target pressures.

I do think braking to the ABS does need to be mastered to KNOW you're in the proper range in the GT4CS, I've got a lot of data to support that.

Most drivers who are not pros don't brake quickly enough, hard enough initially or consistently throughout the brake zone.

This is assuming you're trying to shed more than 50-60 mph. As Frank says, there are plenty of brake zones where you don't need near that much speed shed, therefore no need to bump up against threshold braking.

But in a class where the power is the same, and the weight is the same, and the driver talent is pretty even (over a small range, pros excluded) for a good portion of the field, working on braking technique can be the difference in many finishing spots up the order.

mglobe

From a physics point of view, I think that pressure would be a function of pedal travel. But I'm not sure that absolute pressure should be the metric for effective braking. How/when you get to maximum pressure would be a better metric. And I would think that ABS activation would be a function of more than just brake pressure. Velocity, loading/car attitude, track surface, tire condition, temperature,... all come into the equation.

jakermc

To ask the question a different way:

Suppose a brake pedal moved a total of 3 inches. Would you expect max PSI to be reached in the first inch of travel, second inch, or final 1 inch of travel (before trailing off)? Or would it hit max PSI in all three phases of pedal travel?

I know I am slow on initial application to reach my personal peak PSI, I can see that in the shape of the graph. But is my lack of max PSI vs a Pro also a result of that same slow application (speed - a fast twitch muscle response) or is it my inability to push hard enough on the pedal at max pedal throw towards the end of the braking exercise (strength - a slow twitch response). Is the shape of the graph and the lack of PSI both telling me the same thing (initial application will change both data points) or is it two different and distinct problems I need to solve.

I understand all of the braking theory and know what I need to work on, but trying to decipher what the data sensors are telling me so that I can properly understand the changes in the data as I work on improving.

Thanks for all the insight!

ProCoach

A great question.

I would expect maximum pressure to be reached at or near the end of the pedal travel. This is why it's such a challenge.

Most drivers press the pedal to what they THINK is maximum travel, only to "find" more travel when someone spins in front of them, or their "sphincter sensor" goes off the scale, for whatever reason...

Acceleration maximum is never nuanced. You go to the stop, or not. Brakes? Not so much...

In brake pressure, you are measuring the outcome of the pressure on the pedal, without regard to movement (which is often expressed in pedal POSITION). Pedal position IS important, especially indexed to brake pressure, to SEE if the pedal is "fading" or changing in relative position to pressure, for instance if the fluid boils and becomes less compressible.

I think your challenge is two-fold, as you've pointed out. Your application speed is too slow and your max pressure is not, in fact, maximum pressure...

Matt Romanowski

Can you post up an image of what you are doing in a brake zone? That will help us to see what you are doing and how to fix it.

Regarding position vs pressure. They are related. The pedal is connected to the master cylinder (think of a syringe with fluid in it). When you hit the pedal, it forces fluid out and down the brake lines to the caliper. In simple terms, the caliper is another syringe. The fluid pressure pushes on the caliper pistons and forces them to the pads, gripping the rotor and slowing you down. So, you need to change the position of the brake pedal to build pressure, but position is not what is important for what you are working on.

If you are not building sufficient pressure (versus someone else in your car), then you need to push harder regardless of where you think the pedal position is or should be (assuming your car doesn't have some other problem). Pedal position is an indicator of system compliance- fluid compressing (fade), brake pads wearing out, calipers or spindles flexing, pedals not rigid enough, etc. What you are worried about is pressure, not position.

Does that make some sense?

boxer-11

With the caveat that I'm no expert at this, I did find this paper quite interesting and useful when trying to work on getting brake pressure traces to improve. First mission, get some green trace I was able to set up the math channels suggested in the paper in i2pro pretty easily and make myself a worksheet just to think about braking. Probably overkill but I find this kind of thing fun to think about.




I also don't mean to derail the thread but in this paper it talks about the time it takes to get to max pressure from zero -- it seems to suggest that 200 miliseconds is a good yardstick. You're driving a GT4CS which is probably sprung and damped racecar style so perhaps that's a reasonable goal. I do wonder if it's possible that this is actually _too_ fast for a street car that doesn't have quite the same stiffness to it. So maybe for the experts -- would a street GT4 and a GT4CS have the same goal in terms of how fast you get that initial pressure rise to reach max??

ProCoach

What is most important from the OptimumG paper is the concept of "Brake Aggression," or the period of time it takes to get to the maximum potential of the measure under the drivers control, brake pressure.

My information shows that most all club racers, and especially HPDE drivers even "advanced/instructor level," are FAR, FAR slower than even the stated less-than-optimal .330 second rise time cited in the article. Some take .8-1.0 second or MORE to reach maximum pressure. All of which is not good, not optimal and pretty indicative of lazy feet and a lazy mind.

Now, some of this is because in an underdamped street car (or lightly modified homologated and tightly controlled race car), it can take some time for the pitch change to load the front tires to ACCEPT the peak brake pressure, before going into ABS. This is one of the contributing factors to "ice pedal."

The GT4CS was constructed loosely around GT4 regulations. In essence, a lightly modded street car that is homologated with minimum mods, so this platform is NOT the stable platform that exists as the subject of OptimumG's data.

A GT/LM 911 RSR? Yes. A GT4 or Group N car? No.

I'll look back and see, but if you're under .4 seconds (400 milliseconds) to max pressure EVERY time (except for when the car is finishing being laterally loaded), you're in the ballpark.

ProCoach

So, I just looked at the quickest IMSA LMP-C pole lap at Sebring and another LMP-C lap below the lap record at VIR. Both show between .45-.47 seconds (450-470 milliseconds) between application and max brake pressure for a majority of brake zones (all without significant lateral loading at application). These are purpose-built cars, with serious aero (much more than any GT car). They're also incredibly stiffly sprung (to hold them off the ground), enough to **** blood on a rough race track. FAR more stable and less "squishy" than the GT4CS.

I'll look now at pole time laps for GT4CS cars over the next few days, but I am quite sure they're not as fast as that.

This is why it's SUPER important to take away the concepts of "brake aggression" from the OptimumG paper, practice in testing the GT4CS WHAT the max pressure is that you can register (without regard to corner entry speed, JUST focus on how quickly you can make the car slow down in a straight, checking first that there is no one behind you) and build incrementally.

Braking is the most difficult individual skill execution, because no other is as adversely affected by the perceived limits, requirements and adjustments of the driver than this one...

ProCoach

Oops! My mistake!

Looks like target time for GT4CS, without too much drama and in a straight line at initial application, is .24-.25 seconds (240-250 milliseconds). This is from data from Road Atlanta, NOLA, VIR, The Glen and Road America. Even across the board.

Ok, well that's your target!

boxer-11

Good info, Peter -- thanks! Are you standing by .5 secs for street cars after looking at the GT4CS data??

Not sure this is legit or not but being a kinesthetic learner, one thing I did with a motec set up was dry run brake application to get a feel for what looks right. To do this I sit in the car (engine running, since I have boosted brakes), hook up a laptop to the MoTeC dash, got to Online->Monitor and pick the brake pressure channel, and then use the oscilloscope screen -- what that does is give you a scrolling view of brake pressure real time...so far as I can tell it's like watching the brake pressure trace in a logged file only real time (poor man's telemetry view I guess). I had to wedge the Surface Pro I used in my field of view up on the car dashboard so I could be in a normal-ish driving position. I think that practicing going from throttle pedal to brake with a nice sharp rise time -- which you can see easily on the oscilloscope graph -- has helped me produce better looking results on the track. But to the OP question specifically: perhaps that's a way to figure out how the pedal works in the GT4CS without having to get on the track proper??

[ although you can clearly see from the graphs in the post above that there's still work to do...since looking at this I re-worked my shift lights to show me sequenced low pressure lamps so I can work on smoothing out that last little bit of release where I tend to just slide off the pedal early...but I digress).

ProCoach

Well, a majority of street cars I work with don't have MoTeC, but for now I will stick with that .5 or less is good for a street car.Going to look more closely at this.

Great test and great practice using Online! Awesome use of the technology, to "teach" your foot what it feels like. jakermc, that's a good plan, too.

I've used SLM's a fair bit to show brake pressure rise. Problem is people can get quick on the pedal, but then take too much time to get to MAX pressure...

Matt Romanowski

That makes far more sense than .42 seconds. I was curious on your first post, this on enot so much.