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Why do you think a brake pedal gets soft after repeated hard braking?

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Old 09-26-2003, 08:02 AM   #1
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Default Why do you think a brake pedal gets soft after repeated hard braking?

So during one run session at Road America, I used my brakes VERY, VERY, VERY HARD.

I drove deep into the corners on the straights and really cranked on the brakes. They worked incredible. Yes, they did.

After I did that for two laps, I figured I could control the car better with less brakes, and I could actually go faster with less pressure and slightly earlier braking points.

However, my brake pedal started getting soft, i.e. increased brake pedal travel to get the same braking effect.

The brakes were hotter than ever, and when you go back to the class room, people ask the same thing "why is my pedal getting soft"?

Everybody's answer is "you have air in your system", which I found very hard to believe. There are not as many techies at a club event as you might find here.

So I didn't do anything to my brakes, and they came back the next run session, I figure it was because it had time to cool down.

My instructor asked me on my next run if my brakes were fine, I said yes, and he asked if I had stainless steel braided lines. I told him, only in the front, the rears were still stock (although I have braided lines for the rear in the trunk of the car [not installed], but I guess that doesn't count).

Then I thought about it. With all the heat that the brakes are creating on a course like Road America (my tire pressures would go from 41 PSI to 51 PSI during the course of one run group, 25 minutes), the brake lines could be expanding under intense pressure and heat during hard braking.

Then the car sits for 2 hours and it cools down and so do the lines. The brake pedal feel returns (but not like it is on the street).

Then I drive home and leave the car overnight, now the pedal is very, very firm again with the track pads on.

So, do you think it's the brake lines that expand under extreme heat and pressure?

If so, now I really understand why great race car drivers use the brakes as little as possible, keeping their heat as low as possible, so as not to have these problems.

My instructor said that great drivers can take street cars on the track with stock brakes and not overheat them because they know how to conserve them (and carry momentum through corners).

pw

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Old 09-26-2003, 08:24 AM   #2
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Default It makes perfect sense. Under hard braking you probably got some extreme heat in there.

I wouldnt be suprised if the calipers and their pistons expanded also.
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Old 09-26-2003, 08:30 AM   #3
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Default I think you answered your own question;)

Get the SS rear lines in and whilst your at it, flush the lines with SRF.
Geeash $100 for hydraulic fluid? Should provide about two grins a day for a year, or about 14 cent a grin.
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Old 09-26-2003, 09:16 AM   #4
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Default What kind of brake fluid are you using?

Maybe your brake fluid is boiling? There are several fluids available with dry temp ratings up to 590 degrees.
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Old 09-26-2003, 09:18 AM   #5
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Default I was using ATE gold.....good stuff, I don't think it was boiling

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Old 09-26-2003, 10:05 AM   #6
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Default Some brake cooling ducts might help

Worked for me on my old M3. However it sounds like you're not experiencing brake fade, just expansion of the hydraulic lines, so maybe new lines will do the trick.
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Old 09-26-2003, 10:15 AM   #7
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Default Your brake fluid is boiling.

That brake engineer guy at StopTech spells it out .. you need to change your brake fluid before and after every track day.

Once you boil it, it's future boiling point is 170f lower than it was before.
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Old 09-26-2003, 10:21 AM   #8
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Default If you dont change fluid before track day, all the water in your system boils nice n eaaasy.

unfresh brake fluid boils at least 170f lower than if it's fresh, due to water.

Check "dry" and "wet" boiling points of your fluid.

Dry = new, Wet = old.
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Old 09-26-2003, 11:48 AM   #9
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Default I don't necessarily agree with you

I might of had fluid boiling, but I'm not convinced of it. I think the components overheated and the flexible brake lines had flex in them, causing the brake system to be "compressable" versus "non compressable".

But you could be right.

I use ATE fluid and I did a complete change out four months ago.

Thanks for your opinion, will have to think about it some more.

Paul
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Old 09-26-2003, 01:32 PM   #10
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Default I would think the fluid is boiling ...

Relative to JKay's posts, I have two books of Carroll Smith's (I believe Carroll passed away recently ??) at home and will try to dig up some diagnosis info for you over the weekend, as well as reference Circle Track magazine (900hp, two tons, short track = brake problems). I'll also keep in mind the input of others' on possible causes.

In general, I think boiling fluid is the obvious answer. Usage of SS brake lines could help, I don't think it would be the root cause.

However, I am also waiting for Randy's input :-).

The heat build-up of slowing down a fast, two ton A8 from the long straights of RA will be extreme. Your brake modification is going to create heat that has to be transferred off to somewhere ... air, fluid, rim, tire, etc.
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Old 09-26-2003, 01:42 PM   #11
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Default If that's the case, I just need to become a better driver and use the brakes less!

Which I determined is the thing to do. You wouldn't believe how hard the car was stopping.....just hard to believe.

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Old 09-26-2003, 06:08 PM   #12
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Default Agree with mclarenm8d, Jkay, and JayS8 ...

I think boiling brake fluid is the primary cause of the soft pedal. It is also possible that the rear rubber brake lines are getting very hot, swelling under the heavy hydraulic pressure, and contribute to the problem. Replacing those rear lines with braided ss lines will eliminate the latter problem, but I'd make a small wager that you'll still get a soft pedal at a track like Road America.

Even fresh ATE gold probably isn't enough to eliminate the soft pedal, there's just too much heat being dumped into the front rotors. Don and I were in a garage next to the "Black Dog Racing" Corvette at a Grattan SCCA race last year. In the typical race weekend, bleeding their brakes after every track session, they literally went thru a case of Castrol SRF, which they said was normal! I watched them bleed their brakes once; the fluid came out black with small pieces of the caliper's piston seals in it.

Also, I read an article on Porsche brakes a while back that said they rate their brakes in HP, and that the brakes should be able to dissipate 4x the HP of the motor. That sounds like the correct way to look at the issue. You accelerate down a long straight for 15 sec, and except for friction and aero losses, all the motor's HP goes into kinetic energy that has to be absorbed by the brakes at the end of the straight in ~3 sec ... That's a lot of heat, and I can see why Porsche says you should have 1200 HP brakes.

Other than always having fresh brake fluid (you don't want to make the problem worse with a low boiling wet fluid), I think JayS8's cooling duct suggestion is the best long term solution, especially for someone with a 300 HP car at your level. BTW: My larger front rotors, with their greater mass, give me a little more margin of protection, but I also had a soft pedal in the last lap of Tuesday's 3rd track session coming down into Turn 5. For a winter project, I plan to come up with some kind of brake duct for my car that'll be ready for Mid-Ohio.

As you can see, Mid-Ohio can also be hard on brakes ...
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Old 09-26-2003, 06:46 PM   #13
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Default boiling brake fluid would raise the pressure in the system, and the pedal would be firmer

so i don't buy the theory at all... even before boiling, the fluid would expand and the pedal would be higher...

i think it's hard to belive that the stainless lines would expand under heat, but the rears, well that seems logical..
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Old 09-26-2003, 08:49 PM   #14
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Default The bubbles are compressible, and ...

with increasing pedal pressure, pedal travel is wasted just making the bubbles smaller.
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Old 09-26-2003, 09:05 PM   #15
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Default Sorry; that's dead wrong.

You can't compress most liquids. A liquid is in it's most "dense" form.

When you boil something, you turn it into a gas, and gasses are very compressable, so ... your pedal gets mushy.

The water which migrates into all our brake systems "magically" via osmosis and whatnot also drastically reduces the boiling point of the fluid .. by ~200F in some cases.
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Old 09-26-2003, 09:09 PM   #16
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Default It seems counter-intuitive, but bigger brakes are for bigger heat sinks, not more braking "powah"

So yeah, Randy's bigass brakes hand the advantage to him in more ways than you think.
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Old 09-26-2003, 11:02 PM   #17
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Default I think your instructor was wrong on that point. ...

The better driver needs more, not less, brake.

The energy you put in the system (i.e., HP) has to be taken out somewhere -- either mechanical friction, aero drag, or the brakes ... that's it!

If you run a lap on a road course slow enough that you don't use the brakes at all (like a cool down lap), well ... that's pretty easy on brakes. All the HP is being used to overcome mechanical friction, including tires, and the relatively low aero drag.

Anything beyond that, though, and the brakes have to make up the difference. The better the driver, the more time he spends at full throttle, and the more energy that has to be removed by the brakes. Aero drag and friction increase also; but IMHO, the faster you go, the more work the brakes have to do.

Even the weight of the car isn't really a factor, but the HP of the motor is if you use it! The faster you're going at the end of a straight, the more energy the brakes have to remove. E=mv2, but more "m" means less "v". Since only HP determines how much energy gets dumped into the brakes, and the better driver uses more of the available HP, I believe he's harder on brakes than the novice.

Don is much harder on my S8's brakes than I am (we know who's the better driver), and you're improving very quickly and starting to stress your brakes. I expect it won't be too long before you melt your Pagid 4-4 Orange pads and have to move up to either the Pagid 14 Black or the Ferodo DS3000.
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Old 09-27-2003, 04:40 AM   #18
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Default What I'm saying is I have to be more aware of carrying momentum through corners

Which is much easier on the brakes. Scrubbing speed off with brakes versus carrying it through the corner by being smooth is definitely the faster way.

It does sound like my brake fluid was boiling.

pw
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Old 09-27-2003, 06:30 AM   #19
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Default Agree, and that was Don's parting criticism of my driving. ...

In my effort to late apex for turn 5 and set up for a fast 2nd gear corner exit up that steep hill, I was overslowing the car. Don said that since I was finally getting apexing and exiting more or less correctly, I should start trying to carry more speed thru turn entry. He couldn't see it, but I was making the same mistake at 3 and 14 trying to get fast exits onto those long straights.
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Old 09-27-2003, 07:05 AM   #20
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Default I wonder if using "thermal grease" (like on your computer CPU) between the aluminum wheel and ...

the brake disk rotor would help bleed off more heat to the wheel, faster.
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Old 09-27-2003, 07:05 AM
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