Why do you think a brake pedal gets soft after repeated hard braking?
09-26-2003, 01:42 PM
#11
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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.
pw
pw
09-26-2003, 06:08 PM
#12
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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 ...
<img src="http://pictureposter.audiworld.com/17157/alcon_cracked.jpg">
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 ...
<img src="http://pictureposter.audiworld.com/17157/alcon_cracked.jpg">
09-26-2003, 06:46 PM
#13
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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..
i think it's hard to belive that the stainless lines would expand under heat, but the rears, well that seems logical..
09-26-2003, 09:05 PM
#15
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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.
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.
09-26-2003, 09:09 PM
#16
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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.
09-26-2003, 11:02 PM
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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=mv<sup>2</sup>, 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.
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=mv<sup>2</sup>, 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.
09-27-2003, 04:40 AM
#18
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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
It does sound like my brake fluid was boiling.
pw
09-27-2003, 06:30 AM
#19
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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.
