Explain Brake fade?

NeX

NeX

You're after my robot bee
Club Member
hi all,

can someone explain to me brake fade?

i was messing with some hydro brakes and i can't see how it is possible...

i googled it and google says:

brake fade is where the brake fluid exceeds its boiling temp by excessive use, after boiling, bubbles form that can be compressed which means the pedel can move without applying any extra force to the pads...

as first i just excepted this to be true but then i thought, a brake system is completly sealed and full of 100% oil with no air. so there is no room for the oil to expand into. how can the oil form bubbles when it is already under pressure in a sealed enviroment?

basically you can't compress fluids but according to google if you heat the fluid and apply pressure then you can make it smaller... but in theory it should get bigger not smaller....

i'm sure i'm missing something, any petrol heads care to explain?

thanks
 
It can relate to friction material in the pads and shoes. Its basically when the heat builds up and causes the friction material to change properties and become less "grippy" if you know what i mean

Brake Fade

Hope this explains it for ya mate.
 
Yep google is exatly right.....if youve herd of the cat and fiddle road (i know sonic has) me and a friend were going down to macDs and a idiot in a scooby decided he wanted a race. We were in a corsa sxi lowerd 17inch wheels, it was allright. we were trammin abit and the scooby was still behind us..when we finaly got down in to mac he said....ohh **** ive got no brakes..at first i thought he was jokeing then i saw his foot to the floor....so i grabbed the hand brake and pulled it on slightly and we pulled over. I knew what brake fade was allready because i work in a garage. we had to sit for 5-10 mins waiting the the brakes to cool down before he got his pedal back, and his discs were glowing red.

So yeah google is right my friend....when you expirence it you will never try it again lol...scary stuff. Everything just gets so hot it does make the brake fluid bubble slightly so that theres air in the system and thats why your pedal goes to the floor.
 
ive always believed inthe theory K10 daz put up. when hot the pads change their properties, and they tend to glaze over slightly causing them to slip against the discs rather than grip them, and when they get TOO hot thats when the discs glow red and warp :eek:

edit - the boiling fluid does explain the peddle to floor arrangement lol
 
there is a couple of reasons why brake fade occurs, the most common is that when your really giving your brakes a hammering your brakes heat up, the organic stuff in your brake pads start to give of a gas which forms a cushion between your brake disc and your pad which give you that really crappy brake pedal. Its essentially the same theory as aquaplaning except its with gas and not liquid. this is why drilled and grooved disc help because is kinda slices the gas cushion and helps stop it occuring in the first place.

others are that the the resins that are in the brake start to melt and lubricate the disc however ive never encountered this before (can someone confirm) is just what ive read.

no doubt someone will go into more detail as they always does but thats the basic explanation. Its not like im an automotive engineer or anythin.............oh wait.
 
thanks people,

i get how brake fade happens, i got it in the smart car when i tried to go from 110 to 0 in about 100 meters... the pedel dropped and i had to run the lights :(

don't do it kids...


but what i don't understand how you can get gas in the brake fluid? there is no room for the flud to turn to a gas, its under too much pressure. and if the pedel goes to the floor then that must mean that the fluid is compressing... which is impossible. it cant turn to a gas and compress to a smaller size than it was when it was a fluid...
 
It's not the fluid that compresses when you're pedal reaches the floor, it's the air bubbles in the fluid.

Brake fluid is hydroscopic, which means it absorbs water - not sure what brake fluid itself is made up of, but go boil some water and watch the air bubbles form on the bottom.

Water is just hydrogen and oxygen ... and your brake lines give plenty of room for expansion, given enough pressure
 
CAT 20T said:
It's not the fluid that compresses when you're pedal reaches the floor, it's the air bubbles in the fluid.

Brake fluid is hydroscopic, which means it absorbs water - not sure what brake fluid itself is made up of, but go boil some water and watch the air bubbles form on the bottom.
Click to expand...


yea but when i push my brake pedal in my car it doesn't go to the floor because the fluid can't compress. so if it turns to a gas (which is where the particals are further apart and take up more space) then how can that gas compress MORE than it did when it was a fluid?

oh and how can there be extra room for expansion? if it expanded then it would increase the pressure which would either push your brake pedel up or put more force on the pads...

and if it expands and then gets squashed back to the size of a liquid then the brake pedal shouldn't move at all....

oh and if that sounded a bit arsey it wan't ment to, i just don't understand :doh:
 
when the brake fluid expands through boiling there is plenty of room for it to expand to, there is always a little swell in the flexible hoses, even the braided steel ones, but there is also the free space in the master cylinder, this is one of the reasons for the min-max levels on the master cylinder. As an added extra I found a story on brake fade that I found funny,
BRAKE FADE
I have seen too much of this, having spent 5 years at the bottom of a 13 km hill with 15% grade and continuous switchbacks. Two phenomena contribute to brake fade, one is the fact that the coefficient of friction of most substances gets lower at high temperatures, and that most liquids will boil at some temperature, and that gases compress, while liquids do not. When you use the brakes to decelerate 3,000 or 4,000 or 7-8-15,000 lbs of vehicle, they get hot. Very hot. Under normal circumstances this would be no big deal, the heat that builts up in the pads, rotors, and calipers will slowly radiate back to the air flowing over them as the car continues down the road. But you aren't going down the road, you are back on the brakes, doing more decelerating for the next switchback. Instead of cooling off, your brakes are getting hotter. And hotter, and hotter. . .As the pads and rotors get hotter the friction material of the pads starts to separate. The binding agent starts to boil off from the surface of the pad, plating out on the rotor as a dark, paintlike film...coefficient of friction approaches zero, pedal gets hard, but no braking action. Your pupils dilate to 10 mm and your body goes into fight-or-flight mode, adrenalin courses through your system. But the car just goes faster.... You shift down, now you are standing on the brake pedal with both feet, around this time, the temperature of the brake fluid in the calipers usually reaches it's boiling point and the pedal just sinks to the floor. Your pupils reach 12 mm, your sphyncters contract to pinpoints, somehow you manage to stop the car. There is smoke coming from behind your front wheels, maybe fire. You put out the fire and have lunch. After things cool off you sit in the car and try the brake pedal, it feels almost normal. Congratulations, you've just experienced, (and survived) brake fade. (You've also just flash-fried your front brakes, figure on new everything to fix it properly.)
 
lol thats a good story :D

ok if there is space for the gas to expand in the flex in the hoses etc why isn't that space taken up by the fluid already?

if you are not moving and you have just got in your car and you push the brake pedal as hard as you can then surely the tubes etc expand equally until they burst.

so if you are slamming your brakes on and putting as much pressure on them as you possibly can then the tubes would be at their maximum expansion...
 
Sorry Nex I'm too drunk to think about it any more, I'll think about it tomorrow and post a reply if someone else doesn't cover it first. Or you could accept the fact that that's what happens to the fluid and it's all done by pixies in there magic pixie shoes. :upside: :D :upside:
 
sonic said:
Sorry Nex I'm too drunk to think about it any more, I'll think about it tomorrow and post a reply if someone else doesn't cover it first. Or you could accept the fact that that's what happens to the fluid and it's all done by pixies in there magic pixie shoes. :upside: :D :upside:
Click to expand...

LMAO :D ah that makes it all sooo clear :D

right ok, maybe i'm explaining it badly

lets say you have a tube. the tube is filled with brake fluid with no gas of any kind in it. like a properly bled brake system

the tube is completly sealed the fluid inside cannot leave the tube and nothing can get in. like a brake system

just for now lets say that the tube can't expand or flex. brake system has a slight amount of flex but it has a limit, if it didn't then every time you braked your brake hoses would inflate like a balloon.

you heat the tube and the fluid inside. like applying the brakes and the friction generating heat

the fluid doesn't boil because when something boils it turns to a gas and there in no space for the gas to expand into.

the pressure would increase equally with the heat as the fluid tries to turn to a gas. if this was a brake system then this would mean that the pedal would push back against you and the pad would be pushed harder against the disk.

eventually the tube would explode under the pressure, which obviously doesn't happen to your brake system

i can see how brake fade can happen with the pads changing etc, but there is no way you can get the brake fluid to turn to a gas.
 
Well nex... this is my gcse explaination.

When you heat anything up, it has a boiling point right?? Beyond this it becomes a gas. Right??

In a solid the molecules are like:

0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0

Hard to compress, they are already rigid against one another. In a gas the bonds between the molecules have broken down and the molecules float about like: (but occupy the same physical area, presumably)

images


Yeh... well the gaps in between the molecules can now be closed up and the same number of molecules compressed into a smaller volume.

Someone will definitely correct me... and this was the way my chemistry teacher taught us... he was rubbish!!
 
Spot on Turtle, Now it comes flooding back, (well it has been 13 years since I was at college) yep, when the brake fluid becomes a gas after boiling, it breaks the links between the atoms making it all a lot 'squishier' hence the compression of the fluid in the brake hoses and the drop on the pedal. If you think about it, its like getting a stick of celery, quite rigid wouldnt you say, then boil it in water and it goes all soggy, this is due to the breakdown in the molecular bonds therefore reducing its rigidity.
 
lol yes... BUT

fluid cannot turn to a gas no matter how hot it is if there is no space for it to expand.

water boils at 100c

but if the water is under pressure then the boiling point goes up...

you can keep water a liquid at 1000c if you keep it under pressure. just like gasses turn to a liquid when you increase the pressure around them.
 
The primary cause of brake fade is pad fade rather than gassing in the brake fluid.

It's complex to explain:
- It's generally localized boiling that forms the compressible gas, pressure has little effect on this.
- Water in the fluid has a much lower boiling point and can easily stay as a gas no matter what pressure.
- Your thinking with a sealed container isn't totally correct, but isn't relevant either.

Also, another effect is that silicone based fluids (DOT5) do actually become progressively more compressable as they approach boiling point.

DOT3, 4, and 5.1 are all glycol based, and are hygroscopic so will combine with water by absorption. Only DOT5 doesn't do this.
 
Andrew said:
The primary cause of brake fade is pad fade rather than gassing in the brake fluid.

It's complex to explain:
- It's generally localized boiling that forms the compressible gas, pressure has little effect on this.
- Water in the fluid has a much lower boiling point and can easily stay as a gas no matter what pressure.
- Your thinking with a sealed container isn't totally correct, but isn't relevant either.
Click to expand...

could you explain alittle bit more on that?

1. how can the pressure have little effect on the boiling water?
2. how can the water become a gas when there is pressure acting on it?
3. even if water has a lower boiling temp than the oil, it still won't boil if its under pressure.
4. the brake system must be sealed (when braking) for the system to work, if it is not sealed then there is a way for the fluid to leak.
5.as long as the system is sealed then the water in the brake fluid has no way to boil...

i know i'm wrong cos i've done lots of research that all says gas appears in the brake system but no one is able to explain how it can form...
 
Sonic is spot on....when your braking hard these magical little pixies pop out and sprinkle fairy (Gas) not dust all over the place and thats why your brakes dont work.... Honestly :p

lol Nahhh
 
5ilver Micra said:
Sonic is spot on....when your braking hard these magical little pixies pop out and sprinkle fairy (Gas) not dust all over the place and thats why your brakes dont work.... Honestly :p

lol Nahhh
Click to expand...


lol well suprisingly your not far off.

i have looked into it more and Andrew is right...

its the water in the fluid that turns to a gas and for some unknown reason that i still don't understand in IS able to turn to a gas because something to do with the thearmal energy braking the bonds between the molicules *spell

they are then able to compress back together but not to form a liquid but some kind of new substance (fairy dust) that is a gas but smaller than it was when it was a fluid!?!?

some kind of loop hole in physics that i don't understand........ argh!
 
It's not easy to explain.

The localized boiling can happen at very high pressures as long as it can come into contact with a hotspot. It's not the same as the whole substance boiling.

You've also got to take into account the pressure in the system is only high under braking. When you come off the pedal, the fluid can boil in the conventional sense. It won't just turn back into a liquid the second you press the pedal (both in the sense that it takes time, and you can't just reverse phase changes like that).

You've got a common misconception that water can't turn to a gas under pressure. The pressure in the braking system isn't nearly high enough to stop water becoming steam under all conditions.

Yeah, it's a flaw of physics and chemistry teaching in schools - they teach you very basic laws as if they hold under all conditions, all the time, whatever the case. In reality most of it is either false, or comes under the effect of other laws that dominate in the real world. Welcome to the word of thermodynamics, heat transfer and so on...
 
You break hard, break fluid gets hot remove foot of brakes system pressure drops fluid is able to expand back into the reservoir due to boiling, push breaks now appears you have air in them and they hit the floor..

Get it? The fluid does have room to displace, when your foot is not on the break it is possible for it to go back the other way through the system.
 
personally i think that this is a load of shi** if gas got into the braking system it wouldnt be what is generally accepted as brake fade would it, it would be gas in the braking system which would be a spongy pedal. This isnt the case when you get proper brake fade or what is generally known and accepted as brake fade.

oh and liquid does compress, albeit a negligable amount in this case but it does compress.
 
Ed said:
You break hard oil gets hot remove foot of brakes system pressure drops oil is able to expand back into the reservoir due to boiling, push breaks now appears you have air in them and they hit the floor..

Get it? Oil does have room to displace, when your foot is not on the break it is possible for the oil to go back the other way through the system.
Click to expand...

so the foot to the floor thing would only happen if you were pumping the brake, not if you just held your foot in place under braking?

thanks soo much Andrew :), i thought i was going mad, but what you are saying kinda makes sense, i have to get used to the idea that things are different when working at a very small level...

is there a name for the gas when it has been squashed so that the molicules are the same distance apart as a liquid?

i know if a fluid has molicules the same distance apart as a solid then its known as a glass but i can't find anything about the same thing between gasses and liquids...

it does explain why you can spray lighter gas into a pool of fluid before it turns to a gas... it needs to re absorb the heat energy before it can become a gas... :)

59 bhp is more than enough said:
personally i think that this is a load of shi** if gas got into the braking system it wouldnt be what is generally accepted as brake fade would it, it would be gas in the braking system which would be a spongy pedal. This isnt the case when you get proper brake fade or what is generally known and accepted as brake fade.

oh and liquid does compress, albeit a negligable amount in this case but it does compress.
Click to expand...

gas isn't getting into the system, its already there as water.

and yes liquid does compress, even solids compress alittle bit (at 0 degrees kelvin) but its commonly regarded that liquids don't compress because it requires lots of pressure and the amount is almost un mesurable.. (according to the physics book in front of me hehe :p)
 
so the foot to the floor thing would only happen if you were pumping the brake, not if you just held your foot in place under braking?
Click to expand...

The effect would be much less. I have never had the fluid boil in a car of my own, however I have had break fade to varying degrees many times. Usually pad fade will happen before the fluid exceeds its operating specifications.
 
Ed said:
The effect would be much less. I have never had the fluid boil in a car of my own, however I have had break fade to varying degrees many times. Usually pad fade will happen before the fluid exceeds its operating specifications.
Click to expand...

ah i see :)

its nice to be properly educated finally.... about 5 years too late grr
 
59 bhp is more than enough said:
personally i think that this is a load of shi** if gas got into the braking system it wouldnt be what is generally accepted as brake fade would it, it would be gas in the braking system which would be a spongy pedal. This isnt the case when you get proper brake fade or what is generally known and accepted as brake fade.
Click to expand...

The SAE acknowledge that brake fade can be caused by gas forming in the brake fluid, both from water and the fluid itself. I guess all the funding into the 120 odd papers about it was just for a laugh?
 
If you go go-karting, and keep the break on while you drive around you will get brake fade, that is due the the brake fluid boiling not the pads... 80+% of the time afaik
 
Most karts I have seen have cable brakes? In any case prolong periods of breaking allow the fluid to heat up, however short sharp burts are more likely to kill the pads.
 
At the end of the day brakes do fade when they overheat and the best way to combat this is to use a non hydroscopic brake fluid accompanied by good quality pads and vented and drilled discs.
 
Asraf said:
If you go go-karting, and keep the break on while you drive around you will get brake fade, that is due the the brake fluid boiling not the pads... 80+% of the time afaik
Click to expand...

are you sure? i would have thought that the pads in go karts would be pretty pants... i can't remember cos its been a long time since i went go karting but don't the wheels just lock up which would mean they wouldn't normally get hot?

sonic said:
At the end of the day brakes do fade when they overheat and the best way to combat this is to use a non hydroscopic brake fluid accompanied by good quality pads and vented and drilled discs.
Click to expand...

the end :)


i have learn more in the past 2 hours than the 7 years i spent as school!
 
NeX said:
is there a name for the gas when it has been squashed so that the molicules are the same distance apart as a liquid?
Click to expand...

Well, the thing is that the fluid is made up of water and brake fluid. They behave differently, one can be a liquid and the other gas. Even if it is only brake fluid, you can have bubbles of gas at very high pressure inside of the liquid (due to surface tension and heat transfer). Think of a pan of water just before it boils - you see bubbles of gas forming.

NeX said:
i know if a fluid has molicules the same distance apart as a solid then its known as a glass but i can't find anything about the same thing between gasses and liquids...
Click to expand...

Distance between molecules is not what determines the solid/liquid/gas phases. I'm not sure where you got that definition of glass from, it sounds like it is following the common misconception that glass is a fluid. An amorphous solid (glass is one of these) has quite a lot in common with a liquid though.


NeX said:
and yes liquid does compress, even solids compress alittle bit (at 0 degrees kelvin) but its commonly regarded that liquids don't compress because it requires lots of pressure and the amount is almost un mesurable.. (according to the physics book in front of me hehe :p)
Click to expand...


Solids compress at any temperature, just a very, very small amount. Some liquids are very compressible... actually DOT5 is pretty compressible in real terms...
 
Ed said:
Most karts I have seen have cable brakes? In any case prolong periods of breaking allow the fluid to heat up, however short sharp burts are more likely to kill the pads.
Click to expand...

Not the place i go to, or the previous one, but yeah older karts do use cables.
 
I know on mountain bikes the brakes stop working because of pads, not boiling fluid. Totally irrelevant to cars though.
 
a sticky caliper let me feel this one side wore down and to the metal heated everything then all the breaks faded. was not fun and meant new brakes funny cos it passed the mot a week before

and nex surely there is room for it to expand as there is air in the brake fluid resivoir
 
Andrew said:
Distance between molecules is not what determines the solid/liquid/gas phases. I'm not sure where you got that definition of glass from, it sounds like it is following the common misconception that glass is a fluid. An amorphous solid (glass is one of these) has quite a lot in common with a liquid though.
Click to expand...

thats what i'm having trouble finding out, a proper definition between solids liquids and gases.

i was told that glasses are like fluids because how the atoms are arranged... something like that, its related to the differents between carbon and dimond i think...
Andrew said:
Solids compress at any temperature, just a very, very small amount. Some liquids are very compressible... actually DOT5 is pretty compressible in real terms...
Click to expand...

ah :) what i ment to say was that if you compressed a solid to the point where the molecules were not moving then it would be at 0 degrees kelvin...
is that right?

paul_k10_Lx said:
and nex surely there is room for it to expand as there is air in the brake fluid resivoir
Click to expand...

yea but the resivoir is disconnected from the braking system when you are braking, the tube to the resivoir is blocked by the cylinder when you push the brake pedal
 
NeX said:
thats what i'm having trouble finding out, a proper definition between solids liquids and gases.

i was told that glasses are like fluids because how the atoms are arranged... something like that, its related to the differents between carbon and dimond i think...
Click to expand...

It actually isn't a really simple definition. Just stick with the ones you've got, and forget about how close the molecules are.

NeX said:
ah :) what i ment to say was that if you compressed a solid to the point where the molecules were not moving then it would be at 0 degrees kelvin...
is that right?
Click to expand...

I don't think it works like that. I don't really know, but compression on a big scale probably wouldn't cause that effect.
 
Of course gases turn to liquid if they are compressed enough. They don't do it iunless you compress them enough though do they? You can try to compress air as much as possible, but without cooling it, it won't turn into a liquid. It's the same with steam in a brake line - you will be hard pushed to apply enough pressure to cause it to turn back into water, although it could just about be possible.
 
What we need....

are fly by wire brakes with vented ceramic disks and F1 style pads. Bye bye brake fade :glance:

And 0 kelvin cant really happen in reality either!
 
Turtle said:
What we need....

are fly by wire brakes with vented ceramic disks and F1 style pads. Bye bye brake fade :glance:

And 0 kelvin cant really happen in reality either!
Click to expand...

As long as you dont get a problem with the electrics. lol

I think some of the mercedes Slr's have cable brakes instead of fluid lines
 

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