steve_s

I'm probably being thick but what causes brake fade. Is it the disks overheating, pads overheating or the fluid boiling, or is it a combination of all three.

regards

steve s

Scooby370beast

Hi i had the same problem with my single **** calipers and discs.Anyway get some 2nd hand 4 pots out of an STi V and u definately need bigger discs thats whats causing it-Small discs

DavidRB

Depends on the type of brake fade!

Pad/disc fade
This is where either the pads or the discs overheat and their surface glazes over. This results in a "hard" brake pedal, where the brake pedal feels normal or harder than normal. Sometimes, the pedal just feels hard because you try to compensate for the lack of braking by pressing harder. If the discs overheat, they can warp which might show itself as a pulsing brake pedal (not the ABS!) and/or a "pulsating" squealing sound from the brakes.

Fluid fade
This is where the brake fluid overheats and starts to boil, causing bubbles in the brake lines. This results in a spongey pedal that goes right to the floor.

Pad "knock off"
Not heat related, this is caused by the pads being pushed back into the calipers and can happen after hitting something while on the brakes. This results in a spongey pedal, but goes away if you pump the pedal repeatedly two or three times.


Bigger discs help because the bigger surface area (& volume) dissipating the same amount of energy results in lower temps. Then there are bigger discs, more calipers, higher quality brake fluid, higher quality pad & disc materials, etc., etc., etc. ...

steve_s

just curious cos at last service had prodrive disks and pads fitted, was carefull with them at first then at the weekend gave it some down a winding hill for a couple of miles and they were fading by the bottom. nearly didn't stop. disks were glowing orange too.
they are a mega improvement over standard disks/pads but they obviously have there limits....

thanx
steve

sempers

OK, DavidRB beat me to the draw on the fade types, however, one comment remains - most pads need some running in - I believe it's normal (competition) practice to bring a set of pads up to high temp, then let them cool (brakes off so as not to warp the disks). They seem to be much better thereafter.

- Mark.

Pete Croney

I hope David won't mind me adding a few bits...


There are a number of occasions when the brakes won't work at full efficiency, but only one is "fade". And it is...

The only cause of fade is when the resin that binds the pad material together over heats. Instead of being a binding material, it develops properties similar to KY jelly. Its called fade, because the retardation properties fade away as the heat builds up.

Fade can be reduced/eliminated by changing to...
a larger disc,
a disc with a bigger cooling gap between the plates,
a pad material with a higher temperature rating,
an aluminium calliper.

Not actually fade, as full pad/disc friction can be available, but pedal travel may be very long, as though the brake pedal has become flexible. In reality, the fluid itself has become flexible/compressible.

To reduce/eliminate change to a fluid rated at least DOT4 (the wet boiling point is far more important than the DOT rating), and/or change to an aluminium calliper which will lose heat far more readily.

Again, not fade as full disc/pad friction exists. The most common cause of pad knock off is the heat from the discs causing the hub grease to boil. This grease takes up a certain amount of tolerance in the wheel bearings. With the grease turned to liquid, the amount of bearing play increases. As the driver negotiates a bend after braking, the disc moves out of line and presses against the pads. At the disc edge, this movement can be as much as 2mm in either direction.

On a 4 pot calliper, the pistons move in and the fluid that was in them goes round the calliper and the opposing pads move out to take up the gap. On a 2 pot calliper, the piston moves in and the fluid has no where else to go than back into the master cylinder reservoir. When you next press the brake pedal, it falls to the floor, as you are merely reflooding the piston, not actually pressing the pad against the disc. If you are getting pad knock off, the pedal can be pumped and full braking will be available. Look for signs of hub grease smeared on the wheels.

To reduce/eliminate it... use a disc with better cooling such as turbo vaning and ideally one with an alloy "bell" in the centre.

DavidRB

Picky picky.

steve_s

thanks guys for your answers.
so I presume you can't eradicate completely. just take it more easy on the brakes...

steve s

Andy Winskill

What are the Prodrive disks & pads. I didn't think you could get any form of Prodrive upgrade without going for one of their "packs".

On a track day (MY00 Turbo) I didn't have any fade, but I'd changed the fluid to 5.1.

Mike Tuckwood

BRAKE FADE

There are three types of brake fade. The first is the conventional burning off of resins as the pads are being broken (bedded) in. This is known as green fade and occurs when gases from burnt resins are trapped between the pads and rotors. When this happens the pedal feels firm but the car will not stop.

The second type of fade is when the pads are forced to work outside their temperature continuum. This is the point at which the resins burn off rapidly, and the pad has accelerated wear.

The third type of fade is glazing. This is when the resins, which soften during active braking, then cool and solidify on the face of the pads. This glazed surface is hard and slick, and will not give the same coefficient of friction as an unglazed or new pad.

steve_s

andy..
at the last service dealer upgraded to prodrive grooved disks as standard monkey metal jobs kept on cracking. seems to be norm on my99 impreza's.
Service manager also suggested that upgraded pads be fitted too, i think they are DS2000's but stand to be corrected.

steve

DT

Pete,
Re your quote:

"Fade can be reduced/eliminated by changing to...
a larger disc,
a disc with a bigger cooling gap between the plates,
a pad material with a higher temperature rating,
an aluminium calliper"

What advantage would the Pagid Group N discs and pads offer re performance then, apart from a better pad? Curious cos I need to improve my standard brakes and can't afford the Brembo/AP options.

Cheers DT

Andy Winskill

steve_s,

If you don't mind me asking how much was the brake upgrade? What I'm really interested in is more initial bite on the brake peddle for a small outlay of cash. I've a MY00 so I've got the same 2-pots which seem to hold up quite well to light track use (with different fluid).

If I end up keeping the Impreza (considering getting a Westfield) then I think I may go down the big brake route....

Cheers,
Andy

Pete Croney

DT

You would be using a pad with a much higher operating temp (so much better fade resistance) and would need to match this to a heat treated disc that can also cope with these temperatures.

If you just fit hit temp/co-efficient pads, the standard discs will warp and/or crack from the heat generated.

Mike

I ignored bedding in and have always considered glazing a result of overheating the pads. A glazed pad has a greatly reduce co-efficient of friction all the time, not one that will change unexpectedly.

Fade, by definition, is a transition between states.

DT

cheers Pete

SteveB

iTrader: (1)

Steve s

I can't explain like Mike or Pete the technical differencies in the various types of fade but how I based my thinking on the upgrade I chose.
For me the biggest influence/cause of the brake pedal extending to the floor(fade?) was the Calipers. Being a big chunk of cast iron and not disapating the heat very well. I came to this conclusion after doing various drives with others that had various pad/disc upgrades (twisty b roads - Squirt/brake/squirt/brake etc)and getting similar soggy pedal effects like me(smoke and flames at times even with 4 pots), so came to the conclusion that these upgrades would help a little but note ultimatley stop the fade as I wanted. I also didn't want to spend my money twice!

I'm sure you'll get people saying what an excellent upgrade pads and discs are but for me the increase in performance wasn't good value compared to the full upgrade. I chose the Alcon full kit and have never had anything but a firm pedal that gives a lot of confidence. I'm sure the Ap/Brembo kit is the same.

The choice is yours.

Steve.

steve_s

andy
the price of the pads was about £150 (i'll be sourcing them from elsewhere next time - I think the cheapest I've seen them is about £90)
Disks were £280 + vat four the pair but I got Nearly £200 discount for the cracked originals.
thats the reason I chose the prodrive upgraded disks.
Intial bite is not that different to original pads but when they warm up after a couple of applications they inspire so much more confidence than the originals. Pulls up really well.
hope this helps

steve