markocosic
04-19-2007, 01:37 PM
Hey all,
Below is part of a response from Edward Little - Research Manager for Racing and Motorcycles Division at Federal-Mogul Operations Italy - to a couple of questions about brake pad materials. I thought they might be useful and he's ok-ed publishing them here - cheers Edward!
Thanks for you enquirey. The answers to the questions you ask
could literally fill a book, so I'll try to be brief and address your
direct questions:
1. If there wasn't any more to it, I can't see any reason for
wanting anything other than the DS3000s, and would like to know what
other factors come into play.
A high friction compound like DS3000 has one main advantage: it
delivers a high friction coefficient (thus is more efficent at
translating input pressure into brake torque) across a wide
temperature range, from approx 100?C to 600?C. It may not be the
material of choice for a number of reasons however: friction at
ambient temperature can be low. Below 200?C it will wear the brake
discs very heavily. As a high friction compound it will be more prone
to generate brake squeal. As a hard compound, it may tend to transmit
imprefections in the disc/disc set-up into perceptible vibration. If
the limiting factor in braking is tyre-track adhesion, then there is
no need for high friction and it may lead to wheel lock.
Therefore, if heat resistance and high mu is not a pre-requisite,
another "kinder" compound may be more suitable.
2. Elastic stiffness. However it's measured, generally a
"stiffer" pad will have the following advantage: firm, short pedal,
and the following disadvantage: it will not conform to the disc (and
disc imprefections, transient or otherwise) as well as a softer pad.
This can lead to vibration (as discussed in 1) and localised contact
which can lead to localised heating which can lead to disc damage (in
extreme case to a transition in the perlite iron structure of the disc
to martensite, which is brittle and can crack).
3. Hysterisis. Certainly a pad will demonstrate measurable
hyterisis when tested under static conditions for, for example,
compressibility. The service environment of a brake system is so
complex that I am not aware of any work that has examined the effect
of hysterisis on the performance of a pad. On the basis of no
experience, I would say: a) any hysterisis effects would be more
evidenced in subtle phenomena such as squeal and excited vibrations
which are of less importance in a race car and b) the hysteresis of a
pad w.r.t. time is several orders of magnitude slower than say, the
millisecond and faster changes in environment brough about by the
revolution of a brake disc. That being the case the pad may be
thought of not to be exhibiting hysteresis,, seeing as it would be
effectively constant over the timesacle of any dynamic event with
which it might interact. Only my idea, as I say.
With respect to pad choice, given the details you supply : I'd
say, from the Ferodo range for the fronts, I'd try the following:
DS3000 if you don't have lock-up issues and you can live with a
shortened disc life (as the low temps you will be working at the disc
wear will be fairly high). FER4003 if you are prone to lock up.
DS2500 if you feel your car is well braked and you want a more
"normal" feeling pad. DS2500 would be my recommendation for the
rears.
[I've found little information in the public domain about the temperature limits of pad compounds] ...in terms of temperature what would be your recommended absolute maximum limit?
Answer: depends on how it's measured, but let's say with a thermocouple fitted that measures temps as the vehicle is moving (as opposed to say IR probe (much faster) or a contact probe used when the car comes back into the pits (much slower): You will start to get fade if temperatures stray above 600?C and stay there. That doesn't mean the discs will suffer, but the pads probably will.
The discs will be a standard grey 'flake' iron (~220MPa tensile strength), so presumably we would want to avoid temperatures above ~700?C at all costs to avoid phase-changes?
Answer: Generally speaking it's not the absolute temp that casues the (irreversible) phase change to martensite, it's a rise to above ~700?C (casing a transition from ferrite to austenite) followed by a rapid quench. This does not allow diffusion out of the trapped carbon and can cause the martensite transition. A slower cooling will result in carbon diffusing out in a controlled way and the austenite will revert to ferrite. So it's not a case of "not at all costs above 700?C" but of course avoiding >700?C will remove the possibility absolutely. Getting above that temp you may, if unlucky, see martensitic transition if the cooling is quick. Martensite is more brittle and small areas of it can act as seed for disc cracking.
Presumably DS2500/DS3000 pads will start to fade at this temperature, but will they survive this temperature without permanent damage?
Answer: Yes, more or less. Pads can fade but unless they take a fearfull beating they should be good to go again once they get back down to a reasonable operating temp (in fact we found that, providing you don't get them too hot too quickly initially, they will work better subsequent to their first exposure to fade. This is one of the reasons to do a good bedding).
He has also written a short article entitled Performance Requirements of a Racing Brake Pad which will be of interest, but can't be copied here as others now own the copyright. A search on the title and author should find it though.
Some other notes/thoughts: When you do hit the ~700C phase-change limit in the iron, the crystal structure switches from a body-centred-cubic (BCC) to a face-centred-cubic (FCC). FCC is bigger than BCC, so the disc will increase in volume around that temperature. Once get a hotspot and it'll soon get disproportionately hotter - due to it standing proud of the disc surface and generally takign a beating as a result. This is not the end of the world (the iron will recover if the cooling rate is slow enough), but if the cooling rate does end up too high (cold brakes and the hotspot rapidly being cooled by the 'core' disc material, or external quenching by water etc) you'll form that martensitic region and your discs will probably begin to crack.
If I can be cheeky, I reckon a public plug for Ferodo for taking the time out to offer advice is in order - the only firm we have found in the area actually willing to discuss what it sells and the one that'll be getting our business! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif
Below is part of a response from Edward Little - Research Manager for Racing and Motorcycles Division at Federal-Mogul Operations Italy - to a couple of questions about brake pad materials. I thought they might be useful and he's ok-ed publishing them here - cheers Edward!
Thanks for you enquirey. The answers to the questions you ask
could literally fill a book, so I'll try to be brief and address your
direct questions:
1. If there wasn't any more to it, I can't see any reason for
wanting anything other than the DS3000s, and would like to know what
other factors come into play.
A high friction compound like DS3000 has one main advantage: it
delivers a high friction coefficient (thus is more efficent at
translating input pressure into brake torque) across a wide
temperature range, from approx 100?C to 600?C. It may not be the
material of choice for a number of reasons however: friction at
ambient temperature can be low. Below 200?C it will wear the brake
discs very heavily. As a high friction compound it will be more prone
to generate brake squeal. As a hard compound, it may tend to transmit
imprefections in the disc/disc set-up into perceptible vibration. If
the limiting factor in braking is tyre-track adhesion, then there is
no need for high friction and it may lead to wheel lock.
Therefore, if heat resistance and high mu is not a pre-requisite,
another "kinder" compound may be more suitable.
2. Elastic stiffness. However it's measured, generally a
"stiffer" pad will have the following advantage: firm, short pedal,
and the following disadvantage: it will not conform to the disc (and
disc imprefections, transient or otherwise) as well as a softer pad.
This can lead to vibration (as discussed in 1) and localised contact
which can lead to localised heating which can lead to disc damage (in
extreme case to a transition in the perlite iron structure of the disc
to martensite, which is brittle and can crack).
3. Hysterisis. Certainly a pad will demonstrate measurable
hyterisis when tested under static conditions for, for example,
compressibility. The service environment of a brake system is so
complex that I am not aware of any work that has examined the effect
of hysterisis on the performance of a pad. On the basis of no
experience, I would say: a) any hysterisis effects would be more
evidenced in subtle phenomena such as squeal and excited vibrations
which are of less importance in a race car and b) the hysteresis of a
pad w.r.t. time is several orders of magnitude slower than say, the
millisecond and faster changes in environment brough about by the
revolution of a brake disc. That being the case the pad may be
thought of not to be exhibiting hysteresis,, seeing as it would be
effectively constant over the timesacle of any dynamic event with
which it might interact. Only my idea, as I say.
With respect to pad choice, given the details you supply : I'd
say, from the Ferodo range for the fronts, I'd try the following:
DS3000 if you don't have lock-up issues and you can live with a
shortened disc life (as the low temps you will be working at the disc
wear will be fairly high). FER4003 if you are prone to lock up.
DS2500 if you feel your car is well braked and you want a more
"normal" feeling pad. DS2500 would be my recommendation for the
rears.
[I've found little information in the public domain about the temperature limits of pad compounds] ...in terms of temperature what would be your recommended absolute maximum limit?
Answer: depends on how it's measured, but let's say with a thermocouple fitted that measures temps as the vehicle is moving (as opposed to say IR probe (much faster) or a contact probe used when the car comes back into the pits (much slower): You will start to get fade if temperatures stray above 600?C and stay there. That doesn't mean the discs will suffer, but the pads probably will.
The discs will be a standard grey 'flake' iron (~220MPa tensile strength), so presumably we would want to avoid temperatures above ~700?C at all costs to avoid phase-changes?
Answer: Generally speaking it's not the absolute temp that casues the (irreversible) phase change to martensite, it's a rise to above ~700?C (casing a transition from ferrite to austenite) followed by a rapid quench. This does not allow diffusion out of the trapped carbon and can cause the martensite transition. A slower cooling will result in carbon diffusing out in a controlled way and the austenite will revert to ferrite. So it's not a case of "not at all costs above 700?C" but of course avoiding >700?C will remove the possibility absolutely. Getting above that temp you may, if unlucky, see martensitic transition if the cooling is quick. Martensite is more brittle and small areas of it can act as seed for disc cracking.
Presumably DS2500/DS3000 pads will start to fade at this temperature, but will they survive this temperature without permanent damage?
Answer: Yes, more or less. Pads can fade but unless they take a fearfull beating they should be good to go again once they get back down to a reasonable operating temp (in fact we found that, providing you don't get them too hot too quickly initially, they will work better subsequent to their first exposure to fade. This is one of the reasons to do a good bedding).
He has also written a short article entitled Performance Requirements of a Racing Brake Pad which will be of interest, but can't be copied here as others now own the copyright. A search on the title and author should find it though.
Some other notes/thoughts: When you do hit the ~700C phase-change limit in the iron, the crystal structure switches from a body-centred-cubic (BCC) to a face-centred-cubic (FCC). FCC is bigger than BCC, so the disc will increase in volume around that temperature. Once get a hotspot and it'll soon get disproportionately hotter - due to it standing proud of the disc surface and generally takign a beating as a result. This is not the end of the world (the iron will recover if the cooling rate is slow enough), but if the cooling rate does end up too high (cold brakes and the hotspot rapidly being cooled by the 'core' disc material, or external quenching by water etc) you'll form that martensitic region and your discs will probably begin to crack.
If I can be cheeky, I reckon a public plug for Ferodo for taking the time out to offer advice is in order - the only firm we have found in the area actually willing to discuss what it sells and the one that'll be getting our business! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif