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FarQue

Original Poster:

2,017 posts

84 months

[news] 
Thursday 27th September 2012 quote quote all
Looking for 'accepted wisdom' regarding running in of new discs and pads on a 997 (steel discs). Is it a case of just running around in the car normally or should I be braking as hard as possible from 100mph 5 times on the trot to run 'heat cycles' through the discs, as has been suggested to me.

Any valid opinions appreciated. Ta.

Porkupine

1,604 posts

51 months

[news] 
Thursday 27th September 2012 quote quote all
FarQue said:
Looking for 'accepted wisdom' regarding running in of new discs and pads on a 997 (steel discs). Is it a case of just running around in the car normally or should I be braking as hard as possible from 100mph 5 times on the trot to run 'heat cycles' through the discs, as has been suggested to me.

Any valid opinions appreciated. Ta.
From what I have been told it is the opposite. Don't brake too hard for the first 30-40 miles in order to allow the pads to bed to the discs. (Obviously unless you have to brake hard in emergency)

RatBoy M3CSL

1,457 posts

82 months

[news] 
Thursday 27th September 2012 quote quote all
..I had a new set all round on mine, they were poor to start with, just drive them normally, no hard stops, and after about 2-300 miles you will see the discs shine all over with the machining marks disappearing, the braking will get better and better, mine are superb now, but immeadiatly after fitting I thought they had put weetabix in the calipers by mistake.. wink

Track pads you need to heat cycle and do a set of stops.. a road set seem to benefit from the slower method..

Edited by RatBoy M3CSL on Thursday 27th September 08:17

huytonman

257 posts

80 months

[news] 
Thursday 27th September 2012 quote quote all
you need to bed the pads in by doing what you suggested which is to brake hard 10-20 times from a reasonable speed. if you dont do this there is a danger of creating a glaze on the disks which will cause shuddering and that cant be removed apart from through skimming (its an increibly hard substance). When people refer to warped disks its usually because of glazing not actual metal bending. I take this approach with all new pads and disks and havent had any problems (including after I took delivery of my 981).
There is plenty of article about this if you do a search.
Keith

Sunnysidebb

1,341 posts

53 months

[news] 
Saturday 29th September 2012 quote quote all
Ok might have sold the POrk but I'm still reading and just cant resist.

A mechanic from RED showed and told me how to bed in new discs and pads so they would be top notch for an event.
If both pads and discs are new then do 20 to 30 hard stops to de bur the discs, let them cool by just driving then start breaking from high speed repeatedly untill you get brake fade. Do not stop the car but drive steadily untill the brakes have cooled. Sopping immediately will cause a cool spot on the disc where the pads are and increase the risk of warping.you only need to do this once.
The way to tell the pads have bedded in correctly is that the edge of the pads which is normally black should have a 3 to 4 mm colour change to grey (ish) That shows the heat has dissipated into the pads and they are at their best for a competition.But.....max performance is required and not long life!
I have always done my brakes this way and they were great for competition. When I didn't do any bedding in the first time I changed the discs and Pads on my 911 ( I didn't have the time) within a week the buggers had glazed and were knackered.

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cmoose

25,755 posts

115 months

[news] 
Saturday 29th September 2012 quote quote all
huytonman said:
if you dont do this there is a danger of creating a glaze on the disks which will cause shuddering and that cant be removed apart from through skimming (its an increibly hard substance).
Keith
Have often wondered about this. If you glaze your disks, do they then last forever. This is the implication of the above. Not entirely buying it.

Sunnysidebb

1,341 posts

53 months

[news] 
Saturday 29th September 2012 quote quote all
cmoose said:
huytonman said:
if you dont do this there is a danger of creating a glaze on the disks which will cause shuddering and that cant be removed apart from through skimming (its an increibly hard substance).
Keith
Have often wondered about this. If you glaze your disks, do they then last forever. This is the implication of the above. Not entirely buying it.
Good question, its only ever happened to my 911 and never to another car???????
But in the absence of the correct terminology I'l go along with glaze untill some one else shows different???????

FarQue

Original Poster:

2,017 posts

84 months

[news] 
Saturday 29th September 2012 quote quote all
Interesting stuff chaps - keep it coming. I haven't driven the car since the new bits were fitted (due to work etc), but I have had time to consider where best locally to do the 'speed-up, brake-hard routine several times...

huytonman

257 posts

80 months

[news] 
Saturday 29th September 2012 quote quote all
OK..."glaze" was a simplistic explanation, heres one for the cynics (and yes it probably would make the disks last for a long time but they wouldnt do their job very well). Cast iron is an alloy of iron and silicon in solution interspersed with particles of carbon. At elevated temperatures, inclusions of carbides begin to form in the matrix. In the case of the brake disk, any uneven deposits - standing proud of the disc surface - become hotter than the surrounding metal. Every time that the leading edge of one of the deposits rotates into contact with the pad, the local temperature increases. When this local temperature reaches around 1200 or 1300 degrees F. the cast iron under the deposit begins to transform into cementite (an iron carbide in which three atoms of iron combine with one atom of carbon). Cementite is very hard, very abrasive and is a poor heat sink. So instead of glaze I should have said cementite...

Slippydiff

6,381 posts

109 months

[news] 
Saturday 29th September 2012 quote quote all
Sunnysidebb said:
Ok might have sold the POrk but I'm still reading and just cant resist.

A mechanic from RED showed and told me how to bed in new discs and pads so they would be top notch for an event.
If both pads and discs are new then do 20 to 30 hard stops to de bur the discs, let them cool by just driving then start breaking from high speed repeatedly untill you get brake fade. Do not stop the car but drive steadily untill the brakes have cooled. Sopping immediately will cause a cool spot on the disc where the pads are and increase the risk of warping.you only need to do this once.
The way to tell the pads have bedded in correctly is that the edge of the pads which is normally black should have a 3 to 4 mm colour change to grey (ish) That shows the heat has dissipated into the pads and they are at their best for a competition.But.....max performance is required and not long life!
I have always done my brakes this way and they were great for competition. When I didn't do any bedding in the first time I changed the discs and Pads on my 911 ( I didn't have the time) within a week the buggers had glazed and were knackered.
The above is perfect for competition pads, NOT so for "normal" road pads. Textars respond well to a gradual increase in pedal pressure after the first 100 or so miles. Once you've got 250-500 miles on them, they should be perfectly (and fully) bedded.

mrdemon

5,955 posts

151 months

[news] 
Saturday 29th September 2012 quote quote all
People buy new cars every day and just drive them, they seem to always work :-)

I would not worry to much and just drive it.

Yu can follow pagid,s web site if you wish, but I have had loads of cars and people say all sorts regarding what to do and what not to do.

Slippydiff

6,381 posts

109 months

[news] 
Saturday 29th September 2012 quote quote all
huytonman said:
OK..."glaze" was a simplistic explanation, heres one for the cynics (and yes it probably would make the disks last for a long time but they wouldnt do their job very well). Cast iron is an alloy of iron and silicon in solution interspersed with particles of carbon. At elevated temperatures, inclusions of carbides begin to form in the matrix. In the case of the brake disk, any uneven deposits - standing proud of the disc surface - become hotter than the surrounding metal. Every time that the leading edge of one of the deposits rotates into contact with the pad, the local temperature increases. When this local temperature reaches around 1200 or 1300 degrees F. the cast iron under the deposit begins to transform into cementite (an iron carbide in which three atoms of iron combine with one atom of carbon). Cementite is very hard, very abrasive and is a poor heat sink. So instead of glaze I should have said cementite...
I suspect you're referring to pad transfer, not "glaze". If a disc and pad are not bedded in correctly, and in the case of a competition pad that means getting the friction compound up to the temperature at which that the resins and binding/bonding agents will stabilize and in the process allow the pad surface to "transfer" on to the face of the disc.

If this process (which Sunnyside has described) is not carried out correctly (or indeed at all) the transfer layer will not exist or will be inconsistent, this will lead to what most people assume to be warped discs, it's not, it's an inconsistent layer of pad material (transfer layer) on the disc faces, this in turn manifests as judder (because as the disc rotates, it presents a face alternately with good and poor sections of transfer layer)

If the the pad is not heat cycled sufficiently, the transfer layer will not be released from the pad onto the disc face. THEN the pad will glaze, and in turn cause the disc begin to overheat in localised areas. These areas then form into cementite, which as you've pointed out is incredibly hard. Once this has happened, the disc will be unserviceable, as no amount of surface skimming or grinding will remove the cementite.

HTH.


Edited by Slippydiff on Saturday 29th September 18:57

ChrisW.

1,820 posts

141 months

[news] 
Saturday 29th September 2012 quote quote all
Interesting, I also had a problem with Performance Friction pads when replaced with Pagid ....

What bedding-in process would you recommend ?

And is this different for PCCB's ?

Rockster

1,144 posts

46 months

[news] 
Sunday 30th September 2012 quote quote all
FarQue said:
Looking for 'accepted wisdom' regarding running in of new discs and pads on a 997 (steel discs). Is it a case of just running around in the car normally or should I be braking as hard as possible from 100mph 5 times on the trot to run 'heat cycles' through the discs, as has been suggested to me.

Any valid opinions appreciated. Ta.
5 times from 100 mph is a bit overkill in my opinion.

While you can just drive the car around and let the pads/rotors bed in on their own the brakes will not deliver their best stopping until that bedding in occurs.

Also there is a risk small but real that a hard stop can result in uneven material deposition from the pads to the rotors which will be felt as brake pedal pulsing thereafter.

What I was taught early on and what I have done is whenever I buy a new car or fit new pads or new pads/rotors to an existing car is to bed in the brakes.

A hard braking event from highway speed short of ABS trigger or before ABS short of wheel/tire lock up -- never a stop from speed during in the brake bedding in! -- followed by several more of these then followed by a long enough drive to let the brakes cool down is enough. While a few more might improve things I can feel the braking improve and by the 3rd slow down I'm happy.

And so based on the brake life my cars deliver so are the brakes.

Sincerely,

Rockster.

Ozzie Osmond

16,221 posts

132 months

[news] 
Sunday 30th September 2012 quote quote all
huytonman said:
you need to bed the pads in by doing what you suggested which is to brake hard 10-20 times from a reasonable speed. if you dont do this there is a danger of creating a glaze on the disks which will cause shuddering and that cant be removed apart from through skimming (its an increibly hard substance).
Where do these myths come from? As someone else has said, new cars are bought every day and they work.

The only time I've ever glazed discs was in a single rapid deceleration on the autobahn - not in a Porsche - which must have overheated them. After that the brakes tended to judder and you could see the discs had a strange blue colouration. I thought I'd destroyed the brakes.

But here's the point. After a few hunderd miles of normal driving the discs cleaned themselves up and I never had any further trouble.

jfp

358 posts

109 months

[news] 
Sunday 30th September 2012 quote quote all
Slippydiff said:
Sunnysidebb said:
Ok might have sold the POrk but I'm still reading and just cant resist.

A mechanic from RED showed and told me how to bed in new discs and pads so they would be top notch for an event.
If both pads and discs are new then do 20 to 30 hard stops to de bur the discs, let them cool by just driving then start breaking from high speed repeatedly untill you get brake fade. Do not stop the car but drive steadily untill the brakes have cooled. Sopping immediately will cause a cool spot on the disc where the pads are and increase the risk of warping.you only need to do this once.
The way to tell the pads have bedded in correctly is that the edge of the pads which is normally black should have a 3 to 4 mm colour change to grey (ish) That shows the heat has dissipated into the pads and they are at their best for a competition.But.....max performance is required and not long life!
I have always done my brakes this way and they were great for competition. When I didn't do any bedding in the first time I changed the discs and Pads on my 911 ( I didn't have the time) within a week the buggers had glazed and were knackered.
The above is perfect for competition pads, NOT so for "normal" road pads. Textars respond well to a gradual increase in pedal pressure after the first 100 or so miles. Once you've got 250-500 miles on them, they should be perfectly (and fully) bedded.
Quite right slippy. Normal road pads and discs - just drive, and within a few hindred miles they will be fine..Competition based materials need a 10 or so series of snubs from 0.5 G up to 1.0 g from upwards of 70mph down to walking pace. Until they go through a fade (some Pagids have 2) they are next to useless. After the fade then just drive gently to clean the discs a tad, park up, let the baking finish and then thrash.

cmoose

25,755 posts

115 months

[news] 
Sunday 30th September 2012 quote quote all
Ozzie Osmond said:
Where do these myths come from? As someone else has said, new cars are bought every day and they work.

The only time I've ever glazed discs was in a single rapid deceleration on the autobahn - not in a Porsche - which must have overheated them. After that the brakes tended to judder and you could see the discs had a strange blue colouration. I thought I'd destroyed the brakes.

But here's the point. After a few hunderd miles of normal driving the discs cleaned themselves up and I never had any further trouble.
This is generally my experience. I've often had brakes getting more and more juddery and rumbly on track days as the day went on. A week or two later after some typical road miles? Back to normal.

I'm highly dubious of this idea involving cosmically hard materials impervious to wear. Has anyone actually experienced disks that got "glazed" and went on to last 100s of thousands of miles?


Slippydiff

6,381 posts

109 months

[news] 
Sunday 30th September 2012 quote quote all
cmoose said:
I'm highly dubious of this idea involving cosmically hard materials impervious to wear.


Why ?

See item 5 here :

http://bremboaftermarket.com/en/Car_Disc_Indicatio...

cmoose said:
Has anyone actually experienced disks that got "glazed" and went on to last 100s of thousands of miles?
Discs don't glaze. It's the transfer layer. Pads can glaze if they're not bedded in correctly ie the resins aren't cured/burnt off by heat cycling.

Friction is the mechanism that converts dynamic energy into heat. Just as there are two sorts of friction between the tire and the road surface (mechanical gripping of road surface irregularities by the elastic tire compound and transient molecular adhesion between the rubber and the road in which rubber is transferred to the road surface), so there are two very different sorts of braking friction - abrasive friction and adherent friction. Abrasive friction involves the breaking of the crystalline bonds of both the pad material and the cast iron of the disc. The breaking of these bonds generates the heat of friction. In abrasive friction, the bonds between crystals of the pad material (and, to a lesser extent, the disc material) are permanently broken. The harder material wears the softer away (hopefully the disc wears the pad). Pads that function primarily by abrasion have a high wear rate and tend to fade at high temperatures. When these pads reach their effective temperature limit, they will transfer pad material onto the disc face in a random and uneven pattern. It is this "pick up" on the disc face that both causes the thickness variation measured by the technicians and the roughness or vibration under the brakes reported by the drivers.

With adherent friction, some of the pad material diffuses across the interface between the pad and the disc and forms a very thin, uniform layer of pad material on the surface of the disc. As the friction surfaces of both disc and pad then comprise basically the same material, material can now cross the interface in both directions and the bonds break and reform. In fact, with adherent friction between pad and disc, the bonds between pad material and the deposits on the disc are transient in nature - they are continually being broken and some of them are continually reforming.

There is no such thing as pure abrasive or pure adherent friction in braking. With many contemporary pad formulas, the pad material must be abrasive enough to keep the disc surface smooth and clean. As the material can cross the interface, the layer on the disc is constantly renewed and kept uniform - again until the temperature limit of the pad has been exceeded or if the pad and the disc have not been bedded-in completely or properly. In the latter case, if a uniform layer of pad material transferred onto the disc face has not been established during bedding or break-in, spot or uncontrolled transfer of the material can occur when operating at high temperatures. The organic and semi-metallic pads of the past were more abrasive than adherent and were severely temperature limited. All of the current generation of "metallic carbon", racing pads utilize mainly adherent technology as do many of the high end street car pads and they are temperature stable over a much higher range. Unfortunately, there is no free lunch and the ultra high temperature racing pads are ineffective at the low temperatures typically experienced in street use.

Therefore - there is no such thing as an ideal "all around" brake pad. The friction material that is quiet and functions well at relatively low temperatures around town will not stop the car that is driven hard. If you attempt to drive many cars hard with the OEM pads, you will experience pad fade, friction material transfer and fluid boiling - end of discussion. The true racing pad, used under normal conditions will be noisy and will not work well at low temperatures around town.

Ideally, in order to avoid either putting up with squealing brakes that will not stop the car well around town or with pad fade on the track or coming down the mountain at speed, we should change pads before indulging in vigorous automotive exercise. No one does. The question remains, what pads should be used in high performance street cars - relatively low temperature street pads or high temperature race pads? Strangely enough, in my opinion, the answer is a high performance street pad with good low temperature characteristics. The reason is simple: If we are driving really hard and begin to run into trouble, either with pad fade or boiling fluid (or both), the condition(s) comes on gradually enough to allow us to simply modify our dri

Regardless of pad composition, if both disc and pad are not properly broken in, material transfer between the two materials can take place in a random fashion - resulting is uneven deposits and vibration under braking. Similarly, even if the brakes are properly broken, if, when they are very hot or following a single long stop from high speed, the brakes are kept applied after the vehicle comes to a complete stop it is possible to leave a telltale deposit behind that looks like the outline of a pad. This kind of deposit is called pad imprinting and looks like the pad was inked for printing like a stamp and then set on the disc face. It is possible to see the perfect outline of the pad on the disc.

It gets worse. Cast iron is an alloy of iron and silicon in solution interspersed with particles of carbon. At elevated temperatures, inclusions of carbides begin to form in the matrix. In the case of the brake disk, any uneven deposits - standing proud of the disc surface - become hotter than the surrounding metal. Every time that the leading edge of one of the deposits rotates into contact with the pad, the local temperature increases. When this local temperature reaches around 1200 or 1300 degrees F. the cast iron under the deposit begins to transform into cementite (an iron carbide in which three atoms of iron combine with one atom of carbon). Cementite is very hard, very abrasive and is a poor heat sink. If severe use continues the system will enter a self-defeating spiral - the amount and depth of the cementite increases with increasing temperature and so does the brake roughness.

PREVENTION

There is only one way to prevent this sort of thing - following proper break in procedures for both pad and disc and use the correct pad for your driving style and conditions. All high performance after market discs and pads should come with both installation and break in instructions. The procedures are very similar between manufacturers. With respect to the pads, the bonding resins must be burned off relatively slowly to avoid both fade and uneven deposits. The procedure is several stops of increasing severity with a brief cooling period between them. After the last stop, the system should be allowed to cool to ambient temperature. Typically, a series of ten increasingly hard stops from 60mph to 5 mph with normal acceleration in between should get the job done for a high performance street pad. During pad or disc break-in, do not come to a complete stop, so plan where and when you do this procedure with care and concern for yourself and the safety of others. If you come to a complete stop before the break-in process is completed there is the chance for non-uniform pad material transfer or pad imprinting to take place and the results will be what the whole process is trying to avoid. Game over.

In terms of stop severity, an ABS active stop would typically be around 0.9 G’s and above, depending on the vehicle. What you want to do is stop at a rate around 0.7 to 0.9 G's. That is a deceleration rate near but below lock up or ABS intervention. You should begin to smell pads at the 5th to 7th stop and the smell should diminish before the last stop. A powdery gray area will become visible on the edge of the pad (actually the edge of the friction material in contact with the disc - not the backing plate) where the paint and resins of the pad are burning off. When the gray area on the edges of the pads are about 1/8" deep, the pad is bedded.

For a race pad, typically four 80mph to 5 and two 100mph to 5, depending on the pad, will also be necessary to raise the system temperatures during break-in to the range that the pad material was designed to operate at. Hence, the higher temperature material can establish its layer completely and uniformly on the disc surface.

Fortunately the procedure is also good for the discs and will relieve any residual thermal stresses left over from the casting process (all discs should be thermally stress relieved as one of the last manufacturing processes) and will transfer the smooth layer of pad material onto the disc. If possible, new discs should be bedded with used pads of the same compound that will be used going forward. Again, heat should be put into the system gradually - increasingly hard stops with cool off time in between. Part of the idea is to avoid prolonged contact between pad and disc. With abrasive pads (which should not be used on high performance cars) the disc can be considered bedded when the friction surfaces have attained an even blue color. With the carbon metallic type pads, bedding is complete when the friction surfaces of the disc are a consistent gray or black. In any case, the discoloration of a completely broken in disc will be complete and uniform.

Depending upon the friction compound, easy use of the brakes for an extended period may lead to the removal of the transfer layer on the discs by the abrasive action of the pads. When we are going to exercise a car that has seen easy brake use for a while, a partial re-bedding process will prevent uneven pick up.

The driver can feel a 0.0004" deposit or TV on the disc. 0.001" is annoying. More than that becomes a real pain. When deposit are present, by having isolated regions that are proud of the surface and running much hotter than their neighbors, cementite inevitably forms and the local wear characteristics change which results in ever increasing TV and roughness.

Other than proper break in, as mentioned above, never leave your foot on the brake pedal after you have used the brakes hard. This is not usually a problem on public roads simply because, under normal conditions, the brakes have time to cool before you bring the car to a stop (unless, like me, you live at the bottom of a long steep hill). In any kind of racing, including autocross and "driving days" it is crucial. Regardless of friction material, clamping the pads to a hot stationary disc will result in material transfer and discernible "brake roughness". What is worse, the pad will leave the telltale imprint or outline on the disc and your sin will be visible to all and sundry.

The obvious question now is "is there a "cure" for discs with uneven friction material deposits?" The answer is a conditional yes. If the vibration has just started, the chances are that the temperature has never reached the point where cementite begins to form. In this case, simply fitting a set of good "semi-metallic" pads and using them hard (after bedding) may well remove the deposits and restore the system to normal operation but with upgraded pads. If only a small amount of material has been transferred i.e. if the vibration is just starting, vigorous scrubbing with garnet paper may remove the deposit. As many deposits are not visible, scrub the entire friction surfaces thoroughly. Do not use regular sand paper or emery cloth as the aluminum oxide abrasive material will permeate the cast iron surface and make the condition worse. Do not bead blast or sand blast the discs for the same reason.

The only fix for extensive uneven deposits involves dismounting the discs and having them Blanchard ground - not expensive, but inconvenient at best. A newly ground disc will require the same sort of bedding in process as a new disc. The trouble with this procedure is that if the grinding does not remove all of the cementite inclusions, as the disc wears the hard cementite will stand proud of the relatively soft disc and the thermal spiral starts over again. Unfortunately, the cementite is invisible to the naked eye.



cmoose

25,755 posts

115 months

[news] 
Sunday 30th September 2012 quote quote all
tl;dr?

Slippydiff

6,381 posts

109 months

[news] 
Sunday 30th September 2012 quote quote all
cmoose said:
tl;dr?
So you're highly dubious as to the existence of cementite, yet given the evidence that it does indeed exist, you've chosen not to read it ? Why bother posting your doubts in the first place ? confused

http://www.pistonheads.com/gassing/topic.asp?h=0&a...
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