Cracked AP disc :( => solution :)

[Franzino]

I have a BMW Z4M that I equipped with a full AP racing BBK in 2011. I bought this set brand new with my local AP dealer for street driving and doing some occasional trackdays (mostly Nurburgring). With my first set of front AP discs I did two years of street and track day driving and never had a big problem, then they were worn down and it was time to replace them.

9-10 months a go my Belgian AP dealer fitted new front AP discs on my car. With the new AP discs (CP-3581 / G8) I only did 3-4 tourist trips to the Nurburgring and 1 trip to Spa. The car is not used on track from november till march...so the new discs did not see a lot of hard use. Before stopping the car; I always do a cool down run without breaking, to let the brakes cool before parking the car. My car is also equipped with some extra brake cooling ducts for extra air.

After my last trackday my right front AP discs looked liked this.





This is not normal and it's unsafe to still use this disc! I'm always using the same brake pads and using the same brake method as in the past. Never had any problems before with my previous set of AP discs! The crack on the edge made 1 disc useless...while the front disc are only 20-30% worn down and are still in really good shape (besides the damage).

In my opinion this is not normal for a high quality AP discs, that costs this much money. In my opinion I had bad luck and I received a faulty manufactured AP brake disc (this can happen). I hope AP can help me fix this issue and give me a solution.

Me and my other trackday buddies are all using AP racing on our track day cars and we love them... Trackday enthusiast talk a lot about their modifications and their experiences online. We are hoping AP can give an explanation on how this happened and a goodwill solution from AP to fix this problem. It's absurd to already have to buy a new set of two AP discs again so soon. If AP racing does not want to replace the discs, then I will not take the gamble with AP again and I will buy a set Performance Friction front discs (they promise no cracking).

Any opinions about Performance Friction discs vs AP racing discs? They fit perfectly on the AP racing bell and have identical size..

 

[Beedub]

think its the pagids your using and tbh the incredible amount of track time you do, you live on the ring!!! lmao!! but i agree, that shouldn't happen as these AP rotors.... and they are NOT cheap , whats strange is your first set last 2 solid years with the use you do, so for this set to get to this point which is , imo dangerous ( this rotor WILL fail if further hard use happens) AP should replace, thats some serious heat to crack a rotor like that.

 

[Franzino]

think its the pagids your using and tbh the incredible amount of track time you do, you live on the ring!!! lmao!! but i agree, that shouldn't happen as these AP rotors.... and they are NOT cheap , whats strange is your first set last 2 solid years with the use you do, so for this set to get to this point which is , imo dangerous ( this rotor WILL fail if further hard use happens) AP should replace, thats some serious heat to crack a rotor like that.

My first set lasted 2 year (+ 20 track days) with the same Pagid brake pads… When the discs are at their minimum thickness I replaced them. There were the small thin hairline cracks, but nothing like this. It's true I use the brakes hard for what they are built, but these new discs only did a handful of trackdays in comparison with my first set.

 

[Beedub]

think its the pagids your using and tbh the incredible amount of track time you do, you live on the ring!!! lmao!! but i agree, that shouldn't happen as these AP rotors.... and they are NOT cheap , whats strange is your first set last 2 solid years with the use you do, so for this set to get to this point which is , imo dangerous ( this rotor WILL fail if further hard use happens) AP should replace, thats some serious heat to crack a rotor like that.

My first set lasted 2 year (+ 20 track days) with the same Pagid brake pads… When the discs are at their minimum thickness I replaced them. There were the small thin hairline cracks, but nothing like this. It's true I use the brakes hard for what they are built, but these new discs only did a handful of trackdays in comparison with my first set.

i agree these rotors have failed, no doubt imo... this is a racing brake system and should easily withstand what your using it for, End of the day thats what you've purchased it for, i think AP will replace the rings for you no problem tbh

 

[Franzino]

think its the pagids your using and tbh the incredible amount of track time you do, you live on the ring!!! lmao!! but i agree, that shouldn't happen as these AP rotors.... and they are NOT cheap , whats strange is your first set last 2 solid years with the use you do, so for this set to get to this point which is , imo dangerous ( this rotor WILL fail if further hard use happens) AP should replace, thats some serious heat to crack a rotor like that.

My first set lasted 2 year (+ 20 track days) with the same Pagid brake pads… When the discs are at their minimum thickness I replaced them. There were the small thin hairline cracks, but nothing like this. It's true I use the brakes hard for what they are built, but these new discs only did a handful of trackdays in comparison with my first set.

i agree these rotors have failed, no doubt imo... this is a racing brake system and should easily withstand what your using it for, End of the day thats what you've purchased it for, i think AP will replace the rings for you no problem tbh

I have send a email to AP headquarters in the UK with pictures and the story about what happened... I have made numerous post on forums on how great the AP brakes worked on track; it's only fair to also post when there is a problem.

 

[Lower]

Is it just one disc on one side or both?

That disc has been seriously overheated. Are you using different pads?

If it's just one side I'd say you probably have a sticky caliper that is not releasing properly.

 

[Franzino]

Is it just one disc on one side or both?

That disc has been seriously overheated. Are you using different pads?

If it's just one side I'd say you probably have a sticky caliper that is not releasing properly.

I always use Pagid RS-14 (front) and RS4-4 (rear). Never had any problems in the past with this set-up and a lot of CSL owner use the same set-up. The damage is only on one side of the disc. When the car is on a car jack I can turn the front wheels without the brake pads touching the discs.
Normaly my front AP discs look like this (on both sides). From my experience almost every AP racing front disc that has been used intensive on track looks like this (with some small thin hairline cracks appearing)...



My car has extra brake cooling and never had any problem with fading.

 

[Ewazix]

That's some serious heat cracks there! Just a couple of thoughts, firstly as you say they should be up to the job but cracking of the outer face is common and can occur prematurely following high speed use if the casting has unintended microscopic inclusions (slag) or the cast cooling wasn't right, so it could well be down to the maker.

Another possibility is a brake balance issue caused by the system pulling too much energy on the front, or by uprated (better) brakes or pads being fitted to the front only. This can raise the max temperatures generated on the front by the margin needed to take the rotor over the critical threshold and induce the stress required to crack it after a few high speed applications. So it would be worth checking balance performance and matching the disc/pad specs F & R.

As a matter of interest improved cooling of only one part/surface of a rotor over another (by uprated cooling for example) can cause more differential stress and actually make cracking more likely. It's the size of the temperature difference between the big cooler centre 'hat' and the hot outer disc surface that generates the tension rather the outright max temperature. Likewise the heavy centre fins ironically induce more restrictive stresses to the disc and a radius for stresses to focus around. The cut grooves introduce another tight radius focussing the stress on the vulnerable bottom edge of the rotor where the cool centre hat meets the hot disc, hence the crack running between the two radius points. It will usually be the outer disc that cracks as a result.

I'd guess it's a combination of some sort of F/R braking performance imbalance combined with a poor casting batch (or a bloody heavy right boot )

 

[ga41]

The only experience of Performance Friction discs i have is through a friend who has a set on his CSL with his AP Racing kit. He's very happy with them and so far have lasted at least as good as the original AP discs. The most cost effective solution for you would be if AP sends you a new set so here's hoping!

 

[tertius]

I have PF discs on my AP kit and am very happy with them. However, I think you need to be sure there isn't an underlying issue with the brakes - e.g. the sticking caliper piston possibility or similar.

Extra brake ducting would never offset the heat generated from a sticking caliper.

Perhaps some temperature tape would tell you if one caliper were running hotter than the other?

 

[Franzino]

That's some serious heat cracks there! Just a couple of thoughts, firstly as you say they should be up to the job but cracking of the outer face is common and can occur prematurely following high speed use if the casting has unintended microscopic inclusions (slag) or the cast cooling wasn't right, so it could well be down to the maker.

That is what I also think…that I had bad luck with a bad manufactured disc. I'm now discussing the issue with my local AP racing importer.

Another possibility is a brake balance issue caused by the system pulling too much energy on the front, or by uprated (better) brakes or pads being fitted to the front only. This can raise the max temperatures generated on the front by the margin needed to take the rotor over the critical threshold and induce the stress required to crack it after a few high speed applications. So it would be worth checking balance performance and matching the disc/pad specs F & R.

So what's in your opinion the best way to check the brake balance?

I have a AP racing kit front and rear for optimal brake balance. The first year I had the AP racing brakes I used Pagid RS4-2 all round… The brakes were fine, but the pads did nog have enough bite and could not handle the heat of intensive trackdays and sometimes the pads would leave deposit on the discs. After talking to some specialists; they recommended to use Pagid RS-14 front and RS4-4 rear. Some of the reason why the advised this set-up for me.

Pagid

"For the Z4 (and for BMWs in general) a very good track set up is RS14 (black) front pads and RS4-4 (orange) rear pads. A lot of pro teams do that.

Due to the weight transfer during braking the front brakes do most of the job, especially on a front engine car. In our experience the brake balance is better when using less friction on the rear axle. On an ABS car you won't feel so much difference but it is better to stay out of the ABS especially with a none racing ABS.

With the original brake there is less dynamic weight transform (shifting weight to the front axle and reduce it on the rear axle) because stopping power is just not as good. So the OE brake is set up for these road conditions. On the track, in relation to road use, you can use more front brakes and therefore want to have a more aggressive front pad. For instance in race cars with a balance bar master cylinder system drivers turn more brake bias to the rear in slick conditions (e.g. rain) because there is less dynamic weight transfer and consequently more rear brake can be used."

The guys from Simpson Motorsport (UK) have the same opinion;

On most BMWs you will find that the AP 4 pot caliper is a little large for the rear, and that a softer pad will generally work better in the rear with the APs.

The result was great! I thought my AP set-up was good….after fitting the RS-14 + RS4-4 I thought the complete AP set-up was really awesome! With changing the brake pads in 2012 I brought everything on the 100% "I like" level. Everything I wanted more about the AP brakes is now there. I have an increase in initial bite (on street and track), a little more braking power, higher working temperatures on track (650°C => 800°C), slightly better modulation and less ABS interference!
I used this RS-14 + RS4-4 set-up for the last two years and I'm still using it (never had any problem). Now the new front discs fitted cracked after my last Spa visit when nothing was changed on the brake system the last two years…only two new front AP discs 10 months a go.

As a matter of interest improved cooling of only one part/surface of a rotor over another (by uprated cooling for example) can cause more differential stress and actually make cracking more likely. It's the size of the temperature difference between the big cooler centre 'hat' and the hot outer disc surface that generates the tension rather the outright max temperature. Likewise the heavy centre fins ironically induce more restrictive stresses to the disc and a radius for stresses to focus around. The cut grooves introduce another tight radius focussing the stress on the vulnerable bottom edge of the rotor where the cool centre hat meets the hot disc, hence the crack running between the two radius points. It will usually be the outer disc that cracks as a result.

The extra cooling is NOT directed on a certain point of the disc. It's the whole wheelarch/disc that gets extra cooling so the general temperature of the brakes drop, not a certain area on the disc.

 

[Lower]

Don't forget that as you get more experienced on track you will get faster and therefore work the brakes harder. What worked last year won't necessarily work this year.

 

[Franzino]

Don't forget that as you get more experienced on track you will get faster and therefore work the brakes harder. What worked last year won't necessarily work this year.

That's true… But even so; this crack is to big to be normal.

 

[exdos]

From all the things I've read about BBKs and the comparison with BMW OEM single pot brakes is that the BBKs are supposed to be able to handle heat all day long without fade, largely due to the callipers being made from aluminium alloy. The fact that the steel (i.e. Iron alloy) disks have cracked before the aluminium alloy callipers have melted due to high temperatures, suggests that this particular failed disc has an inherent casting defect, which has ultimately produced a large crack. Although it has taken a fair bit of use before the defect has appeared, this doesn't defeat the argument that this is a manufacturing defect: the defect will only appear with reasonable use.

If you had bought the disc in the UK, from your description of use, you would be able to claim that the goods were not of satisfactory quality, and you should receive a pair of new front discs as replacements. I expect that under EU law, the same applies in Belgium.

 

[Franzino]

From all the things I've read about BBKs and the comparison with BMW OEM single pot brakes is that the BBKs are supposed to be able to handle heat all day long without fade, largely due to the callipers being made from aluminium alloy. The fact that the steel (i.e. Iron alloy) disks have cracked before the aluminium alloy callipers have melted due to high temperatures, suggests that this particular failed disc has an inherent casting defect, which has ultimately produced a large crack. Although it has taken a fair bit of use before the defect has appeared, this doesn't defeat the argument that this is a manufacturing defect: the defect will only appear with reasonable use.

If you had bought the disc in the UK, from your description of use, you would be able to claim that the goods were not of satisfactory quality, and you should receive a pair of new front discs as replacements. I expect that under EU law, the same applies in Belgium.

We have to wait and see...I'm now discussing the issue with my local AP racing importer. For now it's going to right way. AP racing UK contacted them in my name…so that's a good start

 

[Beedub]

Fast heavy cars + proper use = cracked rotors and monster heat.

i would guess your car has done more laps on the ring than any other on this forum, your a quick driver, using a proper racing compound brake pad I'm guessing rd-29 or 14s?. AP may well say under those circumstances, s**t fails, id HOPE they replace them however, these brakes have taken a heavy hike in price recently too, they are serious money. I'm still on your side, they should replace the rotor ring for free imo.

a quick google search see's this is much more common than you'd think on much more expensive platform with similar to our aftermarket BBK multi-piston setups..

http://www.gtr.co.uk/forum/155874-pictures-brake-discs-cracked.html
http://www.gtr.co.uk/forum/208801-alcon-400mm-discs-cracked.html
http://www.gtrlife.com/forums/topic/65443-cracks-in-brake-discs/

i fully expect mine to go the same way, but the sprint circuit use my car see's much shorter laps, so the brakes get a beating for only for 3-4 laps at a time of of a small track ( they are hard on the brakes however and i regularly see lots of smoke etc)

i do think you should halt posting on further sites UNTIL you get your answer... have you called the Tech department, maybe this will get a quicker answer as tbh that car is going no-where withe the rotor in that condition.

 

[Ewazix]

Franzino, not sure if I read the vibe wrong but my comments were intended as helpful observations based on my previous career in electro-mechanical handling systems (handling nuclear fuel rods, submarine weapons, industrial products etc) specialising in the Quality Assurance of high temp/pressure applications and safety systems (including braking).

It looks like you've got it all covered, but get the cheque book you may need ceramic discs :evil:

 

[Franzino]

Franzino, not sure if I read the vibe wrong but my comments were intended as helpful observations based on my previous career in electro-mechanical handling systems (handling nuclear fuel rods, submarine weapons, industrial products etc) specialising in the Quality Assurance of high temp/pressure applications and safety systems (including braking).

It looks like you've got it all covered, but get the cheque book you may need ceramic discs :evil:

It was a good thing that you asked those questions to me That was also one of the reasons I started this topic to get input from others on what maybe could have happend or what to do. For example; I did not think about the possibility of a sticky caliper, so I went out and checked in on my car. The brake cooling was a realy good point, but I thought about that when they installed the extra air cooling.

The thing I said; about checking the brake balance was a real question to you. Do you know a good way to check this? I can only do it when driving and braking with the car and then feel what the car is doing.

 

[exdos]

Have any of you considered the cryogenic treatment of your brake discs before fitting them? see: http://www.195below.co.uk/

For a few years now, I've bought my brake discs over a month before fitting them and I put them at the bottom of our chest freezer and leave them there until I fit them. I got this idea after reading about the way that BMW built its Formula 1 engines in the past, by using engine blocks which had done over 100,000 miles in road cars (i.e. multiple heat treatment cycles) and then leaving them outside in the snow over winter (i.e. cryogenic treatment) to ensure that the crystal structure of the metal block was optimal before engineering it to become a race engine. Likewise, Swiss clock/watch makers also leave their metals in cold conditions before using them. Whether or not putting my brake discs in the freezer increases their hardness and/or durability, I don't know, but it can't do any harm.

 

[Ewazix]

Franzino, not sure if I read the vibe wrong but my comments were intended as helpful observations based on my previous career in electro-mechanical handling systems (handling nuclear fuel rods, submarine weapons, industrial products etc) specialising in the Quality Assurance of high temp/pressure applications and safety systems (including braking).

It looks like you've got it all covered, but get the cheque book you may need ceramic discs :evil:

It was a good thing that you asked those questions to me That was also one of the reasons I started this topic to get input from others on what maybe could have happend or what to do. For example; I did not think about the possibility of a sticky caliper, so I went out and checked in on my car. The brake cooling was a realy good point, but I thought about that when they installed the extra air cooling.

The thing I said; about checking the brake balance was a real question to you. Do you know a good way to check this? I can only do it when driving and braking with the car and then feel what the car is doing.

Cool, without getting too boffin you could theoretically calculate the exact torque generated on the F & R brakes needing weight, wheelbase, tyre dia, rotor dia, speed and stopping time plus the coefficient of friction of the pad the tyres/road surface - which is the problem since they are variable depending on conditions and your ABS is probably going to interfere as well.
You could theoretically measure your brake balance on a brake test bed but that wont be at operating temp and will ignore the variable road surface condition.
Leaving race car style telemetry and an adjustable brake bias set up adjusted for the track, conditions and driver style which is the next step, seriously though the answer is stable ceramic discs or be prepared to replace your cast ones.