# Drilled rotors crack?



## Village_Idiot (Oct 16, 2004)

I need to do a brake replacement again! I was going to upgrade to an R32 kit front (maybe), really just to get the 2-piston calipers and more efficient braking in a similar size/weight package. Also I saw in the Brake FAQ the pad temperatures, that's very useful and thanks to whoever contributed to it!
I have Hawk HP+ now on all four corners with OEM plain rotors. The rotors are completely destroyed now with 40k on the setup and the pads have life left! I need rotors with more bite that will wear the pads more aggressively and give me less fade & better braking power.
I want to upgrade to Ferrardo 2500 pads on all four corners and my ideal setup would be slotted and dimpled rotors but I'm having trouble finding a quality brand at a good price. I'd settle for drilled and slotted rotors but I once upon a time saw photos of a cracked drilled rotor.
*Do drilled rotors actually crack? Do brands make a difference compared to ebay junk?*
Also why are track pads not recommended for a street car? I'd run the Hawk blue or black pads... Is this just a technicality or are there serious safety issues involved with running a full all out track pad on the street??
Thanks!
BTW: This is for my TT (12.3-F / 10.1-R)


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## cndracer25 (Mar 19, 2009)

*Re: Drilled rotors crack? (Village_Idiot)*

yes drilled rotors can crack. but if you are just driving on the strret i would not worry about that. we use the oe replacement on our stock car and after a season they are cracked but that is some pretty serious use. the only reason it is not a good idea to use full on race pads on the street is that the race pad does not work very well cold like 500 degrees or more. so if you are driving down the road and they are not warm and you have to panic stop they probably are not going to stop very well.


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## GT17V (Aug 7, 2001)

track pads aren't really recommended for the street because in order for them to be effective, they really need to be running hot. On the street, where they are relatively cold, the material at those conditions will have poor coefficient of friction, possibly even worse than OE street pads!!!
Are drilled rotors bad? In most cases... yes. In certain cases, no.
In most cases, some random companies will stock whatever blanks they feel like using, then drill the crap out of them, including drilling into the cooling vanes which will signficantly weaken the rotor-- bad. Even the what people believe are "reputable" companies do this to save costs and maximize profit margins (we are suckers for the race look)
The rare cases, where the holes were designed in from the start--- expensive but still reliable..... you'll rarely find that in the aftermarket world.
So... which rotors? Probably go with a slotted rotor to scrub the pads continuously. TireRack sells the Powerslot Cryogenic slotted rotors (which apparently changes the grain structure to make them more durable)


_Modified by GT17V at 10:33 PM 3-23-2009_


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## AP Racing - Chris_B (Jan 30, 2009)

*Re: (GT17V)*

Just a note or two about what I'm seeing posted:
1. Cryo treating does NOTHING to cast iron. It can't. Cast iron does not have the capacity for phase change as there is not enough carbon in it. I've seen the metallurgical tests before and after cryo. No difference in grain structure at all! No surprise there as cast iron has a completely unorganized structure. There is _no way_ to get martensite from it. Cryo treating is supported by some very nice marketing, however....

2. Drilled rotors, if done properly (high quality casting, thermal post-processing, proper finishing, correctly designed hole pattern, radius-chamfered holes, etc.), are not a problem at all for high-performance street driving. Of course, very few companies actually do this correctly. Do NOT try to save lots of money on brake rotors if safety and performance are important to you. Everyone wants to get a good deal, but brakes (and tires) are two areas where cheap stuff can kill you. Can drilled rotors crack? Yes, so can non-drilled rotors. Drilling is useful for the street (give better initial pad bite) but should not be used for serious track duty as they will crack sooner at very high temperatures.
3. HP Plus is not a street compound. It is very hard on rotors when not up to full temperature. Like mentioned above, track pads to do not belong on the street either since they are useless when cold.


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## Warbird436 (Mar 31, 2009)

*Re: (AP Racing - Chris_B)*

As a metallurgist and co-chair of the ASM Cryogenic Processing Committee I have to weigh in on this. Cryogenic treatment has a tremendous effect on cast iron. First of all, cast iron has lots of carbon in it. Much of it shows up as graphite as the ferrous component is saturated with carbon. Steels are iron that have less than about 2% carbon. Everything over that is iron, not steel.
Cast iron can be made to have austenite and martensite, that is it can be hardened. Heat it up to the austenitic zone and quench it. Been there, done that. Martensite would be useless in this application as the temperatures reached by brakes in use would temper it back.
Cast iron does have organization and structure. Brake rotors are usually SAE J431 G3000 cast iron. The microstructure is pearlitic, and if the rotors are good you will not see austenite or martensite. 
As to metallurgical testing of cast iron before and after cryo, you obviously did cross sections and were looking at the microstructure. You were looking in the wrong place. The major affects of cryo are at the atom to atom level. That is why it works on cast iron, copper, aluminum, titanium, magnesium and other metals. If you say it does not work on aluminum you are saying you know better than NASA. They published a paper on it. 
Now, how do I know cryogenic processing works on cast iron? My company has had numerous brake tests run by Greening Labs and Link Labs. Both are the top brake testing companies in the country. The tests were run to SAE 2707 JUL 2004 METHOD B. Cryogenically treated rotors get two to four times the life on this industry standard test. We've also done wear tests with the University of Texas. Same results. 
Why does cryo work? I am not sure, but there seems to be a great deal of things happening as the temperature drops. The solubility of the alloying elements drops. Vacancies are pushed out of the lattice work. Iowa State University is finding changes in carbides. Also, we believe that the crystal lattice structure is becoming more perfect.
As to cryo being supported by "very nice marketing", thanks! We are trying hard. It is also supported by the Cryogenic Society of America. Cryogenic Society of America is a non-profit society for cryogenics, the art and science of extremely low temperatures, almost absolute zero. http://www.cryogenicsociety.org. Another supporter is ASM International, which is the professional society of metallurgists and materials scientists.
Sorry if I went a bit long, but it is a complicated subject. 
F. J. Diekman


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## AP Racing - Chris_B (Jan 30, 2009)

*Re: (Warbird436)*

Oops! You caught what I mistyped regarding cast iron, which should have read - "Cast iron does not have the capacity for the desired phase change as there is too much carbon in it." Sorry 'bout that!
Regardless, rather than get deep into a metallurgical discussion on a non-technical forum, I still stand by the actual laboratory results I've seen. Those series of many, many dyno tests did not reveal the benefits touted for cryo treatment -- at least in real world tests. If it does help, it wasn't found there.
However, if you can post links to the data you are suggesting, I'd be more than happy to read it as a comparison. I am particularly interested in exactly how the test method was applied, which cast iron was used (not all rotors are made from GS3000) and how results presented.
I do understand that cryo treating is very useful for lots of materials in lots of applications. I am yet to be convinced that cast iron brake rotors are one of those applications.
Thanks!


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## Warbird436 (Mar 31, 2009)

*Re: (AP Racing - Chris_B)*

Chris_B

The phase change you were looking for was the retained austenite to martensite shift. It will not happen in pearlitic cast iron because the microstructure has no austenite. The abundance of carbon is irrelevant.
The rotors we had tested were cast from J431 G3000. They were tested by independent labs. One lab we used was Link Engineering (www.linkeng.com) and the other we use is Greening (www.greeninginc.com/gtlbraketesting.htm ) These are well known labs. The tests were a standard SAE test used in the braking industry. (SAE J2707 Jul 2004) You can look that up on the SAE website. Greening tells us we have tested more rotors with them than all other cryogenic companies combined. Our results were two to four times the life of untreated brakes that were run through the same test. You can see a graphical representation of some of tests at http://www.metal-wear.com/Cryo....html. As far as real world tests are concerned, I have many customers who race, and many customers who are police units that are getting better results than the standardized lab tests allude to.
The question comes up, what were you looking for as benefits? You allude to dyno testing, were you testing brakes or engines? What tests did you perform? How were the parts you tested cryo treated? I have seen many "cryo treatment" systems that immerse the part in liquid nitrogen or that spray the part with liquid nitrogen. These can reduce the life of the part being treated. 
I've listed the test standard, the labs that performed the tests, & generalized results. Let's discuss the tests you have done to come to your conclusions.


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## IJM (Jul 30, 2001)

*Re: Drilled rotors crack? (Village_Idiot)*

Back to the OP's question, here'a picture I took of a drilled rotor used for one 30-minute session on track. Notice where it cracked.
















In addition to track pads not gripping when used at street temperatures, you'll also find they'll eat your rotors for lunch and deposit large quantities of corrosive dust all over your wheels and even your paint.


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## AP Racing - Chris_B (Jan 30, 2009)

*Re: Drilled rotors crack? (IJM)*

IJM,
Thanks for posting a very good example of an inferior casting and a poorly done machining job. That is exactly what one should expect when using that type of rotor at the track.
1. Rotor blank: Low quality core tooling (1-sided), inclusions and voids. God only knows what material it was made out of. Maybe "Sweepalloy" -- whatever they swept up off the floor from yesterday's jobs...
2. Incorrect drill pattern: Notice how all the holes are in the same radial location? This will lead to terrible grooving. The only good point is that since they cracked so soon, you won't have to worry about that! Also, the edge distance to the OD is too short, especially for inferior, non-heat-treated castings.
3. Hole chamfer: Single-angle chamfer where a radius chamfer should be used. This leaves too large of a stress raiser.
Would a quality drilled rotor have cracked at the track? Yes, eventually, but not that quickly. I don't recommend using drilled rotors of any type on track cars, but this is a prime example of a "show car" rotor being asked to perform, which it clearly can't do.
We do agree completely that track pads are no good on the street. If someone is serious about tracking their car, they need to change pads when they get there.


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## AP Racing - Chris_B (Jan 30, 2009)

*Re: Drilled rotors crack? (IJM)*

WB,
I will study this later this month as time permits. As for the results I've seen, again, I did not conduct that testing. It was done by a multi-national brake pad manufacturer on the latest dyno equipment. They chose not to post the results (yet) as it would have potentially damaged a strategic business relationship, so I can not post any links (although I wish I could!).
I'll see if I can get another look at their test methods. All cryo work was done by four leading aftermarket service companies that perform cryo treatment of brake rotors, per their advertisements. The methods were not prescribed to them as they were left to do what they say they do. Many pairs were sent from a coded control batch to each vendor. Part of the lot was not sent out for treatment, and the test engineers were not told which received cryo and which did not.
You may choose to believe none of this, and that is fine. However, even though I am still not convinced, I am an engineer first and a skeptic second. If I can find credible information that leads me to a different conclusion, I will happily change my mind. I'm not like one of those militant greenies that believes in man-made global warming even though credible science has yet to prove it!


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