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I need ROTORS: Slotted vs. Drilled vs. OEM ?????

128K views 176 replies 64 participants last post by  jimmy154 
#1 · (Edited)
Ok so it is time to get new rotors/pads and I have been researching what to purchase. I have read through quite a number of threads here but have yet to come to a conclusion. What exactly are the benefits of slotted or drilled rotors compared to OEM? Here are a couple tidbits of info I came across while searching:

Directly off of UUC's website...
"Slotting: there are many reasons to slot rotors. Primarily, slotting aids in removal of water and debris on the rotor surface, evacuation of hot pad "off gassing" vapor, and provides a visual wear indicator.

Details: These slots do not extend all the way to the edge of the rotor for a very specific reason; this maintains the structural integrity of the rotor. Other brands with slots all the way to the end have created "stress risers" where cracking may develop. No such problem exists with this design. Additionally, directional slotting enhances effectiveness.

NO DRILLING! Drilled holes in rotors often used for "show" are prone to cracking and catastrophic failure. The holes create stress risers which will lead to this sort of failure. Additionally, drilled holes do not improve cooling, rotary-vane rotors lose efficiency when drilled because cooling air does not not pass through the rotor circumferentially but is instead prematurely evacuated by the holes.. "


Hmmm if cross drilled rotors do no improve cooling and are prone to failure then why would any BBK use them?


Also here is more info found when scanning ebay (link below)...
"Brembo OE "blank" rotors offer better performance than Cross-drilled rotors. Cross-drilled rotors might look "cool", but what are they really doing for performance? Brake rotors were first "drilled" in the 40's and 50's because early brake pad materials gave off gasses when heated to racing temperatures, which is called "gassing out". These gasses then formed a thin layer between the brake pad face and the rotor, effectively lowering the coefficient of friction. The "cross drilled" holes were implemented to give the gasses a place to escape. Today***8217;s brake pad materials do not exhibit the same gassing out problems as these early (old) pads. Also, there are many companies that sell "cheap" cross-drilled and slotted rotors. They do this by purchasing Brembo blanks and having them cheaply altered by a third party. Doing this sacrifices the rotors structural integrity, and can be a huge safety risk. If you contact Brembo, they will not recognize these discs after they have been altered. These rotors are known to crack and warp very quickly. If you want the actual Brembo cross drilled and slotted rotors you will be paying much more. That's why these blanks are perfect; they perform better, do not tear through pads, are priced much lower, and are much safer."
http://cgi.ebay.com/ebaymotors/Brembo-Brake-Rotors-F-R-Set-E46-BMW-323-325-328_W0QQcmdZViewItemQQcategoryZ33564QQihZ011QQitemZ320087492957QQrdZ1

So right now I am deciding between the following rotors:

1. Stoptech Cross Drilled Rotors

2. Stoptech Slotted Rotors

3. Cross Drilled Brembos (stated above are most likely punched by a 3rd party)

4. UUC Slotted Rotors

5. OEM Rotors



I really like the look of the cross drilled rotors but do not want to purchase them at the risk of performance and safety. Hopefully someone can enlighten me. Rob maybe? Thanks in advance :)
 
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#48 ·
Some more info found on another forum posted by a guy who works for Brembo -
However, one intersting item is that all of our X-drilled holes are exactly that....drilled.....even the Porsche ones. Its a bit of an urban legend that Porsche "casts-in their holes" for strength. Truth is, when I started with Brembo I thought the same thing. I mentioned it to our Italian resident Applications Engineer. I was kinda surprised myself. He laughed and added, "People seriously think we cast in all of the holes? Wow, I'd love to see the mold for that! No, actually they are all drilled and chamfered post-casting. "
spoke with one of the Apps Engineers from Italy and he confimred what was said. The rotors are designed by Porsche and cast/machined by SHW. Brembo supplies the Caliper and the knuckle, and assembles the wheel end (thus the reason for the wheel end assy on my desk).

He said that in the case of Porsche, even the caliper is a build to print. In other words, Porsche Engineering designs all of the components, but has manufactuing done by SHW and Brembo.

He also added a few comments/corrections on the rotor design itself. The "half-vanes" are there soley for preventing crack propagation. The holes are not actualy through the vanes (something we verified with a paper clip with the one on my desk). The holes follow the same curve as the vanes and are placed extremely close to the vanes, but don't actually go through the vane itself. Finally, he added that even SHW does not cast the holes in, they are indeed machined and chamfered. he said he is not aware of anyone who manufactures cast-cross-drilled rotors on a mass-production level.
And in response to the question - "what is your opinion on X Drilling of rotors?"
My personal opinion is with today's materials its mostly for bling. there are some slight benifits to it, but there are also some negatives. There have been Brembo/Porsche tests to show that x-drilled rotors increases first effectiveness in wet conditions. Assuming there is enough mass for thermal capacity of the application, X-drilling can save a slight amount of weight (although any unsprung, rotating mass savings help) and offer small increases in cooling speed. However, they are prone to crack more, especially propagating from the drilled holes. This can be accounted for with a proper rotor design (i.e., pillar and hole location/orientation) to prevent small cracks from propagating all the way to the disc edge. If the rotor is designed correctly and sized properly for the application cracking is minimal. That being said, many designs on the market (I won't name any specifically) arent designed properly (arrangement to prevent crack propagation), machined properly (poor or no chamfering), and are under-sized for the application (thermal mass). This is especially true of those popular in the ***** community.

Now, with all of the previous being my personal/professional opinion, Brembo understands that there are both advantages and disadvantages to X-drilled rotors. We leave it to our customers discretion as to which is used (drilled/slotted/solid-faced). In most OEM situations, it comes down to Marketablility.
 
#50 ·
Just to keep adding to the knowledge - from all the research I've done, if you want to get drilled rotors look for two things as indicators of good quality.

First, it is critical that the holes are chamfered (beveled). This reduced the stress riser effect and will help to mitigate cracking. It's also a good indicator that more thought and effort has gone in to it than simply "Me make hole."

Second, it is preferable that the drilling is done by the manufacturer of the rotor. This ensures that the rotor was originally designed to be drilled one and has the appropiate structural and metalurgical properties.
 
#51 ·
So we seem to have agreed, based upon the data here, that drilled rotors will fail before a flat disk rotor of more or less equal quality.

The question is, can I do drilled rotors, purely for looks, and drive safely down to starbuk's without having the disk explode unexpectedly some morning causing me to spill my grande latte all over my myrtle trim ?
 
#56 ·
If the budget is tight, I'd recommend skipping the slotted rotors and spending a little bit more on upgrading the pads. Slotted rotors seem to run at about a 25% to 100% premium over just the solid rotors - which works out to spending about $50 to $200 extra. Performance wise, you'll get a lot more bang for the buck spending some or all of that on pads.
 
#62 ·
Personally, I'd be skeptical until I could find out who the actual manufacturer is (it's not MimoUSA).

Also, don't believe the "no cracking" claim. The thing is ALL drilled rotors crack. On good ones, they develop small hairline surface cracking around the holes, but they never join or get to the disc edge before the rotor is worn out. On the not-so-good ones, the cracks join from one hole to the next, and/or run out to the disc edge, then if continued to be used, fail and break apart. But they all crack.
 
#73 · (Edited)
The circular logic is inapplicable.

Have you seen the casting apparatus for a BMW composite manifold? No, and you're not likely to.

Here's a better parallel for you: years ago, there was a technical piece published about the special materials and methods used to make it. It is no longer available, yet clearly it is done.

I think you missed my previous point about why a manufacturer might not want to discuss certain aspects of some product for reasons of negatively impacting their other products... especially as the "premium" product they make is privately-labelled for the vehicle manufacturer.

- Rob
 
#74 ·
Have you seen the casting apparatus for a BMW composite manifold? No, and you're not likely to.

Here's a better parallel for you: years ago, there was a technical piece published about the special materials and methods used to make it. It is no longer available, yet clearly it is done.
It's clearly done and it's existence is often cited even if the details of the method are not disclosed. For example -
Lots of engine pictures here - http://www.worldcarfans.com/photos.cfm/photoID/3040718.001/bmw/bmw-6-cylinder-engine-in-depth
Paper on intake manifold casting here - http://www.kspg-ag.de/pdfdoc/kspg_produktbroschueren/p_intake_manifolds_e.pdf
"Magnesium/aluminum composite construction" in the N52 cited here - http://www.bmwboard.com/articles/view.asp?linkid=428
Long article on the N52 here - http://www.worldcarfans.com/news.cfm/newsID/2040718.001/page/1/lang/eng/country/gcf/bmw/bmw's-new-six-cylinder-engines-in-depth
SME paper "Composite Manifold for BMW V8 Engine Wins Award" here http://www.sme.org/cgi-bin/get-press.pl?&&20021634&TE&&SME&

Bottom line - if it exists, it can be found (and quickly too, I might add)
 
#77 ·
Nonsense.

As I posted, I proved to myself that it can be done - but that doesn't mean I have to prove it to you.

- Rob
In that case I have rotors with cast in holes I'd like to sell you, honest...:eeps:

Any interest in bridge? It's not that far from you...
 
#83 ·
'twas not my intent to do so. I thought it clear that we have been discussing much more than just the SHW floating rotors.

The general question was - if I want to buy rotors with cast in holes, who do I call and how do I know what I am getting?

Is there a different answer than what you posted before?
 
#97 ·
Couple of interesting things to note about that -
Both links are to distributors, not manufacturers. It is unclear who actually makes the rotors. It's a reasonably safe bet that the Porsche rotors in the second link were originally made by SHW. And as noted earlier, SHW drills their rotors. FWIW, you will frequently see Porsche crop up as having rotors with cast in holes, but the more you look in to it, the less you will find. My guess for how the rumor started is that Porsche does in fact make some rotors with holes without a subsequent drilling process. These however, are their high end carbon/ceramic/I-really-don't-know-what rotors. However, they aren't exactly cast in the same way you would think of casting in terms of normal rotors. Anyway, my supposition is that what started out as true for some specific exotic rotors, came to be misinterpreted as applicable to all Porsche rotors.

I applaud your efforts and encourage you to continue. But I'll give you the hint to start looking a little deeper at who is actually making the rotors and what they claim, as opposed to what is claimed by people merely selling them.
 
#98 ·
I saw something about the Porsche rotors being cast with dimples, and then the dimples being drilled out, but I'm no expert on their stuff. Regardless, I don't think you really get your money's worth when trying to get "cast-hole" rotors vs buying regular drilled and/or slotted rotors off the shelf. If you want something you know will hold up to extreme abuse, get carbon-ceramic Porsche brakes, or one of the equivalent aftermarket kits that are now being released.
 
#99 ·
I think I may have read that same (or a similar thread) as well. If I recall correctly, the story went something this -
* Porsche rotors are cast with holes!
* Well, maybe they aren't really cast with holes, but they're cast with dimples!
* Well, okay they aren't cast with either holes or dimples after all....
But it took a lot longer to get there in real life.
 
#100 ·
#102 ·
That's some interesting info. I wonder to what extent their is regional variation in what's stocked as "OEM". For example, I don't recall ever seeing the Textar brand sold or advertised in the states.

It might be a branding thing. Textar is part of TMD Friction which also markets the Pagid and Mintex brands. But I'm also not sure I've ever seen either of those two brands on a rotor, just on pads.
 
#103 ·
In the belief that if something is worth doing, it is worth doing obsessive-compulsively, I've been reading up on rotor design and metallurgy.

In regards to distinguishing a higher quality rotor from a lower quality one, here's some things I've learned:

* curved vanes and pillar vanes are about 20%+ more efficient at shedding heat than straight vanes

* a higher carbon content in the rotors is good since it heats it transfer heat faster and also inhibits crack propogation

* alloys are good, especially with the higher carbon content rotors. Some of the main interesting ones are
** molybdenum helps maintain strength, improves resistance to heat cracking and signifigantly increases thermal capacity
** copper increases strength, has marginal benefit on corrosion resistance, and moderately increases thermal capacity
** chromium also helps maintain strength and improves resistance to heat cracking, but can decrease thermal capacity
** nickel has similar properties to copper
** titanium - increases the coefficient of friction, but this can result in accelerated wear and lower overall performance

As a couple reference points, the main alloying elements in OEM BMW rotors are chromium and copper. The higher end rotors also tend to have a higher carbon content. In comparison, Porsche rotors tend to be high carbon with chromium, copper, and also molydenum.

Of course, when comparing your various rotor options, you'll likely have a hard time finding out too much about the metallurgy. But in the case that you do, you can now have a better idea of what it all means.
 
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