924Board.org

[Stu2j]

OK, I give up.

What is the significance of "drilled rotors" or "cross drilled rotors" as opposed to plain ole "rotors"

Thanks.
_________________
-Stu
924 owner since 1988
924S owner since 2002
Click here to view them:

[Fast924Turbo]

Vaughan is probably a good person to talk about this topic. If I recall, he's been close to some research on that area.

[Lizard]

cross drilled rotors are supposed to help dry the routers in wet weather. they apperantly eat brake pads though. and for track use they crack and brake.
do a search on cross drilled rotors
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3 928s,

[Stu2j]

Ok, I found the other discussion and that confused things nicely. Basically, Vaughan says that they provide nothing and basically suck in any application.

However, a minimal amount of web research reveals a great deal of sellers with various performance claims and apparently the drilled rotors sell very well for a variety of applications.

I'm not saying that Vaughan is wrong and I am certainly not going to argue with a racing engineer but while I was able to turn up a lot of information, I wasn't able to turn up anything that supported his conclusion.

Is the entire brake industry in the dark?? What gives?
_________________
-Stu
924 owner since 1988
924S owner since 2002
Click here to view them:

[924RACR]

Not a racing engineer - unless you mean an engineer that races - but rather a brake engineer. Work for Bosch. Lizard's pretty much given the executive summary of what's been said in the past.

Of course, I don't stand to make any money off of them, so I'm not going to be all hot for them.

The claim is generally that they improve cooling, if anything is said. From what I've seen recently, they don't even make any much more claims like that. Anyway. There is no improvement in airflow for cooling with cross-drilled rotors. They don't make enough of a diff in mass to improve rotational inertia.

Alright, down to the bare bones, I worked on a project for a car that was supposed to have cross-drilled rotors stock. They finally had to toss the idea (late enough in the project to really screw us up in ABS/TCS/ESP development) because they had suffered too much of a loss of brake output (Newton-meters of torque per bar of brake pressure aka Nm/bar) with the cross-drilled holes. Another lower trim level of the car had the identical brakes (and otherwise identical car) with the same rotors, not drilled. Same pad material in both cars. You can't get a more back-to-back comparison. That lower level car had no brake output issues, it had a higher output with no holes.

In driving the car for 2 years in development, we ate pads, they groaned and grumbled horribly, never really stopped well (and was scary when we took it on track and got the brakes hot - horrible brake fade)...

They won't help you any. Your money is better spent on good quality pads.

If you can list more detailed reasons why you might want them, I could perhaps tell in more detail why they're no help (and what you should do), but the usual explanation is fade resistance, to which I answer - stock rotors with pads appropriate for your application are all you need. I run old stock rotors with full race pads, and have never experienced brake fade on any track. Without having to add cooling ducts. A good "semi-met" autox type pad works great for the street; I use the PBR/Axxis Metal Masters for my street cars and really like the pedal feel and bite, and they warm up pretty quickly.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Stu2j]

It's not that I want the cross drilled rotors. It is simply that you see so many of them for sell with the ads that talk about the great performance blah blah blah.... and thus my curiosity.

I did find the below item from Baer racing which supports your conclusion. They explain the origins of the cross drilling and I like the part where they offer it only as a "cosmetic" option. I must be completely lost as I can't see how holes in the rotors improve the appearance of the car (racing look?).

___________________________________________________________

Although there are some companies which sell cross-drilled rotors as an actual performance upgrade, in our extensive testing we have seen no improvement to be had by simply crossdrilling stock rotors. This is why Baer has developed EradiSpeed™ rotor upgrades for a variety of applications. Although it is true the crossdrilling, the slotting, or for that matter the zinc surface washing, are cosmetic enhancements, EradiSpeed™ rotor packages also feature rotors with thicker cheeks to provide more heat sink capacity in the fire path of the rotor. Also, they all feature directional vanes for greater pumping efficiency, as well as a two-piece design where the hat, or hub/hat section of the rotor is CNC machined from a solid billet of aluminum and is then fixed to the rotor ring using National Aviation Standard (NAS) stainless hardware. In other words, the EradiSpeed™ is much more than just the most visually appealing direct replacement rotor, it is the only upgrade of its type which can actually deliver the benefits of greater heat absorption, increased durability and lighter total weight.

In racing, crossdrilling was designed to alleviate a problem known as out-gassing. In some of the older pad compounds, when the pads reached elevated temperatures consistent with performance or racing use, the binder (that’s the material that holds the friction material in place) boiled off, producing a gas. This gas would build up between the rotor and the brake pad, effectively keeping the pad from directly contacting the rotor. The holes provide a relief path for these gasses, as do slots, so the pad can once again contact the rotor. Crossdrilling was NOT designed to facilitate cooling.

Although Baer offers crossdrilling as an option on their systems, it is offered as a cosmetic option only. However, with an EradiSpeed™ rotor upgrade, unlike a cosmetically altered stock replacement rotor, you will benefit from improved durability, greater heat sink capacity, lighter total weight and the visual excitement of a 2-piece, aluminum centered, crossdrilled, slotted and zinc washed appearance.
____________________________________________________________

I also found something about the Porsche 911 standard rotors that have the holes. Apparently, these are ok because Porsche doesn't drill the rotors but instead actually casts the rotors in this manner. Porsche's explanation is to improve wet brake performance. They make no claims as to making the brakes cooler. In fact, it appears that Porsche made the holes smaller and enlarged the rotor to minimize the heat problem.

Well, once again I learned something new and saved money by not investing in a gimic item that apparently improves nothing and can actually do harm.
_________________
-Stu
924 owner since 1988
924S owner since 2002
Click here to view them:

[924RACR]

Yup yup yup to all that. Good info from the Baer site; all quite correct, particularly about the origination of cross-drilling, and how it's merely cosmetic. Nice to see, further, that they use a thicker rotor to compensate for the otherwise reduced thermal mass. Brake fade is only combated by increasing cooling (adding brake ducts or revised rotor/wheel design for better airflow) and by increased thermal mass.

I'd thought that more recently Porsche had stopped even using rotors with cast holes, since even those crack.

As for the project I worked on, the use of cross-drilled rotors was initiated by the marketing guys, if that tells you anything.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Lizard]

of course you could always upgrade to the late style 944 suspension and then get the 996 $20k ceramic rotors
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3 928s,