Crossdrilled Rotors, What You Should Know Before Buying.

[Crossbow]

Why:
This post/thread is to address the concerns regarding the use of cross-drilled in street vehicles on the track or road course.

Summary:
(For lazy people, and for those who want to start arguing in the least amount of time spent)

Cross drilling a rotor weakens the integrity of the rotor, while providing for very little, if any performance benefits. In extended high temp braking situations (track use), with track pads and track rubber, cross drilled rotors can present a serious danger due to thermal fatigue, eventually cracking and undergoing catastrophic failures. This happens with all manner of cross-drilled rotors, whether they are drilled, "cast", cryotreated, or even multi-thousand dollar special Porsche/Ferrari brakes.

Do realize that this argument is primarily against cross drilling rotors for performance applications. On a street car the braking hardware will never reach the type of temperatures seen at a road course, and thus will be unlikely to encounter the same type of failures.




What is Cross Drilling?

Cross drilling is the process in which holes are chamfered, drilled, or "cast" from a blank rotor. This was originally done to help vent gasses that occurred between the brake pad and the brake rotor which occurred during the initial developments of pad technology. In the current day and age this is no longer necessary as pads are now engineered to produce very little if any gas. Even so, some pads include a small slot to help vent any gases that may occur, further eliminating the need for any type of venting of gasses on the rotor surface itself.

The primary argument of cross drilled rotor salesman is an increase in "cooling" effectiveness. The holes supposedly allow air to travel in and around the rotor surface helping to cool it. This could not be farther from the truth


What are the Negatives?

The rotor's primary function is to act as a heat sink, absorbing heat energy caused by the friction between the pad and rotor surface during braking. The rotor does this to allow the pad to function within its thermal envelope. If too much heat is produced, the pad material can overheat and glaze, reducing braking effectiveness. The larger/thicker the rotor, the more heat it can absorb before this happens. Increasing the size of the rotor, or changing the pad compound, has a similar effect to extending pad effectiveness in a track situation.

Cross-drilled rotors drill holes in the rotor surface to "enhance cooling." If anyone of you have taken physics class, you'll know that metal is better at transferring heat then air. When you cross-drill a rotor, you remove mass from the rotor, reducing its capacity for absorbing heat. You effectively downsize the rotor by doing this, making it less effective. The other major negative impact of doing this is the reduction in overall rotor strength. The various holes lighten and weaken the rotor, as well as causing variance in heat dispersion around the holes. These cause thermal stresses, which lead to fatigue in the metal, and eventually cracks. Overtime these cracks spread through the rotor and can result in the following failures. Below is a Brembo rotors, as photographed by myself, at virginia international raceway.


(This is a brembo off of an STI after 2 lap sessions with track pads, photo taken by myself)

Chamfering does help prevent this, but you can only ever chamfer 2 of the 4 sides of the rotor. You are still left with 90 degree edges on the inside, providing for an excellent stress point.


Are there any Benefits?

The primary benefits of cross drilled rotors are in unsprung weight and rally applications. In a situation where unsprung weight is critical to the performance of the vehicle, xdrilled rotors can shave off ounces of weight to give you an edge over the competition. (Think sport race bikes). In rally applications, the holes allow dirt/water to be pushed through to allow for a clean braking surface.

Many manufacturers will claim "better pad bite" and "fresh pad surface" as the holes will continuously grate away at the exposed pad surface, preventing glazing. Of course what they fail to mention is that you can get exactly the same effect from a slotted or dimpled rotor.

Of course, they also look cool, which is the main reason they exist in the first place!


If This is All true, Why Do Sports Cars Use These Systems?

Aesthetics and racing pedigree. Bigger wheels, bigger brakes, and lots of holes...these are much more aesthetic modifications then performance minded ones. Larger wheels tend to be heavier (to maintain strength) which increases unsprung weight, which reduces acceleration, handling, and braking ability, while decreasing tire availability (especially in R comps). Some companies actually specialize in making smaller rims for some major performance vehicles, just so the owners don't have to purchase custom tires for their outlandish rim configurations. (Fiske is one of them). The larger rotors tend to have the same negatives, as increased size/mass adds to the unsprung weight increases...which is one of the reasons they get so many holes drilled in them. (Less mass = less weight basically the counter to the increased braking size).

Larger wheels, larger rotors, and xdrilled holes sure do look cool, but that doesn't mean it’s the best solution. Don't mix aesthetics with performance! You can search most of the manufacturer forums, (like the C6 forums, Porsche and Audi Forums etc) for many examples of rotor failure, cracking, and premature pad wear from this aesthetic bling.


I've Heard that Brembo/Porsche Uses Cast Holes in Their Rotors. Is This True?

Brembo/Porsche casting their rotors with holes in them is an internet myth. It was started a long time ago, and no one ever went about trying to disprove it. Well Todd at TCE got tired of the internet rumors and put up a challenge. Anyone who can prove that Brembo/Porsche casts their holes into the rotor (with evidence) gets 400 dollars.

TCE offers $400 bounty - Corner-Carvers Forums

No one has yet to meet the challenge.

If you need any more evidence, check brembo's own faq.
Brembo Frequently Asked Questions

Are discs with cast-in-place holes better than cross-drilled discs? Brembo has extensively studied and tested cross-drilling versus casting the holes in place and found no significant effect on performance or durability.

This is why you see "cast" in quotes throughout the article. Brembo drills their own rotors.






Ha, I got you now! Why does X/Y Race team use xdrilled rotors?

The first thing to ask the race team is who is sponsoring the rotors. More times than not, there is a particular company giving them rotors for free, as long as they state they are using their product. Given the choice between having to field your own replacement rotors from your own budget, vs. using a manufacturer’s rotor for free...almost all small race teams will go for the free solution. Even so, if you take a general look at most teams (like in the SCCA World Challenge Mazda6's), you'll find that blank or slotted rotors are much more the norm then xdrilled ones.




So What's the Best Upgrade to My Braking System?

Tires
The primary upgrade to your braking system should always be tires. Tires are what stop your car. Brakes just stop the wheels, which the tires are attached to. It is the connection between your vehicle and the road that determines the primary braking friction. Before you can fully utilize your stock braking system, you've got to completely saturate it, which hardly ever happens on street rubber. So your first stop to better braking is tires. Go to autocrosses and track events and talk to drivers. You'll generally find they aren't using the most well known brand names, two examples off the top of my head are the Hankook Ventus Z212 and the Falken Azenis RT-615. Both excellent rubbers, both much cheaper than most of the name brand tire selections. Sometimes you can find sales on outgoing tires that are excellent, but are just cheaper because they're being replaced by a new brand, like the Dunlop Direzza Z1.

Pads
After tires, you look to your pads and fluid. All brake pads that ship with manufacturer vehicles are fit only for certain temperature ranges. At the track, or in other extreme situations, they can overheat and glaze, reducing effectiveness. Picking the right pad for your application is ESSENTIAL. You do not want to go to the track with street pads, and you don't want to run track pads on the street! Track pads require a minimum temp range before they begin to bite. At cold temps, they have little if any bite at all, and can be extremely dangerous to run on the street. (They also make your car sound like a garbage truck when cold). Stick to street pads on the street, and track pads on the track!

You'll also want to look into replacing the stock brake fluid with something able to handle the increased temperature of track braking. You want to make sure that if you are reaching track temps that your fluid can also handle these temps...otherwise you'll lose consistency in pedal feel as your stock fluid boils away.

Fluids
Vavoline Synpower (good cheap fluid, good for light track use)
Motul RBF600 (good track fluid)
GS610 (Insane track fluid, expensive)
ATE Super Blue Racing (Stains your lines, cheap, good medium level fluid)

Ducts/Vents
Yup, we still aren't to rotors or calipers! Providing a decent brake duct system can greatly enhance the effectiveness and consistency of a stock braking system. This especially works well with directional vaned rotors.

Rotors:
Increasing the mass of a rotor will increase its maximum thermal capacity. A rotor is basically a large heat sink, so a bigger one can handle more heat and help reduce the temps that the pad has to deal with. Of course you can skip running a larger rotor (up to a point) by running a more aggressive pad. Generally the best rotor is a set of blank rotors purchased when you pick up a set of track pads. Some companies will even give you a discount!

If you really want some aesthetic differences in the rotor surface, look for slotted rotors instead of cross-drilled. Many manufacturers provide these now, and these rotors provide all the benefits of a cross-drilled rotor with non of the negative side effects. (preventing pad glazing, looking cool) It is still possible to crack a slotted rotor, but it's far less likely then a cross-drilled.

Caliper:
You generally get into caliper/rotor upgrades when you get into actual racing...as in not driver education, but racing for rank/position/etc. Larger calipers allow for larger pad surfaces on the rotor, as well as providing for more distributed clamping force (better use of the pad). These types of upgrades don't make a massive difference and are usually more for consistency then any of the other mentioned upgrades.

Why didn't I mention stainless steel lines yet? Mainly because these tend to be more of an upgrade in pedal feel then actual performance upgrade. SS brake lines prevent stock rubber lines from expanding under heat, and keep the pedal feel consistent throughout track laps. (As long as the fluid doesn't start boiling).


References

Stoptech Brake Articles
StopTech : Balanced Brake Upgrades

The Truth About Brakes
Mazda6Tech - The Truth About Brakes

Grassroots Motorsports on Braking
Braking Systems in Plain English

Scr Motorsports Rotors
Those Poor Rotors

Ultimate Discussion/Bash Fest on Xdrilled Rotors
Nissan Enthusiast Forums - Crossdrilled/slotted rotors-why they are like this, like that-final discussion

Additional Discussion on Altima Thread
Cross-Drilled Rotors Anyone? - Corner-Carvers Forums

Discussion on Truth About Brakes Article
Brake article I wrote, publishing? - Corner-Carvers Forums

A Reference I'd like to list here, but can't send you a link to, is all the people I've met and driven with at Virginia International Raceway. My instructors and friends have taught me more about driving and vehicle systems then any number of web articles or books. Nothing replaces seat time, and I highly recommend everyone to attend advanced driver training!

[NYpest]

great stuff

[det_ms3]

i've seen the stress cracks on street driven cars as well and actually only a little contour so i decided i'd rather go with slotted after that. but this was informative.

[Crossbow]

Yes, the cracking can occur on street cars as well. (See C6 vettes for examples), but it is less likely. I decided not to make a sweeping damnation against xdrilled rotors, as they aren't horrible on the street, they just aren't really a performance upgrade, just aesthetic based.

[flashfrenzy]

Great write up. I hate cross-drilled rotors. I have a hard time believing they are as popular as they are. I guess the people using cross-drilled rotors are the same type of people running BPVs in VTA mode....

[det_ms3]

Originally Posted by Crossbow Yes, the cracking can occur on street cars as well. (See C6 vettes for examples), but it is less likely. I decided not to make a sweeping damnation against xdrilled rotors, as they aren't horrible on the street, they just aren't really a performance upgrade, just aesthetic based.

yea i understand alot of times its personal preference on the rotors. me personally i think slotted are enough for me.

[flashfrenzy]

Originally Posted by det_ms3 yea i understand alot of times its personal preference on the rotors. me personally i think slotted are enough for me.

I never understood the aesthetics thing. I don't think I've ever looked at a Porsche and thought, "Damn those are some nice lookin' rotors!".

[ptperformance]

Good stuff by it looks like there is nothing on cryo treating. I would never run crossdrilled unit on the street even if it was a track car. To date (I have seen a bunch of them) I have yet to see a crossdriled rotor not crack on a daily driven car. Not one. Sloted or wave groves are the only way to go if you street the car. Cryo treating really helps with keeping the rotors true with no warping.

[ptperformance]

Most race cars throw them away after the race or have them cryo treated after every event.