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Thread: What’s the deal with Cross Drilled rotors?

  1. #1
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    What’s the deal with Cross Drilled rotors?

    It seems that many people feel as though cross drilled rotors are unnecessary for a street car due to the idea that on the street the need for extra cooling is unnecessary. Well, I for one am pretty darn good at getting my brakes hot on canyon roads. Aside from DOT 4 fluid and upgraded pads, is there really a downside to a brand name cross drilled set of rotors over quality blanks on the street? I have Brembo blanks in the rear but am considering stoptech fronts. Any input from those with drilled rotors?

    Full disclosure, not interested in the E46 330i big brake upgrade, aside from the improved look I definitely don’t need that much improvement and the unsprung weight isn’t the direction I want to go in.


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  2. #2
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    I wouldn’t know any downside to them.

    Just make sure the holes are cast, not drilled afterwards. That would make them (more) prone to crack.
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  3. #3
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    Any hole is a stress concentration, especially when you're thermally loading that part. The hole itself is going to cool first, and spread outward. So you're thermally expanding/contracting that metal between holes, as opposed to one large disk that heats/warms as one large part.

    Hope that makes some sense.

    I would argue, make sure to get good brake duct routing, then look at drilled rotors. I would even upgrade ducting before going drilled.

    these:
    http://www.bimmerworld.com/Brakes/Pe...iABEgKgxfD_BwE
    Last edited by Moron95M3; 04-16-2018 at 10:29 AM.
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  4. #4
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    Not all DOT4 are created equal. I would look to improve your fluid and pads first. Then look to improve your braking methods.

    I recommend Motul RBF660 brake fluid, and flushing in fresh fluid 2x per year. What pads are you using now?

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    Follow the advice of the racers.

  6. #6
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    I always ended up with stress cracks around the holes and that was on a couple different brands. I run slotted now and have not had any issues.
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  7. #7
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    Drilled rotors are basically only for looks or a tiny bit of weight savings. Most track/race guys run slotted or blank rotors. Slots give you that little bit extra bite and room for dust/gas, and you don't have to worry about your rotors cracking when they get crazy hot like you do with drilled rotors.

    Rotor cooling comes from the huge volume of air sucked into the center of the rotor and out the vanes, not from drilled holes. "Adjusting" your rotor backing plates or putting in ducting is how you get better cooling if that's what you're after. Also, two-piece "floating" rotors (where the hub is aluminum and attached to the iron disc only by some sort of expanding connector) will have better thermal characteristics than the OEM one piece rotors, but they're expensive or usually only found in BBK's. That said, I love my PFC two-piece front rotors.
    Last edited by TostitoBandito; 04-16-2018 at 03:36 PM.
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  8. #8
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    Quote Originally Posted by Nate94 View Post
    It seems that many people feel as though cross drilled rotors are unnecessary for a street car due to the idea that on the street the need for extra cooling is unnecessary. Well, I for one am pretty darn good at getting my brakes hot on canyon roads. Aside from DOT 4 fluid and upgraded pads, is there really a downside to a brand name cross drilled set of rotors over quality blanks on the street? I have Brembo blanks in the rear but am considering stoptech fronts. Any input from those with drilled rotors?

    Full disclosure, not interested in the E46 330i big brake upgrade, aside from the improved look I definitely don’t need that much improvement and the unsprung weight isn’t the direction I want to go in.


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    Everyone has alluded to this, but if your brakes are getting soft, you need to change to better fluid and more aggressive pads - it's not a thing different rotors would fix. Better fluid and more aggressive pads are designed to handle higher heat from more aggressive driving.

    So yes - cross-drilled rotors are ALL downside. If you really need better brakes, upgrade your fluid and pads first. I can't imagine that wouldn't be enough for the street, even for aggressive canyon carving. I have completely stock brakes with ATE fluid and have never had my brakes fade on track. I'm not saying it couldn't happen, but for most normal cars upgraded pads and good fluid is all you'll ever need.

    Do that first if you're having trouble, and then start chasing ducting or other more advanced solutions if that proves lacking.

    -Josh
    1998 M3/4/5 with most of the easy stuff, some of the hard stuff. 227k and running like a top.

  9. #9
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    Every Sportbike I ever owned had cross-drilled rotors. My CBR1000RR could hit 160 from second gear in short order, and slow just as quickly. My 2000 ZX-9R had over powerful 6 piston calipers. The rotors never stress cracked.

    Of course the bikes were 400 pounds or a little more plus the weight of the rider, and the brake rotors were as big as most cars, and free floating. There could be some difference there.

    But my heavy Harley Davidson touring bike also has cross drilled rotors, and are only semi free float, so the weight theory is probably bunk. I have locked the crap out of those, the big bore kit outstrips the stopping ability if you get to aggressive. No stress cracks and no warping there either.

    Heating up the rotors heavily and punching the brakes repeatedly under racing conditions is probably the issue, but I don't race. I just drive for fun. In my case, if I decided to get a set of (quality) cross drilled rotors, I don't think it would be an issue.

    Most people that get them probably like the look behind a larger than stock set of wheels, and aren't likely to push their cars to the limit.
    Last edited by Deus Ex Machina; 04-16-2018 at 11:45 PM.

  10. #10
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    Quote Originally Posted by Team Neverlift View Post
    Not all DOT4 are created equal. I would look to improve your fluid and pads first. Then look to improve your braking methods.

    I recommend Motul RBF660 brake fluid, and flushing in fresh fluid 2x per year. What pads are you using now?
    I currently have pagid reds and will likely get hawk hps, I will try the Motul!


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  11. #11
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    Quote Originally Posted by TostitoBandito View Post
    Drilled rotors are basically only for looks or a tiny bit of weight savings. Most track/race guys run slotted or blank rotors. Slots give you that little bit extra bite and room for dust/gas, and you don't have to worry about your rotors cracking when they get crazy hot like you do with drilled rotors.

    Rotor cooling comes from the huge volume of air sucked into the center of the rotor and out the vanes, not from drilled holes. "Adjusting" your rotor backing plates or putting in ducting is how you get better cooling if that's what you're after. Also, two-piece "floating" rotors (where the hub is aluminum and attached to the iron disc only by some sort of expanding connector) will have better thermal characteristics than the OEM one piece rotors, but they're expensive or usually only found in BBK's. That said, I love my PFC two-piece front rotors.
    Two pieces are a bit out of my price range but looking into better ducting definitely sounds like good bang for the buck


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  12. #12
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    I know you said you don’t want more mass, but the 330i brakes bolt on and are about equal to E36M3 brakes. I have run blanks, drilled, Euro M3 semi 2 piece rotors and never noticed any difference in street driving.

  13. #13
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    Also, I have a hard time believing you'd actually be able to get your brakes hot enough on the street to induce sustained fade. Maybe if you were deliberately doing a bunch of high speed stops in a row like a rotor bedding procedure, but not in any sort of normal driving on public roads. If somehow you do, the first thing I'd suggest is just getting some more heat tolerant and higher performance street pads which won't fade as easily. You probably don't want to hassle with brake ducting on a street car since it can be damaged and can limit steering. You also probably don't want to cut off backing plates unless it doesn't rain where you live.

    Anyways, it's your car but I'd try to steer you away from some of the more drastic racecar type items. If you need to replace your rotors and want to try slotted or drilled rotors go for it. Same with more aggressive pads. That's all I'd suggest for a street car though.
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  14. #14
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    Quote Originally Posted by TostitoBandito View Post
    Also, I have a hard time believing you'd actually be able to get your brakes hot enough on the street to induce sustained fade. Maybe if you were deliberately doing a bunch of high speed stops in a row like a rotor bedding procedure, but not in any sort of normal driving on public roads. If somehow you do, the first thing I'd suggest is just getting some more heat tolerant and higher performance street pads which won't fade as easily. You probably don't want to hassle with brake ducting on a street car since it can be damaged and can limit steering. You also probably don't want to cut off backing plates unless it doesn't rain where you live.

    Anyways, it's your car but I'd try to steer you away from some of the more drastic racecar type items. If you need to replace your rotors and want to try slotted or drilled rotors go for it. Same with more aggressive pads. That's all I'd suggest for a street car though.
    Fair enough, I’m not trying to go the track route, but have some great mountain roads nearby, and make a run weekly to a family cabin about an hour away right now doing some work while the weather is nice. Im cooking brakes trying to hang with my friends Camaro as 1le... he’s not real comfortable with it yet so I can follow fairly close...


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  15. #15
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    As others have said drilled rotors are a gimmick. More prone to cracking near the holes, and you actually have less surface area for the pad to contact, so the braking performance is probably worse.

    If you want a true brake upgrade, just keep your stock rotors. Focus on brake ducting to cool them off, and an upgraded, non gassing pad. The best upgrade you could do, short of a big brake kit, would be to get more aggressive pads.

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  16. #16
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    look at formula 1 brakes. they dont even have slots. the design is basically the same as your stock ones. the f1 just has much more ventilation canals from the center outwards to create more surface area for the air. so, thinking about it this way, you only have to make sure cooling works. and if you cook the stock brakes on public roads until they don't work anymore then idk do whatever you want and try some slotted ones after all, maybe they're good, maybe not

  17. #17
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    Quote Originally Posted by samy01 View Post
    look at formula 1 brakes. they dont even have slots. the design is basically the same as your stock ones. the f1 just has much more ventilation canals from the center outwards to create more surface area for the air. so, thinking about it this way, you only have to make sure cooling works. and if you cook the stock brakes on public roads until they don't work anymore then idk do whatever you want and try some slotted ones after all, maybe they're good, maybe not
    And they have very, very good brake ducting and optimal airflow. They also use composite rotors, so the heat tolerance is quite a bit higher. But yeah, they are essentially blanks.

    As I said earlier I doubt you can really get your rotors intolerably hot (or glowing) on the street unless you're doing something very illegal or have a mechanical failure. Getting rotors legitimately hot requires hard (just short of ABS) braking from 100+ mph, or from slightly lower speeds repeatedly. To give everyone an idea on temperatures, OEM brake pads will typically be effective up to around 600-800F, more aggressive street pads from 800-1100F or so, and track/race pads from 1100-1600F and above. Rotors will have a visible glow in daylight starting at around 1200F (less in the dark).
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  18. #18
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    Quote Originally Posted by TostitoBandito View Post
    And they have very, very good brake ducting and optimal airflow. They also use composite rotors, so the heat tolerance is quite a bit higher. But yeah, they are essentially blanks.

    As I said earlier I doubt you can really get your rotors intolerably hot (or glowing) on the street unless you're doing something very illegal or have a mechanical failure. Getting rotors legitimately hot requires hard (just short of ABS) braking from 100+ mph, or from slightly lower speeds repeatedly. To give everyone an idea on temperatures, OEM brake pads will typically be effective up to around 600-800F, more aggressive street pads from 800-1100F or so, and track/race pads from 1100-1600F and above. Rotors will have a visible glow in daylight starting at around 1200F (less in the dark).
    Maybe my rotors were just turned poorly then, they just seemed to warp quickly and the pagid reds get a bit soft.


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  19. #19
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    I have never faded stock brakes on the street, but I don’t live in an area with unpopulated and police free canyon and mountain roads. I usually don’t get in more than a few hard accelerations and stops in a row. I do change fluid periodically and use pads that are supposedly higher performing than stock. If you look at stopping distance tests, it’s usually close to 10 hard and long stops successively before big brake kits stop much shorter than stock brakes.

  20. #20
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    There are plenty of pretty much vehicle free roads over small foothills and a few mountains near that have no traffic in my area. You have to hit the mountainous roads at certain times to get clean runs, I used to take off work occasionally to ride my sportbike on good days for that reason.

    But as I drive for enjoyment and I am not braking late for quicker times and all that, I use my brakes as little as possible. Even when I am approaching lights that are red, I will lift throttle and gear down to pace the light so I never have to stop at it. It's much more fun to be in 2nd gear already when it turns green if you have an open lane and get the jump on traffic.

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