Rotors: Blank vs Cross Drilled vs Slotted and Warping

There is more misinformation about cross drilled rotors than anything else I can think of on a car. The general consensus seems to be that drilled and slotted rotors offer better performance than “blank” rotors. This is simply not the case.

At one point in time race cars did have cross drilled rotors, and this is probably where the idea that they offer increased performance came from. But if you look at any serious professional race car today, I would be shocked if you found any cross-drilling.

Like everything else, there are advantages and disadvantages to drilling and slotting a rotor. Fortunately, technology has progressed so that there is no longer a need to cross dill rotors and therefore, we don’t have to deal with its disadvantages.

The reason why rotors were drilled in the first place was to relieve the gas that was created when the pad material started to breakdown (burn). Since modern pads don’t gas off any significant amount, this is simply not a concern.

Many people and advertisements claim that cross drilling helps the rotor cool. I’m sure those little holes do help the rotor cool in some regard (possibly not measurable), but the effect in reality is completely insignificant. Furthermore, any benefit of extra cooling is most likely off set by the reduction of the rotors mass due to the drilling which lowers the overall heat capacity of the rotor.

So now that you know that there is no benefit to running a cross drilled rotor, we are left with a major disadvantage. What all of those little holes do is create stress risers and a surface that’s unevenly heated and cooled. The result is that the rotor becomes very easy to crack and makes a catastrophic failure much more likely. The worst situation is when a crack forms and connects between multiple holes – much like a connect-the-dot puzzle. This can lead to a large piece of the rotor breaking free which I can assure you is not good at all.

So why do all those high dollar cars like Ferrari, Lamborghini and Porsche have drilled rotors? Well, because people think it looks cool. The rotors on those cars fail when pushed hard as well, and the professional race teams that run these cars replace them with non-drilled rotors. In my experience, cross-drilled rotors just don’t last as long as a blank rotor. If you ever go to the track and find someone pushing a car hard that has cross-drilled rotors, put your ear near one of his wheels and listen carefully when he gets back to the paddock. You will hear small metallic pings and pops as the rotor cools unevenly. What you will be hearing is the sound of the cracks forming….

So what about slotting?

Slotting serves the same purpose as drilling but doesn’t weaken the rotor as much. Slotting is also advertised to “wipe” the surface of the pad preventing glazing. I don’t know how much of this is true or how much of an effect this has, but  the real world result of this is that your pads don’t last as long. Typically, if you glaze your pads, you have already surpassed the heat range of the material and the pad will most likely have “melted” onto the rotor surface creating an uneven transfer layer. I don’t see how slotting is going to prevent this.

So what really happens when the rotor is “warped”?

When you’re on the brakes and there’s a pulsing sensation or vibration, we typical call this situation warped rotors. Surprisingly, I have placed a dial indicator on so-called warped rotors to find that they have no run-out what-so-ever, and also, on new rotors that have run-out but don’t produce any pulsing.

What does happen is that the layer of pad material on the rotor surface builds up unevenly, and also, the metallurgy of the rotor can change states. The layer of pad material on the rotors surface, if unevenly distributed, will create hot spots. If these spots get hot enough, it can form cementite in the rotors metal – a rough iron carbide formation that creates a lot of friction, but is terrible at dissipating heat. The cementite formation can get so bad and cause so much friction that even when you are off the brake pedal completely, because your pads are always in contact with the rotor ever so slightly, it can create a vibration when driving normally. I have even mistaken this vibration as my tires being out of balance.

Uneven transfer layer buildup and cementite is what produces “warped” rotors, not run-out or distortion. Cementite is a problem with iron rotors, rotors made of other materials like carbon do not suffer from this problem.

Vibrating can also be caused by a crack in the rotors surface. If you have a vibration that only appears during hard or extended braking, it may be a crack. The reason for this is that the cracks opens up when the rotor is hot and closes when it’s cold. You may never even know that there is a crack if you never build heat in the system…

Let me digress a little bit – There is surely some uneven dimensional change (warping) to the rotor if you have a localized hot spot. But this seems to be only temporary and when the rotor cools, it returns to its normal flat state. I suppose you could drive through a puddle with very hot brakes and cause a permanent measurable change, but it must be rare.

If I have my rotors resurfaced, will that fix the problem?

In my experiences, no. When I have had my rotors resurfaced, it only cured the vibrations temporarily. Within a few months, they return even if I haven’t been doing any hard driving. So what gives? Most likely, parts of my rotors had turned to cementite and it was thick enough where resurfacing did not remove it all. Even if there was a small area left after resurfacing, that one spot will create a hot spot which will grow in fairly short order.

Food for thought:

  • So actually there is one benefit to cross-drilling and slotting but I didn’t want to mention it since it’s also insignificant – that benefit is in wet weather. It turns out that drilling and slotting either give a place for water to evacuate like the tread on a tire, or allows steam to gas through kind of like what drilling was intended for. I don’t know what really happens, maybe both. But either way, the initial bite tends to be better in the wet.
  • Many people have such strong convictions about rotor warping that they wont believe anything I say in this post. I dare those people to put an indicator on a ‘warped’ rotor and actually find the distortion in the rotors surface. Rotors do tend to have run-out even when new, but you would never know it as long as the run-out is in tolerance; so, don’t mistake some run-out for warping when the rotor had it from the day it was installed and when the pulsations in the brake pedal only started recently. Also, don’t let some fool mechanic cut your brand-new rotors to ‘straighten them out’ or to get rid of the in-tolerance run-out, this will just shorten the rotors life.
  • In general, cutting rotors is only a band-aid.
  • There is different levels of cross-drilling. Some rotors have many more holes per inch than others. The ones with a high density of holes suffer more than ones like the rotor at the top of the page. The results are the same when pushed hard.
  • On the street, drilled rotors are ok, but can create noise. The thing that kills drilled rotors is fast heating and cooling cycles over a wide temperature range. This is why no one uses them on race cars.
  • The transfer layer on the rotors surface is typically invisible in all but extreme cases. When the pad is overheated, it can leave large visible deposits on the rotors surface.
  • Rotors with curved vanes are significantly more efficient than those with straight vanes.
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43 Comments to "Rotors: Blank vs Cross Drilled vs Slotted and Warping"

  1. jim's Gravatar jim
    November 7, 2012 - 8:33 pm | Permalink

    i have cross drilled and slotted rotars. To there credit I have some things to say; Although logically the physics side about what you said concerning heat dissapation and so forth makes sense i have some food for thought. My front right calliper locked. I drove for 3 days on the high way upto 85 MPH or more with out knowing my brake was locked. On the 3rd day my car started vibrating. After all the excruciating heat abuse that the rotar was put through for 3 days my mechanic put the rotar though a test and it was completly unharmed. I only needed new pads and callipers. My (extremley suprized and knowledgable mechanic said) if it were any other rotar it would have been toast.

  2. David's Gravatar David
    September 16, 2013 - 11:05 pm | Permalink

    My car has had the rotors resurfaced 2 times, and I still get a shake while breaking. It seems like light breaking is the worse at highway speeds. Now I’m looking at getting new rotors and pads, you’re description says to use carbon based rotors to slow the cementite build up. Will aftermarket ceramic pads help keep the build up down, or is OEM the way to go?

  3. Howard Wiseman's Gravatar Howard Wiseman
    December 9, 2013 - 4:12 pm | Permalink

    No luck here with resurfaced rotors. I think the metal has lost its temper.

  4. Dan's Gravatar Dan
    March 5, 2014 - 7:24 pm | Permalink

    I am not sure if you know much about heat transfer or energy. If you are worried about your rotors getting too hot under braking, having crossed drilled rotors WILL help cool your rotors. Heat Capacity is the amount of energy it will take to raise a material by 1 Kelvin or J / Kg*K. The heat capacity of air is ~1000 J/ Kg*K depending on humidity while most metals are much less than that. For instance solid Iron ~100 J/KgK. If you have 10000 Joules of energy then it will increase a Kg of air by 10 Kelvin. While for the same amount of Iron it will go up 100 Kelvin! Saying crossed drilled and slotted rotors give no benefits to cooling is completely wrong! In general increasing surface area will help in cooling. Please do some research before posting, you are trying to discredit all the engineers building sports cars. #imtalkingaboutscience

    • Kevin's Gravatar Kevin
      April 24, 2014 - 9:36 pm | Permalink

      Dan, you seem to be confusing specific heat capacity with volumetric heat capacity. Let’s imagine that a solid iron rotor has 1% of its material removed by drilling. Its heat capacity will drop by almost 1%. The density of iron is very roughly 8 g/cm**3, whereas air’s density at sea level is around 1.2 Kg/m**3. That is equivalent to 0.0012 g/cm**3, or less than 1/6000th of the density of iron. Given your argument that air’s specific heat capacity (energy needed to heat a unit mass) is 10x that of iron, then the air in the holes will have 10/6000 or 1/600 of the heat capacity of the iron that used to fill them (its volumetric heat capacity is very low). Obviously the contribution of the air to the rotor’s heat capacity will be negligible and far less than what the removed iron used to contribute. The heat capacity of the rotor will effectively drop in proportion to the amount of material removed, but since so little material is removed this will not be noticeable.

      Of course, in the real world it is not a stagnant mass of air or iron that cools a hot rotor. Instead it is largely the constant replenishment of the air around the rotor with relatively cool ambient air (that, and radiation (ie, glowing) if the rotor is really hot). Even at low speeds, huge amounts of air are flowing around the rotor and carrying off heat. Providing a larger cooling area by drilling certainly *could* improve the rotor’s cooling, though predicting how much would be very hard to do analytically as the air in this region would be very turbulent. If the air is so turbulent that there is little net flow through the holes, then the extra surface area will not help much as the air inside will just get hot. My main point, though, is that air is significant because it’s always in fresh supply, not because a Kg of air can hold more heat than a Kg of iron.

      Incidentally, one of the best ways to reduce rotor temperatures is not to drill them, but to install brake ducts. If you read instructions for bedding track or racing pads, they will often advise you to cover up brake ducts, but they will not advise a longer or higher-speed bedding process for drilled rotors versus solid. That suggests that either the manufacturers have overlooked drilling, or they consider it less effective than ducting.

    • Steve's Gravatar Steve
      January 3, 2015 - 3:57 am | Permalink

      Drilling actually reduces the surface area and mass of the rotor. Slotting increases the surface area of the rotor but reduces its mass.

  5. jared's Gravatar jared
    June 7, 2014 - 10:53 pm | Permalink

    So, I just read an article on espn’s web site that states that the system that brembo developed after months and months of research starting with finite element analysis and moving to computational fluid dynamics followed by more advanced and rigorous tests for all the NASCAR teams they have their systems on, have slotted rotors. Hmmm… It also states that in the Martinsville race drivers apply thier brakes every 8 seconds for five hundred laps. Two turns x 500 laps = 1,000 applications per race where the rotors reach 1200 degrees with only 8 seconds to cool off. Pretty demanding. But yeah, NASCAR uses slotted rotors. But wait, they give no benefit, just look cool and no race cars use them. …………

  6. Steve Waclo's Gravatar Steve Waclo
    June 24, 2014 - 3:02 am | Permalink

    Hello all,

    There’s an outfit called the Society of Automotive Engineers (SAE) and here is a comprehensive report they did on this topic:

    http://home.wavecable.com/~vtucker/A4/rotors.pdf

    Wish I could cut through the 28 pages and provide some profound insight, but I’m afraid you will just need to read it yourself.

    Best wishes

  7. Kumar's Gravatar Kumar
    October 14, 2014 - 2:09 pm | Permalink

    Hello John and others,

    So for everyday driving on a sedan with squeaking noise and vibrations when breaking: do you recommend replacing stock blank rotors rather than fancy drilled or slotted rotors? Also, what if we just resurface and change break pads? I understood that just resurfacing is not 100% solving the problem, but seem like a cheaper alternative. But is it worth it?

  8. MD Putnam's Gravatar MD Putnam
    October 20, 2014 - 11:45 am | Permalink

    A more comprehensive SAE paper on drilled rotor performance:
    http://papers.sae.org/2006-01-0691/

  9. Mark's Gravatar Mark
    June 1, 2015 - 4:03 pm | Permalink

    Hello John and everyone else,

    I have a Lexus LX-570, my rotors have gotten warped and discolored on multiple occasions and were replaced with factory rotors. Now after the end of my warranty period I took the car to Midas for brakes and rotors. The initial ones they used lasted less than 3 months. To fix the problem they are suggesting slotted and drilled rotors and carbon pads. Any other suggestions?

    Thank you,

    Mark

  10. Wes's Gravatar Wes
    August 13, 2015 - 7:49 pm | Permalink

    Hello John and Everyone,

    This post still seems to be alive so here’s my question. I have a 2010 Mazda CX-9 Touring, and I’ve recently noticed a shimmy in the steering wheel. The issue doesn’t occur under 65mph, but the faster I drive and harder I brake, the more the steering wheels vibrates (most noticeably coming down steep hills at faster speeds). This leads me to believe it is warped rotors… and I never turn a rotor. Always replace as I just don’t think it’s worth the money.

    Here’s my question… I’ve been looking at blanks at my local O’Reilly and they offer their “BrakeBest” rotors. $80 for a pair. Where OEM from factory will be $120 each. I did some research and BrakeBest rotors seem to be manufactured by Bosch (correct me if this is wrong). Is this a decent rotor to purchase? Have you heard of anything good/bad on this brand? Do you have a specific brand you would recommend?

    Thanks,
    Wes

  11. Dean Dupre's Gravatar Dean Dupre
    August 28, 2015 - 12:07 pm | Permalink

    Hi there,
    I have a 2014 Cadillac CTS-V, the front discs ‘warped’ after less than 12000 miles of driving. However, the car is an automatic, and ambient temperature here in Abu Dhabi is generally over 45°C. The discs were skimmed, but the problem has recurred after only covering another 3000 miles.
    The general driving conditions are free-flowing motorways, with the odd few miles in city traffic.
    Should the pad compound be changed to reflect the high ambient temperature?

    Many thanks
    Dean Dupre

  12. Josh's Gravatar Josh
    October 27, 2015 - 3:39 am | Permalink

    Hello John, and others. I’d like to say that I agree 100% with what you’ve said here. I don’t have an SAE certification, or an automotive degree if that even exists, but I do have real world experience. I brake hard, and I’ve used slotted, drilled, slotted and drilled and I’ve noticed that if there is a difference in performance from the fancy rotors versus blanks than it’s either negligible, or so insignificant that it’s not noticeable. With all the physics jargon thrown out on this post I’ll say that the best way to prove something is to go out and test it, not throw numbers, equations, and scientific lingo around. But on a lighter note, I’m still grinning about the argument brought up about NASCAR using slotted rotors during a race. I’m grinning because I didn’t realize NASCAR was racing. I wouldn’t call driving in a counter clockwise circle for hours a motor sport. I feel NASCAR is for the racing flunkies, and for the real race car drivers, to get ready for retirement.

    • Artofkicking's Gravatar Artofkicking
      March 29, 2017 - 10:41 am | Permalink

      So I guess Talking about braking from a sport that actually uses the brakes is somehow not relevant? Seriously if you’re saying drag racing counts where you only use your brakes once (often with a parachute back-up) I feel like the point is missed.

      What works best multiple times…I would hate to round that 100th turn with no brakes. lol

      • CSN's Gravatar CSN
        May 22, 2017 - 1:44 pm | Permalink

        I used to work in NASCAR, as an aerodynamicist. It is absolutely racing, and while all racing formulae are to some degree contrived I’d agree that circle-track racing is more contrived than most. That said, just like drag racing, the talent is finding and staying on the edge of the envelope on any given day. (And in all the other aspects on and off the track of course.)

        As for this nonsense: “the best way to prove something is to go out and test it, not throw numbers, equations, and scientific lingo around”, insomuch as it merits a response: neither on their own are sufficient. Science is broadly a rigorous method of not fooling yourself, and you are always the easiest person to fool. Practical experience is a critical part of doing good science. “Jargon”, when used properly, is a necessary way of being precise and concise to avoid ambiguity.

  13. Andy's Gravatar Andy
    November 16, 2015 - 3:00 pm | Permalink

    Cross drilling rotors is done to reduce their mass – See Newton’s Second law of physics.

    It is the same thing as putting lighter rims and tyres on your car or lightening your flywheel – rotational mass stores inertia, removing rotational mass frees up torque at the expense or power stored in inertia.

    Reducing the weight of the spinning components of your drive line will increase your acceleration as less torque will be required to accelerate, so the torque your engine produces (your power band) will be larger. The downside will be your fuel millage – without the stored energy of the extra inertia, your car will slow down faster when coasting.

    It is this last point that makes lightening your rotors with holes seem like the smart thing to do. However, reducing rotational mass elsewhere and having more contact pad surface on the brakes usually yields better results with out the issues already mentioned.

    Rotational mass has a ration of anywhere between 7:1 – 11:1 over static mass depending on who knows what…

    So, for the sake of argument, lets say that cross drilling removes a quarter of a pound from each rotor:

    .25 x 4 x11 = 11lbs of weight from the car. Hardly worth the issues noted above.

  14. Kevin the Engineer's Gravatar Kevin the Engineer
    November 20, 2015 - 5:33 pm | Permalink

    John Milmont,
    If drilled and/or slotted rotors are worthless ,why do car companies, race teams ect still defy all your “uhmmm wisdom”?
    The only thing that ever stopped a vehicle of mine from warping the front rotors, that came with horribly undersized front brakes was, powerstop replacements (drilled and slotted). schools out.

  15. IngyHere's Gravatar IngyHere
    January 10, 2016 - 12:51 pm | Permalink

    Wow. All this BS about how rotors transfer heat and deform but not a single mention of metallurgy. Steel is not Fe and it’s not Al, either. I understand rotors are made in certain grades of steel, but not all steel is the same and manufacturing process plus blend has a lot to do with product performance. I’ve had rotors made overseas that “warped” in three weeks whereas the same item made under better manufacturing standards lasted indefinitely. A true applied test would take identical rotors from the same batch, same manufacturer, same foundry and apply cross drilling and/or slotting to the same rotors. Then run them on identical cars under similar conditions.

    On another note, where can I read the SAE articles without paying through the nose?

    • Reuben's Gravatar Reuben
      January 17, 2017 - 8:59 am | Permalink

      Yes, I think this is an often overlooked aspect.

      The issue is that people have factory or other cheaper rotors and they warp or crack or whatever and then someone tells them to buy fancy slotted rotors, which turn out to be much better and then they come to the conclusion that the slots must be the only difference and therefore the slots are the key.

      What they are forgetting is that they likely just purchased rotors of an overall higher quality in terms of materials and manufacturing and that’s what’s making the difference.

      I used to be on the slotted bandwagon until I realised how good heavy duty plain rotors were and did some more thorough and realistic thinking about how likely it is that the slots and/or holes would be a noticeable benefit.

      I’ve done track days where I had factory calipers with high end plain rotors and pads and a number of other people (with the same model car and similar power) had fancy Brembo calipers and slotted and/or drilled rotors and mid level pads and I could out brake them all day. i could out brake them all day with practically zero fade.

      My performance was primarily due to the very high temp pads. high performance pads are tougher on rotors so you need higher quality rotors, which like you say, are often slightly different alloys than cheaper rotors. Some of these different alloys are also claimed to have preferable heat transfer properties but my opinion thats probably mostly marketing spin also.

      Next to pad choice and the overall mass and hardness of the rotors, slotting or drilling a rotor would have such a negligible difference in performance ether way that I could never justify the additional cost let alone justify any potential reduction of the structural integrity of the rotors or any increase in wear on my brake pads (good pads aren’t cheap).

  16. Dave W's Gravatar Dave W
    March 26, 2016 - 9:18 pm | Permalink

    So, I drive an ’08 Lexus GS350. I drive it the way I believe it was meant to be driven which means I had to have the transmission & AWD transfer case replaced at exactly 100K. Good on me eh? I’ve been through several sets of brakes including rotors. While this thread has been fun to read, the road tells the real story. I went from factory blanks to slotted only and then to slotted & drilled. I am heading back to high quality blanks with The best ceramics I can find. I change 100% of the fluid every time I do the brakes. Interesting that just today as I was disassembling the RR wheel to replace the bearing assembly and I found my rotor looks precisely like the image above which was a bit of a shock. Applied physics lessons aside (but truly appreciated), Mr. Milmont is correct.

  17. Pips's Gravatar Pips
    July 9, 2016 - 7:05 am | Permalink

    I drive a Jaguar XKR in the UK. I have vented cross drilled rotors. All the holes are full of pad debris. They look cool on the Jag but as all the holes are blocked I fail to see what positive affect the holes could have on cooling. The amount of metal removed by the holes relative to the complete rotor is tiny. Weight saving or change to heat capacity must be minimum.
    The rotors and pads are worn and need replacing but I will be replacing them with quality but blank (solid) rotors. I will report back if unitive a difference.

  18. Roland's Gravatar Roland
    August 3, 2016 - 8:46 pm | Permalink

    I drive a 2011 Altima SR…six speed and fun to drive. Are the OEM rotors cut thinner as I was only able to get two resurfacing turns done in 77k miles!
    Planning to replace them with OEM but Nissan wants $130 each for front rotors and $99 for pads. I work in Austin TX and do a great deal of stop and go driving…also drive our 80 mph toll road often so driving good distances at 85 mph is not uncommon. Had issue trying to post and hoping it works this time.

  19. Roland's Gravatar Roland
    August 3, 2016 - 8:48 pm | Permalink

    ….any suggestions on good OEM quality or better replacement kits?

  20. Haider's Gravatar Haider
    November 28, 2016 - 6:22 pm | Permalink

    An excellent read.
    I think most forget that: drilling/slotting/ducting aside, it’s critical to choose correct and good quality materials appropriate for the application.
    As far as the average driver can take their daily on the road in terms of brake abuse: the single biggest difference in performance will be from pads. Also, putting our egos aside: we’re not race drivers, so our perception of brake performance is inadequate at best.

  21. Simon's Gravatar Simon
    April 8, 2017 - 12:03 am | Permalink

    I drive a 2010 Peugeot 207 CC (don’t laugh, it’s a lot of fun for a small car) but under braking I get a huge amount of pulsing, almost like the ABS is kicking in except slower, plus uneven grab at different speeds under the same brake pressure. I’ve just watched a Youtube vide of someone replacing rotors and pads and it looks like it’s within my skill set (after buying a few more tools), but money’s tight and I need to shop carefully. Am I best off replacing pads, rotors, or both? Back ones only or front too? I don’t drive over 120kph so performance parts aren’t an issue, but I do a stop-start round-trip commute of 60km daily.

    @John Milmont – very well written article, found it via Google after researching plain or slotted rotors on eBay. Thanks!

  22. CSN's Gravatar CSN
    May 22, 2017 - 1:51 pm | Permalink

    Great article but I have to take issue with this:

    “You will hear small metallic pings and pops as the rotor cools unevenly. What you will be hearing is the sound of the cracks forming….”

    Pings and pops come from many parts that are differentially heated and cooled. My solid-rotored E63 M6 was pinging like crazy after coming back from a hard drive recently. Do heat-induced cracks even happen all at once, or grow slowly over time without a sound? I’m not sure, but you can hardly assess damage from normal heat pings. The damage probably occurs immediately after a braking events when airflow at speed cools the rotors far more rapidly than stationary convection.

  23. Jon's Gravatar Jon
    May 23, 2017 - 5:50 pm | Permalink

    So, You do not recommend Ceramic pads, or cross drilled rotors? I replaced my factoy brakes with cross drilled and EBC Red ceramic pads. Broke them in proper( Most people don’t know how to properly break in or “seat” pads) did all this 5 years ago, never had a crack. This brake upgrade stopped the car hot and cold much shorter distance than original. just now replacing w slotted / cross drilled and new ceramic pads. I am a true believer in ceramic pads. I have run many rallyes with this setup and had no problems from the braking system.
    356 hp 08 jetta 2.5 turbo.

  24. Jake B's Gravatar Jake B
    June 20, 2017 - 7:26 am | Permalink

    Y’all need to Google “Ferritic Nitro-Carburizing (FNC) Brake Rotors” … They came with my 2014 Chevrolet Malibu and I can honestly say that with 65K miles I have completely smooth brakes with no warping or vibration. It’s amazing. OTOH, my wife’s Town & Country minivan, even with the Mopar rotors and brake pads only last 25K miles.

    https://www.acdelcotechconnect.com/pdf/MarApr_2012_TechCONNECT_R1.pdf

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