Reference: Slotted/Drilled Rotors Information

Discussion in 'Wheels / Tires / Brakes / Suspension DIY How-To's' started by Handlebars, Oct 8, 2004.

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  1. Handlebars

    Handlebars None Taken. Registered VIP Registered OG 5+ Year Member 10+ Year Member

    Apr 18, 2003
    i do not take credit for this, i am unable to locate the original author. however it is a very good read.

    Here is how it works. The friction between the pad and rotor is what causes you to stop. This friction converts your forward energy into heat (remember: Energy is neither created nor destroyed, it is converted). Now that heat is a bad thing. Yes it is bad for the rotors but it is a lot worse for the pads. A warped rotor will still stop the car - it will just feel like s**t. Overheated pads however WILL NOT stop the car. It is here where the rotors secondary responsibility comes in. Its job now is to DISSIPATE the heat away from the pads and DISPERSE it through itself. Notice that DISSIPATE and DISPERSE are interchangeable? Once the heat is removed from the pad/surface area it is then removed. Notice where the removal falls on the list of duties? That's right - number 3. Here is the list again. Memorize it because I will be using it a lot in this post:

    #1 Maintains a coefficient of friction with the pad to slow the forward inertia of the vehicle

    #2 DISSIPATE the heat

    #3 REMOVE the heat from the brake system

    Let's look more in-depth at each step now shall we? No? Too bad, assclown, we are doing it anyway.

    #1 Maintains a coefficient of friction with the pad to slow the forward inertia of the vehicle:
    This one is pretty simple and self-explanatory. The rotor's surface is where the pads contact and generate friction to slow the vehicle down. Since it is this friction that causes the conversion of forward acceleration into deceleration (negative acceleration if you want) you ideally want as much as possible right? The more friction you have the better your stopping will be. This is reason #1 why BIGGER brakes are the best way to improve a vehicle's stopping ability. More surface area on the pad and the rotor = more friction = better stopping.

    #2 DISSIPATE The Heat:
    Let's assume for a second that the vehicle in question is running with Hawk Blue pads on it. The brand doesn't really matter but that is what I am using as my example. They have an operating range of 400 degrees to 1100 degrees. Once they exceed that 1100 degree mark they fade from overheating. The pad material gets too soft to work effectively - glazing occurs. This means that a layer of crude glass forms on the surface of the pad. As we all know glass is very smooth and very hard. It doesn't have a very high coefficient of friction. This is bad - especially when you're coming down the back straight at VIR at 125MPH. Lucky for us the rotor has a job to do here as well. The rotor, by way of thermal tranfer, DISSIPATES the heat throughout itself. This DISSIPATION lessens the amount of heat at the contact area because it is diluted throughout the whole rotor. The bigger the rotor the better here as well. The more metal it has the more metal the heat can be diluted into.

    #3 REMOVE the heat from the brake system:
    Now comes the step where the rotor takes the heat it DISSIPATED from the pads and gets rid of it for good. How does it do this? By radiating it to the surface - either the faces or inside the vanes. It is here where cool air interacts with the hot metal to cool it off and remove the heat. Once again there is a reoccuring theme of "the bigger the better" here. The bigger the rotor, the more surface area it will have which means more contact with the cooling air surrounding it.

    Now let's look at why cross-drilling is a bad idea.

    First - as we have already established, cross-drilling was never done to aid in cooling. Its purpose was to remove the worn away pad material so that the surfaces remained clean. As we all know this doesn't have much of a purpose nowadays.

    Next - In terms of cooling: Yes - x-drilling does create more areas for air to go through, but remember - this is step 3 on the list of tasks. Let's look at how this affects steps 1 and 2. The drilling of the rotor removes material from the unit. This removal means less surface area for generating surface friction as well as less material to accept the DISSIPATED heat that was generated by the friction. Now because of this I want to optimize step one and 2 since those are the immediate needs. If it takes longer for the rotor to get rid of the heat it is ok. You will have a straight at some point where you can rest the brakes and let your cooling ducts do their job. My PRIMARY concern is making sure that my car slows down at the end of the straight. This means that the rotor needs to have as much surface as possible to generate as much friction as possible and it needs to DISSIPATE the resulting heat AWAY from the pads as quick as possible so they continue to work. In both cases x-drilling does nothing to help the cause.

    Now let's talk about strength - and how x-drilled rotors lack it. This one is simple. What happens to a cast iron molecule when it is overheated? I will give you a little hint - the covalence bonds weaken. These bonds are what hold the molecules together. You do the math - it adds up to fractures.

    So why don't race teams use them if they are so much better? Consistency? Hmmmm . . . no. I am gonna go with the real reason here. It is because of several factors actually. They are as follows but in no particular order:

    - Less usable surface area for generating friction
    - Less material to DISSIPATE the heat away from the pads
    - Less reliable and they are a safety risk because of fatigue and stress resulting from the reduced material

    And what are the benefits? Removal of particulate matter and enhanced heat removal. I gotta tell ya - it is a tough choice but I think I am going to stick with the safe, reliable, effective-for-my-stopping needs solution.
  2. Beelzebubba

    Beelzebubba SubGenius Member Registered VIP Registered OG 5+ Year Member 10+ Year Member

    Oct 8, 2003
    I'm glad the author put in pad there. :thumbs up

    little clarification there...bigger rotors by themselves = greater rotational mass or more flywheel effect and longer stopping distances. Plus the fact that they displace greater mass they are harder to get moving therefore they have a negative effect on acceleration too.
  3. TurboZinc

    TurboZinc New Member Registered VIP 5+ Year Member 10+ Year Member

    Jan 14, 2003
    Phoenix, AZ
    Cross-drilling was used to create a path for any outgassing from the pads to escape from between the pad and rotor surfaces. The outgassing would tend to create a less than ideal mating surface and greatly reduce the friction between the pads and rotors. This is no longer a problem with todays pads though.

    Slotting the rotors was used to remove the used pad material.

    Neither has any real effect with todays technology and products, other than for the aesthetic. Cross-drilling and slotting the rotors both reduce heat dissipation. Also, cross-drilled rotors tend to crack at the leading edges of the drilled holes. These sharp edges create large stress fields which under the rigorous conditions of a track meet can cause the rotors to crack. Usually not a good thing =)
  4. Jack ffr1846

    Jack ffr1846 New Member 5+ Year Member 10+ Year Member

    Nov 3, 2004
    Hopkinton, MA
    Stopping distances are predominantly determined by the tires. If you want shorter stopping distances, get stickier tires. Think about it this way.....if you can lock up your tires, then you've exceeded the maximum possible force needed to minimize your stopping distance. If you floor your brake pedal (without ABS) and can't skid, I'd say you've got other brake problems.

    Outgassing has not been a problem since the '70's. If you have a bunch of 70's pads that have been sealed in an airtight container, then you might find this problem. Otherwise, you will not.

  5. mirahe

    mirahe New Member 5+ Year Member 10+ Year Member

    Jan 6, 2005
    Sorry but i am a little bit in disacording :

    1-The friction is the same in a litlle disc than in a bigger , the diference is the momentum M=F*Reff , if you have a big disc then your Reff is bigger , for this reason you will achieve more momentum to stop the car. example :
    M1=F*R (F=nu*m*g = 1.2 * 400 Kg * 9.8 = 4704 N this is the maximum force that the tire suport before slip , to achieve this force M2=M1 Fb (force of braking) * Reff (radius effective of disc = 4704 If i increase the Reff i need to make less force in the pedal.

    2-Weel more or less the conductivity of the heat is better with more material.

    3-Wheel is not bad , i am agree that drilled discs are not for more cooling , but these drilled clean disc , and the most important improve less unsprung weight .
  6. Redline57

    Redline57 New Member 5+ Year Member

    Jan 7, 2007
    Added info about drilled rotors and why not to use them

    I posted this information on honda-tech but since there all ricers who don't seem focused on performance anymore, I'm on this site, and will share all my information with you.

    The reason for the creation of cross drilled rotors initially was to remove the "gasses" from the brake pads. HOWEVER, most of your modern brake pads (Axxis metal, AEM semi-ceramic) do NOT produce gasses when heating. This was on bad brake pads used in the 1950s and 1960s. Back then, asbestos was also used, and we dont use that either.</p><p nd="4">The other reason is so called heat dissapation. I don't have my physics and thermo books with me, but the logic is that the holes in the rotor are suppose to allow the brake pad to cool. So...air gets into the rotor from the inside of the vents. If you have a back rotor which is solid, air gets into these holes how? If your stopped, you are leaving air inside these holes sandwiched between the pads, thus creating air with a rising temperature. Its increasing in pressure from the heat, which I guess you "could" call a gas that would affect braking. So the cross drilled rotors do not remove any gasses formed by brake pads (because there are none created anymore) but could possibly inhibit the creation of "hot spots".</p><p nd="5">Cross drilled rotors have LESS contact area because of the holes.<br>But if the rotor is cooler, its better, right? Well no, because these rotors are not cooler. THe heat is generated from the pad/rotor contact. What removes heat the most effectively? When stopped or moving, the pad transfers heat into the rotor because its made of cast iron. the rotor has a lot of surface area and even vanes in it. But the little holes allow air in this surface contact, and you can transfer more heat into a solid big ass chunk of cast iron more than you can into the air. Don't believe me? Touch some steam at 150 degrees, then touch a piece of hot metal which is at 150 degrees. Which burns your hand? the metal. So let the heat transfer into the metal, because since it has so much more surface area, dissapates better.</p><p nd="6">Safety!!<br>Cross drilled rotors can crack! I have seen them!!<br><img src="" border="0"></p><p nd="7">Even racing teams will reccomend AGAINST cross drilled rotors:<br><a href="" target="_blank">http://www.livermoreperformanc....html</a></p><p nd="8"><br>Companies that sell cross drilled rotors that are redrilled may not be structurally sound. I have actually seen pictures of rear Integra rotors that have had hairline cracks turn into the rotor actually breaking apart!</p><p nd="9">Do your homework. Even Porsche and Ferrari will admit that the cross drilled rotors they use are for looks. So if you are one of those kids who thinks the little holes look cool, get a name brand drilled rotor like Ferrari does. The REASON Ferrari's 'holed' rotors are alright to use is because they are CAST with the holes in them, so they are not actually drilled into cast iron rotors. Cheap drilled rotors are not safe, and even the good ones are not necessary. Why do Ferrari do it? People THINK they want it, and it sells. If you don't believe me, go into the business world. You will learn that pretty soon, you can sell utter s**t if people THINK its better.</p><p nd="10">Information I gathered from <a href="" target="_blank"></a><br>says:</p><p nd="11"><br>"KVR Crossdrilled Rotors</p><p nd="12">Why should you upgrade to cross drilled rotors?</p><p nd="13">Simply stated, the function of any vehicles brake system is to stop the vehicle. This is accomplished by absorbing the kinetic energy stored in the moving vehicle, and converting it into heat. The friction caused by the brake pad rubbing on the rotor is the source of this heat. The more quickly and efficiently that heat can be absorbed and dissipated, the more quickly and efficiently the car will stop.</p><p nd="14">There are several contributing factors to this heat reduction. One of the most common sources of heat is from the gases produced by the bonding agents of the brake pad burning off. Under severe braking, this can actually produce a boundary layer of gas that pushes the pad away from the rotor, which can lead to excessive brake fade. The cross-drilled holes or slots in a rotor provide an escape path for these gasses (de-gassing or out-gassing are common terms), and allow the pad to stay in contact with the rotor. As well as de-gassing, cross drilling or slotting will provide better wet weather braking as water is swept through the holes, or down the slots.</p><p nd="15">A vented rotor can be viewed as an air fan. When in motion, the vents draw air from the center of the rotor outward. This air flow, over an increased internal surface area, effectively dissipates rotor heat. Cross drilling adds to this air flow, as well as providing additional rotor surface cooling. "</p><p nd="16">This company is just telling you that the rotors may be cooler, however they fail to mention that the holes really do create a more than substantial decrease in surface area, thus less braking, thus less heat created, thus the less heat CREATED will leave the rotors cooler, the holes barely do anything! Its the less braking lowering the temperature!</p><p nd="17">Slotted rotors-<br>Find me a company that uses stock slotted rotors. They remove brake dust, but if you study braking systems, you find that with modern cars, flat blank rotors and semi-ceramic pads, the brake dust causing the rotor to slip on it is almost non-existent. But the brake dust doesnt need all those lines. Notice how most front brake pads (and most back) have that line down the middle to give essentially two bite points. If OEM or racing companies found it to be a benefit, they would do it.</p><p nd="18">PROOF OF IT ALL:</p><p nd="19">Find me an F1 car as of now that uses cross drilled or slotted rotors.<br>They all use full ceramic rotors and ceramic pads. Are they drilled or slotted? No.</p><p nd="20">If they helped the fastest cars in the world, wouldn't they use them? Its basic calculations that show the lack in surface area does not make up for the possible loss in temperarure. They use brake cooling air ducts insted.</p><p nd="21">BIG BRAKE KITS:<br>Some have asked if the big brake kits are worth it. This is sort of a relative question, but the simple answer is no. Regarding the big ones with drilled rotors, if you know that they are cast that way, at least they wont crack. I will still advice against them. <br>In terms of a big brake kit, I have seen some for Civic DX models. Civics have the small pad, small caliper, and a 9.5" rotor. The big rotors are 12" in diameter, ok so the overall diameter is close to that of an Acura RL (1999). But the sweeping area (the area that the pad can grab) is still the same if they use the same caliper and same pad. If you have the same pad and caliper, you are using the same rotor surface, just farther out, so it will increase braking from stock. However, if you were to change knuckles, etc, and get Acura RL caliper (larger piston than your civic DX piston), RL pads (much bigger and taller), and RL rotors 11.8" but much more surface area is touched, then you have a better brake setup because you have OEM parts, and a better grip on more area of the rotor. The downfall is added weight (since big brake kits are usually 2 piece and lighter) but the benefit is that you have so much more stopping. Ok, so the big brake kit will have less unsprung and rotational mass (so a little better accelleration but less braking), but they tend to run over a grand, and you can use OEM parts to build a better setup for half that.</p><p nd="22">IN CONCLUSION:<br>Don't buy slotted or cross drilled rotors, blank are better, and stop better. Physics people, get me my formulas and help me out here.</p><p nd="23">If you must get rotors with designs on them, get the slotted ones by a good company, and DON'T get blank rotors redrilled with little holes all over them. IF you absolutly must have the rotors with holes cause you like em, get them from a company that casts the rotors like that. I have seen rotors break and this is for your safety!
    REMEMBER......Your car will only stop as fast as your tires will allow. All the braking in the world wont work if your tires are bald and on ice!!
    anv2tk likes this.
  7. anv2tk

    anv2tk Respected

    Jan 26, 2016
    Atlanta, GA
    take them to school! :thumbs up
  8. XpL0d3r

    XpL0d3r What Civic? Staff Member Registered VIP Registered OG 5+ Year Member 10+ Year Member

    Mar 8, 2007
    Rochester NY
    Blast from the past!


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