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Old 03-22-2001, 09:42 PM   #1
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1990 Legacy & 97 OBS

Post Rotors: Analysis of Cross-drilled vs. Slotted

I did this up for a reply on USMB last week. Thought it might be useful here too, enjoy..hehe.

OK...I'm sorry, but what they have there is WRONG!



You buy cross-drilled or slotted rotors for performance right? Well they say, "Cross Drilled Discs will last up to twice as long as O.E.M. rotors (depending on your braking style)." That little parenthetical statement should give you a clue! Most people that are going to buy cross-drilled rotors are going to buy them because they drive harder, demand more out of their braking system, hence a more aggressive braking style. I warped my Brembro cross-drilled rotors in about 2 years.... and I was in school...my car sat most of the time.

I agree with these two statements, within a certain extent "40% Better Cooling, 20% better stopping; Improved Wet Braking"

This statement is very misleading, "Lower Brake Temperature Reduces Rotor Warpage” Yes your normal operating brake temperature may be lower, and yes your rotors may cool a little faster, however lower operating temperature and cooling speed aren't major players in warpage. Actually, cooling something faster will warp it faster then a gradual cool down.

Again...very misleading, "Less Brake Fade and Longer Life" Yes they reduce brake fade, by keeping operating temperatures down. However brake fade is more attributed to the compound material on the brake pad itself. Same rotor with different pad material (ex. organic, and semi-metallic or carbon metallic) will exhibit totally different temperature range, braking performance, and life expectancy. The reason OEM brakes fade like hell is the pad compound, it's organic. I could nuke a set of organic pads in no time flat...cause their level of heat tolerance is so much lower than a semi or carbon metallic pad. However they are quieter, and since Joe Public doesn't give a **** about how their car performs, they just want it to go from point A to B, it don't matter. However for the few that want better performance you need to step up to a better pad.


Ok before I look at what they said, one thing should be noted. Look at how one sided their two pages are for cross-drilled rotors...hmmmm.

Ok first thing they say "Enhanced Initial Bite" Sure this gives you may get a stronger "BITE" from the slot, BUT that is NOT what it is there to DO!! The slots on the rotors are to vent the build up of gasses produced in the interface between the rotor and the brake pad material! That is really all they're meant to do. Some brake pads have a slot down the middle, this slot also helps release those gasses. Main thing is, if you have gasses between the rotor and the pad, you loose braking performance, you do not have as solid a direct contact between the two.

"Intended For Track Use" I can say the same for cross-drilled, this statement is TOTALLY OPINION and should not be on a page comparing slotted and cross-drilled!

I'll agree with this "Better Cleaning of Pad Friction Material" Yes over time the pad material will get a glazed surface on it. The slots will help wipe this glaze away, or keep it from forming. However...this is trivial.

Again....more misleading information, "Slotted discs offer cleaning of the friction material (brake pads), but do little in terms of additional heat dissipation. Slotted brake discs do not cool better than cross-drilled discs or even standard discs. The face grooves will slice the brake pad material allowing the pad to bite harder into the disc, therefore causing an increase in disc temperatures. This is recommended for competition vehicles to bring pads and disc temperatures up to optimal operating ranges. (Race cars warm-up engines, tires and brakes for the best possible performance)."

The thing they are pushing in this paragraph is how much better the cooling is with cross-drilled rotors, and that slotted add heat. The main thing I find very misleading is, with ANY rotor-pad combo, you will have heat generation from driving...even if you DO NOT touch the brakes AT ALL. Rotors drag against the pads. You will get heat build up. Your rotors and brakes are designed to work better with a little heat in them. Ever drive you car first thing in the morning on a cold day. The brakes aren't too good when they are cold, so why do you need to excessively cool these rotors that are going to generate heat anyway? (Exception is organic pads...they work decent when cold, but better when a little warmer)



There are several key points that they failed to mention, rotor warpage and the main cause of it. The main cause of warpage in any material, object, whatever is heating it beyond a certain point where plastic deformation occurs, now there’s really no force being put on the rotor that it can’t handle, our force is heat, so think of heat being a force and there is a curve that this material will follow on when it is heated and cooled. Think of warpage when the material is pushed past a certain point along this curve.

Also cyclic loading of a material will also cause failure, or warpage in our case. Heating up and cooling of a rotor is our cyclic loading. Again, the more rapidly you do this, the greater your chances are for warpage. So if you go drive like a crazy person and then park your car the rotors will cool faster then if you drove nicely around for a couple miles to slowly bring them down in temperature.

That brings me to my next point, When you drive your car hard, and park it, the part of the rotor which is exposed to air cools at a different rate then the part that is touching the pad material. The pad material will retain heat a lot longer then the air, so you will get a spot on the rotor where the pad sat that will be warped. A friend of mine who works in motor sports, Indycar to be exact, told me that during a practice session they would go through rotors, i.e. warp them faster then they would if they were in a race. The main reason, like I said above, is the cyclic cooling of the rotors from really hot to cold, and the “hot spot” from the brake pad.

The main point or problem with what they say about warpage has to do with something they talk about, heat dissipation. They say that they have increased heat dissipation with cross-drilled rotors. This is partially true. At lower to medium temperatures the holes in the rotor allow more surface area, and as any good mechanical engineer or thermal scientist knows, you can achieve better cooling with more surface area. The holes in the rotor sort of act like the fins in your radiator, they aid in cooling the rotor. However those holes are a double-edged sword. They actually do more damage then they are supposed to help. When you cross drill a rotor, you take material out of it. Well guess what, that material is what helps that rotor maintain a certain level of performance before it warps. The more material you have the better heat distribution you get through the object. Think of it as I said before, a curve that this rotor follows when it is heated and cooled, and there is a certain plateau or ceiling where warpage occurs. When you remove material from the rotor, you decrease this ceiling. So it actually takes less heat to warp the rotor then it did before you started drilling holes in it. So if you look at the cooling by the holes in comparison to the loss of heat dissipation or heat tolerance that the rotor can handle, you have a balance scale. What is more important? Well In my book, I would want that ceiling to be as high as it could be to ward off the effects of warpage. Who cares if you rotor runs slightly cooler for normal use, and may cool slightly faster. The main thing they were saying was that cross-drilled reduces brake fade. This is 1/10 correct, better pads reduce brake fade.

Another thing, heat flows through the rotor as it heats up. With a normal rotor this heat is evenly dispersed and expelled. With the holes in cross-drilled rotors you get an interruption in this heat flow through the rotor. You again get hot spots or points where the material is discontinuous, and with these discontinuous points you get added stresses or higher temperatures at these discontinuities. So if you would look at a thermal image of a normal rotor you would see an evenly spaced out heat flow in the rotor. With cross-drilled you will see cold and hot spots in certain areas of the rotor. Temperature is a cyclic force and over time, with these hot spots occurring around the holes you end up with cracks, where the material has failed.

One last thing, there is actually a formula to calculate how much material can safely be removed from a rotor without compromising it’s ability to dissipate the heat generated in it. So with larger rotors you can have more or the same amount of holes and it don’t matter, there is plenty of material there. But with smaller rotors there is less material so you either need fewer holes to maintain a safe amount of material to dissipate the heat or you will lower the “ceiling” at which the rotor will warp.


Ok to sum everything up. Cross-drilled rotors are good for fast heat dissipation and reduction in braking gasses, however they are prone to warpage because of their less amount of material, and lower peak temperature tolerance. They are good for racing applications where you need very fast cooling from high-speed stops, and where they don’t care about the longevity of the rotors. They are not practical if you want to get more life out of your rotor. Slotted rotors main advantage is that they help get rid of the braking gasses between the rotor and pad. They are good for mild to medium racing applications and for the performance minded street driver. The longevity will be greater then that of cross-drilled, yet may be a little less then stock. There are also high performance rotors that offer a combination of both slotting and cross-drilling.

I guess now you may be asking what gives him the authority or background to be saying all this. Well first off I have gone through the hassles of cross-drilled rotors myself, and had the down sides of cross-drilling happen to me. Secondly I have researched and found NON-BIAS articles and information on the two types of rotors, as well as consulted several automotive professionals on the topic. Next I am a mechanical engineering major at Penn State, and have had courses in the areas I have talked about, the stresses, cyclic loading, temperature, discontinuities, and heat transfer. So I’m not just talking out of my ass on these things. And finally, like other performance-minded drivers I too want to get the most out of my car, and have looked into the different possibilities and options.

If anyone has any questions comments, feel free to post or email me.


'90 Subaru Legacy L Sedan
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[This message has been edited by legacy777 (edited March 22, 2001).]
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Old 03-22-2001, 10:14 PM   #2
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Good analysis. thats why i am sticking with my STi 4pot brake instead of going slotted or cross drilled.
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Old 03-23-2001, 04:23 AM   #3
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Wow, great info! But I have a question. What's your opinion of Cryotreating X-drilled/Slotted rotors to increase their lifespan and decrease chance of rotor warpage. I'm going for the x-drilled rotors because I like how they look, but I'm going to get them cryotreated to reduce the chance of warpage. Thanks
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Old 03-23-2001, 04:49 AM   #4
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Fido - I think cryo treating is beneficial for lots of things, including rotors. Also good for crankshafts, con-rods, etc.
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Old 03-23-2001, 06:17 AM   #5
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Good analysis on these 2 kinds of rotors. I agree with you all the way.

I had cross-drilled rotors on my Civic, and they warped faster than the stock rotors did. I got the cross-drilled because the stock rotors kept on warping every 10k miles. I thought that by using the cross-drilled rotors, it will prevent me from doing the brake job every 10k. I was very very wrong.
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Old 03-23-2001, 06:33 AM   #6
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Fido: I've heard very good things about the cryo-treatment. I would think that it would help the rotors. The metalurgy behind the cryo treatment is a little beyond my school But from what I know freezing anything down to close to absolute zero does something to the alignment and structure of the material's molecules and such. It helps align everything in a row or certain pattern. I guess a good way to think of it is the freezing just takes the material like a super material...hehe
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Old 03-23-2001, 06:56 AM   #7
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I was reading on AP's or Alcon's web site the other day that two piece (aluminum bell with iron rotor) rotors are more warp resistant than the one piece rotors that are common on street cars.

I assume the bell has a lot to do with warping--after all the bell prevents the rotor from growing and shrinking radially when it heats and cools.

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Old 03-23-2001, 07:02 AM   #8
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I want the x-drilled, but I don't want them to warp in a few months ...If the cryotreatment is worth it, then I'm defintely getting it done. Thanks, and again, this is great info...
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Old 03-23-2001, 09:26 AM   #9
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That was a killer post Josh. Glad to see you joined the i-club! Lots more traffic here than on the USMB. Although sometimes less traffic is good!
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Old 03-28-2001, 03:35 PM   #10
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With X-drilling, the water essentially gets into the holes, and the gasses (air in this case) becomes heated under the pad due to friction and hence "pushes" the water out of the braking surface. I've kinda simplifed the whole process, but all this is simple physics.
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Old 03-29-2001, 12:30 AM   #11
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I have seen the "better wet braking performance" claim for cross drilled rotors before (and maybe slotted too), but can someone explain why that would be the case?
Looking at the KVR web site, it sounds like they are saying because of the better bite from the pads biting into the holes on the brakes, but in a lower traction situation, it seems like having the pads 'catch' on the holes would just be more likely to unsettle the car in a lower traction environment, locking the wheels.
Or is it because the holes are venting the steam created by boiling the water from the rotors?
Steve H.
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