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Guest Message by DevFuse

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Rotors: Analysis of Cross-drilled vs. Slotted

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32 replies to this topic

#1 Guest_Legacy777_*

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Posted 14 March 2001 - 07:39 PM

Ok this is a continuation of this topic pub1.ezboard.com/fultimat...1321.topic about brakes.

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 rump roast 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.

#2 Guest_Lewy80DL_*

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Posted 14 March 2001 - 07:41 PM

I think you are the 'Book of the Month Club' winner!!! Man, are your fingers tired??/

#3 Guest_Legacy777_*

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Posted 14 March 2001 - 07:43 PM

hahaha....I started typing it in the reply window and moved over to word in fear of loosing everything, plus a way to keep it and do sp. I think I started writing that around 6:15 or so....so about hour and a half writing that. :)

#4 Guest_ImprezaP1_*

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Posted 14 March 2001 - 08:02 PM

didnt read the whole thing you wrote but would have to agree. all in all cross drilled is to cool the rotors down faster but because of these holes they are a little weaker and are more prown to breaking and also with the holes gas forms in these holes adding more pressure to the rotor causing it to break and also not work as well. But the slotted is to solve this gas problem but doenst cool the brakes. So what ever your preference is either slotted or crossed or both they all have there goods and bads.

#5 Guest_remarcable_*

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Posted 15 March 2001 - 02:24 PM

Yeah dude, we all knew that already.

Just kidding!! Great job, you should consider trying to get a job writing for some of these automotive review sites!

- Marc 'everything-i-ever-needed-to-know-about-brakes-i-learned-from-Legacy777'

#6 Guest_Legacy777_*

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Posted 15 March 2001 - 03:22 PM

Hey marc thanks.

I never did think about that... I guess that's another option, or do it as side work or something. Then I'd get to play with new toys :) hehe

#7 Guest_VDCHawk_*

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Posted 15 March 2001 - 09:15 PM


How about an Aluminum cross drilled AND slotted rotor. Just saw them in the March Popular Mechanics, up to about $1000 from Baer/Eradispeed. That should add to the book.

#8 Guest_NV Zeno_*

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Posted 15 March 2001 - 09:41 PM

Increased performance usually comes at the cost of lesser reliability. Example: Boring Honda Civic vs. Maserati Diablo. Just my 2 bucks
:edit: Oh, sh*t, that's *Lamboghini* Diablo :P


#9 Guest_Legacy777_*

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Posted 15 March 2001 - 10:57 PM

Keith: Hmmm, aluminum rotors. Honestly, that doesn't sound too good. They will cool a hell of alot faster, but I can't see them standing up to the temperatures as well. I really haven't heard of any....have to see what info I can find on them.

#10 Guest_VDCHawk_*

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Posted 15 March 2001 - 11:29 PM


Try www.eradispeed.com

#11 Guest_Legacy777_*

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Posted 16 March 2001 - 12:02 AM

Keith, I believe you miss read something. The rotor is not aluminum, the hub it aluminum, but the actual stopping part where the pad comes in contact is still iron/steel, with a zinc coating on it.

I don't know if that is what you meant when you posted earlier. I have seen rotors like this before, I would love to get a set :) They look awesome and perform well, however I would be a little reserved to get them for my original stock rotor size.

#12 Guest_czo79_*

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Posted 16 March 2001 - 03:25 AM

so i have refrained from commenting so far...but here goes: i would suggest mintex compound pads (available from SPD tuning) which have been baked in a kiln to off gas, the point being that when they are heated, the do not give off gases, so essentially you have no brake fade, eliminating the need for slotted or cross drilled rotors. They do create a lot of dust and have a tendency to make noise, but if you want a totally streetable pad that offers maximum performance, there is no other way to go. Though i have no experience with them, they also sell cryogenically treated rotors which are supposed to stand up to the wear of the mintex pads much better than other rotors, they are notorious for eating through them. Hope this makes sense, its a little late and I have been at the bar too long, but hey, the american touring car series was on, and i am a sucker. I can't wait till next season when there will be a new WRX on the circuit.
97 Legacy GT

#13 Guest_VDCHawk_*

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Posted 16 March 2001 - 08:33 AM


The magazine says "the rotors are made of aluminum instead of cast iron...". Guess they'll print a retraction next month. I didn't really look at the web site, they are outta my price range.

#14 Guest_Legacy777_*

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Posted 16 March 2001 - 09:27 AM

hey it's cool, yah I think someone did an oopsy. Yah I can believe they're quite expensive. How much did magazine say they were? I know a big brake upgrade that has similar rotors and 4-pots is around 1500 or some where in that ball park.

#15 Guest_VDCHawk_*

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Posted 16 March 2001 - 10:56 AM

It was saying $645 to $945, did not say each or pair, it is also just the rotors, no new calipers.

#16 Guest_Legacy777_*

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Posted 16 March 2001 - 12:32 PM

HOLY CRAP!! That's insane, there ain't no rotor that good. I could see 1000 something for both rotors and 4-pots with brakes....oh well

#17 Guest_miksmi_*

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Posted 16 March 2001 - 12:59 PM


Primo post; very informative. Q: I took my wife's 00 GT wagon (5MT, stock brakes, no mods) to a track (Summit Point, WV). I drove with an instructor for 20 minutes, had a cool down lap (easy on the brakes for cooling), exited to the pit area, stopped, and had to wait for a few minutes while instructors egressed from the cars ahead.

I applied the parking/emergency brake and released the brake pedal, to avoid (I thought) cooking the pads on the rotors. My instructor preferred that I release the parking brake and depress the brake pedal; he said applying the parking brake would cause the rotors to warp.

I think the 3rd generation Legacy platform has a 5th brake, a drum parking brake, distinct from the disc/pad at each wheel. I may be wrong and why the instructor disliked my parking brake application.

Any insight?


#18 Guest_Legacy777_*

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Posted 16 March 2001 - 04:54 PM

Mike, You are right, actually ALL legacies have a drum type parking brake system. My 90 has it. That is why the rear rotor has a larger hub part, there is a small drum brake setup in there.

Thinking about, I would believe that all cars that have rear discs have this drum parking brake setup. The reasoning is you need a completely different system incase one goes bad. If you loose hydraulic pressure, you have a back up mechanical system, i.e. the parking brake with cable.

So yah if you were sitting the parking brake would have been a better option. It is technically not even connected to the rotors themself, it is totally idepedant

#19 Guest_miksmi_*

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Posted 16 March 2001 - 05:23 PM

That's what I thought, Josh. Thanks for the reply.

#20 Guest_VDCHawk_*

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Posted 16 March 2001 - 06:30 PM


Actually some of the rear disc brake cars use a hydralic pump up parking brake. I rented a Buick Regatta once upon a lifetime ago, you had to pump the parking brake pedal.

#21 Guest_Legacy777_*

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Posted 16 March 2001 - 06:37 PM

yah...yah I know what you're talking about, the gm's have them, they're on the floor, but they're not the old-school click type suckers, you just push as hard as you can, no clicking. Then push it again to release.

#22 Guest_mile hi sub_*

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Posted 18 March 2001 - 07:55 PM

Nice article I had been thinking of cross-drilled but will probably pass now after the very good points you made. I have Legacy turbo brakes on my '95 Impreza and that is probaly enough. I really enjoyed watching Sebring yesterday, went there years ago and the rotors glowing in the dark are impressive.

#23 Guest_Sam_*

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Posted 18 March 2001 - 08:20 PM

So whats the best way to keep my rotors from warping? Every Subaru I have owned has had warped rotors. I bought a new OEM set and warped them within a couple of weeks. I am not racing the thing just geting on it every once in a while. Is the OEM material junk? I owned a Ford once and they are notorious for warping rotors, I replaced them with aftermarket and they never warped again.


#24 Guest_mile hi sub_*

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Posted 18 March 2001 - 08:55 PM

Good question I also wonder if there is anything that can be done other than driving around cooling them off before parking. My turbo Legacy Rotors are warped a little. But after whats been said I don't think that cross-drilling or sloting is going to have much of an effect. Thinking back to watching Sebring yesterday they showed several pad changes and I got a good look at the rotors and there were no slots,holes or anything and a friend that knows something about metal said that when they start glowing orange that is around 1000 degrees.

#25 Guest_Legacy777_*

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Posted 19 March 2001 - 12:13 AM

Ok....the biggest problem with subaru's rotors is they are too small. Also with some of the calipers they are a larger single piston design. This gives good braking force, yet it's not a very equally distributed force. The turbo legacies and newer subarus have two smaller pistons rather than one larger. This is better it adds a more equivelant force on the brake pads, and more braking power. Yet the best out there are the 4-pot(piston) calipers. They have two pistons on either side of the rotor, to provide the best braking force distribution, they are actually easier to change then the subaru's. Surprisingly my mom's toyota truck had a set of 4pots on them...pull out two clips, then two pins, and the pads slide out. Very nice design.

Unfortunately the solution to the many commonly warped rotors is larger rotors, however this is at an added cost to manufactures as well as consumers. Since the majority of drivers do not push their cars the manufacturers rather save money. It's a little surprising that they skimped on the 4pots for the US wrx.....oh well, there's always aftermarket upgrades ;)

As per your question about is there anything to do, not really, other than not driving like a mad man and parking it :)

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