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#1 (permalink) Old 01-19-2006, 11:19 AM
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slotted or drilled

Hey y'all-

I'm upgrading brake systems and wonder if anyone has any info about difference between drilled and slotted rotors. My mechanic says drilled are best, my foreign car specialist buddy always talks up the slotted ones.

I am having a hell of a time finding some slotted that fit my 01 325XI. I am asking because I can find drilled ones pretty easy.

Any help would be appreciated.

Current:
2001 BMW 325XI, 60k miles
1993 Ford Ranger, 200k miles

History:
1995 Ford Escort
1991 Ford Mustang
1986 Ford Taurus
1978 Pontiac Firebird
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#2 (permalink) Old 01-19-2006, 03:42 PM
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This has been an on-going debate on which rotor is better. The Cross-Drilled rotor or the Slotted rotor? Let me give you my analysis on these 2 rotors.

CROSS-DRILLED ROTOR OVERVIEW

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 agree with these two statements that were made on a vendor's site, within a certain extent "40% Better Cooling, 20% better stopping; Improved Wet Braking"

The following 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...another very misleading statement, "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 (most are). 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 shit 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.

GAS SLOTTED ROTOR OVERVIEW

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 the above 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).

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#3 (permalink) Old 01-19-2006, 03:43 PM
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MAJOR POINTS MISSED BY THESE TWO ROTORS

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.

CONCLUSION

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. So I’m not just talking out of my ass on these things. 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.

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#4 (permalink) Old 01-19-2006, 04:19 PM
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this is magnificent!

few things to add:

1. it is important to knwo what alloy is the rotor made of. typically, stock rotors for cars in the US are made of softer alloys, which are quiet and light, but warp more and last shorter then hard cast iron ones.
2. if one is rich, one may consider "hat" type rotors, which are made of a light alloy hub centric part, and an outer ring (mounted with expansion capability) of very hard iron mounted on the outside. just like BMW M3 CSL.

these setupt are available for e46 from brembo, along with multipiston calipers. LOTS of money.
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#5 (permalink) Old 01-19-2006, 04:53 PM
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This is exactly the info I was lookin for y'all, thanks a bunch.

I dont race so I wont be getting the drilled rotors. I do believe I fit in the performance minded street driver category. I guess I am going to have to dig for some slotted ones.

The only site I've seen them is approx 105ea. Is that about right?

Current:
2001 BMW 325XI, 60k miles
1993 Ford Ranger, 200k miles

History:
1995 Ford Escort
1991 Ford Mustang
1986 Ford Taurus
1978 Pontiac Firebird
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#6 (permalink) Old 01-19-2006, 06:37 PM
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if you go to TireRack.com, you can find the slotted rotors made by Brembo and PowerSlot.

Brembo is approx. $194 a pair

PowerSlot is approx. $91 - $122 each

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#7 (permalink) Old 01-19-2006, 08:33 PM
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Cross drilled are for show. They might slightly reduce brake fade. I dont know where that article is pullign shit out of it's ass or something but im sure it was written by soemone who was tryign to sell a product.

Slotted is much better at removing heat. It's not as showy though.

For the best braking power, blanks are the best. It has the maximum surface area.


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#8 (permalink) Old 01-19-2006, 10:33 PM
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Originally Posted by ///M Power
Cross drilled are for show. They might slightly reduce brake fade. I dont know where that article is pullign shit out of it's ass or something but im sure it was written by soemone who was tryign to sell a product.

Slotted is much better at removing heat. It's not as showy though.

For the best braking power, blanks are the best. It has the maximum surface

area.
Agreed ! wtf big deal about it ? The most important thing is to get one with a good name on it and they will work just fine under any regular driving conditions even with a max speed.

How about wooden ones? LOL...
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#9 (permalink) Old 01-20-2006, 11:48 AM
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I'm not going to get into any arguement with any of you. I have provided some valid input on both of these rotors. I am not trying to sell anything or discourage you from buying any products. This has been an ongoing debate between the Cross Drilled and Slotted rotors. I was just giving my opinion on these two.

However, I do agree that the best one is the blank rotor because of the maximum surface area and also to get a good brand such as Brembo, PowerSlot, OEM or StopTech to name a few.

I have given my analysis on the two popular rotors. If you think I am pulling shit out of my ass on my input, then so be it. Everybody is entitled to their own opinions or feedback on any products. This is what the forums are all about, helping one another, giving advice, giving our opinions, etc. I am not asking you to like my input. If we all were to think the same way, then we wouldn't be here.


Last edited by AZ325Ci; 01-20-2006 at 11:50 AM.
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#10 (permalink) Old 01-20-2006, 12:36 PM
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No I never said you were pulling shit out of your ass. Perhaps my statement was a bit aggressive, btu I was saying that whoever wrote the article was probably biased because they might be speaking of a product they sell and trying to give it high praises.


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#11 (permalink) Old 01-21-2006, 01:54 PM
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i like the article. in my experience with crossdrilled and slotted, what it said is likely all true. i did have old iorn rotors crack at the holes: we drilled a blank ourselves, and did not chamfer anything.

i now run slotted Powerslots bought from tire rack on my car, they are excellent. 15k, not a sign of warping, and little wear. new pads every 10k though (hawk hp plus).
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