Pro's and Con's of drilled and slotted rotors

[Ryan]

Here's the summed up version:

-Lessened heatsink effect. less metal -> less heat absorption (half of the whole idea). Once your rotor is heat soaked, it works its way into the hub, killing wheel bearings. Racers have known this for a long time. Poor brakes kill wheel bearings.

-Wear out pads -> pads will wear out like they're running over a cheese grater (the pads compress a tiny bit when compressed , but expand that much over the hole/slot, to be ground off on the trailing edge of the hole/slot). This can be thought of a pro, as it does keep the surfaces fresh and hard biting.

-Stress risers -> making a hole in anything creates a stress concentration. Most drilled and slotted rotors are cheap cast blanks with steel grains. A hole in an inconvenient spot is asking for a crack and maybe rotor failure. This will not be a problem with an expensive rotor that was designed to have holes/slots.

-Built on old-technology fad -> old pads used to release a gas that acted as a cushion between the pad and rotor, holes and slots helped evacuate that. Thats not a problem anymore. Also, most pads have slots in them.

-Don't allow for significantly more cooling -> The rotor is moving very fast most of the time, and all of these holes and slots are not vanes, they're along a face or inside the body. They do help, but you will not see a 10% increase in cooling capacity.

-Less swept area -> less braking strength. Those holes are removing the area where the pad touches the rotors, which reduces braking strength. This is a small effect, but there none the less.

The Pro's

-The are slightly lighter. Less rotating mass makes for a quicker car. Also less unsprung weight is healthy for any car.

-Well designed (read, expensive) drilled and slotted rotors can provide increased cooling at the expense of pad life.

-They look cool.


Here's a good read that goes more in depth:

I thought this was a nice read.

Cliffnotes: Slotted/drilled = for looks except for a few rare cases which shouldn't apply if you're on this forum.

QUOTE(Taken from a sticky at Celicatech)
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First, lets get some physics. Tell me how a heatsink with less mass will cool better? You do realize that a brake rotor acts as a large heatsink to transfer heat from the brake pads to the rotor. The heat generated from pads has to go somewhere and so it transfers to the rotor and caliper.

Porsche claims: "Discs are cross-drilled to enhance braking in the wet. The brakes respond faster because the water vapour pressure that builds up during braking can be released more easily."

They have said nothing about enhancing normal braking circumstances and the larger diameter rotors probably make up for the lack of material present in a smaller cross drilled rotor.

From Wilwood's website:
QUOTE
Q: Why are some rotors drilled or slotted?
A: Rotors are drilled to reduce rotating weight, an issue near and dear to racers searching for ways to minimize unsprung weight. Drilling diminishes a rotor's durability and cooling capacity.

Slots or grooves in rotor faces are partly a carryover from the days of asbestos pads. Asbestos and other organic pads were prone to "glazing" and the slots tended to help "scrape or de-glaze" them. Drilling and slotting rotors has become popular in street applications for their pure aesthetic value. Wilwood has a large selection of drilled and slotted rotors for a wide range of applications.

As for the porsche rotors, a few notes from a forum I frequent:
QUOTE

1) The holes are cast in giving a dense boundary layer-type crystalline grain structure around the hole at the microscopic level as opposed to drilling which cuts holes in the existing grain pattern leaving open endgrains, etc, just begging for cracks.

2) The holes are only 1/2 the diameter of the holes in most drilled rotors. This reduces the stress concentration factor due to hole interaction which is a function (not linear) of hole diameters and the distance between them.

3) Since the holes are only 1/2 as big they remove only 1/4 as much surface area and mass from the rotor faces as a larger hole. This does a couple of things:

It increases effective pad area compared with larger holes. The larger the pad area the cooler they will run, all else being equal. If the same amount of heat is generated over a larger surface area it will result in a lower temperature for both surfaces.

It increases the mass the rotor has to absorb heat with. If the same amount of heat is put into a rotor with a larger mass, it will result in a lower temperature.

3) The holes are placed along the vanes, actually cutting into them giving the vane a "half moon" cut along its width. You can see that here:

This does a couple of things:

First, it greatly increases the surface area of the vanes which allows the entire rotors to run cooler which helps prevent cracks by itself.

Second, it effectively stops cracking on that side of the hole which makes it very difficult to get "hole to hole" cracks that go all the way through the face rotor (you'll get tiny surface "spider cracks" on any rotor, blank included if you look hard enough).

That's why Porsche rotors are the only "crossdrilled" rotors I would ever consider putting on my car.

BTW, many of the above features are not present in older Porsche brakes. The above is for "Big Reds" and newer.

This is quite different from the standard drilled rotors you get from brembo/kvr/powerslot/"insert random ricer parts brand name here" brake rotors.

Further proof of the uselessness of cross drilled rotors are found here:

Those Poor Rotors

QUOTE
Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the 40’s and 50’s, not a whole lot. Rotors were first ‘drilled’ because early brake pad materials gave off gasses when heated to racing temperatures – a process known as ‘gassing out’. These gasses then formed a thin layer between the brake pad face and the rotor, acting as a lubricant and effectively lowering the coefficient of friction. The holes were implemented to give the gasses ‘somewhere to go’. It was an effective solution, but today’s friction materials do not exhibit the same gassing out phenomenon as the early pads.

For this reason, the holes have carried over more as a design feature than a performance feature. Contrary to popular belief they don’t lower temperatures (in fact, by removing weight from the rotor, the temperatures can actually increase a little), they create stress risers allowing the rotor to crack sooner, and make a mess of brake pads – sort of like a cheese grater rubbing against them at every stop. (Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it.)

The one glaring exception here is in the rare situation where the rotors are so oversized (look at any performance motorcycle or lighter formula car) that the rotors are drilled like Swiss cheese. While the issues of stress risers and brake pad wear are still present, drilling is used to reduce the mass of the parts in spite of these concerns. Remember – nothing comes for free. If these teams switched to non-drilled rotors, they would see lower operating temperatures and longer brake pad life – at the expense of higher weight. It’s all about trade-offs.


From Stoptech:

QUOTE
Which is better, slotted or drilled rotors?

StopTech provides rotors slotted, drilled or plain. For most performance applications slotted is the preferred choice. Slotting helps wipe away debris from between the pad and rotor as well as increasing the "bite" characteristics of the pad. A drilled rotor provides the same type of benefit, but is more susceptible to cracking under severe usage. Many customers prefer the look of a drilled rotor and for street and occasional light duty track use they will work fine. For more severe applications, we recommend slotted rotors.

That almost sounds like an excuse to use cross drilled rotors, and for your street car which probably is never driven on the track, the drilled rotors are fine, but as Stoptech states, they will crack and are not good for severe applications.

From Baer:

QUOTE
"What are the benefits to Crossdrilling, Slotting, and Zinc-Washing my rotors?

In years past, crossdrilling and/or Slotting the rotor for racing purposes was beneficial by providing a way to expel the gasses created when the bonding agents employed to manufacture the pads...However, with today’s race pad technology, ‘outgassing’ is no longer much of a concern...Slotted surfaces are what Baer recommends for track only use. Slotted only rotors are offered as an option for any of Baer’s offerings."

Then from Grassroots Motorsports:
QUOTE
"Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the '40s and 50s, not a whole lot. Rotors were first drilled because early brake pad materials gave off gasses when heated to racing temperatures, a process known as "gassing out." ...It was an effective solution, but today's friction materials do not exhibit the some gassing out phenomenon as the early pads. Contrary to popular belief, they don't lower temperatures. (In fact, by removing weight from the rotor, they can actually cause temperatures to increase a little.) These holes create stress risers that allow the rotor to crack sooner, and make a mess of brake pads--sort of like a cheese grater rubbing against them at every stop. Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it...Slotting rotors, on the other hand, might be a consideration if your sanctioning body allows for it. Cutting thin slots across the face of the rotor can actually help to clean the face of the brake pads over time, helping to reduce the glazing often found during high-speed use which can lower the coefficient of friction. While there may still be a small concern over creating stress risers in the face of the rotor, if the slots are shallow and cut properly, the trade-off appears to be worth the risk. (Have you looked at a NASCAR rotor lately?)

And then, let's check out what was said on the aforementioned Altima thread [[[ Long thread at altimas.net that was deleted by that server. it is hosted here ]]]:

QUOTE
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 Einstein: 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---. 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. Does that make sense Ace? Good. Let's move on.

#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 I am 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. Make sense? This isn't rocket science here d00d.

#3 REMOVE the heat from the brake system:
Now comes your favorite part of the process. This is what you thought DISSIPATION was. It is ok. I will allow you to be wrong. This is 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 veins. 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. Got it? Good.

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. Explain again just how drilling away material/structure from a CAST product DOES NOT weaken it? Since you are obviously a man of great knowledge and experience surely you have seen what can happen to a x-drilled rotor on track right? Yes it can happen to a non-drilled rotor as well but the odds are in your favor when pimpin' bling-bling drilled y0! Since you are also an expert on thermodynamics why not explain to the group 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 boys and girls. 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 her chodeboy. 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 Tex.

For me , it will always be cheap rotors and good pads.

[Sleeper6]

Ryan, I really appreciate the info its definatly a good read. On a side note though I think you should change it to not to buy drilled rotors as there are still benifits to running slotted rotors IMo if you use a quality rotor.

[Ukrman]

great myth buster

[Inodoro Pereyra]

That's a good read, but, actually, most of it is incorrect.

1. The heatsink effect is not proportional to mass, but to surface. Cross drilled and slotted rotors have more surface than flat rotors, therefore they dissipate MORE heat than them, not less.

2. Slotted rotors are MEANT to increase pad wear. The whole purpose of the slots is to shave the pads, to always keep their surface rough, therefore increasing friction. Slotted rotors are meant to increase stopping power (and they do), not pad life.

3. Stress risers: that's the only real reason not to buy CROSS DRILLED rotors. An excellent solution, widely used in amateur racing, is to buy slotted rotors (much stronger than cross drilled ones), and make a hole in the outside end of each slot, so the slot itself does the degassing of the pads.

4. Slotted and cross drilled rotors are not built on a fad, they're built on racing technology. Pads did, and still do, release gases when they overheat, and their surface did, and still does get polished upon contact with the rotor. The problem with them (as well as with most things based on racing technology, used on street cars) is that users don't know what they're doing. In racing, pad longevity is not a concern, braking efficiency is.

5. The boundary layer over a flat rotor is laminar, hence, low pressure, and has a tendency to easily disengage from the surface of the rotor. Slots and holes change the nature of the boundary layer to turbulent, which increases the air pressure over the surface, therefore increasing heat transfer. Furthermore, the layout of the slots is meant to increase air centrifugation, greatly increasing airflow over the braking surface of the rotor. Once again, slotted and cross drilled rotors are built on racing technology, and therefore thoroughly tested. If they're used for racing, is because they work.

6. The (slightly) less swept area of the cross drilled/slotted rotors is easily offset by using bigger pads/more efficient calipers, which would cause flat rotors to overheat and fail.


Cross drilled and slotted rotors have been designed for use in disciplines where the longevity of rotor and pads is not important. A race car driver may hit the brakes more than 1000 times within an hour, and his LIFE depends on the brakes performing flawlessly each and every time. For THAT PURPOSE, cross drilled/slotted rotors are far superior than flat rotors. Expecting the rotors to perform differently than they're designed is the failure of the user, not the technology.

[Josh]

I agree with Inodoro on this one.

I have been running slotted cross drilled rotors on several cars for street/strip use for over 10 years now with great results. I have only ever cracked one rotor and that was a none vented rear rotor. The problem is that most vented slotted rotors you buy were SOLID to start and they drill and machine them after. they were never designed to be slotted or drilled.

Not to mention most street bikes like my ZX10 come with cross drilled and or slotted rotors from the factory for the better part of a decade.

IMO it comes down to the quality of the parts you are using and the application. For instance I am running EBC greenstuff and Stoptech slotted rotors on my 8 all the way around. now if the pads are cold its like stopping on ice you just keep going no matter how hard you step on the brake. Most would take this as they are horrible, but that's not the application they were designed for. If you are on and off the brakes lets say in rush hour traffic, they are overly effective. They will grip and will stop on a dime, they are designed for high heat applications from constant braking. I think it comes down to misconceptions and peoples misinterpretations.

[wytbishop]

I have only one comment.

The amount of enregy an object can absorb is a function of its mass. the amount of energy an object can dissipate is a function of its surface area. The pertinent question with respect to brake rotors is, does the additional surface area of the slots/holes increase the dissipation of heat energy by a greater degree than the change in mass reduces its ability to absorb heat?

I don't know the answer but that is the only correct question regarding heat. You can all bicker over the other stuff.

My opinion is that the brakes are just about the only part of the car where I appreciate a little bling. Slotted and drilled rotors look cool.

[MrMazda92]

This reinforces my choice of slotted > drilled by no small margin... Still, it gives me yet another late night research topic.

[MrMazda92]

IMO it comes down to the quality of the parts you are using and the application.

This applies the every single aspect of mechanics, and is a VERY wise statement. Cheap parts(Cheap, not VALUE) yield cheap results. Value parts on the other hand... Those are just Badass to the core.

[Mooneggs]

I agree with never buying drilled rotors ever again for the mx-3. I've seen them crack and I agree with the statements... unless you are buying a high end vehicle (like a porsche) you probably aren't going to find the quality engineering needed to actually produce benefits. For all intents and purposes standard blanks and decent pads are all you need for the mx-3 for 90%+ of the people on this forum.

I am using slotted as I think it does help with the high performance pads I'm using and the aggresive driving with all the autocross I've been doing and plus I already bought them and I'm not about to swap them out

[Daninski]

Well Guys, I just got home burned out from doing a 14hr HVAC install with 4 hrs of driving and now after reading all this my last brain cell just fried. Good write up and very good and thought provoking back and forth comments. I think for lot of us on limited income my money is on cheap rotors and ceramic pads. Good nite.

[Nd4SpdSe]

Oddly I agree with Ino and with Wyt's added info.

My Mx-3 had crossdrilled and slotted, never warped or cracked, even got a second set afterwards to replace my worn set. There was a part of me that wanted to get just regular rotors and use the same pards to see if there was any improvement or not going away from slotted and drilled, but the brakes worked so well that I didn't want to risk making it worse for trials sake. Mine were actual Brembo rotors, but the drilling and slotting was done by an aftermarket company.

[MrMazda92]

I think it's highly dependent on quality of parts used, to be honest. R & D is worth a lot more than "ooooooh shineeeeyyy!!!" for real performance.

[stereoking15]

Here is proof of why not to buy slotted and drilled. These are brembo brand. Now I stick to just slotted and dimpled on the front and blanks on the rear.

[Daninski]

Here's some additional info that some may find interesting. http://www.cryogenicsinternational.com/rotors.htm

[mitmaks]

Here is proof of why not to buy slotted and drilled. These are brembo brand. Now I stick to just slotted and dimpled on the front and blanks on the rear.

Is this your car?