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Drilled/Slotted Rotors....the reality...a good read

7K views 58 replies 11 participants last post by  OBZ 
#1 ·
I thought this was a nice read. I got it from another website i visit often.

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)
===========
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 shit. 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.
 
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#2 · (Edited)
Thanks for posting this. You might be attacked for being a, "kill joy", though! :laugh: :bigsmile:

This is exactly what I have been saying. Drilled rotors can crack, are a safety issue and are not necessary. Why anyone would want to, "mod", their car but in doing so reduce its functionality or performance, is just downright stupid in my opinion. :blink:
 
#4 ·
That's why I just got PowerSlot slotted rotors...drilled are bad for the most part...I actually would've bought blanks, but I got such a great deal on the PowerSlot's...couldn't resist.

Great write up OP.
 
#6 · (Edited)
Oh but you're just a negative kill joy! :laugh: :bigsmile:

Gotta go with the crowd man and be like all the other robots walking around, all behaving the same and looking the same! :laugh: :bigsmile:

"Those drilled brake rotors look sooo keeeewwwllll! Everyone will think I'm a real race car driver!" :laugh: :laugh: :bigsmile: :bigsmile:

"Oh...the rotors can crack and fail? They don't work as well as regular rotors? Ahh...who cares! It's how they look that's important!" :laugh: :laugh: :laugh:
 
#7 ·
And when the do fail it will be "How deep your pockets are" that is important LoL! Tow costs, plus new brake parts and collateral damage to surrounding parts, not to mention where they failed imagine going around a turn "a little faster then the posted speed limit" and then oh ou no brakes LoL! no thanks hahaha
 
#9 · (Edited)
"Awe come on man! You're being a kill joy again! Don't tear down other people who like to look keewwll, but in reality are dangerous to everyone else on the road! Go with the crowd dooooddd!" :laugh: :laugh: :laugh: :bigsmile:

"Drilled rotors are MAD KEEEEWWWWLLLL!" :laugh: :laugh: :bigsmile:

"I'm young, I'm not gonna die!" :laugh: :laugh: :bigsmile:
 
#8 · (Edited)
Nice. My rotors were warped, so I got powerslots, but it looks like I made a good choice in getting just slotted rotors. I knew the drilled were prone to cracking. Tho I feel like they're saying there's less material to dissipate heat, but really releasing heat has a lot to do with surface area, which slotted rotors have more than plain. (Which is why heat sinks have all those skinny vanes.) However its barely measurable probably between slotted and not.
 
#14 ·
Didn't mean to start any drama :S, just wanted to inform people that drilled rotors aren't all they are cracked up to be (see what i did there) :p IMO though I would not risk buying drilled rotors when slotted or even blanks (bigger blanks mind you) will give you better stopping power assuming these blanks have vanes. Some people like drilled and can even go through the life of their rotors without any problems and hey all the power to them and to each his own. I'm just saying that if you want "actual" better performance from your brakes, and not just "bling" then people should probably look at something other then drilled rotors.
 
#15 ·
^^ (John Adams) Dude you are so lame, that I have no words for you... I don't give a rats ass how intelligent you are, you can take your rubbers and shove em you know where. I think 3 of your posts in this thread with the EXACT SAME CONTENT are enough of your bull. Tell it to someone who cares, there is such a saying- "To each their own"...

I'm an 18yr old and if you think that everyone my age is a dumbass then I guess you've found yourself a gem here... Lets cut the crap, it was an informative read with a lot of unnecessary commenting by an insufferable know it all..

To the OP, thanks for providing REAL info as why drilled rotors aren't the best idea.
 
#29 · (Edited)
If you get brake rotors and pads from high quality, well known, long standing and trusted brake component companies like Raybestos, Bendix or Wagner, you can't go wrong. These companies have been in business for a long, long time and make original equipment brake pads and rotors for many vehicle manufacturers. Typically, they offer lifetime warranties on their brake pads and rotors that any Advance Auto Parts of Auto Zone store will back up, (provided you made the purchase at their store of course). This is a convenience for you with a capital CONVENIENCE. :)

Properly functioning brakes for a street car are just fine. Probably the best improvement that can be had in shorter braking distances is by improving the performance of the tires by using a tire that grips the road better. This can make a huge difference in braking. In SCCA autocross racing, my 1969 Yenko Camaro, (with its front disc brakes and rear drum brakes), has out-braked much newer high performance cars, such as Corvette's and Mustang GT's with four wheel disc brakes. How does it achieve that? By using high performance tires from B.F.Goodrich that stick like glue! :)

I've been in the automotive industry for 25 years, worked on my own cars for longer than that, am certified in brakes from the Automotive Service Excellence program, (ASE), NAPA and EIS brake certified. So that's my advice, take it or leave it. :)
 
#33 · (Edited)
As for drilled vs slotted rotors, the slotted rotors are a much safer choice.

But if you think about it, although the slotted rotor increases the surface area for cooling purposes, it actually reduces the surface area the brake pads have to actually grab onto the rotor and hold on, slowing down the car. If your driving a street car that doesn't have to worry about brake rotors getting so hot that they glow bright red, (as race cars do on road courses with very heavy brake use), then of what use it it to you? A benefit you can't take advantage of might not be a benefit. It would be like spending money on a car that will do 200 mph, when there isn't any way you could ever drive that fast. It's a benefit with no use, with no purpose. If you are a road course race car driver that applies heavy brake use with every lap, many times a lap, heat will build up in the brakes and cause a problem with brake fade. But with a street car, if you were to drive that hard, that fast, jam on the brakes that forcefully and that often to have the slotted rotors advantage of better cooling be of any use to you at all, I think it safe to say you might be pulled over and arrested before then. :bigsmile:

Keep in mind, unless we are talking about very high end sports cars, (that typically do see road course racing use), slotted, (or worse yet, drilled), brake rotors are not used on regular production new cars, even high performance models. Why? Because they are of no reasonable benefit to the street driven car and can actually cause problems for a street driven car, as I have noted.

Also, reducing the surface area a brake pad has to grab onto the rotor can be significant in the reduction of braking power. With my customers cars, we can't just do what the tech's call a, "pad slap", which is just slapping on brake pads on top of old rotors. That's not a brake job. Old rotors have a microscopically uneven surface from the brake pad wear that was put into them. You can feel it with your fingers, all the ridges and valleys. Installing new pads on top of old rotors actually provides less braking force because the braking surface area, (the contact area between the brake pad and the rotor), is reduced, is smaller, is not perfectly flat, which results in less braking force. The proper way to address this problem is to turn the brake rotors on a brake lathe, (or replaced with new brake rotors), cutting the surface down, removing all the ridges and valleys, so we have a nice new, flat, smooth surface for the brake pads to grab onto, having 100% contact and maximum braking force. I hope you can understand what I mean here. Two flat surfaces are going to be in 100% contact with each other, providing maximum braking, whereas surfaces that are not flat don't have 100% contact and will provide less braking force. This also can reduce brake pad life, because the brake pad is going to become overheated, (because it's not operating at 100% efficiency), and wear out faster.

Now, there are some other issues here that you might want to consider before shipping your money off to an unknown Internet company.

First off, the companies address is listed as a suite number. That sounds shady to me. I've seen news reports of less than credible companies using suite numbers as their address. It's as if they want to hide their real address for some reason. If the company is not reputable, it's an easy matter just to get a different web site address with a different company name and a different suite address, and presto chango, they are back in business! Personally? No thanks. I wont do business with companies that have P.O. boxes or suite addresses for this reason. Way too many red flags waving.

Secondly, the warranty is listed as 3 years or 36,000 miles, whichever comes first. Great. Only problem is, if you need to make a warranty claim because of a warped rotor lets say, (which is very common), the companies web site says the processing time for warranty claims, once received, is 5-10 days. Okay, so you take the brake rotors off the car and ship them back to the company, (their cost or yours, it doesn't say for some reason...that's not comforting!), and shipping will take about 3 days, plus about 3 days back to you. So that's 6 days right there plus the 5-10 days they say they need to process the warranty return, adds up to 11-16 days. Basically two weeks.

Can you be without your car for two weeks? Even if you could, would you want your car out of service for two weeks? I didn't think so. :)

This is one of the reasons I try to encourage people to use their local auto parts stores, because a warranty is a valuable thing to have with anything you buy for your car, but it really means nothing if your car is going to be out of service for two weeks, waiting for the warranted part to be shipped back to you. Here again, we have a benefit, (in this case a warranty), that's of no practical use, so in reality, it's not a benefit at all.

Further, I know someone who works for a very well known shipping company, (the name doesn't matter), and they have told me that the people who actually handle the packages sometimes will almost intentionally try to break whatever it is inside the package, by throwing it around really hard, even if clearly marked, "FRAGILE - HANDLE WITH CARE". Their attitude is, "Well, if it breaks, then it wasn't packaged well enough. That's what the insurance is offered for. Ha, ha, ha". Would you want your car parts bashed around like that?

I once had a very rare and expensive auto hood shipped to me. It couldn't be replaced, as it was the last of its kind. I was extremely lucky to locate it. It was shipped in its brand new original box, clearly marked, "HANDLE WITH CARE". Well guess what the shipping monkey's did to it? They had cut holes in the sides of the box to use for hand holds! The box was very banged up and I was extremely lucky the hood was not damaged. I was pretty hot!

Another thing to factor in is, do you want that package of expensive auto parts delivered to your front door, sitting on the steps, maybe when you're not home, just begging for someone to steal it? Would you leave anything expensive sitting on your door steps, begging for someone to steal it? Another big strike against shipped auto parts.

Another thing to consider, I've never heard of this company in all my years in the automotive industry. They could be perfectly fine, I don't know. How reputable and trustworthy are they? Do they have any Better Business Bureau complaints against them? Contacting the Better Business Bureau will tell you this. Don't just go by the advertising hype on their web site about how great they are and how, "satisfied", their customers are. Of course they aren't going to advertise their customers who are not happy and that could potentially be a lot of unhappy customers. We don't know. It's just some mysterious, Internet based company somewhere in cyberspace that we don't know anything about.

Conversely, your local auto parts stores, (probably an Advance Auto Parts, or Auto Zone), are well known, reputable for quality parts and service, aren't going out of business next week, don't use suite return addresses, are open early in the morning until well into the evening 7 days a week, are nearby and able to honor a warranty on the spot, with no waiting, without your car being out of service for extended periods of time. They have the parts right there and they'll gladly let you inspect them before ever laying out any money. You can check the quality of the parts, see if it they have shipping damage in any way, or rust, or what have you. Can't do that with some mysterious online brake parts supplier. You're stuck with whatever they decide to ship you. With Advance or Auto Zone, if you get the parts home and decide you have changed your mind and don't want them, or you discover hidden shipping damage, you can gladly return them for a full refund or exchange, right away, no waiting for shipping. Whereas the online company you listed has a 15% restocking fee, (nice...for them). They have trained people right at the counter to help you with your brake installation and all the parts and accessories you need to do a clean, long lasting, professional job. If you forget something, or need something additional in the middle of the job, you don't have to send them an e-mail over the Internet and wait and wait and wait, you can go right down to their store and get immediate assistance. The online company doesn't have any cans of brake clean for sale, CRC Disc Brake Quiet, or caliper bolt lubricant, all of which should be used in every brake job. Advance and Auto Zone do though. :) If you do the bakes and find that one of the old brake pads had uneven wear compared to the other brake pads, then you have a sticking brake caliper and it will need to be replaced. Hmm...wait days for the caliper to be shipped to you, with your car out of service, or just go down to your local Advance or Auto Zone store? You know the answer. :)

Sorry this post was a little long, but there was no short and easy way to answer your question, explain this all and have it make sense to you. Long and short of it, for a race car, slotted brake rotors have applications, but in my opinion, for a street car, a regular full surface, full contact brake rotor will do a better job of braking, as all engineers working for vehicle manufacturers would seem to agree, because their vehicles come with full surface, full contact brake rotors and not slotted brake rotors. :)

If you have any other questions, please feel free to ask.
 
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