Why X-Drilled Rotors?

[HADA 11]

This subject is one of my favorites. The subject of cross drilled rotors came up on another message board I frequent and I learned a great deal about brakes, brake rotors and how and what stops a car in motion.

I'll be quoting directly from articles written by James Walker, Jr. a mech.eng. James Walker, Jr. is (or was at the time of writing) an ABS Applications Engineer. I'll probably borrow explanations and pictures from around the net and give the originators credit if and when I can find it. I'm not an engineer so most if not all of what I post here are not original thoughts, rather thoughts borrowed from people that know their tech. I believe many of them are mech engineers also.

Why do I bother? Well I visit and post here to lean about the car I just bought for my wife. I'd like to make contributions to make this board better, so I feel better when I use the information I find here for my benifit.

I've already posted some of this in the Pic/Video Section in this thread.

Here goes!

First off read this if you'd like to understand that forces at work to stop your car.

Braking Systemsin Plain English
by James Walker, Jr. of scR motorsports
...as published in Grassroots Motorsports, Dec 2000

From Team scR


Now the good stuff... (again from James at Team scR )

Crossdrilling

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. 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 todays 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 dont 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. Its all about trade-offs.

Summary

So, whats the secret recipe? Again, there is no absolute right or wrong answer, but like most modifications, there are those which appear to be well-founded and those that look cool. If ultimate thermal performance is your goal, look to what the top teams are running (relatively large, slotted rotors). However, if image is your thing, break out the drillpress and be prepared to replace your brake pads on a regular basis.

 

[Logan]

Good thoughts.
However I am going to argue with you about what F1 and other top spec cars use.
They don't use metal like us mere budget limited mortals.
Carbon is the way to go. You will never see drilling in F1 or rally cars for example as the are all using carbon composite brake components. Completely different technology when it comes to friction...
I agree with you on the point of wear:
Drilling/Slotting WILL drastically reduce the life of your brake pads, because the dust (and gasses!!) are venting out of the way MUCH faster than a normal solid rotor.
Drilling/Slotting is NOT something you want to do to your street car, unless it's a garage queen during the week and a track beast on the weekends WITH racing pads.
Racing pads do still create a gas between the pad and the rotor, but street pads do as well.
Some more than others.
The proper mix of pad material and what the heat in the pads creates is what regular pad manufacturers play with to get nice quiet comfy brakes for gramma in her Tercel.
But when you want to go fast, and then stop, all the extra heat and used pad material/gasses need to go somewhere.
Slotting and drilling just might be the way to go.
I'm talking about drilled rotors from personal experience.
I have a CRX that uses drilled brembo rotors and racing pads. The braking plain sucks until the pads/rotors heat up, but then hold on to your teeth!
(You do see MANY slotted rotors on rally cars that use a metal rotor, they don't drill because they don't want a pebble etc stuck in the hole).
Comments?

 

[MP5Raycer]

see this is a perfect example of why i love this site so much .. her is an awesome mature FACTUAL conversation without the bull in the way for getting points and views accross ... so to Logan and Hada .. keep it on ..this is what makes this site so awesome. thanks for your contributions to this site and on a personal note .. i am considering 4 brand new slotted rotors for my MP5 and i will get them but this is an aweome perspective to look into and i can say I had no idea about some of the things that Hada mentioned so like i said keep it going guys

 

[HADA 11]

However I am going to argue with you about what F1 and other top spec cars use.

No arguement. F1 and WRC are very different. However they are still battling heat. A better place to look for high tech brakes in NASCAR. NASCAR? Yeah. They have to use steel brakes just like we do.

Drilling/Slotting WILL drastically reduce the life of your brake pads, because the dust (and gasses!!) are venting out of the way MUCH faster than a normal solid rotor.

Modern Brake pads don't off gas to the point that it will cause brake fade. The enemy is heat, not gas. So cross drilled rotors are a solution to a problem that no longer exists.

Drilling/Slotting is NOT something you want to do to your street car, unless it's a garage queen during the week and a track beast on the weekends WITH racing pads.

I'd argue that cross drilled rotors are not items you'd want to put on any car that is driven, on the street or otherwise.
The last place you'd want to see drilled rotors are on a car driven on the track.

Rotors play 3 roles. Borrowed from a guy named Drew (I'd rather not post a like to the refference w/o his pernission. I'll post a link if he gives me a tumbs up) I editied the hell out of this as the author was not in a good mood when he wrote it. If I missed anything it's not directed at anyone on the Protege Forums.

#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?

#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 here. 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.

 

[HADA 11]

Oh, and Porsche doesn't use cross drilled brakes. The holes are cast not drilled and the rotor's are ceramic.

Porsche have their problems with hole too;

 

[gb5mx]

What about water dispersion? Would slots on the rotor surface act like the grooves in tires to help disperse water if braking in wet weather? (I'm not arguing, I'm asking).

Disk brakes in general are much more resistant to fading from water than drum brakes... I imagine that the pads squeezing the rotor would quickly overcome any water on the surface, but could grooves help speed up this process?

 

[HADA 11]

The rotors get so hot driving in "normal" wet conditions that the water evaporates.

WRC cars use slotted carbon rotors. We'll all probably agree that WRC conditions are anything but normal.

 

[igdrasil]

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.

The heat will not "dilute" into the metal -> you need more surface area in order to "dissipate" heat into air more efficiently?

But it will also depend on the material properties. I think cooper and aluminum are better in this matter.

 

[HADA 11]

The heat will not "dilute" into the metal -> you need more surface area in order to "dissipate" heat into air.

Read #2 and #3 again. If you still think heat will not dilute into the metal I'll try a different explanation.

 

[Logan]

I think the biggest problem with fading is not the pads or the rotors, it's the heat generated from the friction that is fed back into the braking system causing the fluid to boil.
Doesn't matter how big your brakes are, all that heat is going to wind up in the fluid sooner or later, and fade will come.
Changing to a fluid with a higher boiling point will play a major role in any well executed braking system.

 

[HADA 11]

I think the biggest problem with fading is not the pads or the rotors, it's the heat generated from the friction that is fed back into the braking system causing the fluid to boil.

The correct brake fluid is important. I use Ford DOT 3. I don't have a bottle in front of me but I believe the boiling point is 550 deg. Another consideration is how hygroscopic the brake fluid is. Many of the race fluids have high boiling points but they are very hygroscopic and absorb hymidity for the atmosphere. Once the humitity gets into the brake fluid it's boiling point drops like a rock.

The biggest problem is still the pads and rotors though. If your pads don't generate heat the coefficient of friction is not high enough. So the pads should get pretty hot, but not beyond their range. If your rotors make good heat sinks then the heat shouldn't boil the fluid.

But yeah, good clean brake fluid is important. Good tech on brake fluid and bleeding brakes here.

 

[igdrasil]

Read #2 and #3 again. If you still think heat will not dilute into the metal I'll try a different explanation.

Yeah, maybe Im using the wrong terms (english not my language)...

Anyway, basically we need the accelerate heat transfer, thats all, and the best way is to somehow get the heat, faster, out of there.
We all know that air is a poor heat conductor, but blowing it away will help ...so we need to get enough clearance for air to circulate around object surfaces, if we dont do this, the heat will be conducted into the other attached metal parts, which is good anyway if it does not affect the object preperties performing their jobs (rims, suspension, etc). Another thing is the heat radiation that will heat other objects close to them.
But having all that, if we have more surface area on the rotors, calipers and pads(think a micro-processor heatsink & fan as an example), good clearance and the wind to keep heated molecules out around the metal, heat transfer will accelerate, thus we get lower temps.

Damn what a headache

 

[APEXistud]

Who cares really. None of this really applies to a daily driven protege. Unless you auto cross or participate in track events, you really have nothing to worry about. I have yet see my factory MP3 brakes s*** out on me. From my experience, slotted and dimpled rotors are better and with Project Mu pads, which believe it or not, are available for our Proteges they'd work wonders. Cracking rotors are not really a common thing at least not from my (or anyone I have ever met for that matter) experience. I lived in Michigan for 2 years and did a lot of really stupid, hard driving in the winter. So we all know the affect that heating up brake rotors in freezing temps can do......warpage.

If you're looking to fight cracks on rotors and make rotors last longer, you can always cryo your rotors. Just a thought.

 

[HADA 11]

Anyway, basically we need the accelerate heat transfer, thats all, and the best way is to somehow get the heat, faster, out of there.

I understand what your saying. More air around the heat sink for better cooling. But how do we get that much air to the brakes, that fast? We can't in a practical manner. It would take a huge amount of air to rapidly cool the brakes effectively, and it's not practical on a moving car.

Using your microprosessor and heat sink for example. Think of the size of the processor vs. the size of the fan. Now if you had a processor that ran hotter you could put a bigger fan on. The hotter the processor gets the larger the fan would have to get. You'd reach a point where the whole thing would become a huge package.

What if the processor wasn't hot all the time, what it it only spiked every now and then? A bigger fan would be one option, bigger heatsinks would be another.

Since we can't get more air to a brake rotor that "heat spikes" every so often we provide a larger heat sink via larger rotors.

APEXistud,

Are you running stock brakes and pads? If you are you've pretty much supported the information I've posted. There's isn't a problem with our stock brakes for daily driving. So why introduce a problem with drilled rotors?

If you are going to the track or autocross you may want to look at more argressive brake pads. But you don't need drilled rotors to go with them, it's counter productive. If your brakes overheat you need larger rotors to address the problem, not holes.

BTW - Cryo-treatment has not been proven to help, or hurt braking performance. So cryo-treatment would not help to prevent or reduce the cracking of a drilled rotor.

 

[jared]

another problem with stock braking systems are the stock rubber lines. when these heat up (which under a little harder than normal driving conditions is very possible) they tend to expand, causing more fade.

answer: stainless steel brake lines. I think everyone here can agree to this.

 

[HADA 11]

Yes, braided lines improve feel. They may exagerate master cylinded to firewall flex, but the MC can be braced. If you have braided lines and and a braced MC (if you need it) your brakes should feel better.

 

[APEXistud]

BTW - Cryo-treatment has not been proven to help, or hurt braking performance. So cryo-treatment would not help to prevent or reduce the cracking of a drilled rotor.

By the way it has been document by quite a few different times to have improved the life of brake rotors and in some cases engine internals. But the engine internals is whole other story. If you do a search on Cryo Treatment, you'll find all sorts of information explaining the benefits of Cryogenically treating metals. If you're too lazy to search on google, I've tracked some links for those who seek more info on this......

300 Below

Brake Rotor Comparison

Scientific Explanation

Even the Police do it

After doing all the research I did, before I had my rotors cryo. I was sure that it was worth the investment ($80 for all 4). Last but not least, I did not say that Cryo treatment would improve braking performance. I said it will improve the life of your rotors and it will help fight cracking. But hey everyone is entitled to their opinion. But my statements on Cryo Treatment are fact not fiction.

 

[HADA 11]

I based my post on this comment for scR

Last year we bought 4 rotors. Two were bone stock, and two were subjected to a process know as Cryogenically Treating one of the high-tech buzzwords floating around the paddock. The rotors were run back-to-back on the same track on the same car on the same day with temperatures taken to make sure that they saw the same level of heat. Following the track session, the parts were removed and we had them literally dissected by a materials lab.

I shouldn't have stated that they won't help a drilled rotor. That said, I personally wouldn't consider the information a manufacturer of a product presents as fact. Is their motive to present you information so you can make an informed decision, or is it present information to get you to buy their product?

The WS6 guys did the same thing I'd probably do. They don't use they exact same circumstances for each "test", due to budget no doubt. I really don't think they guy even intended to do a test. The just looked at the two rotors after a period of time and came to a conclusion. I'm looking for a test (same conditions, same car, same track, one cryo rotor, one non-treated rotor) if I find anything other than what I posted above I'll post it here.

I really like the pics of the rotors on the WS6 site.

 

[pr5owner]

i have xdrilled and slotted rotors on my car, and i know they don't do jack but in the looks department... you cant beat them!

these rotors are all for looks since the p5 is slow to begin with and slow with mods.

 

[Brian MP5T]

Dude! 2003