Dear fellow Autopians, welcome to another edition of Ask An Engineer. I got a question a while ago from one of our astute readers who wanted to know why some high performance cars and high performance brake upgrades have rotors full of holes. What’s the point of all these holes? Do they actually improve braking performance or are they only there to look cool? Let’s get into somer nerdy physics to discuss how brake rotors work, and then we’ll delve into whether cross-drilled ones are worth your time.
I’ve been thinking about this question myself for many years, and I figured this would be a good time to find the answers. I thought that rather than develop my own theories, I better contact some experts. I contacted Brembo North America, the North American arm of Brembo S.p.a., and they were kind enough to indulge my questions. And while I wasn’t able to interview any of their engineers directly, they did respond to my questions in writing.
Why Cross Drill?
The term “cross-drilled rotor” refers to a brake rotor that has been drilled with a cross-wise series of holes. Most brake rotors have a smooth un-blemished surface where the brake pads sit:
A cross-drilled rotor, on the other hand, contains a series of holes in this area:
While the term “cross-drilled” suggests the holes are drilled into the disc after it is made, it is more common nowadays for the holes to be cast into the rotor as part of the casting process. Drilling holes can lead to cracks later in the life of the rotor, so most manufacturers create them as part of the initial forming process. This ensures the impact on the strength and durability of the rotor is minimized.
Most car enthusiasts I talked to about this see cross-drilled rotors as a cool upgrade. Some race cars have them so they must be good for the street, right? But beyond the esthetics and the fact that some race cars use them, are there engineering reasons why you should put them on your car?
According to Brembo:
Among other things, cross-drilling provides a pathway for gasses created during the friction process to escape. As most would expect, there is often an accompanying reduction in weight as well. Where aesthetics are concerned, depending on your year of birth or simply where you land on the topic, it’s entirely possible that nothing looks cooler behind a wheel than a cross-drilled brake disc.
Fair enough, but I wanted to know more. Cross drilling is a lot of work and expense to go through just for looks and to let a few gasses escape.
To better understand the impact of cross drilling, we need to dig into some of the engineering behind brakes, and look specifically at the function of brake rotors.
At a very fundamental level, the entire braking system in a vehicle is an energy conversion device. Its entire purpose is to convert the energy of motion onto heat. All objects, when they are in motion, have what is known as Kinetic Energy. It goes back to Sir Isaac Newton’s first law of motion, which states: “An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.”
The second part of that law is key here: An object in motion remains in motion… unless acted on by an unbalance force. By the way, don’t get weirded out by the words “unbalance force.” That’s just engineering speak for a force that pushes on the object in some way.
So, an object that is in motion wants to stay in motion because it has energy that wants to keep it that way.
Mathematically speaking, this “kinetic” energy is:
KE = 1/2 x Mass x Velocity²
Kinetic Energy is 1/2 the mass of the object times the square of its speed. You can think about it as the amount of energy it took to get it up to that speed. If something is heavy, it takes more energy to get it up to speed vs something that is light. And it takes more energy to get something up to a higher speed than a slower speed. Makes sense, right?
Once you have an object in motion, if you want to change that motion, i.e. if you want to change the direction it’s moving in, or you want to speed it up or slow it down, you have to provide the “unbalance” force that Newton was talking about. There are many ways to do this. You could push on the object sideways to make it deviate from the “straight line” it is traveling on. This would be like steering the wheels on a car, causing the tires to generate a sideways force. Or you could push on the car from behind to make it go faster (in effect, increasing its energy). Lastly you could push on it from the front to slow it down.
Pushing on it from the side won’t change the amount of energy the object has, it just changes its direction, and pushing from behind would increase its energy and thereby its speed. Pushing from the front would slow it down. But, you say, the kinetic energy equation contains velocity, so if we slow an object down, aren’t we changing its energy? To which I say, “You are absolutely correct, grasshopper. Very astute of you.” Slowing something down does indeed reduce its kinetic energy because we have reduced its velocity.
But now we get into another fundamental area of physics which is governed by what is known as the Conservation of Energy law. This law states that energy can neither be created nor destroyed. Wait a minute. You’re saying that all the energy I had after drinking that cup of coffee (which has little caloric value) this morning was already there? What about when I get tired? What happened to all my energy? Well, the energy you had this morning came from the steak you ate yesterday, which came from the cow that gave up its life for your dinner. The cow grew from the grass it ate in the fields and the grass came from the sunlight and the rain that fell on it. The sunlight came from the fusion reaction happening in the sun which came from the hydrogen molecules in the sun.
Those molecules came from the gaseous cloud that created the sun a few billion years ago, which came from other stars that blew up a few billion years before that. Eventually, if you trace it back far enough you get to the big bang and maybe even before that. So, all the energy you had this morning and all the energy you will ever have in your lifetime has always existed and will always exist in one form or another.
But, I’m talking here about stuff like hydrogen molecules and grass and energy as if it’s all the same thing. Well, that’s because fundamentally they ARE the same thing, and as proof, I give you Albert Einstein’s famous Theory of Relativity which states:
Energy = Mass x (Speed of light)²
This means that any object is actually nothing more than a bunch of energy, and if you could convert an object into energy, the Theory of Relativity tells you how much energy you could get.
Get To The Point Already
Well, that’s all very interesting, but this article is supposed to be about brake rotors, not high-level physics, so let’s get out of these weeds and back to the good stuff. You’ll see in a minute why we had to take this little diversion.
As we saw earlier, if we want to slow an object down, we need to reduce its kinetic energy. But we also learned that we cannot create or destroy energy. If we then want to reduce the energy of a moving object and we cannot simply destroy the energy it has, we need to do something else with it. We have to send it somewhere else. One thing we could so is transfer the energy to another object. This is what happens in a Newton’s Cradle children’s toy:
The energy of one moving ball is transferred through the other balls until it finds the last ball and gives it motion. That ball then moves until gravity brings it back and the cycle starts over again.
Brake Rotors 101
We could reduce the kinetic energy of a car to nothing by running the car into a tree or brick wall. In that case, the kinetic energy is converted into twisted metal, broken bits, and noise. But that’s not very practical, so that’s where brakes come in.
The brake system takes the energy of motion and converts it not into the motion of another object or into destruction and mayhem, but instead into heat. Brakes slow down a car by taking the cars’ kinetic energy and using it to heat up the brake rotors. And this is where the design and configuration of the brake rotors plays such an integral role.
A car is a pretty heavy thing, and when you multiply even half of that by the square of its speed, that’s a LOT of energy. Converting that into heat means we get a LOT of heat, and that heat has to go somewhere. Normally, there are two places where we can get put heat: we can store it in some object, like the brake rotors, or we can send it into the air.
Storing heat in the rotors means their temperature goes up, which is fine but there is a limit to how hot we can let the rotors get because the hotter they get, the harder it gets to put more heat into them, and eventually they just melt anyway. So, the preferred method is to transfer the heat into the air as quickly as possible, and that’s what brake rotors are designed to do. But sending heat into the air takes time, and when you need to stop a car RIGHT NOW, you don’t have a lot of time. So, the first thing that happens when you hit the brakes is the rotors heat up, and then, as you drive further, the heat gets sent into the air and out the back of the car.
Think of it as pouring water into a bucket that has a hole in the bottom. You want to pour as much water (which is analogous to heat here) through that bucket as quickly as possible. As you pour water in, it starts to leak out the bottom, but if the hole is relatively small, there will come a point where you can’t pour any more water into it and you have to wait until some or all the water drains out the bottom. Brake rotors work the same way.
But there are ways to make the bucket bigger and/or make the hole in the bottom bigger, figuratively speaking.
As you put heat into an object, like a brake rotor, its temperature will go up based on the formula:
ΔT = Q/(M x C)
The change in temperature (ΔT) is equal to the amount of heat you put in (Q) divided by the mass of the object you are heating up (M) times the specific heat of the material the object is made of (C). The specific heat of the material is something that is intrinsic to the material and doesn’t change, no matter how you design the rotor. For example, if the rotor is made of cast iron then the specific heat (C) is equal to 0.46 KJ/(Kg K). Don’t worry about the crazy units after the number 0.46. All you need to know is that this number is the same for all cast iron rotors. If your rotors are made of some other material, like carbon ceramic, this number will be different. The number represent’s the material’s capacity to absorb heat.
What we need to understand about this formula is that if we increase Q, in other words if we increase the amount of heat we put in, the change in temperature will get bigger. Makes sense, right? But what we also need to understand is that if we make the object bigger, i.e., if we make the mass (M) bigger, then the change in temperature gets smaller. In other words, if we put a specific amount of heat into a small, light object, the increase in temperature will be bigger than if we put the same amount of heat into a bigger, heavier object. That also makes sense.
Basically, if we make the rotor bigger and heavier, we have made our bucket bigger because we can put more heat into it before the temperature gets too high and the thing can’t take any more or melts.
The other part of rotor design gets into the way we get rid of the heat once it is in the rotor. Remember how we said we couldn’t destroy energy? Since we can’t just destroy the heat that’s in the rotors, we have to send it somewhere else and that somewhere else is the air. We need to somehow get the heat into the air so it can be sent out the back of the car and away from our rotors. That’s where brake cooling comes in. We need to get as much of our brake rotors in contact with the air so the heat can pass from the rotors to the air as fast as possible. Doing this would be like making the hole in the bottom of our bucket bigger so more water can pass through it more quickly and our bucket drains faster.
Let’s take a look at some rotor designs. Cast iron brake rotors come in two basic varieties: solid and vented.
Notice how the solid rotor has a relatively thin single disc while the vented rotor has two similar thickness discs connected together by a bunch of short posts or fins. These posts form a gap between the discs (called the cheeks) that is open to the inside of the rotor, as you can see here:
Having these gaps allows air to pass between the rotor surfaces and adds area to the back of each surface so more heat can transfer from the cast iron to the air.
Looking at these two different rotor designs, it is clear that the vented rotor contains a lot more material (i.e. is a much bigger bucket) than the solid rotor, and having the gap between the surfaces adds a lot of area (acting like a much bigger hole in the bucket) to transfer heat to the air.
Of course, the gap isn’t very useful if we can’t somehow provide a lot of air to the rotor so it can pass through it. In most street cars, this can be difficult to do since there are a lot of other things in the way: wheel bearings, dust shields, other suspension components. Getting lots of air into that area of the car can be difficult because we really want the air to pass smoothly over and under the car instead of up and inside the wheels.
Any time we divert air from moving smoothly over and under the car adds aerodynamic resistance, and we want to minimize that as much as possible for other reasons. Racecars, on the other hand, are much less sensitive to these aerodynamic problems, and you’ll find lots of methods used to direct air to the brakes to help cool them. This 1982 Lotus Formula 1 car used a plastic duct to direct air through the spindle to the inside of the brake rotors:
Other times you may see large hoses used to direct air to the brakes:
While they are great in racing, these aren’t really practical in normal street cars because the space simply isn’t available inside the wheelhouse area. Racecars have a lot more options in this regard.
We saw earlier how adding a gap between the faces of the rotor adds surface area and allows better cooling. But what if we could increase the surface area even more? That would make the hole in our proverbial bucket even bigger. If we drilled a whole bunch of holes through the rotor, each of these holes would add more surface area and allow even more air to cool it. I asked Brembo if this was true:
In conditions of heavy use, technically speaking cooling benefits of cross-drilling will increase or decrease relative to certain variables such as general disc design, the existence of vehicle ducted cooling, etc. In our years of racing experience, we have encountered certain applications where there has been evidence indicating measurable benefit with regard to operating temperatures. In other cases, we’ve not seen this. Again, any benefits realized tend to be application and usage specific. There are simply too many variables involved to have a “one size fits all” answer here.
But, drilling holes in our rotors removes material and we saw in our temperature formula how important mass is to helping keep the temperature down. I asked Brembo about this as well:
In general, high performance road going, street legal applications this is not a factor. On the other hand, if you have a vehicle which is set up for pure racing conditions while using OE sized brakes, an argument could be made that issues might be possible with a loss of thermal mass. Whichever application you have, it’s important to note that there is a right way (and several wrong ways) to cross-drill a brake disc. Details matter here – discs which have not been cross-drilled using the correct technique bring a level of risk regardless of intended usage.
So, cross drilling CAN be a benefit under the right circumstances. The added holes do add surface area, but that is only useful if we can provide the needed airflow so enough air can move through those holes to make a difference. In a racecar, there are many possibilities to make that happen but in a normal street car, we are stuck with the airflow we have, and it is usually minimal at best. The loss of mass in a rotor could, theoretically, be a bad thing, since mass helps absorb more heat, but experts don’t think that’s significant enough to matter in a street car. So the answer of whether cross-drilled rotors are useful clearly depends on the application.
Dust and Gasses
In the answer Brembo provided to my first question, they mentioned letting gasses escape. I wanted to find out more about this, and also about how the dust that is normally produced during braking is cleared away from the pads. Do cross drilled holes more effectively help clean this dust off the pads?
In our experience, pad life tends be overwhelmingly determined by things like temperature, speed, pressure, and time. That said, any disc which has been machined correctly and provides a pathway for friction dust and gasses to escape, will typically promote more consistent pad performance and overall life in high performance use.
So, cross drilling does help promote better pad life as long as it’s done correctly.
Cross-drilling isn’t the only way to help clear dust, though. Another way to allow gasses and dust to escape is by using slots:
I have used these in my suspension designs in the past and wanted to know which was better, slots or cross drilling?
For the vast majority of street legal vehicles, high performance or otherwise, if you’re looking at a “Brembo” disc I’d say you’re free to simply choose the one you like better. When you get to professional level, purpose-built race cars where all aspects of cooling, thermal capacities, etc. have been optimized, then we tend to find that different slotting designs offer us some level of control with regard to things like pad engagement, release, and overall life. This is why slotted discs tend to be more prevalent among certain realms of the racing world.
In summary then, cross drilling CAN be helpful under the right conditions and in the right suspension and vehicle environment. In general, though, these conditions do not exist in normal street cars so the question then becomes “does it hurt performance” and I think the answer to that is almost certainly: “no.” So it boils down to personal preference. Do you like the look of cross-drilled rotors? If you do, then go for it. You do you. But don’t expect to get a performance benefit. If that brake system upgrade you installed on your Mustang or BRZ with the cross-drilled rotors made the brakes feel better and stronger, it was probably due to some other change that came as part of the kit: bigger rotors or more aggressive pads, for instance.
But even if you didn’t get a performance upgrade, cross-drilled rotors do look cool!
Got a suspension question? Send it to AskAnEngineer@theautopian.com
[Editor’s Note: We made a little mistake (unrelated to the core of the article) in an earlier version. It’s gone now! Mistakes happen. Thanks! – JT]