Brake rotors are probably the most misunderstood component of common brake systems. A lot of it is due to rotor remanufacture’s (notice the ‘re’) trying to hype up drilled rotors, so they can sell them to you for twice what they paid for them blank. It is not uncommon for someone to swear they have increased stopping distances because they switched to a cross-drilled rotor. This is not the case, as drilling does not effect brake torque. I will go into this further, once we determine what exactly the function of rotors is.

What do brakes do?

I am sure everyone knows that brakes stop the car, but the question is how exactly do they stop the car? Simply put, they convert energy. The brakes convert the kinetic energy of the car into heat, via friction, which then slows the car down. Since heat is the key factor here, it is not surprising that lots of brake components usually have something to do with proper cooling and heat ranges.

Brake torque (what he measure stopping power by) is

(Disc radius * clamping force * coefficient of friction) / 12

As you can see, the only way to increase stopping power via rotors is to move to a larger rotor.

What is the rotor made out of?

In most cases, rotors are constructed out of cast iron. Because of this, the coefficient of friction of the rotor itself is rarely discussed, as they are all almost identical. Because its rare for anyone to use anything other than a cast iron rotor on a Honda, I will limit this article to cast iron rotors.

The rotor has two main functions. First it provides a friction surface for the brake pads to be clamped on. Second, it deals with the heat created during braking. There are two aspects to dealing with heat, absorption, and dispersion.

As I stated earlier, brakes function by turning kinetic energy into heat. So obviously the rotor plays a very important role in the braking system. Any flat surface can provide a friction surface for clamping, however not all rotors are created equal in terms of dealing with heat.

Lets look at the two aspects of heat closer.


Heat Absorption

How much heat a rotor can absorb is dependent on its mass. The larger the mass of the rotor, the more heat it can absorb. This is often referred to as “thermal storage”. Obviously more is better when you are only concerned about braking ability. But what happens when you run out of thermal storage? Or your brakes heat up to the point where they can no longer store heat?

This is called brake fade, and one aspect is the rotor being unable to store any more heat. The coefficient of friction of your brake pad is going to drop dramatically, as the rotor can no longer store any more heat and the heat is going to radiate into your fluid, pad, and calipers. It can get so bad that pressing the brake pedal provides little to no decrease in speed. Rotors can also warp, crack or fracture under these extreme heats.

In a performance-driving standpoint, your life is in your brake systems hands. It’s not hard to see that having enough mass on your rotor is important.

Heat Dissipation

Heat dissipation is governed mainly by surface area. Rotors radiate heat through their surface area. Radiation is the transmission of thermal energy via waves. The more surface area a rotor has, the more heat it can radiate. Further, the surface area is constantly being cooled by airflow. Having fresh available airflow to the rotor is very important in dissipating heat. Air cooling the rotor is refereed too as Convection Cooling. Convection is heat transfer via fluid. In brake systems, air can be considered a fluid.
Vanes

This is one of the reasons why we have internal vanes in our rotors. While having two disc surfaces connected by vanes increases surface area (instead of having a solid disc with two open sides, you have two discs and thus four open sides). The vanes are also designed to suck air in while the rotor is moving, absorb heat, and be shot out the other side. The air moving through the vane absorbs thermal energy along its path, and thus cools the rotor.




There are three common types of vanes

Straight Vanes –

Straight Vaned rotors are used on most production vehicles use. They are cheap to make, and can be used on both sides of the car. Straight vanes are exactly like they sound, straight lines going from the center of the rotor to the edge. They provide little increase in airflow through the rotor.




Curved Vanes –

You are more likely to see curved vanes on a carbon rotor used on serious racing vehicles. The vanes are curved, instead of straight. The curves are designed so they act almost like an air pump, sucking in air and pumping it out. The vanes are often overlapped to prevent possible thermal cracking. These are much more expensive to manufacture and must be mounted directionally.




Differential Vanes –

Differential vanes have all different lengths, which are determined by flow testing. The testing discovers what varying vanes allow the greatest volume of air to flow through the rotor, and thus aid in cooling.

As of now, I am unaware of any rotor manufacture that mass-produces any curved or differential vaned rotors for hondas.

So more surface area is better for radiating heat, more mass is better for absorbing heat, and more airflow is better for convection cooling.

Rotor Upgrades

The most commonly seen upgrade on rotors is to move to a drilled and slotted rotor of the same size. What people often fail to realize, is that set of “powerslot” or “powerdrilled” rotors are actually going to decrease performance.

Drilled and slotted rotors were originally designed to battle gasses that were given out when brake pads were heated to extreme temperatures. That gas would get trapped between the pad and the rotor, and become a barrier causing brake fade. The slots and drills gave the gas somewhere to escape. Modern pad technologies have allowed us to use materials that no longer give off gas. Further, quality brake pads have a slot designed into them, to allow any gas or vapors to escape.

The first problem with these rotors is you are sacrificing mass for surface area. While yes, drilling and slotting can increase surface area; it’s always at the sacrifice for mass. On the stock sized rotor, you really need all the mass you can get. As I stated earlier, once your rotor can no longer absorb heat, you are going to have a very hard time stopping.

Some times, the drilling does not even increase surface area… and it can even DECREASE it. When drilling a hole, you essentially create a cylinder. You gain surface area on the cylinder walls, but lose the face.

Surface area = 2*pi*r*h - 2*pi*r^2

If the radius of the drill is equal to the thickness of the rotor, you gain no surface area. If the radius is smaller, you gain surface area. Finally, if the radius is larger you lose surface area.

Example 1:

2 * 3.14 * 8 * 8 – 2 * 3.14 * 8^2 = 401.92 – 401.92 = 0

Example 2:

2 * 3.14 * 6 * 8 – 2 * 3.14 * 6^2 = 301.44 – 225.08 = +76.36

Example 3:

2* 3.14 * 10 * 8 – 2 * 3.14 * 10^2 = 502.4 – 628 = -125.6




If a rotor is to be drilled, it must be done correctly. The edges should be radiused or chamfered (To cut off the edge or corner of; bevel) as well as peened. Finally it needs to be stress tested to insure the drills can hold up to the heat with our cracking.

The most important thing to keep in mind with drilled or slotted rotors is eventually they will fail and crack. There is no way to drill a hole in the rotor with out reducing its structural integrity. You just have to hope you catch it soon enough and you are not heading into a turn at 90mph when it happens.




However, there are some advantages to drilling and slotting when done correctly. Keep in mind, you will not see any worthwhile benefits on stock sized rotors. (Which need all the mass they can get).

The edges of the slots and drills provide enough of a slight lip to increase initial bite of the pad. This is not going to be a dramatic increase in braking power, but it will give a nice initial feel.

Drills and slots can keep the surface of the pad clean. However this is rarely beneficial outside of rally racing.

Drilling and slotting can increase surface area. Again surface area radiates heat, and thus can provide better cooling.

Drilling and slotting does decrease some unsprung weight. This is hardly an advantage, as you are much better off spending money on high quality lightweight rims than something that directly effects your braking ability. Once you have spent all the money possible on lightweight suspension components and rims, then you can consider the very slight decrease in weight of a drilled rotor.

Ultimately, drilled and slotted rotors are worthless on stock sized rims. People often have the misidea that drilled or slotted rotors decrease stopping distance. This is incorrect. As the formula for brake torque shows, drilling and slotting rotors does not increase brake torque. Further, most people buy drilled and slotted rotors for street use, thinking they will increase cooling. Ultimately I don't care who you are, you are not going to continually heat up your brakes to the point of needing more surface area, on the street or durring an AutoX.

The best upgrade is to move to a larger rotor, which not only increase brake torque (due to the increase in leverage), but also greatly increases surface area and mass.