Cross Drilled, Slotted, or Stock?

Many companies advertise high performance braking parts, with one of the most popular parts being brake rotors. Tuners have the choice of choosing either cross drilled rotors, slotted rotors, or staying with the same basic design that the vehicle came with.

Cross Drilled Rotors

Cross drilled rotors were developed initially to allow “degassing” or to “outgas” the brake pads. Older vehicles were equipped with brake pads comprised of a compound that would release a gas when it reached a specified temperature, which induced a problem in braking performance. This gas would become trapped between the pad surface and the rotor contact area, creating a sort of lubrication effect and lowering the overall friction coefficient of the braking system. Keep in mind that friction is what allows a braking system to do its job. The drilled holes in the rotor would allow a place for these gases to escape to. The gases would enter the holes, and then be released to the atmosphere through the vents in the rotor. Another beneficial effect of cross drilled rotors is that air flows through these holes (which increased surface area), allowing the temperature of the rotor to be reduced at a quicker rate. Up to this point cross drilled rotors seem to be ideal; however this is where things change.

Since the early 1950’s automotive manufacturers have stopped using pad compounds that created this gassing effect when temperatures are increased. Since this compound is not in use anymore, there is no longer any gas trapped between the pad and the rotor which renders the “degassing” effect useless. Secondly, when a solid vented rotor is taken and holes are drilled into it, the overall structural integrity of the brake rotor is reduced. While this is not immediately apparent, the overall probability that the rotor will crack in the long run is higher than that of a stock rotor. Cross drilled rotors also have a higher operating temperature than their OEM equivalent. While the air flowing through the holes allow the rotor to cool down quicker, it is important to remember that because so much material of the rotor is missing, it is unable to absorb the same quantity of heat from the pads as it would if there were no holes. Keep in mind that one important characteristic of a brake rotor, is it must dissipate heat from the pads to the atmosphere. When a driver feels a soft pedal due to “overheated brakes” what the driver is actually feeling is overheated pads (we will omit the probability of boiling brake fluid). What brake rotors do is absorb as much heat from the pads as possible and then dissipate it to the atmosphere. While cross drilled rotors are more effective at dissipating the heat to the atmosphere, they are not capable of absorbing the same amount of heat off of the pads. In the end, it can be argued that the pads and rotors end up operating at higher temperatures. Lastly, a cross drilled rotor increases stopping distances when compared to its OEM equivalent. This is primarily due to the fact that the surface area of the rotor is reduced, and there is now less area for the pad to bite on to. Based on all of this info, it is shown that when a cross drilled rotor is compared to its OEM equivalent, the OEM design surpasses the cross drilled design in every aspect. However, there are situations where cross drilled rotors are “acceptable”.

As a quick review of the cons of cross drilled rotors, we have:

- decreased structural integrity
- higher operating temperatures due to less mass
- increased stopping distances due to less surface area

What most manufacturers have done to overcome this is they would increase the overall size of the rotor/pad combination, drilling the holes smaller, and giving them a CAD coating. What this does is it increases the mass of the rotor, allowing more heat absorption from the pad as well as providing more “bite” area for the pads. The oversized rotor may also be thicker as well as having smaller holes, increasing the structural integrity. The cons of going with an oversized rotor are as follows:

- oversized pads and sometimes new calipers must be used to make use of
- increased surface area
- new wheels may be needed
- all four brakes must be upgraded equivalently in order to keep brake bias as originally engineered
- significantly greater unsprung weight, resulting in slower acceleration

When one actually takes a step back and wonders if it is worth it to go through all of this complications, it is likely that most people once informed will decide against it. Especially because the holes are not needed for the pad compounds used in brakes today. However, a pair of CAD plated cross drilled rotors are definitely aesthetically pleasing, and for some people that in itself is enough reason to purchase them. Although they would have a shorter lifespan, for street driving they would suffice. It is just pertinent that the driver/tuner isn’t fooling themselves into thinking that swapping over to a set of cross drilled rotors is a performance upgrade.

Slotted Rotors

Slotted rotors share many of the characteristics of their cross drilled brother:

- “out-gassing” effect (which is useless with current pad compounds)
- increased stopping distances
- increased operating temperatures
- decreased structural integrity

What they do differently than cross drilled rotors, is that they increase initial bite in wet and dry conditions. This is a result of the pad ‘grabbing’ into the slots. This increased bite also increases pad wear and operating temperatures. Slotted rotors are similar in nature to cross drilled rotors, and while they are an upgrade in certain scenarios, they are not in others.

Blank Rotors (OEM Design)

This is ideally the best choice for a street car that is a weekend racer. These rotors are able to absorb heat well and perform consistently while maintaining its structural integrity, especially when a high quality TUV certified rotor is used (such as Brembo blanks). An optimal setup for a weekend racer would be a high quality pad, high temp DOT3 brake fluid (such as Ford DOT3 fluid), and a set of stainless steel braided brake lines, on a set of Brembo blank rotors. This setup will offer a higher friction coefficient, greater resistance to high temperatures, durability, confidence inspiring predictable pedal modulation (depending on pad choice) and does not break the bank. Not to mention that it is also perfectly streetable, and makes for an overall more enjoyable driving experience. There is just one last question we must address, because I am sure someone is thinking this.

Why do some race cars have slotted or drilled rotors?

There are a few reasons that you will see these kinds of rotors on Porsches and other high end vehicles. One important reason is the better initial bite for slotted rotors. Also, to ensure that the rotors are durable as well, they are manufactured with the holes cast into them. Unlike having the rotor start off blank and then getting drilled, having the rotors cast with the holes in them allows them to be more durable than a drilled equivalent. These high end manufacturers also spend large amounts of time engineering their rotors, optimizing rotor thickness, vent direction, hole diameter, rotor material, ect. In the end, this is a completely different rotor than one that was simply taken off the shelves and placed in a drill press. When considering everything that is being said, the user needs to decide on their intentions with their vehicles, and with the information provided make a decision that best suits their needs.


Thanks for reading and good luck!
- Chris De Freitas

Copyright May/05 – Chris De Freitas