When it’s time to replace your brake rotors, you’ll undoubtedly find that there are tons of aftermarket options, particularly if you are interested in increasing the performance (and aesthetics) of your braking system. The most common types of aftermarket rotors are designs that feature cross-drilled holes, grooves, slots, or a combination of these features. These differ from most factory rotors, which are usually solid. In this article, we will explain brake rotor function as well as some of the pros and cons of solid, slotted, cross-drilled, and grooved brake rotors as well as the applications they are best suited for.
Function of Brake Rotors
Brake rotors are critical components that work in concert with other components in the braking system to bring your car to a stop when you depress the brake pedal. The brake rotors rotate along with your wheels, whereas your brake calipers are fixed relative to your vehicle’s suspension. When the brake pedal is applied, your brake calipers clamp your brake pads onto your brake rotors thanks to hydraulic pressure transmitted through your brake lines, which creates the necessary friction between the rotors and pads to slow the rotation of the wheels until your car eventually comes to a stop.
When it comes to designing a brake system, it is relatively easy to design a system with frictional properties capable of bringing the vehicle to the stop quickly. However, brake system design gets much more complicated when intended driving includes repeated frequent aggressive stops, such as the type of braking that is done when driving on a race track or on hilly roads that vary significantly in elevation.
The reason for the increasing levels of design difficulty in braking systems is that with each successive braking event, more and more heat is generated due to the friction between the pads and rotors. A well designed brake system dissipates this heat efficiently, or in other words keeps your brake system cool. The more frequently and more aggressively one stops, the harder it is for a brake system to dissipate this heat build up fast enough. With too much heat build up, braking components may provide reduced performance or outright fail. In the case of a brake rotor, this failure might manifest as warping, cracking, or glazing.
Which Are Best – Solid, Vented, Slotted, or Cross-Drilled Brake Rotors?
When it comes to brake rotors, many people debate the pros and cons of solid, vented, slotted, and cross-drilled rotors. Each has their trade-offs, with the main considerations being braking performance, part longevity, and brake system reliability. Below we break down each type:
In practice, these types of brake rotors are used on the vast majority of light passenger vehicles whose brakes systems are extremely unlikely to be subjected to significant heat stresses. As a rule of thumb, brake systems designed for solid rotors normally operate in a temperature range which is not high enough for friction films or gas cushions to develop under typical driving conditions, even if they are used with metallic or organic friction materials.
Therefore, solid brake rotors provide reliable, consistent, and satisfactory braking performance with most stock brake pads. Perhaps the biggest advantage of solid rotors is that they are significantly cheaper than any other type of brake rotor due to the ease with which they can be manufactured.
Vented rotors consist of two solid discs of metal that are separated by a series of vanes whose function it is to channel hot air from the center of the rotor to the outside edge as a means of cooling the rotor.
Vented rotors are typically used on applications where brake temperatures are expected to be higher than a typical brake system designed for solid rotors. This is common on vehicles that regularly tow heavy trailers, to boats, or otherwise carry heavy loads. Vented rotors are also commonly used on high-end passenger vehicles and high-performance vehicles that are expected to be driven aggressively.
Vented rotors are becoming increasingly common as car performance continues to evolve. Although vented rotors do afford a driver with increased performance, this also comes at increased cost, as vented rotors typically are more costly than solid rotors.
With a few exceptions, slotted rotors (also called grooved rotors) designed for high-performance applications are usually vented in addition. However, in real terms, the primary function of slots in brake rotors is to reduce the formation of brake fade due to glazing to ensure brake system reliability is maintained.
During heavy operation of the brakes that leads to high temperatures, brake pad glazing can become a serious issue. When a brake pad becomes glazed, the heat damages the pad surface by making it smooth (imagine the surface layer on the pad melting), which can cause a reduction in friction between the pad and rotor.
To prevent this from happening, the edges of the grooves/slots in rotors effectively scrape the surface of the pads clean, which removes any glazed material that could contribute to brake fade, thereby reducing degradation of performance due to high heat conditions in the brake system.
While the functionality of slotted rotors makes them sound like a no-brainer, there is a significant trade off. The continuously high rate at which material is removed from brake pads translates into extremely high rates of brake pad wear when using slotted rotors, and thus it is not uncommon to see new brake pads wearing down much more quickly than a comparable pair of vented or solid rotors. Slotted rotors are also usually sold at a premium price that is typically higher than vented or solid rotors.
While some vented grooved/slotted brake rotor designs feature cross-drilled holes, the holes do nothing to improve braking performance if the rotors are not fitted to brake systems that typically operate at extremely high temperatures.
These kinds of temperatures are typical on supercars and highly modified race and track vehicles that can suffer the formation of gas cushions in their brakes. Gas cushions form when high brake temperatures cause some brake pad ingredients to decompose at very high rates. In practice, the gas accumulates in the contact area between rotors and pads, and in severe cases, sufficient quantities of gas can be created to form an opposing force to the brake pressure that is powerful enough to push the pads off the rotor surface.
This phenomenon can result in extreme loss of braking efficiency, with predictable consequences. Much research has shown that the only effective way to mitigate gas cushions is to drill holes cross-wise through rotors, to allow the gas to escape into the space between the two friction surfaces, from where it is vented to atmosphere.
Viewed objectively, and given that cross-drilled holes in brake rotors have a very specific function, there is limited practical advantage to fitting drilled rotors to vehicles whose brakes will never reach those temperatures, even though such rotors might enhance the appearance of a normal roadgoing passenger vehicle. Like slotted rotors, cross-drilled rotors are also typically much more expensive than vented or solid rotors.
The issue of whether or not to install vented, grooved, slotted, or cross-drilled rotors is one that should be approached carefully as each type has both advantages and disadvantages. When weighing the improvement in performance, it’s also important to weigh the initial cost and the impact to longevity of brake system components. Both the intended application of the vehicle and the average temperature of the brakes during use should always be the guiding principles when making this decision. If you don’t understand the nature of your brake system, installing fancy rotors could well cost you significant money for no improvement in braking. In rare case, you may even reduce braking performance slightly depending on your application.
We hope you have found this article helpful. If you are looking for a set of rotors, check out our article on the best brake rotor brands to save yourself some time during the search! Similarly, if you are planning a project involving your car’s brake system, make sure you have the best brake bleed kit and a good caliper wind back tool to make the job as easy as possible!