Which is Better, a Slotted Brake Rotor or a Drilled Brake Rotor?

Slots vs. Drills - The War Rages On

To answer this, we need to have a bit of a history lesson, so let's take a few steps back. A few decades ago, engineers realized that maximum brake performance wasn't possible in certain situations, so they set out to determine what contributed to these incidents of reduced brake performance. Was it the brake caliper, the brake rotor, the brake pads?

What they found was that even the best street or track brake pad material used at that time had a tendency to create gasses as the friction material heated up. These gasses wanted to escape, so they would build up and apply pressure against the brake pad, thereby reducing the amount of pad pressure being applied to the rotor surface. The result of this was a reduction in brake friction and effectiveness. This situation became known as brake pad ‘outgassing’.

To remedy this, people began drilling holes in their brake rotors to allow the gasses to escape more easily without fighting against the brake caliper clamping force. The drilled holes were quite a success at reducing the issues associated with outgassing, as well as providing a minor weight reduction to the brake rotor itself. Additionally, it was found that the holes also provided some benefits when it came to clearing water and other debris from the brake rotor surface.

But, it wasn't all unicorns and rainbows.

Soon, it was found that in high performance and race car brake applications, rotor life was reduced from the drilling process. The holes didn't affect the life of the rotor from a wear perspective, but they did increase the incidents of brake rotor cracking. The holes that were drilled were often cut without much regard for the internal structure of the brake rotor, so more often than not, the holes actually weakened the brake rotor, which is certainly not something you want for your racing brake system.

It was also noted that much of the rotor cracking was emanating from the holes themselves. As it turns out, the holes also provided a temperature differential from the air in the holes, to the rotor’s overall temperature, and this difference invites cracking due to the varying expansion and contraction of the iron. And lastly, one other downside to drilled holes is the overall reduction in actual contact area between the pad and rotor reducing the

overall friction coefficient.

For all but the most extreme scenarios, the benefits of the holes still outweighed the negatives, but there was much room for improvement. So, along came the rotor slot.

The rise of more accurate machining tools and CAD design, along with CNC machines were exactly what was needed. The accuracy and repeatability that comes from modern machines allowed us to cut precision grooves, or slots, into the rotor face. These had the ability to solve the outgassing issue, as well as clearing debris, all without weakening the rotor. Plus there was an added benefit from the slots, they did a great job of deglazing the brake pads. (Which for the sake of this conversation is essentially scraping excess material from the brake pad, revealing newer pad material for maximum friction.) So slots seemed to be the answer, and people began cutting all types of slot patterns into rotors. Straight slots, curved slots, short slots, long slots. Well, you get the idea. But slots had one more skill to share.

Today, we have very powerful tools to help with product engineering, and for brakes, FEA (Finite Element Analysis) simulations have become an essential tool. With this tool we have learned that slots cut into a brake rotor have a role in the way the brake rotor itself transmits and dissipates heat. Which is the rotor’s primary job by the way. So while there are many ways to design an effective slot for outgassing, slotting for maximum heat balance is a different animal.

Maximizing the heat management within the rotor is what drove our development of the TW3 slot design. While it does give a nod to our winged logo, its design is all business. The TW3 slot design is non-directional so it works on either side of the vehicle, and it provides additional benefits by equalizing the temperature across the rotor face. Without this, there can be hot spots on the rotor faces where pad material can build up and lead to an uneven friction surface causing pedal pulsation. Or if left long enough, uneven wear or Disc Thickness Variation (DTV), which also shows up as pedal pulsation. Which most people mistakenly assume is a warped rotor. Sparta’s TW3 slot design reduces these issues leading to longer trouble-free operation of your brake system.

In summary, slotted rotors can provide all of the benefits that a drilled rotor can, with none of the drawbacks. So hands down, a slotted rotors is a better choice from a quality perspective. The only advantage drilled rotors have over slotted rotors is the cost to manufacture them. It is significantly less expensive to drill a rotor than to precision machine slots into one.

We hope this helps you better understand the differences between slotted and drilled rotors.

Keep an eye out for upcoming blog posts where we go into more depth on the features and design elements that make Sparta’s brake products unique.

To get your hands on our TW3 slot design, visit our web store!