Are drilled and slotted rotors better? The truth about brake rotors

Why you shouldn’t install drilled and slotted rotors

When it comes to automotive braking systems, the type of rotors you choose can significantly impact performance, safety, and longevity. Drilled and slotted rotors are often touted as superior to regular rotors, but are they really better? This article will explore the differences between drilled and slotted rotors and regular rotors, examining their advantages, disadvantages, and ideal use cases to help you make an informed decision.

Regular rotors have smooth, flat friction rings and are either solid or vented. On the other hand, drilled and slotted rotors have holes (drilled) and/or grooves (slotted) machined into their friction ring surfaces.

How brake pads and rotors stop your car or truck

A properly operating brake system stops your vehicle by converting Kinetic Energy from the rotor into Heat Energy through friction. The brake pad’s job is to resist the rotation of the rotor, creating heat in the process. and the rotor’s job is to dissipate the heat.

A deep understanding of brake system heat transfer

All automotive braking systems dissipate heat using all three methods of heat transfer:

• Thermal conduction— Heat conduction involves the direct microscopic transfer of kinetic energy between molecules. Any time there’s a difference in temperature between two brake components, the heat will always flow from the region of high temperature to the region of lower temperature. So, the conductive heat in your brakes flows from the hottest portions of the rotor toward the rotor hat and wheel hub on one side and the brake pad backing plate to the caliper body on the other. In most automotive braking systems, thermal conduction removes approximately 25% of the braking heat.

2) Thermal convection— Thermal convection in a brake system is created by airflow and aided by rotor ventilation. The cooling vanes on a ventilated rotor draw air in from the wheel hub area, pull it across the inside areas of the rotor plates, where it picks up heat, and then force it out from the openings on the circumference of the rotor.  In most automotive braking systems, thermal convention removes approximately 35% of the braking heat.

 3) Thermal radiation— Thermal radiation is the heat you feel by holding your hand near a hot brake rotor. Radiant heat is infrared. In most automotive braking systems, thermal radiation removes approximately 45% of the braking heat.

Why drilled rotors aren’t as effective as you think

Drilled brake rotors tout their ability to improve cooling by increasing convention, transferring more heat from the rotor plates to the air flowing through the vented portion of the rotor. As airflow in the vented area flows past the drilled holes, a low-pressure (vacuum) area forms in the holes, drawing in more lower temperature outside air. The outside airflow increases heat transfer through additional thermal convention.

That claim is true as far as it goes, but it ignores the physics of thermal conduction. Since conduction relies on heat transfer through metal molecules, reducing the mass of the rotor by drilling holes reduces its ability to transfer heat, which reduces braking ability.

The studies show that the loss of rotor mass due to drilling and slotting reduces heat transfer more than the convection heat transfer gained by drilling holes in the rotor. The loss of heat transfer directly equates to reduced braking. In other words, drilling holes causes a net loss in heat transfer and less braking.

More importantly, normal street driving doesn’t generate near the heat encountered in track conditions. In other words, for normal street driving, you want more rotor mass.

Drilled rotors provide uneven cooling. That’s why they crack

Hot rotors continue to cool when you’re stopped. As hot air rises through the cooling vanes, the air movement pulls outside air through the holes, creating uneven cooling around the holes, compared to the more solid areas of the rotor. The uneven cooling causes expansion/contraction stresses that crack the rotor. This isn’t rocket science. When you create small cooling holes all around a very hot rotor made out of cast iron you get uneven cooling that cracks the iron.

How about slotted rotors? Do they increase stopping power?

Slotted rotors don’t provide any benefit for street use. Here’s why: Slotted rotors are designed to reduce fade from off-gassing in heavy braking at high speeds for race cars.

The slots vent gas that accumulates between the older asbestos brake pads and the rotor. That layer of gas causes brake fade because the pad is no longer in contact with the rotor. Much like tire hydroplaning, brake fade is caused by the pad riding on a “gas bearing,” which dramatically reduces braking and causes pad glazing.

In addition to venting the off-gassing, the sharp edge on the slots also removes glazing by refreshing the face of the pad. In other words, just like a kitchen mandolin that turns a potato into potato chips, slots cut off a thin layer of pad material. So you always have fresh brake material touching the rotor at the expense of reducing pad life.

How big of a problem is off-gassing?

Early non-organic asbestos (NAO) brake pads that were glued (instead of riveted) to the backing plate used to have a big problem with off-gassing and brake fade. The resins and adhesives in those pads produced gas byproducts when heated to high temperatures in heavy braking. However, NAO pads aren’t used much anymore, and the adhesives have improved greatly since the introduction of ceramic pads.

Braking heat

So here’s the big question is: does stopping with ordinary brake pads and rotors generate enough heat to cause fade? Keep in mind that brake heat dissipates two ways; the majority of the heat is removed due to the pumping/cooling action of the rotor vanes, and the rest through the brake pad and backing plate dissipating heat back to the caliper and brake fluid. When brakes heat up to near the fade point, enough heat has already entered the brake fluid, causing it to boil.

If you want better braking, get better cooling

Rotor vane design is the single most important feature to look for in a brake rotor because the pumping action is far more effective than the slots and holes. Remember, the vanes are a centrifugal fan that sucks air into the rotor near the hub and throw it out the vanes on the rotor’s circumference.

Carmakers spend a lot of money and engineering time designing these “fans” and that’s what makes OEM rotors more expensive. Factory rotors generally have more mass than economy aftermarket rotors as well. More mass, bigger fans, more vanes all add up to better cooling.

Cheap aftermarket rotors don’t duplicate the factory cooling vanes. Instead, they use straight vanes because they cost less to produce. But you won’t get the same cooling.

Brake rotor cooling vane designs from cheapest to most expensive

©, 2016 Rick Muscoplat