Cross Drilled vs Slotted Rotors
Performance upgrades to increase horsepower are all well and good. But, if you can’t slow down all that extra power, you may be cruisin’ for a bruisin’. Braking is a critical component of any vehicle and should also be modified in accordance to the additional power you’re making. Upgrading the rotors to cross drilled or slotted rotors is a common modification. But, what’s the difference between cross drilled vs slotted rotors?
Above shows three different types of rotors. From left to right, they are a smooth rotor found on most stock cars, a drilled rotor, and a slotted rotor.
Before we get into the differences of these types of specific rotors, let’s first go over some braking basics. Since most of us have ridden a bicycle, let’s start with that analogy.
A bicycle’s braking system works in a similar way as your vehicle’s. You squeeze a lever connected to a brake line. That line contracts squeezing together a caliper. That caliper has pads on each side that pinch your rim. The friction caused by those pads pinching your rim are what slows your bicycle to a stop. Let’s compare that to your vehicle’s braking system.
In order to stop your vehicle, you start by pressing the brake pedal. That pedal is connected to a plunger that presses inside a cylinder filled with brake fluid. This fluid compresses inside the cylinder and forces fluid through the brake lines. At the end of these lines are calipers. Just like your bicycle, these calipers compress, but with the force of the fluid instead of a hard line. Instead of against your rim, these calipers compress against a brake rotor which is bolted to a hub connected with your wheel. The friction between the rotor and the brake pads brings the rotor, and in turn the wheel and your car to a stop. You can see how important the fluid in the system is. Anytime you complete brake upgrades that open the system, like replacing brake lines or calipers, you must bleed the brakes.
The greatest variables in the braking system outlined above in terms of performance (assuming equal pressure from your foot) are the pads and the rotors. These two components work together to stop your car. In the process, they generate tons of heat. The most significant benefit of slotted rotors over your stock rotor is how this heat is handled. To get a better understanding of this relationship it’s important to understand the different types of heat transfer. There are three different types of heat transfer occurring when your pads meet the rotors. These include conduction, convection, and radiation.
Conduction occurs when heat flows from high to low. A simple example would be touching a hot engine. The heat quickly flows from the hot engine into your (less hot) hand. Similarly, after burning your hand on the engine block, you put it in ice. This is also conduction as the heat flows from your hand to the ice. The energy flow will always be from hot to cold.
In your braking system, the hottest point will typically be where the rotor meets the pad. This heat will flow from this area from the pad into the caliper and from the rotor into the hub and wheel.
Convection is similar to conduction but is specific to liquids and gasses. Like conduction, heat will transfer from hot to cold. Convection occurs in the braking system as air contacts the rotors. The more air that contacts the rotors, the more heat that can dissipate. There are vanes in the brake rotors which allow airflow and promote convection. Additional airflow can also be had with holes by way of drilled rotors.
Radiation occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed. An example or radiation is heat coming out of a camp fire or the sun warming the earth. Unless your brakes are the same temperature or cooler than the ambient air, they will be radiating heat.
All three of these types of heat transfer are occurring as your braking system is working.
Drilled or Slotted Rotors
According to the braking experts at powerstop, during a typical stop, the heat transfer is about 25% conductive, 35% convective, 40% radiation. For a high temperature, high speed stop, the heat transfer is about 15% conductive, 40% convective, 45% radiation.
From this, you can conclude it becomes more important to beef up the convective heat transfer capacity if you’re driving the car hard at the track. Like most things in motorsports, running cooler is better, and your brakes are no different. If they get too hot, you can have brake fade or even brake failure. Also like most things in motorsports, there are trade-offs.
Having drilled rotors is a great way to promote heat convection in the rotor. It allows more airflow to pass through the vanes which helps cool the rotor. However, this can come at a downside. If you’re really pushing the car at the track there are extreme heat cycles in the brake rotors. This can cause cracking in the weak points of the drilled rotor between the holes.
Slotted rotors will not affect heat transfer but can improve braking output over the stock smooth OEM rotor. As the pad wears away on the rotor, it leaves brake dust. This dust staying on the rotor decreases the amount of friction between the pad and rotor reducing stopping power. The slots are there to channel away this dust and gasses that are a byproduct of stopping.
While drilled rotors are definitely a cool looking option and well suited for street duty, if you’re on the race track, slotted rotors would be a better choice.
Now that you’ve got the brakes taken care of, you might be interested in some suspension upgrades. Check out Coilovers, Air Suspension, and Lowering Springs: An Overview or Linear Vs Progressive Springs: Which is Better for Your Car.