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Drilled Rotors or Slotted Rotors - Which is best?
Perhaps the most debated issue on the subject of brakes is the question, “Should I put Drilled or Slotted Rotors on my vehicle?” This article is intended to explain the advantages and disadvantages of each type of rotor.
The primary job for the braking system is to convert the kinetic energy (moving mass of the vehicle) to heat energy. In order to understand the benefit of drilled rotors, it is necessary to talk about the physics of heat transfer. There are three different types of heat transfer: 1) Convection (increases with air flow), 2) Conduction (increases with mass and contact area so heat transfers from the rotor to the Hub, or the pad then to the caliper piston to the brake fluid), and 3) Radiation (the amount of infrared energy waves emitted by the rotor). The balance between these 3 types of heat transfer is affected by the speed of the vehicle. At lower speeds, the balance is Convection 30%, Conduction 30% and Radiation 40%. At high speeds the balance changes to Convection 35%, Conduction 20%, and Radiation 45%. Therefore, at higher speed, convection heat transfer (air flow) becomes very important to remove heat. Cross Drilling a rotor increases the convection heat transfer of the rotor.
The rotor is designed like a fan to draw air from the center and expel air through the vanes. Heat is the enemy of brake pads because it decreases pad bite. This is called brake fade. The drill holes allow hot air to flow out of the vanes and cool the rotor faster. According to SAE Technical Paper 2006-01-0691 “The Effect of Rotor Cross-drilling on Brake Performance”, the percentage difference in cooling in front rotors between 50mph and 80mph is 7.8% to 8.5%. This would be about 150 degrees reduction in temperature. There can obviously be too few cross drill holes in a rotor, but the study also shows that there can be too many as well. The optimum number of holes is right in the middle of the two extremes with about one hole per cooling vane in the rotor. The study also showed that drilled rotors prevent pad glazing (a contributing factor of rotor warping). One argument is that by reducing the mass of the rotor by cross drilling, you are reducing the cooling ability of the rotor. Don’t be fooled by this argument because the mass from the drill holes constitutes between .2 to .4 lbs. On a 20lb rotor, that equates to 1% to 2% reduction in mass, which has very little affect if any on heat transfer, i.e. maybe 2 degrees Fahrenheit. This is more than offset by the 150 degrees of improved cooling from the drill holes. So this argument does not carry much weight (pun intended).
On the other hand, cross drilled rotors do have some disadvantages. When any rotor is thermally shocked (a rapid change in temperature), it has the possibility of forming a crack, just like dropping ice cubes in hot water. Cross drilled rotors reduce the likelihood of such thermal shock during normal use, but if a drilled rotor does heat up to the point of thermal shock, it will be more likely to crack than a plain rotor. This is because the drill holes provide additional points of origin for cracks to propagate from. In my experience, only the most severe applications cause premature rotor cracks from drill holes. Cracking is highly unlikely for street driving. In general, I recommend drilled rotors for normal street driving. The extra cooling means more brake output and lower risk of rotor warping.
Brake pads throw off a lot of gas and dust. The gas is mainly the resin in the pad turning from solid to liquid to gas from the heat. The main advantage to slotted rotors is to remove the debris and gas to improve the contact between the rotor and the pad. Under a heavy stop or at high speeds, the slots in the rotor will constantly wipe against the pad, keeping a clean contact patch between the pad and the friction surface of the rotor at all times. This is advantageous on a track where the brakes are heavily used, creating large amounts of dust that needs to be evacuated between the pad and the rotor. Slotted rotors also are less likely to crack when thermally shocked compared to cross drilled rotors. Slotted rotors do not provide any cooling advantage over OE replacement rotors. If you think about the heat transfer properties of a rotor, there is nothing about a slotted rotor that increases the heat transfer rate. Slots will also increase pad wear from 10 to 20% faster. I recommend slotted rotors for heavy duty use like track racing, autocross, off-road driving and commercial hauling.
In summary, different rotors are used for different applications. If your vehicle is primarily used for street and highway, then I recommend drilled rotors for maximum brake performance. If you routinely thermally shock your rotors in a racing (or racing-like) environment, use slotted rotors with an aggressive pad compound. No matter what choice you make, performance rotors and pads are an excellent way to upgrade brake performance at a very reasonable cost.