There are many claims as to the benefits of drilled vs slotted rotors on stopping power. This guide is intended to provide some facts about drilled and slotted rotors. As a member of the SAE (Society of Automotive Engineers), I was pleased to see a paper "The Effect of Rotor Crossdrilling on Brake Performance" by two GM engineers published in 2006. They examined three vehicle platforms with cross-drilled rotors vs standard rotors to measure convection cooling capability, fade characteristics, wet braking, pedal feel and lining wear. The result is summarized as follows:
1. For the sports sedan, the coefficient of friction was 21% higher for drilled rotors than standard front rotors at 340F and higher using 15 brake snubs at 62mph. The track simulated 124 mph fade test showed 37% better brake output for drilled rotors. The drilled rotor brake temperature was about 150 degrees cooler.
2. For the performance car, the coefficient of friction was significantly higher for drilled rotors especially at high temperature.
3. Wet braking at high pedal pressure was the same for drilled or standard rotors. Wet braking is not significantly improved by drilled rotors.
4. Pedal force was much more consistent with drilled rotors over the brake temperature range. That is, to stop at the same deceleration rate, the driver does not need to modulate pedal pressure based on different brake temperatures. This reduces driver fatigue and improves brake response.
Totally dependent on the friction material used. A pad with more organics will do this, a high temp semi-met or full met won't show a difference with drilled vs. non-drilled. They wanted to show the greatest benefit and used the worse case situation. I know because I ran those types of tests.
The rotor temp drop is also dependent on where the thermocouple is. OD is cooler, ID is hotter.
Does anyone else feel that a 124 mph fade test is a little not real world?
The authors also reported that drilled rotors prevent pad resin glazing on the rotor. So we now have solid evidence that drilled rotors have benefits over standard rotors. However, I have not found any published paper to show how slots affect brake output. So I reviewed inertial dynamometer tests using ISO NWI 26867 from Link Testing in Detroit with slotted rotors vs standard rotors. The results showed no significant difference in the coefficient of friction during the fade sections, hot stop section or pedal sensitivity portion of the test. My hypothesis is that slotted rotors do not contribute to rotor cooling whereas drilled rotors improve convection heat transfer to cool rotors and reduce brake fade. I should also point out that the pad lining wear for the slotted rotor was very severe during the test, i.e. the pad was chewed up over 20% more than the lining with stock rotors. While I believe that slots will help remove gas and debri from under the pad, I am not sure that this has a significant effect on brake torque for normal street driving. Perhaps the effect of slotted rotors is more significant on the race track, and conversely, I believe that drilled rotors are better for street and highway driving. For most drivers, I recommend drilled rotors over slotted rotors, and this conclusion is supported by the fact that Corvette, Ford GT, Porsche, Mercedes and BMW come with OEM drilled rotors.
Coming from a guy who has a business selling cheap drilled rotors on eBay.
Here’s a short thread from a Corvette forum that I remembered from about a year ago:
Brake motive brake package - Corvette Forum
Slots do not contribute to cooling, they are there for friction material gas removal and can contribute when rotor temps get to 350°F IF
the pads composition starts to degrade at that low of a temperature like some organics do. With good semi-mets, you have to get to 1200-1500°F. Sometimes higher with more race oriented compositions.
Our rotor tests showed that both slots and holes will increase friction material wear unless you use a composition that is stiff with little compliance, the type of pad that was shown in an SAE paper by Ferodo that contributes to rotor hot spotting. One reason that compliant organic or low-met pads won’t show glazing after the test.
Here is another rotor discussion a while back on the WRX forum:
The real deal about cross-drilled and Slotted Rotors - IWSTI.com: Subaru WRX STI Forums
Not that it is the final word on rotors, but that there are so many interpretations.
BMW talk: The Effect of Rotor Crossdrilling on Brake Performance - E46Fanatics
WRX- you know the vehicle with the fake hood scoop because it sells “high performance”. Like factory cars with drilled rotors.
I started a response to this thread a few days a go, but pulled back as this always gets into a P**** contest. I'll add what I had:
I’ve stayed away from this for a while and against my better judgment I’m jumping in.
Slots and grooves in rotors have their place, but not for the reasons that people marketing them usually propose. And as with everything, there’s a dark side.
One of the arguments for the holes is for cooling. Well, lets talk about the rotors on these trucks.
The rotors are built with 4 components - 2 rubbings discs, (cooling) vanes, and a hat. The rubbing discs are where all the heat energy is generated and first absorbed, since deceleration is the conversion of kinetic to thermal energy. Rubbing discs are not that dissimilar to a container of water.
Lets say in a blacksmiths shop we heated up a block of metal about 3” square cherry red and forged it to the exact shape we wanted. We have this half gallon can filled with water sitting next to the bench and throw the metal block into it to cool the block down. The can doesn’t have a lot of water mass so A) the water is going to get pretty hot, and B) the block is not going to cool that much. You’re not going to pick the block up and go through the hot water with you hands in 30 seconds to a minute.
But if you threw that block into the horse’s 50 gal water troth outside the shop, there is sufficient water mass to absorb the heat energy without raising the water temperature too much. While both containers hold the same material, the one with the larger mass has the lower temperature rise. And because the water surrounding the block stays cooler, the block gets cooler as well.
So by boring a lot of holes in the rubbing discs, we’ve reduced their mass, which for a given thermal input will raise the temperature, just like that little pail of water.
But it’s the increased cooling they provide during that all-important stop which offsets this!
Really ……? Surprisingly, there is mostly thermal absorption and moderate cooling during that one important stop. When making a stop, we’re generating heat energy, absorbing heat energy, and as the temperature is rising, we’re starting to dissipate the heat into the air. But during that all one important stop we’re decreasing speed and rotor rotation.
A rotor cools by drawing air into the vane area at the hat or hub region and expels the heated air at its outer diameter by centrifugal force. The vanes are little fans. And as the temperature rises in-stop, the fans are slowing down and not drawing through as much cooling air. You remember the trick Mom taught you; blow over the spoon to quickly cool the hot soup. Velocity is king. Or in the case of Mom, queen.
But I’m going down the mountain pass and not stopping, just keeping the vehicle at a constant speed.
Good point. So here we have the little vane channels acting as the HVAC on a hot day. And just like twirling a ball on a string over your head, it’s the OD of the rotor that pulls the air through the vanes. The volume of air that can be drawn through is limited by the size of the vane opening and it’s rotational speed, and in this example, speed is constant. And just like if you added additional branches on a HVAC trunk, further down the line your decreasing the volume of air moving. The majority of air is being short-circuited, moved through the outer region of the rotor, but not so much in the ID or center. Those regions are going to get hotter then they would if the rotor was less holey. Or is that Holy based on the proclamations?
Holed rotors are fine if they are designed from scratch for them. Race rotors have holes to primarily reduce mass. Even a road race is a series of drag races from one corner to another, and less rotational mass gives you faster acceleration and deceleration. They form stress cracks at the holes due to the extreme thermal cycling they do, but that type of cycling is rarely seen on our trucks.
Slots really do not alter cooling properties at all. Despite the circular path that they are usually made in, with one of the four “walls” open to the atmosphere, air will not channel through.
---This was about half I was going to write.
Honestly, if drilled rotors make you happy, go for it. Same with slots. I'm pretty happy running stock rotors with good pads. Even when I run my truck 1,500lbs overweight.