Brake rotor type? Plain, drilled, Slotted

[67goatman455]

Wondering what the advantages/disadvantages are to the different types of rotors. Plain rotors seem to be cheaper when you need to replace them and there would be a higher surface area i assume. But i can see the slotted/drilled keeping the rotors and pads cooler to help brake fade.

[Ron Sutton]

Hey Scott,

I'll outline my thoughts on the pros & cons of different brake rotor features & everyone can decide for themselves what makes sense for their individual needs. I think if you ask 5 different brake experts you will get 7 different opinions, so decide yourself & go with what makes sense for your goals.


Drilled: In the early years, before we knew better, many racers thought that drilling rotors made them run cooler. Wrong. Brake experts had rotors drilled to give the brake pads boundary layer of gasses a path to escape, while the edges of the holes helped clean the pads of debris somewhat.

For a rotor to deal with the heat generated, having more “mass” increases its thermal capacity. Drilling a bunch of holes in the rotor reduces the rotor’s mass & reduces the thermal capacity of the rotor. If the rotor gets overheated often & for long periods of time … the drilled holes become "stress raisers" that can lead to three problems: Elongation and/or distortion of the rotor, cracks between the holes … and breakage. Drilled rotors still have their place. I’ll explain at the end.

Slotted/Grooved: The grooves in the rotor … often called slots … help "clean" the debris from the brake pad. That’s it. It’s a good thing. The grooves/slots need to run a specific direction. See the illustration below.

J-Hooks: AP brakes offers a neat rotor a lot of road race & short track oval guys use. Instead of a straight slot grooved into the rotor, they have these “J”s machined into the rotor surface. The “J” goes all the way through the rotor, unlike grooved slots. The perform the same function as a slot and “maybe” they’re helping the boundary layer of gasses to escape. See them HERE.

Cooling: is not achieved with slots or drilled holes. There are three ways to cool rotors: Vanes inside the rotor, air ducting, with or without fans, to blow cooler air on the rotor, and the Ultra Cool brake fans that mount on the hub. See them HERE.

Important: I learned long ago, if we cool the rotor optimally, we can run a little less mass in the rotor ... so it's lighter. For this advantage ... I make cooling the rotors a serious priority. I don’t want to provide the rotors “a little cooling” … I want to provide “as much cooling as practically possible” so we can run lighter rotors & reduce that rotating mass.

Vanes: Are the most important cooling feature of the rotor itself. The vanes of the rotor … spinning on the hub axis … create a vortex, sucking air into & through the rotor’s vanes … helping to cool the rotor. These are simply referred to as vaned rotors & they come in a lot of configurations. The details matter here. The air travels from inside the rotor to the outside, not the other way around as is commonly thought. So the vanes must face rearward when viewed from the outer diameter of the rotor.

Straight vanes do not work as well as curved vanes. There are made & sold as an economy product, because they can go on either side & store doesn’t need to stock left & ride side rotors.

Curved vanes are the way to go. Just think of fan blade design. The vanes scoop the air from inside the rotor … suck it through the inside of the rotor … cooling both sides … and out the top.

More curved vanes provide more cooling … and add mass (weight) to increase the rotor’s thermal capacity. This is my preference instead of adding diameter. If we can get a 12” rotor with 48 vanes that weighs around 11# … that would be my preference over a 13” rotor with 32-36 vanes that also weighs around 11# … because the thermal capacity is the same, the 48 vanes cool better & the 12” rotor has the mass closer into the center, so it’s easier to accelerate & decelerate it.

The only advantage the 13” rotor offered (besides cool looks inside the big wheel) is 10% more braking force. But I can get that 10% braking force four other ways (pad, pedal, piston size & M/C size) … and not have the disadvantage of that weight rotating so far out there. A .05 CoF change in brake pad compound can achieve that.

Note: The vaned rotor is shooting hot air onto the inside of the wheel surface. This is where some of the tire’s heat comes from. If the brakes are overheating, this may over heat the tires too. (Seen it MANY times). Another reason to cool the brakes better.

Rotor Width: Making a vaned rotor wider adds several things: More weight & thermal capacity … and better cooling because the vane channels are bigger, flowing more air. Typical rotor widths are .81”, 1.0” 1.1”, 1.25” & 1.38”. Of course you have to have calipers capable of working on the wider rotor. I like to use the wider option as it adds minimal flywheel effect … as compared to adding rotor diameter.

If you’re not familiar with the math …
Changing the weight of a rotating mass is a one-for-one change in “stored energy” … which I think of, and call, “the flywheel effect.” Moving the weight of a rotating mass increases the stored energy by the SQUARE of the radius change. OMG! This is because as you move weight OUT from its rotating axis, you are increasing the weight's circular velocity (speed).

On the track where speeds are up significantly higher … than everyday passenger car speeds … it really matters. The faster we spin a rotating weight, the more energy it stores (flywheel effect). If we double the RPM of a rotating weight, we multiply the stored energy four times. So at twice the speed … say going into a corner at 100 mph instead of 50 mph … everything rotating has four times the flywheel effect. This is why it’s key to have lighter tire & wheel combos & not run larger or heavier rotors than we really need. My belief is to run as large as I need for thermal capacity & no more.

Rotor Diameter: A larger rotor diameter adds several things: Increased braking force, potentially more weight & thermal capacity, increased flywheel effect of moving the weight out from the centerline ... oh, and the cool factor.

Let’s outline braking force gains first:
Going from 10” to 11” adds 12.5% braking force
Going from 11” to 12” adds 11.1% braking force
Going from 12” to 13” adds 10.0% braking force
Going from 13” to 14” adds 9.1% braking force

Why I said potentially increases weight & thermal capacity … is because rotor designs vary. It’s possible to have an 11# 12” rotor & an 11# 13” rotor. If that is the case, you didn’t gain mass (weight) or thermal capacity. But, you still moved the weight OUT & increased the flywheel effect. Of course it is probable that you increased the mass (weight) & thermal capacity … as most 13” rotors are heavily than the same design 12” rotor … typically in the 3-6# range. So then, not only did you move the weight OUT & increase the flywheel effect … you added weight to the flywheel effect. UGLY!

I exhaust every option BEFORE I go larger on diameter. I go more aggressive pad compound, wider rotor, curved vanes, more vanes, better rotor material, no drilled holes and lastly more rotor material. Then, if I still need more thermal capacity than that … think GT/Trans Am road race cars … by all means I’m going up in rotor diameter.

Racing pad science has come a long, long way. No longer are there just soft, medium & hard compounds. The technology of pad compound mixology is very cool and the resulting pad performance today is night & day from 30 years ago.

Knowing the Coefficient of Friction (CoF or CF) of a brake pad … in the operating temperature range you’ll run in … is key to picking a baseline pad to start with in a new brake system. If you do not know how hot your rotors are … on track days … you’re missing out on a key piece of information that I find to be important to tuning the car & coaching the driver. Buy a low cost infrared temp gun & check all 4 rotor temps right after each session. It will tell you a lot of key info.

Most passenger car brake pads range from .32 to .35. Performance street pads range from .36 to .55. Racing pads start at .50 & go up as high as .70 & cost more because of better materials & lower production numbers. Typically, the higher the CoF … the shorter the pad & rotor life. This varies, but be advised.

Let’s outline braking force gains first:
Going from .35 to .40 adds 14.3% braking force
Going from .40 to .45 adds 12.5% braking force
Going from .45 to .50 adds 11.1% braking force
Going from .50 to .55 adds 10.0% braking force
Going from .55 to .60 adds 9.1% braking force
Going from .60 to .65 adds 8.3% braking force

As you can see … making a modest brake pad compound change can have significant effects on the braking force. Easily as much as a 1” rotor diameter increase.

I always look at pad compound graphs when I can get my hands on them. It shows the "personality" each pad has ... like how different temps affect it. I love that Wilwood shares them in their catalog & online. And it frustrates me when a company won’t share them, because the only way to truly know how a compound will behave is to spend money & time testing their pads. Many brake companies have given, or offered to give, me free brake pads to test.

In the past, I’ve tested brake pads … a lot. But life is short & time is valuable. A test day can cost thousands of dollars & wears my race car out (engine, trans, diff, tires, etc.) costing thousands of dollars more. If a brake pad company won’t give me info today, I simply pass.

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