Explanation of Cross Drilled Rotors

Separating fact from fiction



I and many of my autocrossing and road racing friends have been trying to separate the marketing hype from the product and explain the existence of Eradispeeds or any cross drilled and/or slotted brake rotors. Mostly, there seems to be a misunderstanding of the reasons for drilling holes in a perfectly good rotor. Cross drilled rotors have somehow become the "magical cure" for your brakes. Many would like you to believe that they will help you stop faster, they will wear better, stay cooler, and they can magically avoid warping. Unfortunately all of these things are false. But, since the guys who make these products spend more on advertising than I do (ok, I don't), it is easy to fall into that trap.



Broken down to the most basic physics (don't worry, no serious math involved here). You will begin to see the reasons they can't do the things they are touted to do.



The "basic" lesson:

Ok. If I am trying to boil a pot of water with 2 gallons of water in it. It will take a lot of heat to get all of that water up to boiling temperature. Right? Now, if I remove 1/2 the water and with it, 1/2 the mass used to absorb heat, it will now be easier to get that water to boil since there is less mass to absorb heat. Make sense?

Ok, If I take a 3 lb chunk of metal and a 6 lb chunk of metal and set them both over a small torch, which one do you think will heat up first? The lighter mass will see a more rapid temperature rise (it will get hotter, quicker). Correct?

Now, if I take a brake rotor that weighs 20 lbs and begin making "panic stops" with it, it's temperature will increase. Easy enough.

If I now take the same rotor, drill holes in it reducing its weight to 17 lbs and make the same "panic stops", would it not make sense (from the above examples) that the lighter rotor will end that stop with a higher temperature? Less mass to absorb heat will cause a more rapid temperature rise.

Many will say "cross drilled rotors cool better". Hmmm. If you really look at it, how much air do you think can blow through those little holes? Add the fact that they are spinning at a reasonably high speed and the cooling argument becomes even less believable. In fact, the holes could, in theory, disrupt the natural draw of air from the center of the rotor to the outside edge. That is the flow that actually cools the rotors.

Now, some of the aftermarket drilled/slotted rotors are in fact heavier than the stock parts they replace. That is good for heat absorbtion, but you still have the problem of "holes". Where there is a hole in the rotor surface, there is no friction. So, the effect is similar to reducing the size of the brake pad because less of it can touch the rotor (to make friction) at any given time.



This is from Baer racing (I don't think it is on their site any longer).

From Baer Brake Systems
What are the benefits to Crossdrilling, Slotting, and Zinc-Washing my rotors?
In years past, crossdrilling and/or Slotting the rotor for racing purposes was beneficial by providing a way to expel the gasses created when the bonding agents employed to manufacture the pads began to break down at extreme temperatures. This condition is often referred to as “green pad fade” or “outgassing”. When it does occur, the driver still has a good firm brake pedal, but simply little or no friction. Since this normally happens only at temperatures witnessed in racing, this can be very exciting!

However, with today’s race pad technology, ‘outgassing’ is no longer much of a concern. When shopping for races pads, or even ultra-high performance road pads, look for the phrases, “dynamic surface treatment”, “race ready”, and/or, “pre-burnished”. When these or similar statements are made by the pad manufacturer, the pad in question will likely have little or no problem with ‘outgassing’. Ironically more pedestrian pads used on most streetcars will still exhibit ‘outgassing’, but only when used at temperatures normally only encountered on the racetrack.

Although crossdrilling and/or slotting will provide a welcome path to expend any gasses when and if they develop, it is primarily a visual enhancement behind today’s often wide-open wheel designs.

Crossdrilling offers the greatest gas relief pathway, but creates potential “stress risers” from which cracks can occur. Baer’s rotors are cast with crossdrilling in mind, from the material specified, to curved vanes, behind which the holes are placed to minimize potential crack migration. Slotted surfaces are what Baer recommends for track only use. Slotted only rotors are offered as an option for any of Baer’s offerings.

From Brembo:


From Brembo
Why use drilled or slotted discs?
Drilling or slotting discs aids the disc in several ways:
The edges of the slots or holes continuously clean and refresh the pad surface as well as providing increased brake "bite". Additionally, they prevent gasses from collecting between the pad and disc interface.
The disc is lightened, thereby decreasing its rotational inertia.
Improved ventilation increases the disc's ability to shed heat, resulting in cooler operating temperatures.

Wilwood:

From Wilwood
Q: Why are some rotors drilled or slotted?
A: Rotors are drilled to reduce rotating weight, an issue near and dear to racers searching for ways to minimize unsprung weight. Drilling diminishes a rotor's durability and cooling capacity.

Slots or grooves in rotor faces are partly a carryover from the days of asbestos pads. Asbestos and other organic pads were prone to "glazing" and the slots tended to help "scrape or de-glaze" them. Drilling and slotting rotors has become popular in street applications for their pure aesthetic value. Wilwood has a large selection of drilled and slotted rotors for a wide range of applications.

And Stoptech:


Discs that have been drilled through with a non-intersecting pattern of radial holes. The objects are to provide a number of paths to get rid of the boundary layer of out gassed volatiles and incandescent particles of friction material and to increase "bite" through the provision of many leading edges. The advent of carbon metallic friction materials with their increased temperatures and thermal shock characteristics ended the day of the drilled disc in professional racing. They are still seen (mainly as cosmetic items) on motorbikes and some road going sports cars. Typically in original equipment road car applications these holes are cast then finished machined to provide the best possible conditions by which to resist cracking in use. But they will crack eventually under the circumstances described in another section (see Cracking). Properly designed, drilled discs tend to operate cooler than non-drilled ventilated discs of the same design due the higher flow rates through the vents from the supplemental inlets and increased surface area in the hole. That's right, inlets. The flow is into the hole and out through the vent to the OD of the disc. If discs are to be drilled, the external edges of the holes must be chamfered (or, better yet, radiused) and should also be peened.



The reason:

Rotors were originally drilled to eliminate something known as "green fade". The best way to explain "Green fade" is to relate it to an air hockey table. The puck is suspended on a cushion of air that prevents it from touching the table, this reduces the friction between the puck and the table.
A long time ago, pads were made with the best resins we had available. Many of those resins would produce gas as they cured. When a pad was used the first few times, the heat would "cure" the resin which would cause it to produce vapors. This was known as "out gassing". The vapors would build up between the pad and the rotor and lift or "force" the pad away from the rotor (like the puck in air hockey). This caused the brakes to be very ineffective, even though they were not yet at the maximum rated operating temperature. The holes were drilled to allow that gas a place to escape. So, it is correct to say that rotors were cross drilled to eliminate fade, but not for the reasons you would think. The good news is that today's resins no longer suffer from these problem and the modern race pads are so good that this is really no longer an issue. So, by cross drilling rotors, you will only manage to shorten the lifespan of that rotor (it now has less surface area to wear against the brake pad and will wear more quickly as well as a reduction in weight that will cause the brakes to operate at a higher temperature).

Another problem with cross drilled rotors is the potential for cracking around the holes. The holes become a stress point in the cast iron that can more readily allow cracks to form in the rotor surface. This requires that you pay close attention to the rotor surface for signs of cracking. Some small cracks, known as "surface checking" are acceptable, but anything that resembles a crack would be a reason to replace that rotor. When looking at slotted rotors keep in mind that the slots should not be milled off of the edge of the rotor. This is a great place for cracks to form, and they will. The slot should be ball milled in the rotor face and originate and terminate on the surface of the rotor without exiting the rotors edge. The goal is to eliminate sharp edges that cause stress risers on the rotor surface. This will reduce the possibility of cracking. If you see slotted rotors with slots that are milled off the edge of the rotor, shop for another brand. Slots that are not cut through the edge of the rotor are a good sign that the manufacturer of that rotor knows what they are doing. This is a good indicator of parts made by a brake company and not a machine shop that happens to drill and slot rotors.

Many years ago, when I ordered my first brake kit from Baer Racing, they told me that drilled rotors would typically last 20% less than an equivalent solid rotor. This was why they always recommended solid rotors for extreme use. Baer has changed their stance on this since discovering there was a large amount of money to be made selling "Eradispeeds" (they are very pretty brake rotors). This change in their marketing strategy has caused me to feel that they have gone from "supporting the racing community" to "making maximum money". And, I guess I can't blame them.....

If you are truly looking for upgraded braking performance for your car. I suggest, as a first upgrade, that you leave the stock size rotors and upgrade the pads. Try a set of Hawk HPS pads or something from Larry at Carbotech Engineering (http://www.carbotecheng.com) <http://www.carbotecheng.com)>. You'll think you put "big brakes" on all 4 corners (compared to stock).

If you must go bigger, look at any of the various brake upgrades available from many major manufacturers. Brembo, Baer, and many others will have what you need, if you need to upgrade. There are larger kits and they increase in both cost and braking ability. Only your needs and your budget are the limit.

Tires:

I'd also like to take a moment to address the impact that tires have on braking performance.

While it is true that the tires have the "last word" with the pavement when it comes to how the car accelerates, turns, and brakes. There are a few things that tires can and can't do. While it is true that a car on wide, sticky tires should and will out brake the same car on thin, non-grippy tires (all other things being equal). And, while tires are extremely important. They become less important on the 5th or 8th stop or the 3rd lap of a road course. By then the brakes (depending on rotor size, cooling, pads, etc) may be so hot that they can't lock up the skinny little tires, much less the wide, grippy ones. This is where upgrading the brakes pays dividends. This is not meant to minimize the tires role in braking performance. Simply, you must remember to weigh the tires limits against the brake systems limits. You are working with a "package" and changes in one can impact the other. So, while tires will be the limit on the first few stops, they will play a less important role as the brake system temperatures increase. Remember, just because you can lock up the tires on the first stop does not mean that you can't benefit from brake system upgrades.


Class dismissed.

Have fun!



The abbreviated version:

For those who want the short version.

Eradispeeds will not slow your car down any better than stock rotors. They are built to look pretty in the driveway. Iron has a very similar coefficient of friction and it is all about the same. A rotor only has so much metal in it. Metal absorbs heat, metal with holes in it, is lighter than metal without, this makes it lighter, this will allow it to absorb less heat. So, you will not see any miracles from a set of Eradispeeds. Catchy name, shiny parts, great marketing, but they are still "just rotors". Pads...the pads will make a difference (and for much less money). Rotors are still a wear item, and for the money they are asking, I'd hate to run aggressive pads that chew up rotors on slotted, drilled or Eradispeed rotors. However, those same upgraded pads will stop your car and very well. So, I'd buy $60 rotors and $100 pads, instead of $20 pads and $800 rotors. You'll be happier with the result.

Be safe!