Why Cross Drilled Rotors Are BAD
First off, how do brakes work? Simple, friction and torque is created when the pad comes into contact with the rotor. Now, whenever a force is applied to a spinning object, torque is created. What happens to the torque? The interface between the road and the tire reacts to this torque and opposes the motion of the object in motion (car), eventually bringing the car to a stop. This is something a lot of people are missing. If your car can lock its brakes up, then bigger brakes will not help you stop faster. That’s right, those big brakes will not improve your braking distance. If you want to stop better, then get better tires.
Ok, so brakes don’t stop the car, then why do people upgrade their brakes? Although it is the tires that stop the car, the brakes still play a big role in the stopping of a car, especially a race car. The first reason for upgraded brakes is to improve friction. Why improve friction? Well, if your brakes use friction to create the torque that stops your car, then less force needs to be exerted onto your brakes to make them stop the car. For example: if a person simply were to upgrade to a set of brake pads with a higher friction material, then that person would have to apply less force to his brake pedal in order to make the car stop in the same amount of time. Increasing the sweeping area of the brake pads can create additional friction. For example, upgrading to larger rotors, larger calipers and thus, larger pads increases the surface area of the rotor which comes into contact by the pad. Basically, you’ve created a larger friction area, and that friction as you read earlier will eventually help the tires stop the car. Ok, what is the purpose behind getting those nice big four piston calipers? Well, as we said those calipers usually house larger pads. The other reason is that with more pistons, more force can be exerted onto the rotor (creating that friction and torque). Basically, the extra pistons are saving your leg from having to mash on that brake pedal so hard. On the race track that means you can better modulate your brake pedal to allow the driver more control of the car.
The second reason for upgrading brakes is heat dissipation. Imagine this scenario: Your new pads have a maximum operating temperature of 1000*. After a couple of laps your brakes exceed this maximum temperature. What happens? Simply put, the pads fade. Essentially the pads become too soft to function properly. This is where glazing happens. Glazing is where a thin layer of crude glass forms on the pad. Glass obviously has a very low co-efficient of friction, which would then be also very bad for stopping the car. Overheating the brakes can also cause the brake fluid to expand, stretching brake lines etc, but we won’t get into that here. So how do we prevent overheating? Mass is the answer. That is the mass of the rotor. The more mass that a rotor has, the more heat that the brakes can dissipate from the area where the pads contact the rotor, preventing the pads from glazing through exceeding their maximum temperature. So now we know, a bigger rotor (with more mass) will help dissipate heat.
Third reason for upgrading brakes is to further remove heat from the brake system. Obviously, the heat that is dissipated from the contact area of the pad and rotor has to go somewhere. This one is very simple. Surface area removes heat by transferring it to the surrounding air. More surface area means that a larger portion of the now heated rotor is in contact with the much cooler surrounding air, which cool your whole brake system down. How do you get more surface area? Bigger rotors.
Ok, you’re probably wondering why cross-drilling rotors is a bad thing after I just spent tons of your time with a rundown of your braking system. First thing, drilled rotors were NEVER intended to cool the brake system. Their original purpose goes back to the ’50’s and ‘60’s where brake pads were prone to something called "gassing-out". The gasses form a very thin layer between the rotor and the pad, lowering the co-efficient of friction. The holes in the rotors basically gave that gas a place to go. Now, you think about it, and you realize that the holes in the rotor do create more areas for the air to come into contact with the surrounding air (third reason for upgrading brakes). But in doing this, you are hurting the much more important reasons for upgrading (reasons one and two). By drilling those holes you are removing important contact area between the pads and the rotors (reason one). Less contact area equals less friction. Drilling those pretty holes in the rotor also further reduces the rotors ability to dissipate heat from the pad to rotor contact area. By putting holes in the rotor, you are removing some of the mass of the rotor. Remember, more mass equals better heat dissipation. Heat dissipation means less glazing.
So now we went over how cross-drilling hurts the first two functions of brakes, while only slightly improving the third function (least important). Let’s talk about another weakness inherent in drilling rotors. That is strength. Cross-drilling rotors drastically reduces the strength of the rotor. The big problem is when you create the sharp edges from drilling. Each hole creates four sharp edges, the front and back of the rotor and on the inside of it. Cracks love to form from sharp edges. Cracks are the rotors way of relieving the stress from all that heat that is created by the brake system. The other problem with cross-drilled rotors is inconsistent cooling. The areas immediately surrounding the hole in the rotor cools much more rapidly than the areas that are not. The result is that the area which is cooled faster than the rest of the rotor, which causes cracking when the area around the hole contracts sooner than the rest of the rotor.
Just for kicks, I will quote some big names when then say that cross-drilling is a bad idea:
From
Wilwood's website:quote:
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.
From
StopTech's website:quote:
Which is better, slotted or drilled rotors?
STOPTECH provides rotors slotted, drilled or plain. For most performance applications slotted is the preferred choice. Slotting helps wipe away debris from between the pad and rotor as well as increasing the "bite" characteristics of the pad. A drilled rotor provides the same type of benefit, but is more susceptible to cracking under severe usage. Many customers prefer the look of a drilled rotor and for street and occasional light duty track use they will work fine. For more severe applications, we recommend slotted rotors.
From Rennsport:
quote:
Porsche used cross-drilled rotors with mixed success. The Zimmerman rotors used on the 930 Turbo brakes were very soft and crack prone, even though the holes were chamfered. The later Brembo rotors will also show some wear when used with most performance brake pads. Some of the best rotors in the world are made in Britain by AP and Alcon. Those rotors, when properly adapted, seem to be superior to the Brembo ones in terms of wear and crack resistance. All drilled rotors will crack sooner or later if overheated. Slotted rotors are more durable in this regard however they are heavier. One reason that most large iron rotors are cross-drilled is to save some unsprung weight. Since nice big, light, carbon rotors cost $1000 each, saving some weight without bankruptcy, is important. Plus, carbon rotors possess very little friction until they reach 500-600 degrees F.
From
Baer Brakes:quote:
Q: I don’t want to spend the money for a complete brake upgrade. Do you offer cross drilled rotors to work with my factory brakes?
Yes. Although there are some companies which sell cross-drilled rotors as an actual performance upgrade, in our extensive testing we have seen no improvement to be had by simply crossdrilling stock rotors. This is why Baer has developed EradiSpeed™ rotor upgrades for a variety of applications. Although it is true the crossdrilling, the slotting, or for that matter the zinc surface washing, are cosmetic enhancements, EradiSpeed™ rotor packages also feature rotors with thicker cheeks to provide more heat sink capacity in the fire path of the rotor. Also, they all feature directional vanes for greater pumping efficiency, as well as a two-piece design where the hat, or hub/hat section of the rotor is CNC machined from a solid billet of aluminum and is then fixed to the rotor ring using National Aviation Standard (NAS) stainless hardware. In other words, the EradiSpeed™ is much more than just the most visually appealing direct replacement rotor, it is the only upgrade of its type which can actually deliver the benefits of greater heat absorption, increased durability and lighter total weight.
In racing, crossdrilling was designed to alleviate a problem known as out-gassing. In some of the older pad compounds, when the pads reached elevated temperatures consistent with performance or racing use, the binder (that’s the material that holds the friction material in place) boiled off, producing a gas. This gas would build up between the rotor and the brake pad, effectively keeping the pad from directly contacting the rotor. The holes provide a relief path for these gasses, as do slots, so the pad can once again contact the rotor. Crossdrilling was NOT designed to facilitate cooling.
Although Baer offers crossdrilling as an option on their systems, it is offered as a cosmetic option only. However, with an EradiSpeed™ rotor upgrade, unlike a cosmetically altered stock replacement rotor, you will benefit from improved durability, greater heat sink capacity, lighter total weight and the visual excitement of a 2-piece, aluminum centered, crossdrilled, slotted and zinc washed appearance.
Well, that’s just about all I have to say about cross-drilled rotors. Now go out and buy some sticky tires and a nice big brake package with solid rotors and enjoy your improved stopping power. Hope this article helps.
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