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Old 04-14-2006, 09:05 AM   #1
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Brake information and educational materials

I found this article on the Mazda6 Tech forum. Its a very good summary by a moderator on that forum about brakes in general. If you're a gluten for punishment read "the famous altima brake thread" referenced at the end. I believe I referenced that thread several years ago. Its a very painful (but informative) discussion wherein an individual (also a vendor on the altima forum) catagorically stated the positive virtures of drilled and slotted rotors. Several altima forum members challenged his comments. In the end there were participants from just about every major car forum on the web including some major race car builders participating. The results of those discussions (that's putting it politely) is summarized below. Its long but worthwhile reading.

"The Working of Brakes"

Over the past several years I have seen many myths perpetrated by the main stream. The purpose of this article is to dispel some of those myths while explaining basic concepts. Through the course of this article you will learn about how brakes work. You will also learn the advantages and disadvantages of cross-drilled, slotted, and vented rotors. Lastly, you will learn about brake bias.

There is a common fallacy out there that increasing your brake pad size in terms of swept area will increase the stopping power of your car through greater friction. From a standpoint ignoring operating temperatures this is in fact false. The force of friction is determined by physics as the force down on the object times the coefficient of friction. As such there is no surface area in the friction equation. However, the temperature of the pad varies throughout its use changing the coefficient of friction at each point along its temperature slope in a non-linear/non-progressive manner. As such it is possible that a larger pad will change the friction force favorably given pad makeup. It certainly will change the amount of time before the brakes enter the proper range and when they leave the range. It will also influence when and how long it is at the peak performance point. Meanwhile, modifying the pad material can change this operating range. As such the affect of increase in pad size on braking friction would depend on the makeup of the pad. Also note that the only way to modify the force down is to change the brake piston force (by size changes or number for example).

This does not mean that a larger brake pad does not help braking! The benefit of a large brake pad comes into effect when you consider thermal dissipation. The larger the pad the more this thermal temperature (created by the interaction between the pad and rotor) is spread amongst a pad. This means less temperature is concentrated at one point on the pad and the rotor absorbs more heat. This decreases the likelihood that the pad itself will heat beyond operating temperature. If the pad were to go beyond operating temperature it would glaze over resulting in brake fade. Furthermore, a larger pad results in a longer service life of the pad since there is more pad material to consume.

**Note: This is not to say that a huge pad is the way to go. I am simply telling you the benefits of a bigger pad. Do not. I repeat do not buy a huge pad thinking that will be the end all. However, consider a pad with a better material makeup for a large difference.

Cross-Drilled /Slotted Rotors

The second thing you can do to improve your brake performance is often to go to a larger rotor. We all know that this gives the rotor further ability to dissipate heat away from the pads through itself and through the air (conductive and convective heat transfer). So obviously a larger pad, a larger rotor, or both result in better brake performance by avoiding brake fade.

But what about cross drilled or slotted rotors? Well the common belief in the main stream is that somehow slotted or cross-drilled rotors allow for better performance by handling heat. This is 100 percent false. The individuals involved in such fallacies mention that air through the holes or slots work to cool the rotor (convective heat transfer into the air from the rotor). The issue is that from physics we know that metal transfers heat better then air by a significant amount. As such the larger mass of the rotor becomes more important then the larger surface area of the rotor in any situation other then the optimal. Cross drilling and slotting rotors are not optimal manners of creating metal to air transfer through larger surface areas. There is not much airflow through the holes or slots. Furthermore for cross drilling the holes will fill with brake dust in effect lowering the cooling ability of the rotors vanes they pass through.

Rigidity

From the information above we can glean that the rotor begins to work as a heat sink. Now by cross drilling or slotting we are decreasing the overall amount of metal to transfer this heat to. Clearly we are decreasing performance of the rotor to dissipate heat amongst itself. Furthermore, the holes of a cross-drilled or slotted rotor decrease the area of the pad that contacts the rotor. This concentrates the heat more on certain areas of the pad, which has similar effects to that of using a smaller pad. As such the pad heats up more quickly.

We are also damaging the brakes structural rigidity. The iron in a brake rotor is made of a crystalline structure. By drilling holes in said surface we cut the end grains creating a situation that breeds cracks. Furthermore, even if we were to cut the rotors correctly to avoid cutting the end grains structural rigidity is still decreased. The temperature around the holes will be slightly less then that of the entire rotor leading to temperature stress. Moreover, the decreased mass will result in lowered rigidity.

Advantages

So what do cross drilled and slotted rotors accomplish? The main original purpose of slotted and cross-drilled rotors was to vent gases that buildup between the pads and the rotors. However, this reasoning is no longer valid. As the years have gone by pads have been designed that produce very little gas. Furthermore many pads come with groves in themselves that allow for the removal of any minor gas that is created. A slotted or drilled rotor always decreases the rotors capability to dissipate heat amongst itself. A slotted or drilled rotor will also clean off the brake pad as it passes the slots at the expense of faster pad wear. As such there are benefits for rally and dirt tracks. Furthermore, the slots or holes themselves can serve to wipe off the top layer of glaze that tends to appear on your brake pads. Some racers say this last part is beneficial while others question whether the slots will fill before the deglaze affect is ever helpful. I have yet to determine the answer to this question.

The answer of slotted and cross-drilled rotor usefulness seems to lie with whether the benefit of cleaning the pads outstrips the loss in heat dissipation. In terms of cross drilling there are so many costs that nothing is accomplished beyond perhaps giving you a certain bling look. In a motorcycle or other extremely light vehicle the decrease in rotational inertia and unsprung mass might perhaps be useful (once other more efficient avenues are exhausted). However, in a street car or race car the speeds and weight of such vehicles will make the relatively miniscule decrease be outweighed by the need for more heat dissipation. Slotted rotors, meanwhile, share the positives of cross drilling but notably are slightly less subject to the costs. They do not impede airflow through the rotors vanes, nor do they have as large an affect on structural rigidity. Therefore, the need for slotting depends on your application.

Vented or Vaned

So what do ventilated rotors accomplish? Well, the concept is that they will help cool the rotors. We discussed earlier that giving up mass for surface area to gain cooling of the rotors should only be done when optimal. Vanes are the optimal method of achieving these goals. The rotors are designed to increase surface area and to flow air in the middle of the rotors. The increased surface area to the air clearly provides for more cooling from the air at the cost of mass. So why does this method work while the others fail? The first reason is that a ventilated design flows a lot of air through a rotor. A ventilated rotor acts as a centrifugal pump sucking air into the rotors. This is why rotors with curved vanes provide better braking.

A slotted or cross-drilled design will flow very little air under heavy braking. As such the vanes of the ventilated system are far more efficient. Moreover, air moves through the center of the rotor cooling the rotor more evenly and efficiently. Furthermore, the ventilated design does not decrease the contact patch of the pad on the rotor. Finally, the design has different structural rigidity qualities then that of a cross-drilled or slotted design.

Brake Bias

So now you know that increasing your pad size and rotor size will help to stop your brake fade. You also know that swapping the pad, increasing the rotor size, or increasing the force of the pistons on the pad can increase your stopping force at the tires. Finally, you have learned to stay away from cross-drilled and look very closely at whether to use a slotted rotor.

So does that mean it is time to go get that fancy front brake kit for your car? Well, potentially no again. The first thing to consider is that in any braking setup the tires are the ultimate limiting piece. You cannot stop faster then your tires allow you to stop, ever. As such, if your car can lock it’s tires under braking consistently then better brakes will not improve your braking performance. (I stress the consistent part, as brake fade must also be combated.)

Furthermore, most people understand the idea of brake bias, but fail to understand its application. A typical car is setup with the front brakes being far more effective then the rear. Now the first thing we must realize is that from a dynamic stand point your car should have stronger front brakes. When you brake physics transfers more weight to the front axle that must be accounted for. However, in this dynamic state we also have brake bias. Your typical street car is slightly dynamically biased towards the front. This leads to the front tires locking up before the rear tires under heavy conditions. Such a situation is obviously not optimal for a car stopping quickly.

You want the stopping bias to be roughly equal given the acceleration you are traveling at (please note that the bias depends on the acceleration of the vehicle). When you have a front bias you get a more stable stop (as opposed to a rear bias where a lock can cause spins), but you also get further forward weight transfer and longer stopping distances. Most cars stock come with a minor front bias for the layman. So it is clearly discernable that by going with a bigger front brake kit you are possibly increasing your stopping distance if you do not equally modify the rear brakes, change your pads, change your tires to ones that do not lockup, or set the clamping forces lower on the front brake. Without making such changes the larger effective radius can lead to an earlier lockup of the front wheels.

For further information please try:

Ruiz, Stephen and Smith, Carroll. “Brake Systems and Upgrade Selection”
McCready,Tom and Walker, James. “Brake Bias and Performance”

http://www.stoptech.com/whitepapers/...erformance.htm

Corner-Carvers.com altima brake thread discussion.
http://corner-carvers.com/forums/sho...drilled+rotors

Corner-Carver discussion on this article.
http://corner-carvers.com/forums/sho...threadid=14827

The famous Altima brake thread.
http://www.altimas.net/forum/showthr...7&pagenumber=1

Additional thanks to Dennis for writing this informative article!

Edited/Updated (Wednesday, 21 February 2007)
New Brake Tech Articles:
Rotors do not warp!



Last edited by WD40; 02-21-2007 at 10:35 AM.
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Old 04-14-2006, 05:48 PM   #2
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Thanks Gary!
I appreciate the write-up.

In fact, this is now a "sticky".

Do you want it left open for comment, or "close" it , and just leave it for informational purposes.
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Old 04-17-2006, 01:23 PM   #3
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Leaving it open or closing it is totally up to you. I'm just hoping people would just read it. I think they would be able to make their own informed (rather than hearsay) decisions if they would (and may learn something too!). Thanks for considering it worthy educational material.
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Old 10-13-2006, 05:58 PM   #4
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My vote would be to leave it up. Seems a lot of people need to learn this. The guys from StopTech know their stuff.
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Old 01-06-2007, 12:30 PM   #5
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The famous Altima thread is down... BOOO! Anyone summerize?
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Old 01-19-2007, 10:05 PM   #6
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^
That's what the original post was; a summary of the Altima brake thread.
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Old 01-19-2007, 10:33 PM   #7
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wow that's a lot of good info. **kicks himself for buying cross drilled/slotted rotoer** lol
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Old 02-21-2007, 10:36 AM   #8
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Updated original post to add additional tech article.
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Old 03-28-2007, 11:44 AM   #9
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Holy info.. thanks!
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Old 04-04-2007, 01:11 PM   #10
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damn! i bought brembo slotted brakes but i go the big ol hawkpads too bad there going to wear out faster :(
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