# "The Working of Brakes" - Geezer



## 95GolfIIIGL (Jul 7, 2005)

*This needs to be in the FAQ*

Because it's not and it definitely needs to be there. This is the most comprehensive and scientifically sound description of the affects of different types of rotors on brake fade, brake wear, and stopped efficacy. Unfortunately, it didn't answer for me my question of slotted or OEM rotors, but it did answer a lot of other questions. here is the link http://www.focusfanatics.com/f...81789 to the forum I found the info in, and I will copy the info in word format directly into here. I will then get off my lazy butt and find out how to get this put into the FAQ. Hope this is actually useful to someone.
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"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.
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Again the link to where I found this is above, this is NOT my writing, but someone has done a tremendous job with it, and I thought it was a great myth killer.


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## AP Racing - Chris_B (Jan 30, 2009)

*Re: This needs to be in the FAQ*

Speaking of busting myths, there are some in this post. Let's hit one of them head-on...
General Motors, Porsche, Mercedes-Benz, BMW and other have done extensive research in the area of rotor surface geometry. As a cure for insomnia, one should peruse SAE papers on this topic. Here is where cross-drilling rotors is beneficial:
a) Pad bite: The combination of a reduction in swept area along with the extra leading edges leads to a noticeable (and measurable) increase in the braking torque generated by a cold pad. Overall, cross-drilled rotors produce more brake output (brake torque) than plain face rotors, especially when pushed hard (high line pressures).
b) Cooling: It has been shown (via controlled experimentation) that cross-drilled rotors cool better at higher temperatures and at higher speeds than non-drilled rotors (from 8.8% to 20.1%). At lower temps and lower speeds, there isn't much difference.
c) Fade resistance: While not much difference is shown, cross-drilled rotors provided a slightly higher resistance to fade and had more stable temperatures and brake torque.
d) Vapor degassing: This is most evident when the rotors have become wet as the drilled holes provide an escape path for water vapor. The data is more exaggerated at higher pad pressures than at lower pressures. Pad degassing has become, as stated above, essentially a non-issue.
e) High temperature pedal feel: Slightly less pedal force and pedal travel were seen with cross-drilled rotors compare to plain face rotors in low speed fade test. High speed fade test showed less of an improvement.
f) Pad wear: At lower temperatures, virtually no differences were recorded. At higher temperatures and track abuse, cross-drilled rotors resulted in an increase in lining wear, although peak temperature wear was lower than pads running on plain face rotors. There was less of a friction reduction with cross-drilled rotors as the plain face rotors showed an increase in pad surface glazing.
g) Noise: Cross-drilled rotors exhibited a whirring noise not heard when using plain face rotors (no surprise there!).
h) Crevice corrosion: Cross-drilled rotors showed a tendency to suffer from corrosion in the holes. (Note: The rotors used in the GM test were not treated in any way.)
i) Fatigue life (cracking): In the most extreme high speed abuse tests and race track conditions, the cross-drilled rotors showed more thermal fatigue and stress cracks. At lower temperatures and at normal cycles, there was no difference found. It was important to note that rotor shape, vane design and plate mass appeared to have an effect on fatigue life.
j) Pad glazing: Under some race track conditions, the plain face rotors produced pad glazing that did not occur when using cross-drilled rotors.
k) Brake judder: Plain face rotors were found to exhibit more hot roughness (felt as brake judder) than the cross-drilled rotors. The cross-drilled variants also did not show heat spotting where some of the plain face rotors did.
Reference: SAE paper #2006-01-0691, Antanaitis and Rifici (General Motors), presented at SAE World Congress April 3-6, 2006.


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## teutoned (Jul 29, 2008)

*Re: This needs to be in the FAQ (AP Racing - Chris_B)*

i was under the impression that vanes and venting also acted as dampening to reduce harmonic vibrations/noise under braking??


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## AP Racing - Chris_B (Jan 30, 2009)

*Re: This needs to be in the FAQ (teutoned)*


_Quote, originally posted by *teutoned* »_i was under the impression that vanes and venting also acted as dampening to reduce harmonic vibrations/noise under braking??

The primary function of rotor vanes is for cooling by moving air between the friction plates. Nothing cools brakes better than substantial air flow. That said, the vanes can be arranged in such a way that NVH issues are completely damped or at least mitigated.
Engineers who do brake system development are constantly striving to reduce the harshness and pad squeal that are so annoying to the driver. A lot of times the pad is blamed, but an inadequately designed system can resonate without much help from the pad. Of course, the pad is the most reasonable item for an end user to change in search of noiseless bliss.


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## teutoned (Jul 29, 2008)

*Re: This needs to be in the FAQ (AP Racing - Chris_B)*


_Quote, originally posted by *AP Racing - Chris_B* »_
Engineers who do brake system development are constantly striving to reduce the harshness and pad squeal that are so annoying to the driver. A lot of times the pad is blamed, but an inadequately designed system can resonate without much help from the pad.

a good reason not to skimp on rotors and/or to avoid the rotor made in china? venting and vanes do not match OE configuration?


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## AP Racing - Chris_B (Jan 30, 2009)

*Re: This needs to be in the FAQ (teutoned)*


_Quote, originally posted by *teutoned* »_a good reason not to skimp on rotors and/or to avoid the rotor made in china? venting and vanes do not match OE configuration?

20 years ago I would have said yes for sure. However, there are some quality rotors being made in China today -- but certainly NOT most of them! Volkswagen of China (and GM and Ford etc.) use Chinese iron almost exclusively. Buicks made in China (much, much better cars than than American Buicks, by the way!) use almost all locally sourced materials.
In fact, many of the production cars being sold in the USA have rotor manufactured in China (or India or Vietnam....). Now, these are made to _strict guidelines and quality standards_, not like much of the aftermarket garbage being sold at discount auto supply stores. There are very few aftermarket companies bringing in mostly higher quality rotors from Asia, and their business is doing very well right now. They will follow the OE component as closely as possible, right down to the vane design. Some of them even have a sister company that made the OE piece!
A good thing about this economic speed bump we are getting over is that weak companies with weak products are going away. This is a good thing.


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## GTijoejoe (Oct 6, 2001)

*Re: This needs to be in the FAQ (AP Racing - Chris_B)*


_Quote, originally posted by *AP Racing - Chris_B* »_Speaking of busting myths, there are some in this post. Let's hit one of them head-on...
General Motors, Porsche, Mercedes-Benz, BMW and other have done extensive research in the area of rotor surface geometry. As a cure for insomnia, one should peruse SAE papers on this topic. Here is where cross-drilling rotors is beneficial:
a) Pad bite: The combination of a reduction in swept area along with the extra leading edges leads to a noticeable (and measurable) increase in the braking torque generated by a cold pad. Overall, cross-drilled rotors produce more brake output (brake torque) than plain face rotors, especially when pushed hard (high line pressures).
b) Cooling: It has been shown (via controlled experimentation) that cross-drilled rotors cool better at higher temperatures and at higher speeds than non-drilled rotors (from 8.8% to 20.1%). At lower temps and lower speeds, there isn't much difference.
c) Fade resistance: While not much difference is shown, cross-drilled rotors provided a slightly higher resistance to fade and had more stable temperatures and brake torque.
d) Vapor degassing: This is most evident when the rotors have become wet as the drilled holes provide an escape path for water vapor. The data is more exaggerated at higher pad pressures than at lower pressures. Pad degassing has become, as stated above, essentially a non-issue.
e) High temperature pedal feel: Slightly less pedal force and pedal travel were seen with cross-drilled rotors compare to plain face rotors in low speed fade test. High speed fade test showed less of an improvement.
f) Pad wear: At lower temperatures, virtually no differences were recorded. At higher temperatures and track abuse, cross-drilled rotors resulted in an increase in lining wear, although peak temperature wear was lower than pads running on plain face rotors. There was less of a friction reduction with cross-drilled rotors as the plain face rotors showed an increase in pad surface glazing.
g) Noise: Cross-drilled rotors exhibited a whirring noise not heard when using plain face rotors (no surprise there!).
h) Crevice corrosion: Cross-drilled rotors showed a tendency to suffer from corrosion in the holes. (Note: The rotors used in the GM test were not treated in any way.)
i) Fatigue life (cracking): In the most extreme high speed abuse tests and race track conditions, the cross-drilled rotors showed more thermal fatigue and stress cracks. At lower temperatures and at normal cycles, there was no difference found. It was important to note that rotor shape, vane design and plate mass appeared to have an effect on fatigue life.
j) Pad glazing: Under some race track conditions, the plain face rotors produced pad glazing that did not occur when using cross-drilled rotors.
k) Brake judder: Plain face rotors were found to exhibit more hot roughness (felt as brake judder) than the cross-drilled rotors. The cross-drilled variants also did not show heat spotting where some of the plain face rotors did.
Reference: SAE paper #2006-01-0691, Antanaitis and Rifici (General Motors), presented at SAE World Congress April 3-6, 2006. 

Awesome! I'm glad Chris posted this, there is a lot of information and in my experience for these statements to become true. But be aware that not all slotted or cross drilled disks are the same, the overall concept is here to prove pro's and cons.

_Quote, originally posted by *AP Racing - Chris_B* »_
The primary function of rotor vanes is for cooling by moving air between the friction plates. Nothing cools brakes better than substantial air flow. That said, the vanes can be arranged in such a way that NVH issues are completely damped or at least mitigated.
Engineers who do brake system development are constantly striving to reduce the harshness and pad squeal that are so annoying to the driver. A lot of times the pad is blamed, but an inadequately designed system can resonate without much help from the pad. Of course, the pad is the most reasonable item for an end user to change in search of noiseless bliss.

In addition to freq. analysis for disk design, the same is done on calipers to try and optimize specific freq. regions that have been known to increase noise occurances. 
Unfortunately noise prediction is close to impossible, especially on a brake system, sometimes you think or know you have good parts but assemble and miss match them together and you can find noise issues now that weren't there before. For OEM's this is very fustrating and of course the largest concern for warranty... we blame the environment


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## spitpilot (Feb 14, 2000)

*Re: This needs to be in the FAQ (AP Racing - Chris_B)*


_Quote, originally posted by *AP Racing - Chris_B* »_
20 years ago I would have said yes for sure. However, there are some quality rotors being made in China today -- but certainly NOT most of them! Volkswagen of China (and GM and Ford etc.) use Chinese iron almost exclusively. Buicks made in China (much, much better cars than than American Buicks, by the way!) use almost all locally sourced materials.
In fact, many of the production cars being sold in the USA have rotor manufactured in China (or India or Vietnam....). Now, these are made to _strict guidelines and quality standards_, not like much of the aftermarket garbage being sold at discount auto supply stores. There are very few aftermarket companies bringing in mostly higher quality rotors from Asia, and their business is doing very well right now. They will follow the OE component as closely as possible, right down to the vane design. Some of them even have a sister company that made the OE piece!
A good thing about this economic speed bump we are getting over is that weak companies with weak products are going away. This is a good thing.

I agree....OEM quality parts can be made anywhere in the world....providing you have rigorous QC involved...but...from what I've seen..many parts comin from China lack that involvement! I ordered an "Empi" new axle..recoginzed the name as a quality supplier from air cooled VW daze..the part was garbage..rubber boots were crudely molded, axle shaft was bare steel







and who knows what quality of the CV joints was..sent that back and rebuilt my axle! http://****************.com/smile/emthup.gif Same thing happend with motor mounts I bought...recognized name...Febi...parts in the box looked like crap...again crude rubber and poorly fabed steel components..with a paint job that looked like they let the afternoon kindergarten class have at it!


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## GTijoejoe (Oct 6, 2001)

*Re: This needs to be in the FAQ (spitpilot)*


_Quote, originally posted by *spitpilot* »_
I agree....OEM quality parts can be made anywhere in the world....providing you have rigorous QC involved...but...from what I've seen..many parts comin from China lack that involvement! I ordered an "Empi" new axle..recoginzed the name as a quality supplier from air cooled VW daze..the part was garbage..rubber boots were crudely molded, axle shaft was bare steel







and who knows what quality of the CV joints was..sent that back and rebuilt my axle! http://****************.com/smile/emthup.gif Same thing happend with motor mounts I bought...recognized name...Febi...parts in the box looked like crap...again crude rubber and poorly fabed steel components..with a paint job that looked like they let the afternoon kindergarten class have at it!









HAHAHAHA







Sorry but I found that funny. 
It is true, only because the parts are made in China doesn't mean they are crap, it totaly depends on the quality control. 
Labor rate in china is very very cheap, and china has alot of foundaries to make casted parts. 
We have some videos of industry labor workers in factories, its slightly humorous but very sad to see what type to danger these ppl work in everyday. ie. when your hand gets smashed in the press, good thing you still have another one to keep your job to support your family


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## spitpilot (Feb 14, 2000)

*Re: This needs to be in the FAQ (GTijoejoe)*


_Quote, originally posted by *GTijoejoe* »_
HAHAHAHA







Sorry but I found that funny. 
It is true, only because the parts are made in China doesn't mean they are crap, it totaly depends on the quality control. 
Labor rate in china is very very cheap, and china has alot of foundaries to make casted parts. 
We have some videos of industry labor workers in factories, its slightly humorous but very sad to see what type to danger these ppl work in everyday. ie. when your hand gets smashed in the press, good thing you still have another one to keep your job to support your family








 The conditions you describe are exactly like US....in the 1800's and early 1900's!


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## GTijoejoe (Oct 6, 2001)

*Re: This needs to be in the FAQ (spitpilot)*


_Quote, originally posted by *spitpilot* »_ The conditions you describe are exactly like US....in the 1800's and early 1900's!























Thats true, but not the 21st century. Ppl arn't actually standing inside a giant press and ducking their head as it comes down....thats reality


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