Which is better a cross drilled or slottd rotor?
Is it really a problem if the cross drilled rotor start to crack from the holes?

 

actualy, i could go into this quite in depth, i am going to save myself the time, since i saw this on another message board (it is a mustang board, but darn good tech there). and it is said sooooooo much better than i could say it.


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Brake Fallacies.
Dennis Cook
The Working of Brakes
There is a common fallacy out there that increasing your brake pad size will increase the stopping power of your car through greater friction. 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. The only way to change the coefficient of friction is to modify the material or design of the brake pad. 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). It also increases the effective radius of the force acting on the rotor. Since actual brake torque equals force of pistons* effective radius * coefficient of friction of the pad. So obviously a larger pad, a larger rotor, or both result in better brake performance by avoiding brake fade. It is also obvious that a larger rotor will give you a larger brake torque (stopping force). 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 works 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. (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 is not an optimal manner of creating metal to air transfer through larger surface areas. There is not much airflow through the holes.) As such 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. 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. Lastly, the holes of a cross-drilled rotor decrease the area of the pad that contacts the rotor. This concentrates the heat more on certain areas of the pad (similar to the idea of using a smaller pad where the pad heats up more quickly).
So what do cross drilled and slotted rotors accomplish? Well cross-drilled does not do anything for a car but perhaps give 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 serve a few purposes. The main original purpose of a slotted rotor 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. So does that mean that slotted rotors fall to the same fate as cross-drilled? Well, no. A slotted rotor always decreases the rotors capability to dissipate heat amongst itself, but they have their applications. A slotted rotor will clean off the brake pad as it passes the slots. As such it works really well for rally and dirt tracks. Furthermore, the slots 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 brake usefulness seems to lie with whether the benefit of cleaning the pads outstrips the loss in heat dissipation. This question depends on your application.
Ventilated or Veined
So what do ventilated rotors accomplish? Well, the concept would be the same as the idea for cross-drilled/ slotted that have been previously perpetrated. However, there are a few distinct differences. 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 veins provide better braking. A slotted or cross-drilled design will flow very little air under heavy braking. As such the veins 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, 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 brake dive 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 actually most likely increasing your stopping distance if you do not equally modify the rear brakes (or set the clamping forces lower on the front brake to make up for the front brakes increased effective radius).

 

I will put it this way. I put cross drilled rotors on the front on my 1LE Z. They dont seem much better than stock, until you really have to jam on the brakes, then you know where all the money went. Wait, I got mine for free, but you get my point.

 

they make rotors that are drilled and slotted. id get both.

 

Has anyone done a direct comparison of the options, or just replacing shot rotor with new ones? If you take old stock rotors and replace them with anything new then they will be better, if they're not then go get your money back!

Firstly I would have thought that you should have a look and see what pads are recommended for what rotors. If a pad is intended for use with non-slotted/cross-drilled then don't get slotted or cross-drilled as the pad has no use for them.

If your looking at ultimate GT racing performance then cross drilled is never recomended as far as I've found. Folks use cross-holed, ie. there are holes through to stop gas build-up during 200mph to 40mph stops etc, but they are NOT drilled but cast that way, as drilling causes stress risers.

As was mentioned above, the application has a lot to do with what is "best". If it's just "heavy street use", then cross drilled rotors seem to look the best, and for most folk thats what really matters.

If it ain't just looks your after then forget cross drilled and decide whether you want the best stopping power or whether the added scrubbing effect of the slots would be of advantage.

Si.

ps. Remember when you get them to rigourously follow the bedding in instructions, if you don't then the above discussion was a total waste of your time and ours.
:lala:

 

Good info. Dewey!

Basically, on a street car, drilled rotors don't do much but look good and you won't see a huge improvement (if any) over a full rotor. Better brake pads will make a much bigger differnece. Unfortunately, many people will still go for the hype that the manufacturers advertise regarding cross-drilled rotors.

Some brands of drilled rotors aren't too expensive and if you want that look, then by all means go for it. Just don't think your car will stop like an Indy car.

 

two words - 'Bling, Bling'

 

Go with both





I went with them because they were just as expensive as replacement full rotors. So far no cracking, warping, or even rust and it's been almost a year

 

where ya get, i want me sum

 

did any of you guys read that post from above?

 

cross-drilled and slotted rotors will do absolutely no good. Its outdated technology. Take my advice, I've seen plenty of cracked rotors (most being cross drilled). You won't see a performance gain. Get plain old rotors and some good pads.

BTW- The above post was actually written by a mustang guy named cobra in a Altima.net message board.

 

all the heat dissipation talk in that post is great, but the reason for cross drilling is not to disspate heat....

its to allow the gasses to escape from the heated pad and rotor....as the pad is pressed into the rotor, and heat is created, gasses are released (this depends on the type of pad material as well) the holes give the gasses someplace to go, as well as brake pad material that is worn off the pad......otherwise it wouldn't be able to escape as quickly and create a buffer between the pad and the rotor.

also,

"There is a common fallacy out there that increasing your brake pad size will increase the stopping power of your car through greater friction. 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."

this is only true when the consideration is around a point

secondly,


"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 is also incorrect.....by increasing the size of the brake pad, you thereby reduce the force per unit area, more heat is not absorbed by the larger pad.....

in fact, less heat per unit area is created in the first place....resulting in cooler brake temps.

the same goes for a rotor, althouth a larger rotor WILL help diissipate heat, it also produced less heat in the first place per unit area.