Cheapest good "dimpled" rotors?
12-18-2004, 06:21 PM
#2
1st Gear Member
Join Date: Aug 2004
Posts: 147
RE: Cheapest good "dimpled" rotors?
There was a good discussion over at corner-carvers.com I cant link the page but a guy wrote article Ill post part of it for you. If you need the article search there forums at the bottom of page one or top of two it say brake article I wrote
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.
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.
12-18-2004, 07:53 PM
#3
6th Gear Member
Join Date: Jan 2004
Posts: 5,836
RE: Cheapest good "dimpled" rotors?
Damn, even if that's just a copy of a post.
Still a great post.
Glad I didn't opt for Drilled/Slotted.
However I did get a great deal on some slotted rotors+pads from one of the vendors on this site.
Can't remember the name of the place at the moment
But I got some Powerslot Rotors w/ Hawk Performance pads for like 200 bux I think?
Ahh, it was New Era Performance, and I got them for 269.99
A great deal from what I've looked around for.
Still a great post.
Glad I didn't opt for Drilled/Slotted.
However I did get a great deal on some slotted rotors+pads from one of the vendors on this site.
Can't remember the name of the place at the moment
But I got some Powerslot Rotors w/ Hawk Performance pads for like 200 bux I think?
Ahh, it was New Era Performance, and I got them for 269.99
A great deal from what I've looked around for.
12-19-2004, 06:01 AM
#4
3rd Gear Member
Thread Starter
Join Date: Sep 2004
Location:
Posts: 837
RE: Cheapest good "dimpled" rotors?
Hehe, yah, long way to say it, but I wasn't really specific, I'm getting rid of my cross-drilled rotors, and want dimpled or just slotted instead. My reason for wanting the slots is one of the reasons mentioned there, to clean off the pad as it passes over. Not really for performance or anything, but I heard it increases the life of your pads, not too sure if that's true, but that's why I want them.
Anyway, if you have a good recommendation for a replacement, dimpled, slotted, or not at all, please share, any help will be appreciated.
Nice copy&paste though, pretty informative.
Anyway, if you have a good recommendation for a replacement, dimpled, slotted, or not at all, please share, any help will be appreciated.
Nice copy&paste though, pretty informative.
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