Installed Zimmerman Slotted rotors last week, a few questions-->
07-24-2006, 01:00 PM
#11
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Actually, no.
A braking system is just a heat sink. When you reduce the amount of rotor mass, you're reducing it's ability to absorb heat. By cross drilling/slotting you're also reducing the contact area between the pad and the rotor...a double whammy if you're trying to improve your braking. With VERY few exceptions, all other things remaining the same, adding slotted or crossdrilled rotors will REDUCE the system's ability to dissipate heat which actually makes matters worse.
As an analogy, let's say you have a Pentium processor and you tend to run some fancy simulation that makes it get really hot and you want to try overclocking it. Do you think overclocking it will be easier or harder if you snip off 5-10 of the cooling fins from the heat sink on top of the processor?
But it sure does look cool. :-/
As an analogy, let's say you have a Pentium processor and you tend to run some fancy simulation that makes it get really hot and you want to try overclocking it. Do you think overclocking it will be easier or harder if you snip off 5-10 of the cooling fins from the heat sink on top of the processor?
But it sure does look cool. :-/
07-24-2006, 01:21 PM
#14
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erm, no.
if you wish to look at a rotor as a heat sink, and take cooling into effect, you're close to right. but close only counts in horse shoes, hand grenades, and napalm.
I've studied a great amount about airflow and thermal dynamics and have designed some very small profile embedded devices that require substantial cooling. the part that you got right is that less contact area between friction surfaces=less heat generated. a rotor absorbs a minimal amount of heat from the drivetrain, as a matter of fact, the rotor is a heatsink for itself. there is less heat generated because of the fact that there is less surface contact, but fade is caused by brakes being too warm, heat retention is a bad thing, why do you think they vent rotors? don't get me wrong, brakes work better warm, but not hot. the slots do very little to affect cooling, they're more for deglazing purposes. they do add to cooling, and provide a limited amount of improvement in braking. also hot rotors warp and have microfractures more easily, both of wich can cause cracked rotors.
additionally, a heat sink's purpose is to remove heat from it's source and transfer it into the ambient area around it, thus more surface area=better cooling. the problem with rotors is that they have to be very durable, materials that are good at transferring thermal energy are not too durable (2 of the better ones are copper and silver, both very soft metals). a heatsink with more fins is better than one with less, provided it is designed properly; pack fins to tight and the hot air has limited room to escape, actually countering the effects of the added fins. some of the best processor coolers on the market offer dozens more fins and yet requite no active cooling (read: fans) because of the metallic composition, the number of fins, and the appropriate spacing of the fins.
I'm not trying to school you here, but I just want to make sure the right info is out there. slots do add to cooling, but the added cooling is often neglegible. the more important aspect of a rotor is the vent design.
I've studied a great amount about airflow and thermal dynamics and have designed some very small profile embedded devices that require substantial cooling. the part that you got right is that less contact area between friction surfaces=less heat generated. a rotor absorbs a minimal amount of heat from the drivetrain, as a matter of fact, the rotor is a heatsink for itself. there is less heat generated because of the fact that there is less surface contact, but fade is caused by brakes being too warm, heat retention is a bad thing, why do you think they vent rotors? don't get me wrong, brakes work better warm, but not hot. the slots do very little to affect cooling, they're more for deglazing purposes. they do add to cooling, and provide a limited amount of improvement in braking. also hot rotors warp and have microfractures more easily, both of wich can cause cracked rotors.
additionally, a heat sink's purpose is to remove heat from it's source and transfer it into the ambient area around it, thus more surface area=better cooling. the problem with rotors is that they have to be very durable, materials that are good at transferring thermal energy are not too durable (2 of the better ones are copper and silver, both very soft metals). a heatsink with more fins is better than one with less, provided it is designed properly; pack fins to tight and the hot air has limited room to escape, actually countering the effects of the added fins. some of the best processor coolers on the market offer dozens more fins and yet requite no active cooling (read: fans) because of the metallic composition, the number of fins, and the appropriate spacing of the fins.
I'm not trying to school you here, but I just want to make sure the right info is out there. slots do add to cooling, but the added cooling is often neglegible. the more important aspect of a rotor is the vent design.
07-24-2006, 02:05 PM
#15
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I disagree........
I run slotted/dimpled rotors on my pickup, I have noticeably better braking ability. The only downside, I am now using a ceramic pad with them, and it definetly has shortened the life of the rotor.
07-24-2006, 03:21 PM
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actually, yes.
Yes, I'm fully aware of what happens to heat sinks when you remove mass. I'm also fully aware of what happens when you increase the surface area over which the cooling fluid (air) can transfer heat to, and what happens to the heat transfer rate when you increase the velocity of that fluid by removing stagnation in the fluid circulation, which is exactly what the purpose of cross drilling is.
Look at any motorcycle brake system. They use very light rotors compared to a car, and a lot of cross drilling. This dissappates heat.
You need to go and study some more heat transfer theory. Clipping off pins on a heat sink is only minimally impacted due to the loss of mass. The large impact is due to the loss of surface area over which to transfer heat to the cooling fluid, again air. Actually, if I can dig it up I have a program that calculates optimum number, size and density of heat sink pins per unit area for a given heat flux.
I have performed these calculations in my carreer as a mechanical engineer. Yes, there is an optimum solution of mass versus surface area versus air flow. Loosing the surface area that the pads act upon in a rotor due to cross drilling or slotting really does not impact the brake efficiency. Braking power is the force applied times the distance over which it acts, so if you really want to increase braking power, you could either a) apply more force at the calipers (which really is limited by the amount of heat generated, the limiting factor being the materials used and by the potential to boil the brake fluid (not a good option really), or b) increase the diameter of your brake rotor to achieve a larger distance over which that force acts. This is a much more viable option, and why large brake kits are a good upgrade. Ever seen 'wave' rotors on a motorcycle? This essentially do the same thing, increasing the distance while maintaining a set diameter for the rotor. The major difference is that the instantaneous force vector changes with each wave on the rotor, but the sum of these vectors results in a larger vector in the correct direction, opposite to the direction of rotation.
Really folks, I'm not making this stuff up, I'm a professional engineer... don't **** with a jedi master!
Also
Look at any motorcycle brake system. They use very light rotors compared to a car, and a lot of cross drilling. This dissappates heat.
You need to go and study some more heat transfer theory. Clipping off pins on a heat sink is only minimally impacted due to the loss of mass. The large impact is due to the loss of surface area over which to transfer heat to the cooling fluid, again air. Actually, if I can dig it up I have a program that calculates optimum number, size and density of heat sink pins per unit area for a given heat flux.
I have performed these calculations in my carreer as a mechanical engineer. Yes, there is an optimum solution of mass versus surface area versus air flow. Loosing the surface area that the pads act upon in a rotor due to cross drilling or slotting really does not impact the brake efficiency. Braking power is the force applied times the distance over which it acts, so if you really want to increase braking power, you could either a) apply more force at the calipers (which really is limited by the amount of heat generated, the limiting factor being the materials used and by the potential to boil the brake fluid (not a good option really), or b) increase the diameter of your brake rotor to achieve a larger distance over which that force acts. This is a much more viable option, and why large brake kits are a good upgrade. Ever seen 'wave' rotors on a motorcycle? This essentially do the same thing, increasing the distance while maintaining a set diameter for the rotor. The major difference is that the instantaneous force vector changes with each wave on the rotor, but the sum of these vectors results in a larger vector in the correct direction, opposite to the direction of rotation.
Really folks, I'm not making this stuff up, I'm a professional engineer... don't **** with a jedi master!
Also
07-24-2006, 03:33 PM
#18
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NO! Bah... again, study some more heat transfer and fluid dynamics
Might I suggest 'Intro to Heat Transfer' by Incropera and Dewitt? (Mechanical Engineering text used at MIT and other engineering colleges).
By adding the cross drilled holes, you are removing fluid flow restriction, which will increase the fluid velocity, which will increase the heat transfer rate. Also, if the fluid (air) was only to be drawn in at the rotor edge, and travel along the entire radius of the rotor to the center where it is dissapated, then a temperature gradient would be established in the rotor where the heat transfer is greatest where the 'delta T' is greatest, at the point where the cold air enters, with less heat transfer towards the center of the rotors when the air has been heated.
furthermore, brake rotors are not like a turbine fan wheel with the blades going inward. The 'blades' on the rotors only go in about an inch in the casting. What they really achieve is creating some turbulance, not really a balanced flow in the classical sense. The cross drilling helps to eliminate stagnation within the rotor of the cooling fluid (air).
Believe me, it is a valid, raceproven concept.
Most people may never need it though, and will be perfectly happy driving around on stock rotors and never know the difference.
But for those of us who drive a bit more spirited, who may go canyon carving or over very steep mountian passes like I have in my state, slotted rotors help to keep from glazing the pads, and cross drilled help dissapate heat and prevent fade.
Cross drilling can definately introduce stress risers in the rotor material... no question. The cross drilling must be done correctly, with a properly cooled drilling process that doesn't re-temper the material and create residual stresses, and with properly chamfered edges on the holes to help relieve the stress around these risers.
By adding the cross drilled holes, you are removing fluid flow restriction, which will increase the fluid velocity, which will increase the heat transfer rate. Also, if the fluid (air) was only to be drawn in at the rotor edge, and travel along the entire radius of the rotor to the center where it is dissapated, then a temperature gradient would be established in the rotor where the heat transfer is greatest where the 'delta T' is greatest, at the point where the cold air enters, with less heat transfer towards the center of the rotors when the air has been heated.
furthermore, brake rotors are not like a turbine fan wheel with the blades going inward. The 'blades' on the rotors only go in about an inch in the casting. What they really achieve is creating some turbulance, not really a balanced flow in the classical sense. The cross drilling helps to eliminate stagnation within the rotor of the cooling fluid (air).
Believe me, it is a valid, raceproven concept.
Most people may never need it though, and will be perfectly happy driving around on stock rotors and never know the difference.
But for those of us who drive a bit more spirited, who may go canyon carving or over very steep mountian passes like I have in my state, slotted rotors help to keep from glazing the pads, and cross drilled help dissapate heat and prevent fade.
Cross drilling can definately introduce stress risers in the rotor material... no question. The cross drilling must be done correctly, with a properly cooled drilling process that doesn't re-temper the material and create residual stresses, and with properly chamfered edges on the holes to help relieve the stress around these risers.
07-24-2006, 05:00 PM
#20
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Better than the stock rotors.......
Dodge trucks are notorious for lousy braking ability. I found that by simply upgrading to the slotted and dimpled rotors (EBC) that it greatly improved the quality of my braking ability. I installed these rotors, Nov. 3rd, '03, they are still on there, though will need to be replaced this year, after 36,000 miles. My factory rotors had to be replaced twice in the first 42,000 miles.