#1
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Vented Discs
On 2009-06-19, Michael Press wrote:
In article , Ben C wrote: On 2009-06-19, wrote: SJM who? wrote: [...] 1. To cool the disks 2. To vent gases between pads and disks 3. To allow disk heat expansion/contraction without subtle warp 1. Drilled disks have less cooling surface than solid ones. If you drill a disk it has a higher surface-area to volume ratio than it did before. So I'm not sure what you mean. You are reducing the area of the bit that's directly conducting heat away from the pads. But heat will conduct from there to the inside surfaces of the holes whence it will be cooled by the air. So it's difficult to say which is theoretically better. Denote by h: thickness of the disk r: the radius of the drilled hole the change in surface area = 2.pi.rr - 2.pi.r.h You've only counted one end of the cylinder that you drilled out, and you're considering only the change in surface area, not the change in surface area to volume ratio. See http://groups.google.co.uk/group/rec...d90ad43f00c793 so the surface area increases if r h and decreases if r h. Almost (if you count both ends, the turning point would be 0.5h), but the surface-area to volume ratio always increases, whatever the hole size. Having said all this though, I think you're right that surface area is a more important consideration than surface area to volume ratio. The volume affects the heat capacity, but disks (on cars) heat up quickly and then reach a thermal equilibrium where radiation matches heat generated by friction. So we don't really care a lot about the heat capacity. (Disks on trains, for example, may work mainly as heat sinks instead, like bicycle rim brakes.) Now you must show that the heat transfer out of the walls of the hole is as great as that off the surface of the disk. The air speed through the hole decreases with the radius. Good luck. I guess another question to ask is is heat dissipation typically limited by pad-disk conduction or by disk-air radiation/convection? If the former, you don't want holes (for cooling). If the latter, maybe you do. |
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#2
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On 2009-06-23, Phil W Lee phil wrote:
Ben C considered Tue, 23 Jun 2009 06:12:28 -0500 the perfect time to write: On 2009-06-19, Michael Press wrote: In article , Ben C wrote: On 2009-06-19, wrote: SJM who? wrote: [...] 1. To cool the disks 2. To vent gases between pads and disks 3. To allow disk heat expansion/contraction without subtle warp 1. Drilled disks have less cooling surface than solid ones. If you drill a disk it has a higher surface-area to volume ratio than it did before. So I'm not sure what you mean. You are reducing the area of the bit that's directly conducting heat away from the pads. But heat will conduct from there to the inside surfaces of the holes whence it will be cooled by the air. So it's difficult to say which is theoretically better. Denote by h: thickness of the disk r: the radius of the drilled hole the change in surface area = 2.pi.rr - 2.pi.r.h You've only counted one end of the cylinder that you drilled out, and you're considering only the change in surface area, not the change in surface area to volume ratio. See http://groups.google.co.uk/group/rec...d90ad43f00c793 so the surface area increases if r h and decreases if r h. Almost (if you count both ends, the turning point would be 0.5h), but the surface-area to volume ratio always increases, whatever the hole size. Having said all this though, I think you're right that surface area is a more important consideration than surface area to volume ratio. I don't think surface area is relevant at all - for a given caliper force, reducing surface area just increases pressure on the remaining area in proportion (so if the caliper exerts 200lb pressure, you can utilise that as 200psi over a square inch, or 100psi over 2 square inches) the resulting friction is going to be nearly identical. I wasn't thinking of the friction force, but of having a larger area to conduct heat into. But in many applications you will get a higher friction normal force with a larger area-- that's why racing cars have wide tyres. [...] There must be a limit on how much of the disk you can cover with pad before the ability of the disk to pass heat to the air is compromised, although with vented disks the limit would be higher. Yes, and since most disks _are_ vented, it implies that the disk-air boundary was the limiting factor. |
#3
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Ben C writes:
On 2009-06-19, Michael Press wrote: In article , Ben C wrote: On 2009-06-19, wrote: SJM who? wrote: [...] 1. To cool the disks 2. To vent gases between pads and disks 3. To allow disk heat expansion/contraction without subtle warp 1. Drilled disks have less cooling surface than solid ones. If you drill a disk it has a higher surface-area to volume ratio than it did before. So I'm not sure what you mean. You are reducing the area of the bit that's directly conducting heat away from the pads. But heat will conduct from there to the inside surfaces of the holes whence it will be cooled by the air. So it's difficult to say which is theoretically better. Denote by h: thickness of the disk r: the radius of the drilled hole the change in surface area = 2.pi.rr - 2.pi.r.h .... Now you must show that the heat transfer out of the walls of the hole is as great as that off the surface of the disk. The air speed through the hole decreases with the radius. Good luck. I guess another question to ask is is heat dissipation typically limited by pad-disk conduction or by disk-air radiation/convection? If the former, you don't want holes (for cooling). If the latter, maybe you do. Holes through the disk increase turbulence. Greater turbulence increases heat, mass, and momentum transfer. Correlations for turbulent transfer typically include the effect of surface roughness -- sharp edged holes are the equivalent of a very rough surface. Even though there may be no net flow through the holes, there will be enough flow in and out of them to cause the air inside the holes to quickly approach the conditions just outside. Improved heat transfer through increased turbulence may well help to cool the disk. Also, the contents of the holes provide a bulk flow of air past the *pad*. Without holes, when not braking, there is only a narrow space for air to flow past the pad, which makes convection cooling very slow. When braking there is no space for air to flow and cooling can occur only by conduction, to the disk or to the caliper. With holes there is significant flow of air past the pad, whether braking or not. The air inside the holes will be well-mixed, and quickly approach equilibrium with the surface of the pad. Once past the pad, air in the holes will be exchanged with the free stream of air flowing past the disk, thus cooling the pad. The above is just intuition on my part, I have neither calculated nor experimentally verified any of it, nor do I have any experience designing brakes. |
#5
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Nate Nagel wrote:
[...] Well, there's no point in drilling a clutch, because except when starting from a dead stop it should be either in or out; that is, there's no slipping so no heat buildup. You can slip a clutch? -- Tom Sherman - 42.435731,-83.985007 I am a vehicular cyclist. |
#6
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In article ,
Ben C wrote: On 2009-06-19, Michael Press wrote: In article , Ben C wrote: On 2009-06-19, wrote: SJM who? wrote: [...] 1. To cool the disks 2. To vent gases between pads and disks 3. To allow disk heat expansion/contraction without subtle warp 1. Drilled disks have less cooling surface than solid ones. If you drill a disk it has a higher surface-area to volume ratio than it did before. So I'm not sure what you mean. You are reducing the area of the bit that's directly conducting heat away from the pads. But heat will conduct from there to the inside surfaces of the holes whence it will be cooled by the air. So it's difficult to say which is theoretically better. Denote by h: thickness of the disk r: the radius of the drilled hole the change in surface area = 2.pi.rr - 2.pi.r.h You've only counted one end of the cylinder that you drilled out, Area of a circle is ? Answer: pi.rr Area of the two ends of a right circular cylinder is 2.pi.rr. and you're considering only the change in surface area, not the change in surface area to volume ratio. That is so. I am not considering the surface to volume ratio. See http://groups.google.co.uk/group/rec...d90ad43f00c793 so the surface area increases if r h and decreases if r h. Almost (if you count both ends, the turning point would be 0.5h), but the surface-area to volume ratio always increases, whatever the hole size. Having said all this though, I think you're right that surface area is a more important consideration than surface area to volume ratio. The volume affects the heat capacity, but disks (on cars) heat up quickly and then reach a thermal equilibrium where radiation matches heat generated by friction. So we don't really care a lot about the heat capacity. (Disks on trains, for example, may work mainly as heat sinks instead, like bicycle rim brakes.) Now you must show that the heat transfer out of the walls of the hole is as great as that off the surface of the disk. The air speed through the hole decreases with the radius. Good luck. I guess another question to ask is is heat dissipation typically limited by pad-disk conduction or by disk-air radiation/convection? I do not know. Heat is mostly generated in the pad, is transferred to the rotor by conduction, and is mostly dissipated from the rotor by convection. If the former, you don't want holes (for cooling). If the latter, maybe you do. -- Michael Press |
#7
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#8
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#9
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On 2009-06-24, Still Just Me wrote:
On Tue, 23 Jun 2009 20:49:34 -0400, Radey Shouman wrote: Also, the contents of the holes provide a bulk flow of air past the *pad*. Without holes, when not braking, there is only a narrow space for air to flow past the pad, which makes convection cooling very slow. When braking there is no space for air to flow and cooling can occur only by conduction, to the disk or to the caliper. With holes there is significant flow of air past the pad, whether braking or not. The air inside the holes will be well-mixed, and quickly approach equilibrium with the surface of the pad. Once past the pad, air in the holes will be exchanged with the free stream of air flowing past the disk, thus cooling the pad. This might apply in a system with a center vented rotor. The holes would not cool the pads in a single, non vented disk configuration except by virtue of the fact that they might lower the rotor temperature while it's not between the pads. Even with the vented rotor, the flow to the pad is minimal. I suspect that if the holes have any cooling effect in either configuration, it's when the rotor is NOT between the pads. But the rotor's always between the pads? |
#10
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Vented Discs
On 2009-06-24, Michael Press wrote:
In article , Ben C wrote: On 2009-06-19, Michael Press wrote: In article , Ben C wrote: On 2009-06-19, wrote: SJM who? wrote: [...] 1. To cool the disks 2. To vent gases between pads and disks 3. To allow disk heat expansion/contraction without subtle warp 1. Drilled disks have less cooling surface than solid ones. If you drill a disk it has a higher surface-area to volume ratio than it did before. So I'm not sure what you mean. You are reducing the area of the bit that's directly conducting heat away from the pads. But heat will conduct from there to the inside surfaces of the holes whence it will be cooled by the air. So it's difficult to say which is theoretically better. Denote by h: thickness of the disk r: the radius of the drilled hole the change in surface area = 2.pi.rr - 2.pi.r.h You've only counted one end of the cylinder that you drilled out, Area of a circle is ? Answer: pi.rr Area of the two ends of a right circular cylinder is 2.pi.rr. Sorry, my mistake, you are quite right. |
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