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#51
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Carbon Brake Tracks With all this talk about cfrp....
Tim McNamara wrote:
I think it's likely for example that bicycle disk brakes dissipate energy to the air much better than rim brakes. I'm curious as to what the mechanism of this would be. Because they get hotter which may more than make up for their lower surface area. Here are the calculations where I estimated the heat capacity of a disk to be a bit less than half that of a rim: http://groups.google.co.uk/group/uk....1d32532f671264 It doesn't really show anything except that a rim brake may well just work as a heat sink, and that if a disk brake also worked that way, it would suck a lot more. My conclusion is that disk brakes must work by dissipation if they work at all and are therefore a better choice for long drags downhill. Disk brakes can be heated much higher than a rim brake because there is no tire attached. You could make a disk brake glow red from heat if you wanted. Rim brakes need to dissipate heat quickly to avoid blowing off the tire, disk brakes can dissipate heat slowly and it doesn't matter. I matters. At some temperature steady stat4 must be reached or the disk would melt. Therefore, dissipation rate must be at least as good as for rim brakes, although at a higher temperature. I have not heard from anyone who, with a disk brake, descended a mountain pass like the Stelvio, but I'm sure it has been done. The question is whether the disk got visibly red hot. I am doubtful of the technology, because I have not yet found any manufacturer at InterBike that could explain why there are holes in the disks and why they are made as flimsy as they ar... other than to save weight. A rim brake has a higher surface area, but doesn't get hot enough (or if it does the tyre blows off). Aluminum rims easily get hot enough to boil water. We've had reports of observing this posted to the newsgroup- I've never seen it myself, but the hills round here are only 600-700 ft gain/loss from base to top, and the roads are straight enough to not require significant braking. Indeed, but I think much hotter than that and you may get tyre problems. A disk on the other hand can get much hotter provided you make the pad material out of some appropriate material. Angel Rodriguez reported putting adhesive heat-sensitive stickers on his tandem rims and finding that the rims got up over 200F very quickly on steep descents requiring heavy braking. I make that 93C, a bit less than the boiling point of water. How hot does a disk get? Dunno, but it could easily be more than twice that. I said 200F because I was confident about that and I didn't feel like searching my bookshelves for Rodriguez's book. My recollection was that his rims heated above 250F in a surprisingly short time of hard braking with two riders on the tandem, like 45 seconds of braking on a steep descent, but I'm not confident in my recollection. The numbers have been quoted in the newsgroup before, though; perhaps Carl "the Google" Fogel can ferret them out. I'll assure you that it is more than 212°F because I generated steam while braking into each hairpin turn on the Nufenen Pass (CH) that blew with a loud hissss from the valve stem hole of a front wheel that had taken on water in stream crossings. That is, until the rim was dry. Fiamme red label rims and Clement Campionato del Mundo tires! Jobst Brandt |
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#52
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Carbon Brake Tracks With all this talk about cfrp....
Tom Kunich wrote:
D'ohBoy wrote: But I am hoping they will be sufficient to handle the max 2-4 mile downhills we have around here that I don't brake on anyhoo. You seem to be missing what's being said. IF you need to stop quickly with carbon rims you cannot. There's no place for the heat energy to go save to melt the rims or the pads since the carbon cannot conduct the heat away effectively. Carbon-epoxy composite is a wildcard with regard to its ability to transmit and reject heat. The thermal conductivity of the fibers themselves varies according to their quality and what kind of original organic material was used to make them. Basically they straddle the same two-orders-of-magnitude range of thermal conductivity that structural metals do, from worse than the least conductive stainless steel to better than silver (the most conductive metal). The epoxy resin matrix in which the fibers are encapsulated has poor thermal conductivity similar to that of other familiar plastics, and comprises a variable portion of a CFRP mixture. This resin can be filled with other powdered materials to change its thermal characteristics. The bottom line is that between the huge conductivity variation between fibers themselves and the huge variation between fiber-resin mixtures, you can't make a valid generalization about how well CFRP dissipates heat. I think it's fair to say that just about any mixture would be harder on brake pads than aluminum, but the thermal conductivity of a CFRP composite can vary by a factor of more than 100. Chalo |
#53
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Carbon Brake Tracks With all this talk about cfrp....
Ben C? wrote:
I think it's likely for example that bicycle disk brakes dissipate energy to the air much better than rim brakes. I'm curious as to what the mechanism of this would be. Because they get hotter which may more than make up for their lower surface area. Here are the calculations where I estimated the heat capacity of a disk to be a bit less than half that of a rim: http://groups.google.co.uk/group/uk....1d32532f671264 It doesn't really show anything except that a rim brake may well just work as a heat sink, and that if a disk brake also worked that way, it would suck a lot more. My conclusion is that disk brakes must work by dissipation if they work at all and are therefore a better choice for long drags downhill. Disk brakes can be heated much higher than a rim brake because there is no tire attached. Yes exactly. You could make a disk brake glow red from heat if you wanted. Rim brakes need to dissipate heat quickly to avoid blowing off the tire, I don't think they do dissipate heat very quickly. In normal use it's OK for them just to soak it up because they have a high heat capacity. If put heat into them continuously because you're keeping them on all the way down a mountain, the tyre _does_ blow off. As I said, they must or they would melt on heavy braking descents. Weighing only a few grams, their heat capacity is less than that of an aluminum 700c rim. I think this is a difference in semantics. They get hotter but do not store more energy than a rim. As hot as they must get, radiant cooling also plays a role beside the convective cooling. disk brakes can dissipate heat slowly and it doesn't matter. Well there is still going to be a temperature at which they stop working properly. Actually for bicycle disks you read stories of the fluid boiling, a problem that has been pretty much eliminated in cars with modern brake fluids. Yes, but no one I have met can tell me anything about the limits. As often occurs in the bicycle business, no significant testing is performed and documented. Just talking to manufacturers at interbike, gives me doubt about many of these brakes. Take for instance among cars and trains. Only "Sport" cars have disks with many transverse drilled holes... line most bicycle disks. I am sure these are vestiges of the "gas bearing brake fade scenario" that is entirely BS. The holes allow the gas emitted from friction materials to escape instead of causing brake fade, as the faithful believe. Jobst Brandt |
#54
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Carbon Brake Tracks With all this talk about cfrp....
Chalo Colina wrote:
But I am hoping they will be sufficient to handle the max 2-4 mile downhills we have around here that I don't brake on anyhoo. You seem to be missing what's being said. IF you need to stop quickly with carbon rims you cannot. There's no place for the heat energy to go save to melt the rims or the pads since the carbon cannot conduct the heat away effectively. Carbon-epoxy composite is a wildcard with regard to its ability to transmit and reject heat. The thermal conductivity of the fibers themselves varies according to their quality and what kind of original organic material was used to make them. Basically they straddle the same two-orders-of-magnitude range of thermal conductivity that structural metals do, from worse than the least conductive stainless steel to better than silver (the most conductive metal). The epoxy resin matrix in which the fibers are encapsulated has poor thermal conductivity similar to that of other familiar plastics, and comprises a variable portion of a CFRP mixture. This resin can be filled with other powdered materials to change its thermal characteristics. The bottom line is that between the huge conductivity variation between fibers themselves and the huge variation between fiber-resin mixtures, you can't make a valid generalization about how well CFRP dissipates heat. I think it's fair to say that just about any mixture would be harder on brake pads than aluminum, but the thermal conductivity of a CFRP composite can vary by a factor of more than 100. So why, as often, is nothing measured and documented on these characteristics of rims? Why is the bicycle industry so lacking in science? If there was good thermal conductivity, no special brake pads would be needed as a start. Where do we go from here? Jobst Brandt |
#55
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Carbon Brake Tracks With all this talk about cfrp....
On Jan 23, 9:54*am, wrote:
I'll assure you that it is more than 212°F because I generated steam while braking into each hairpin turn on the Nufenen Pass (CH) that blew with a loud hissss from the valve stem hole of a front wheel that had taken on water in stream crossings. Water boils at 196°F at the top of that pass (2478m above sea level) and 205°F at the bottom (1159m). Tom Ace |
#56
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Carbon Brake Tracks With all this talk about cfrp....
On Jan 23, 10:54 am, wrote:
Tim McNamara wrote: It doesn't really show anything except that a rim brake may well just work as a heat sink, and that if a disk brake also worked that way, it would suck a lot more. My conclusion is that disk brakes must work by dissipation if they work at all and are therefore a better choice for long drags downhill. Disk brakes can be heated much higher than a rim brake because there is no tire attached. You could make a disk brake glow red from heat if you wanted. Rim brakes need to dissipate heat quickly to avoid blowing off the tire, disk brakes can dissipate heat slowly and it doesn't matter. I matters. At some temperature steady stat4 must be reached or the disk would melt. Therefore, dissipation rate must be at least as good as for rim brakes, although at a higher temperature. I have not heard from anyone who, with a disk brake, descended a mountain pass like the Stelvio, but I'm sure it has been done. The question is whether the disk got visibly red hot. I am doubtful of the technology, because I have not yet found any manufacturer at InterBike that could explain why there are holes in the disks and why they are made as flimsy as they ar... other than to save weight. Radiative energy loss goes as area * T^4 where T is absolute temperature (eg degrees Kelvin) of the disk or rim, assuming the disk radiates roughly as a blackbody, while conductive energy loss should go linearly, as area*(T-T_ambient). So if a disk gets very hot, it is possible (I haven't calculated the actual numbers) that the radiative loss dominates. This doesn't tell you whether the rate of energy loss is greater for a disk or rim at low T. At equal temperatures the rim should still dissipate energy faster because it has more area. In order to assert that a disk brake would be better for long drags downhill one would need to know something about the temperature at which the disk stops working as effectively (probably has to do with glazing the pads?) and actually calculate the energy losses from conduction and radiation to see how hot you could expect the disk to get. Ben |
#58
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Carbon Brake Tracks With all this talk about cfrp....
"Ben C" wrote in message
... Well there is still going to be a temperature at which they stop working properly. Actually for bicycle disks you read stories of the fluid boiling, a problem that has been pretty much eliminated in cars with modern brake fluids. I think you may be attributing rather too much to "modern brake fluids". Well I've heard various reasons why they don't just use car fluid or something like it: it would damage the seals they use (but why not use different seals?); and it strips the paint if you spill it on your pride and joy-- but that applies equally to cars. Actually bikes use similar brake fluid to cars and motorbikes. Either conventional DOT fluid (which is the yeuchy stuff), or mineral oil - but the latter is also used in some cars (hint - they have a lever to make them go up and down, and are known for having good brakes :-) ). Mineral oil isn't hygroscopic, which I regard as a positive point - it means it doesn't go off when exposed to the atmosphere. For that reason alone I'd prefer it if my bike brakes used it. And as you should know, boiling fluid in car brakes isn't the fluid, it's water in the fluid, typically absorbed from the atmosphere. So it seems there's no advantage to be gained from using DOT-style fluids. |
#59
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Carbon Brake Tracks With all this talk about cfrp....
Jobst Brandt wrote:
Chalo Colina wrote: The bottom line is that between the huge conductivity variation between fibers themselves and the huge variation between fiber-resin mixtures, you can't make a valid generalization about how well CFRP dissipates heat. *I think it's fair to say that just about any mixture would be harder on brake pads than aluminum, but the thermal conductivity of a CFRP composite can vary by a factor of more than 100. So why, as often, is nothing measured and documented on these characteristics of rims? *Why is the bicycle industry so lacking in science? * I think the bicycle industry is traditionally lacking in science, depending instead on a long history of trial and error resulting in a kind of genetic fitness for purpose. I'm not sure that is still the case for the sorts of very expensive exotic parts we're discussing here. It is true, though, that the manufacturers of such parts are not generally forthcoming with properties data or test results. If the manufacturer of expensive CFRP rims (just as an example) had lots of good and informative data about their rims that they could share with the public, there are a few reasons they might not want to do so. They might fear that differences from common rims, where they are not to the advantage of their product, could expose them to extra liability claims. They might fear that such data would give a marketing advantage or even technical assistance to their competitors. And they might not want their more astute buyers looking at the prospects for real improvement over what they already have and doing cost/benefit analysis. Nonspecific promises of technical benefits, if believed, do the manufacturer more good than hard data, because the significance of the benefits can become inflated in prospective buyers' minds. We see this effect every single time someone makes a part out of CFRP that was not previously made of that material-- at the outset, magical properties and unrealistic advantages are ascribed to the new gadget. There is also a likelihood that scientific principles have been applied, and some testing has occurred, but that the testing methods are not rigorous. Rigorous testing costs time and money. If the development budget or schedule doesn't support good conclusive and presentable testing, then a small manufacturer will do just what he can to make himself confident that the product is good. At that point, you don't have data, you have "we put the thing between 2x4s in the hydraulic press and it didn't break until we mashed it really hard." In other words, you can be confident in the product without having hard numbers or even without wanting to share the details of why you are confident in it. If there was good thermal conductivity, no special brake pads would be needed as a start. * That's not necessarily true. Even if a CFRP rim had spectacularly good thermal conductivity equivalent to aluminum, you might still need to use a pad specially formulated to be non-abrasive to the particular materials in the rim's surface. Chalo |
#60
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Carbon Brake Tracks With all this talk about cfrp....
Ben C? wrote:
I think it's likely for example that bicycle disk brakes dissipate energy to the air much better than rim brakes. I'm curious as to what the mechanism of this would be. Because they get hotter which may more than make up for their lower surface area. Here are the calculations where I estimated the heat capacity of a disk to be a bit less than half that of a rim: http://groups.google.co.uk/group/uk....1d32532f671264 It doesn't really show anything except that a rim brake may well just work as a heat sink, and that if a disk brake also worked that way, it would suck a lot more. My conclusion is that disk brakes must work by dissipation if they work at all and are therefore a better choice for long drags downhill. Disk brakes can be heated much higher than a rim brake because there is no tire attached. Yes exactly. You could make a disk brake glow red from heat if you wanted. Rim brakes need to dissipate heat quickly to avoid blowing off the tire, I don't think they do dissipate heat very quickly. In normal use it's OK for them just to soak it up because they have a high heat capacity. If put heat into them continuously because you're keeping them on all the way down a mountain, the tyre _does_ blow off. As I said, they must or they would melt on heavy braking descents. Weighing only a few grams, their heat capacity is less than that of an aluminum 700c rim. I was talking about rims. My theory is that disks must dissipate because their heat capacity is so low, but that in normal use rims don't need to dissipate because their heat capacity is high enough just to soak up the energy. My experience is that they cool rapidly and that switching speed control braking between front and rear appears to keep maximum temperatures loser than using both brakes equally. I find such decisions difficult to evaluate without instrumentation. I think this is a difference in semantics. They get hotter but do not store more energy than a rim. They may well store less. As hot as they must get, radiant cooling also plays a role beside the convective cooling. Yes. disk brakes can dissipate heat slowly and it doesn't matter. Well there is still going to be a temperature at which they stop working properly. Actually for bicycle disks you read stories of the fluid boiling, a problem that has been pretty much eliminated in cars with modern brake fluids. Yes, but no one I have met can tell me anything about the limits. As often occurs in the bicycle business, no significant testing is performed and documented. Well I've heard various reasons why they don't just use car fluid or something like it: it would damage the seals they use (but why not use different seals?); and it strips the paint if you spill it on your pride and joy-- but that applies equally to cars. Prima donnas. Jobst Brandt |
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