|
|
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. |
Vented Discs
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. |
Vented Discs
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. |
Vented Discs
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. |
Vented Discs
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 |
Vented Discs
|
Vented Discs
|
Vented Discs
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? |
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. |
Vented Discs
On 2009-06-24, wrote:
Ben C? sniped: [...] 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. This is beginning to sound like a "stress relieving" thread. If the effects are so scientifically definable, why are most disks of disk brakes not cross-drilled? Most road cars in normal use don't have such big problems with brake cooling, and the holes have side-effects the normal user doesn't want like faster-wearing pads and possibly cracked disks in what a normal user would consider a low mileage. What means: "Such a problem" on roads whose legal limit is 70mph? These cars are not racing, at least not legally. Actually in my experience you get brake fade from repeatedly braking hard into tight corners without long straights in between for them to cool down. In a road car, fast driving on twisty roads such as when doing a road rally can overheat the brakes. Speed probably never goes above 70. I think your claim of effectiveness of the puny suggestions of aerodynamic devices on the rear of production sedans, or even sports cars, needs some supporting data. I don't have any data, but you can google for it. Here's a link for example: http://www.automobilereviews.com/audi/tt_2000.html Note that there is no speed limit on many German autobahns-- you only got this aero problem at about 125mph according to that article. [...] I think you will find that civilian sedans formerly crashed from brake fade as trucks, still equipped with drum brakes still do judging from warning road signs before steep descents in North America and Europe. What you present is a compendium of auto enthusiast magazine lore. Sure, drum brakes were nearly always worse. But disk brakes do still fade (I've experienced it myself). [...] Some of those spoilers do do something even for road cars. Don't know if you get the Audi TT in America, but a few of them spun off the autobahn while changing lanes at 100mph because the back end of the car experienced aerodynamic lift. The problem was solved by adding a small spoiler. Like the Porsche trunk lid that is automatically raised slightly at speeds above about 70mph to affect an aerodynamic appearance. Yeah, I never understood that. You want the picnic table deployed when you're parked, not at 70+mph. It's also a good way of advertising to plod that you're speeding. You may not have noticed, but that part of the car is swathed in eddy currents which deposit dust and tire spray on the car, for which it has a windshield wiper. If you believe in the merit of spoilers on sedans, you are susceptible to much folk lore. No, I do think most of them are cosmetic actually. |
Vented Discs
"Ben C" wrote in message ... 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? Ben, I think Still Just Me meant that the cooling, if any, happens to the portion of the rotor that is not covered by the pads at any particular time - the pads cover perhaps 1/6 of the rotor circumference. Kerry |
Vented Discs
In article ,
says... On 2009-06-24, wrote: Ben C? sniped: This is beginning to sound like a "stress relieving" thread. If the effects are so scientifically definable, why are most disks of disk brakes not cross-drilled? Most road cars in normal use don't have such big problems with brake cooling, and the holes have side-effects the normal user doesn't want like faster-wearing pads and possibly cracked disks in what a normal user would consider a low mileage. What means: "Such a problem" on roads whose legal limit is 70mph? These cars are not racing, at least not legally. Actually in my experience you get brake fade from repeatedly braking hard into tight corners without long straights in between for them to cool down. In a road car, fast driving on twisty roads such as when doing a road rally can overheat the brakes. Speed probably never goes above 70. Certainly true in my experience. The only time I ever suffered brake fade was in a Lada (front disks - undrilled, read drums) when I drove fairly rapidly down a very steep winding hill. Total distance about 3 km and max speed below 60 _kph_. Nasty brake-fade at the bottom. Mike |
Vented Discs
Hi,
Radey Shouman wrote: Improved heat transfer through increased turbulence may well help to cool the disk. *IF* you wanted to achieve effective cooling by venting, you'd have to do it like the railway disk brakes do it: use double discs with venting channels between them. A lightweight version might be to cut slots into the discs from inside to outside, so that the disc would look like a "comb". These slots would have to be "diagonal" so that the centripetal force acting on the air trapped in the slots forces the air outwards, resulting in a steady airflow. Looking at the low radial speeds on a bike, this would probably not work well. The accepted solution to the cooling problem seems to be "larger discs" - and since disc brakes are quite effective, a large disc might be un-manageable by the (inexperienced) rider, so by reducing the effective surface area, the disc brake gets "softer"... If you're afraid of heat on disc brakes, you need maximum-size discs which would be the wheel rims - so get a rim brake for maximum heat capacity... ... 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. Actually the amount of heat transferred from pad to "air-in-hole" will be so small, you can simply ignore it. The duration of contact between air and pad is almost zero, so the effect is almost zero, too. Stop trying to find a reason for the holes in discs - as I stated in my other post, the most probable reason is "looking cool" - the only technical effect of the holes probably is cleaning the pads - and the disc linked in the original post (with the huge holes) would be unsuitable for that because of the hole arrangement. If the holes had any sensible effect, rims would have holes on the braking surface, because rim brakes are large-diameter disc brakes... And they have no holes... Actually, rim brakes do seldom heat to critical temperatures according to (1). (1) sorry, german only: http://fahrradzukunft.de/fz-0603/0603-11.htm The author used temperature-measuring strips on the rims and found that extreme temperatures (long downhill passages with all-time breaking and lots of luggage) leads to failure of tube patches beyond 80°C and fading due to brake pad overheating beyond 110°C. Ciao... .. |
Vented Discs
On Jun 25, 9:49*am, Bernhard Agthe wrote:
If the holes had any sensible effect, rims would have holes on the braking surface, because rim brakes are large-diameter disc brakes... And they have no holes... And no, you can't bore experimental holes in my rims! Actually, rim brakes do seldom heat to critical temperatures according to (1). (1) sorry, german only: http://fahrradzukunft.de/fz-0603/0603-11.htm The author used temperature-measuring strips on the rims and found that extreme temperatures (long downhill passages with all-time breaking and lots of luggage) leads to failure of tube patches beyond 80°C and fading due to brake pad overheating beyond 110°C. Gee. I know the RBT readers are supercyclists who wear their underpants over the rest of their clothes, but how many 1. Are capable of going to places where it is possible to induce such extreme conditions? 2. Haver ever cycled or will ever cycle in such places? 3. Having reached there, are stupid enough to cycle in such a dangerous manner as to induce such conditions? 4. Having induced such conditions, are stupid enough not to notice? 5. Having noticed, are so thick as not immediately to take the necessary counter-measures? Hell, consider this sequence of facts. Jobst reported the other day that he got up to 50mph or 80kph down some notorious mountain in California. If his brake pads faded, never mind failed to stop him (or melted!), you can bet we would have heard of it. I needed truck assistance to get up to 100kph, and had to stop in a hurry afterwards to avoid crossing a dangerous intersection (or, as bad, hitting the back of my own truck). Neither disc brakes nor roller brakes got too warm to touch.. I didn't at the time have a bike with rim brakes but I do now and often ride the brakes down long descents so as not to speed ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard braking from over 50kph at the bottom of a hill makes no impression either, as I've reported here before. Sounds to me like this German test proves that existing bicycle brakes are plenty good enough, and that venting is an affectation, just like you say. On the other hand, if holes make the fashion victims think they stop cooler, let the disc brake manufacturers take their money. And who knows, maybe some fashion victims will ride and brake hard enough (or believe they do) to get real value out of narrow diagonal slots designed to increase airflow, the only kind that can be said even theoretically to work. I doubt though that objectively venting works perceptibly at bicycle speeds and weights. In conditions where bicycle brakes are genuinely stretched to the point of heating up, say descending an Alp, discs should be made larger rather than vented. But disc brakes are probably not the right choice anyway. If such extreme, extended use is intended, discs should swapped out for hydraulic rim brakes, because the rim is by definition the longest circumference metal surface you can fit to a bicycle wheel, passing through a lot more air for every revolution of the wheel than a disc ever can.. Andre Jute "The brain of an engineer is a delicate instrument which must be protected against the unevenness of the ground." -- Wifredo-Pelayo Ricart Medina "Lateral thinking" -- Edward de Bono |
Vented Discs
On Jun 25, 2:38 pm, Andre Jute wrote:
snip Gee. I know the RBT readers are supercyclists who wear their underpants over the rest of their clothes, but how many 1. Are capable of going to places where it is possible to induce such extreme conditions? 2. Haver ever cycled or will ever cycle in such places? 3. Having reached there, are stupid enough to cycle in such a dangerous manner as to induce such conditions? 4. Having induced such conditions, are stupid enough not to notice? 5. Having noticed, are so thick as not immediately to take the necessary counter-measures? I have seen bicycles that set their brakes on fire stopping after a straight line run on level ground, at Battle Mountain NV. I have personally experienced tire failure due to brake heat riding down Big Cottonwood Canyon east of SLC UT. Hell, consider this sequence of facts. Jobst reported the other day that he got up to 50mph or 80kph down some notorious mountain in California. If his brake pads faded, never mind failed to stop him (or melted!), you can bet we would have heard of it. I needed truck assistance to get up to 100kph, and had to stop in a hurry afterwards to avoid crossing a dangerous intersection (or, as bad, hitting the back of my own truck). Neither disc brakes nor roller brakes got too warm to touch.. I didn't at the time have a bike with rim brakes but I do now and often ride the brakes down long descents so as not to speed ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard braking from over 50kph at the bottom of a hill makes no impression either, as I've reported here before. Sounds to me like this German test proves that existing bicycle brakes are plenty good enough, and that venting is an affectation, just like you say. The best reason for disc brakes has nothing to do with heat. Rim brakes use the rim as a friction surface, which destroys the rim over time. Disc brakes remove that source of wear from a soft aluminum or carbon fiber rim to something designed to be a consumable item. Mountain bikers and bike tourists can wear out a rim in just a single season of riding in muddy conditions as the pads and the dirt combine to form an abrasive slurry applied under high pressure against the rim. The same issues apply to cyclocross in which the UCI recently banned the use of discs. I guess the "purity" of the sport out weighed actually making it affordable to run over extended periods of time without costly parts replacement. |
Vented Discs
On Jun 25, 4:10*pm, Opus wrote:
On Jun 25, 2:38 pm, Andre Jute wrote:snip Gee. I know the RBT readers are supercyclists who wear their underpants over the rest of their clothes, but how many 1. Are capable of going to places where it is possible to induce such extreme conditions? 2. Haver ever cycled or will ever cycle in such places? 3. Having reached there, are stupid enough to cycle in such a dangerous manner as to induce such conditions? 4. Having induced such conditions, are stupid enough not to notice? 5. Having noticed, are so thick as not immediately to take the necessary counter-measures? I have seen bicycles that set their brakes on fire stopping after a straight line run on level ground, at Battle Mountain NV. I'll leave that one to the local scoffjaws, who're just engaging motor- drive. I have personally experienced tire failure due to brake heat riding down Big Cottonwood Canyon east of SLC UT. Why don't you tell us how long and steep this ride is, how irresponsibly you rode down there, how the tire failed, how you determined that it was due to brake heat. Note that I never said it is impossible, merely that the circumstances are rare and avoidable. Hell, consider this sequence of facts. Jobst reported the other day that he got up to 50mph or 80kph down some notorious mountain in California. *If his brake pads faded, never mind failed to stop him (or melted!), you can bet we would have heard of it. I needed truck assistance to get up to 100kph, and had to stop in a hurry afterwards to avoid crossing a dangerous intersection (or, as bad, hitting the back of my own truck). Neither disc brakes nor roller brakes got too warm to touch.. I didn't at the time have a bike with rim brakes but I do now and often ride the brakes down long descents so as not to speed ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard braking from over 50kph at the bottom of a hill makes no impression either, as I've reported here before. Sounds to me like this German test proves that existing bicycle brakes are plenty good enough, and that venting is an affectation, just like you say. The best reason for disc brakes has nothing to do with heat. Rim brakes use the rim as a friction surface, which destroys the rim over time. Disc brakes remove that source of wear from a soft aluminum or carbon fiber rim to something designed to be a consumable item. I agree with you. If you read the archives, you will find me saying so. Mountain bikers and bike tourists can wear out a rim in just a single season of riding in muddy conditions as the pads and the dirt combine to form an abrasive slurry applied under high pressure against the rim. The same issues apply to cyclocross in which the UCI recently banned the use of discs. I guess the "purity" of the sport out weighed actually making it affordable to run over extended periods of time without costly parts replacement. Well, here we drift out of agreement again. I think discs and their maintenance are too expensive, though it is true that wheel rebuilding because the rim is worn through is also very expensive. But the solution is roller brakes. I have Shimano's 70/75 series rollerbrakes on my Cyber Nexus Trek, and they're every bit as good as disc brakes, but cost a tiny fraction in maintenance. You can see my Trek Smover with these excellent enclosed brakes at: http://www.audio-talk.co.uk/fiultra/...%20Smover.html Andre Jute I'm not cheap, I'm poor! |
Vented Discs
Opus aka DC wrote:
[...] I have seen bicycles that set their brakes on fire stopping after a straight line run on level ground, at Battle Mountain NV.[...] Presumably the full streamliners that exceed 120 kph. -- Tom Sherman - 42.435731,-83.985007 I am a vehicular cyclist. |
Vented Discs
On Thu, 25 Jun 2009 07:38:19 -0700 (PDT), Andre Jute
wrote: (snip) Hell, consider this sequence of facts. Jobst reported the other day that he got up to 50mph or 80kph down some notorious mountain in California. If his brake pads faded, never mind failed to stop him (or melted!), you can bet we would have heard of it. I needed truck assistance to get up to 100kph, and had to stop in a hurry afterwards to avoid crossing a dangerous intersection (or, as bad, hitting the back of my own truck). Neither disc brakes nor roller brakes got too warm to touch.. I didn't at the time have a bike with rim brakes but I do now and often ride the brakes down long descents so as not to speed ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard braking from over 50kph at the bottom of a hill makes no impression either, as I've reported here before. (snip) Data point: 220 lb. rider, reasonable 700c rims (Alex DM-18). Coming down Gannett Hill http://www.mountainzone.com/mountain...sp?fid=6276756 the road isn't too straight and there is a deep ditch on either side. I kept speed down to 20 mph. Both rims were very hot at the bottom. How hot? Wish I knew. It wasn't boiling, but uncomfortable to the touch. The temperature measuring strips are a good idea. Now scale the hill up to Rocky Mountain size... |
Vented Discs
Hi,
Opus wrote: I have seen bicycles that set their brakes on fire stopping after a straight line run on level ground, at Battle Mountain NV. I have personally experienced tire failure due to brake heat riding down Big Cottonwood Canyon east of SLC UT. Sure this is possible, though the article I linked earlier says it's unlikely in "normal operation" for most riders. And there's ample warning to prevent brake overheating on long downhill passages - just look in the manual ;-) Apart from that, you can do a lot to keep your brakes cool by not breaking steadily but in intervals. And most brakes don't fail catastrophically. When you notice brake fading, you should stop and wait for the brakes to cool ;-) I needed truck assistance to get up to 100kph, and had to stop in a hurry afterwards Be careful with that - rather find a nice downhill section than use a truck... Sounds to me like this German test proves that existing bicycle brakes are plenty good enough, and that venting is an affectation, just like you say. He concludes that rim brakes do take a lot of abuse and it's unlikely they'll fail catastrophically. He is an experienced long-distance touring cyclist and he doesn't see any special danger in (correctly set up) rim brakes, even on long descends. The best reason for disc brakes has nothing to do with heat. Rim brakes use the rim as a friction surface, which destroys the rim over time. Well, usually the rims will take a lot of breaking before they disintegrate. At least when you use pads that are not too "abrasive"... You know, I once saw a disintegrated rim, but they used very narrow pads (which reduces the area of friction quite much)... Though I do remember an article (by J.Forrester?) on coaster brakes overheating quickly and failing catastrophically... So I'd guess, any type of small drum brake needs some cooling capacity... Remember the cooler on SRAM's iBrake? Disc brakes remove that source of wear from a soft aluminum or carbon fiber rim to something designed to be a consumable item. You could mount "discs" to your rims - at least if you build such a system... But I guess that nobody would do that ;-) rim. The same issues apply to cyclocross in which the UCI recently banned the use of discs. I guess the "purity" of the sport out weighed actually making it affordable to run over extended periods of time without costly parts replacement. Well, I'm not so happy with UCI technical rules - they should at least allow for a "special bike" category - for example, allow recumbent bikes for one week out of the Tour de France - or allow 'bents on time-trial races - I do understand the rule about racing-style handlebars in group races (they allow very good control), but not any rules on bike fit, frame form or whatever. What's the problem if somebody uses a smaller front wheel, disc brakes or curved stays? At least parts of the rules are not strictly necessary, so I'd rather see them eased... Ciao... .. |
Vented Discs
On Jun 25, 4:59 pm, Andre Jute wrote:
snip I have seen bicycles that set their brakes on fire stopping after a straight line run on level ground, at Battle Mountain NV. I'll leave that one to the local scoffjaws, who're just engaging motor- drive. These were human powered bikes capable of 80km/h for an hour and sprints in excess of 110 km/h, not assist bikes. I have personally experienced tire failure due to brake heat riding down Big Cottonwood Canyon east of SLC UT. Why don't you tell us how long and steep this ride is, how irresponsibly you rode down there, how the tire failed, how you determined that it was due to brake heat. Note that I never said it is impossible, merely that the circumstances are rare and avoidable. I don't have the info on that ride, and from what I have seen on Google the road has been changed considerably since I did the race with my friends. The tire exploded off the rim when the bead melted and caused the tire to escape the rim. I was riding steel rims with centerpull calipers in the mid 1970s and the tires were rated to 95 PSI. We would ride our bikes to the ski resort at the top of the canyon, recover and then race to Wasatch Boulevard at the bottom of the canyon. There used to be a C-store there and whoever got to the door of the C-store first was the winner, bike had to be upright on the kickstand so you couldn't just ride up to the door you had to stop and park the bike like a normal person who had just exceeded the national speed limit on a 10 speed bicycle. Nobody had tires go down on the runs, just in the parking lot at the C-store, and sometimes they would fail spectacularly, as in flip the bike over and set it back upright on the seat and handlebars. When the tubes were replaced after the blowouts the tire almost every time would not seat the bead because of bead damage. Sometimes the damage was so easily spotted we didn't even try to reinstall it we just bought another tire. Now we crashed a whole bunch of different ways, but we never had a tire let go from the rim on the road. I had some scary moments on that bike with the steel rims in the rain, but never had a problem in the dry. That race down Big Cottonwood Canyon would wear out a new set of pads if you didn't save your brakes by sitting up with your hands on top of the bars before the turns |
Vented Discs
On Jun 25, 10:24 pm, Tom Sherman °_°
wrote: Opus aka DC wrote: [...] I have seen bicycles that set their brakes on fire stopping after a straight line run on level ground, at Battle Mountain NV.[...] Presumably the full streamliners that exceed 120 kph. -- Tom Sherman - 42.435731,-83.985007 I am a vehicular cyclist. aka DC? Who pray tell is DC. I'm Opus and have been using that name since 1997 |
Vented Discs
On Jun 26, 5:13*pm, Opus wrote:
On Jun 25, 4:59 pm, Andre Jute wrote:snip I have seen bicycles that set their brakes on fire stopping after a straight line run on level ground, at Battle Mountain NV. I'll leave that one to the local scoffjaws, who're just engaging motor- drive. These were human powered bikes capable of 80km/h for an hour and sprints in excess of 110 km/h, not assist bikes. I have personally experienced tire failure due to brake heat riding down Big Cottonwood Canyon east of SLC UT. Why don't you tell us how long and steep this ride is, how irresponsibly you rode down there, how the tire failed, how you determined that it was due to brake heat. Note that I never said it is impossible, merely that the circumstances are rare and avoidable. I don't have the info on that ride, and from what I have seen on Google the road has been changed considerably since I did the race with my friends. The tire exploded off the rim when the bead melted and caused the tire to escape the rim. I was riding steel rims with centerpull calipers in the mid 1970s and the tires were rated to 95 PSI. We would ride our bikes to the ski resort at the top of the canyon, recover and then race to Wasatch Boulevard at the bottom of the canyon. There used to be a C-store there and whoever got to the door of the C-store first was the winner, bike had to be upright on the kickstand so you couldn't just ride up to the door you had to stop and park the bike like a normal person who had just exceeded the national speed limit on a 10 speed bicycle. Nobody had tires go down on the runs, just in the parking lot at the C-store, and sometimes they would fail spectacularly, as in flip the bike over and set it back upright on the seat and handlebars. When the tubes were replaced after the blowouts the tire almost every time would not seat the bead because of bead damage. Sometimes the damage was so easily spotted we didn't even try to reinstall it we just bought another tire. Now we crashed a whole bunch of different ways, but we never had a tire let go from the rim on the road. I had some scary moments on that bike with the steel rims in the rain, but never had a problem in the dry. That race down Big Cottonwood Canyon would wear out a new set of pads if you didn't save your brakes by sitting up with your hands on top of the bars before the turns Wow! An impressive ride. One would hope that components moved on a bit from them. -- Andre Jute |
Heat soak on stoppping, was Vented Discs
On Jun 26, 2:57*am, somebody wrote:
On Thu, 25 Jun 2009 07:38:19 -0700 (PDT), Andre Jute wrote: (snip) Hell, consider this sequence of facts. Jobst reported the other day that he got up to 50mph or 80kph down some notorious mountain in California. *If his brake pads faded, never mind failed to stop him (or melted!), you can bet we would have heard of it. I needed truck assistance to get up to 100kph, and had to stop in a hurry afterwards to avoid crossing a dangerous intersection (or, as bad, hitting the back of my own truck). Neither disc brakes nor roller brakes got too warm to touch.. I didn't at the time have a bike with rim brakes but I do now and often ride the brakes down long descents so as not to speed ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard braking from over 50kph at the bottom of a hill makes no impression either, as I've reported here before. (snip) Data point: *220 lb. rider, reasonable 700c rims (Alex DM-18). *Coming down Gannett Hillhttp://www.mountainzone.com/mountains/detail.asp?fid=6276756 the road isn't too straight and there is a deep ditch on either side. I kept speed down to 20 mph. IOW you were riding the brakes almost constantly. Both rims were very hot at the *bottom. *How hot? *Wish I knew. *It wasn't boiling, but uncomfortable to the touch. *The temperature measuring strips are a good idea. Makes me wonder if "uncomfortable to the touch" is anywhere near the failure of patches due to overheating beyond 80 degrees C found by the the German researcher cited by Bernhard. Now scale the hill up to Rocky Mountain size... Did you read about Opus's ride on steel rims and narrow tyres with bead failure on stopping? (Everything I've reported is on relatively wide rims or very wide rims, and with tyres between 37 and 60mm wide.) Again, it makes you wonder about heat build-up when you stop moving, when the air no longer flows around the rim, IOW whether the danger isn't the hard ride but cutting off the airflow. Maybe those recumbent riders with fairings aren't doing their brakes much good either. Andre Jute Curioser and curioser |
Heat soak on stoppping, was Vented Discs
In alt.rec.bicycles.recumbent wrote:
Andre Jute wrote: Maybe those recumbent riders with fairings aren't doing their brakes much good either. Tires generally lift off while rotating under load because it requires flexing of the tire bead o the rim to creep off.. Unless a tire is already well on its way to blow off when the bicycle stops, the tire doesn't come off. On a recent ride in Sonoma County, California recently I experienced two blowoffs that occurred on steep downgrades when I stopped to take photos. The first on my front tire occurred after I had stopped about 2/3 of the way down a short but steep downhill on Stewarts Point-Skaggs Springs Rd. between Tin Barn and Annapolis Rds. for the express purpose of letting my rims cool, and the other on the rear tire was near the bottom of Meyers Grade where I had stopped to take photos. In the latter case the blowoff occurred shortly after I had resumed the descent following a photo stop; in the former case, I had come to a complete stop before the tire blew. http://tinyurl.com/q889to I also suffered a third blowoff earlier that day while I was dragging my rear brake holding a camera in my free hand to record a video clip. To Mr. Jute's point, aerodynamic fairings allow less aerodynamic braking, requiring the brakes to be used more, resulting in hotter rims. My experiences are not too unlike those of a moderately heavy tandem pair. Although I probably end up replacing my pads and rims slightly more often than an upright bike rider of similar weight, wear is not the main issue. Rim heating causing tire blowoff is. Finding the right tire, tire pressure, and rim is the key to avoiding blowoffs. -- Bill Bushnell http://pobox.com/~bushnell/ |
Vented Discs
Opus the Poet wrote:
On Jun 25, 10:24 pm, Tom Sherman °_° wrote: Opus aka DC wrote: [...] I have seen bicycles that set their brakes on fire stopping after a straight line run on level ground, at Battle Mountain NV.[...] Presumably the full streamliners that exceed 120 kph. -- Tom Sherman - 42.435731,-83.985007 I am a vehicular cyclist. aka DC? Who pray tell is DC. I'm Opus and have been using that name since 1997 Oops, wrong C. -- Tom Sherman - 42.435731,-83.985007 I am a vehicular cyclist. |
Vented Discs
Still Just Me... writes:
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. Holes that communicate with central vents would certainly have a centrifugal pumping feature, as would the radial slots described somewhere up thread. I think that even holes in a solid disk would significantly increase air flow past the pad, since without them air flow is near zero. The disk is normally rotating, so there are two phases, when between the pads the air in the holes is heated by the pads, and when not between them the holes exchange air with the cooler free stream around the brake. In order to cool the pads both are necessary, which explains why clutch disks would not benefit from similar holes. |
| All times are GMT +1. The time now is 11:49 AM. |
Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
CycleBanter.com