#21
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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 |
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#22
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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 |
#23
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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 |
#24
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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 |
#26
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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. |
#27
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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. |
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