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On Tue, 24 Aug 2004 12:26:31 -0400, Frank Krygowski
wrote: Mike Jacoubowsky wrote: For those of us who have forgotten our high-school physics, could you give a few examples of absolute vs Celsius (or Fahrenheit) temperatures? In the real world, we might conceivably see a tire/tube start the day at 40 degrees (F) and reach a peak of well over 100 (F), possibly 150? To tell you the truth, I don't honestly know just how hot the air in a tube might get on a very long, very steep descent with ambient air temp at 90 degrees. However, that's a real-world example that some of us experience. OK, the relationship is (P1*V1)/T1 = (P2*V2)/T2 where P and T refer to absolute pressures and temperatures, V is volume. If volume is (reasonably) assumed constant, it cancels out, and you can rearrange to: P2 = P1 *(T2/T1) But again, P & T have to be measured on absolute scales. They're usually not, so you have to convert. For metric measurements like the originally quoted Celsius, the absolute temperature scale is Kelvin. To convert Celsius to Kelvin, add 273.15 If you're starting with degrees Fahrenheit, the corresponding absolute scale is Rankine. To convert Fahrenheit to Rankine, add 459.67 (or alternately, convert Fahrenheit to Celsius and use the Kelvin scale.) For pressures, your gage measures "gage pressure," psig. To convert to absolute pressure (psia) add the pressure of the atmosphere, 14.7 psia. So, 40 deg. F = 499.67 deg R 150 deg F = 609.67 deg R 90 psig = 104.7 psia and P2 = 104.7 psia*(609.67/499.67) = 127.7 psia But now we have to get that absolute pressure converted back to gage pressure. Subtract 14.7 and you get P2 = 113 psig. So in general, it's not as bad as you might think. There are commercially available stick-on temperature indicators that record maximum temperatures of the surface to which they're stuck. http://www.tempil.com/Tempilabel.htm ISTR someone checking tandem rims on mountain descents with these things, but I don't recall what the maximum temperature was. Long response, wow. I think the main thing is that the tire/tube combination is much more sensitive to temperature than the change in air pressure. Rubber like materials don't fare well over about 150 F. Hot pavement with braking on a long descent can cause material troubles. Bill Baka -- Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/ |
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#12
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Frank Krygowski writes:
For metric measurements like the originally quoted Celsius, the absolute temperature scale is Kelvin. To convert Celsius to Kelvin, add 273.15 If you're starting with degrees Fahrenheit, the corresponding absolute scale is Rankine. To convert Fahrenheit to Rankine, add 459.67 (or alternately, convert Fahrenheit to Celsius and use the Kelvin scale.) For pressures, your gage measures "gage pressure," psig. To convert to absolute pressure (psia) add the pressure of the atmosphere, 14.7 psia. So, 40 deg. F = 499.67 deg R 150 deg F = 609.67 deg R 90 psig = 104.7 psia and P2 = 104.7 psia*(609.67/499.67) = 127.7 psia But now we have to get that absolute pressure converted back to gage pressure. Subtract 14.7 and you get P2 = 113 psig. So in general, it's not as bad as you might think. Actually things are worse than you think because rims get much hotter than suspected. My first introduction to how high rims get was with steam generated from water in the rim on a mild descent with hairpin turns at the end of several straight runs. The experience also showed that on exiting the turn, steam stopped escaping which revealed how fast air cooling reduces rim temperatures. That rim heating is significant has been experienced by tandem riders who ride in mountains as well as singles on steep roads such as those in Austria, typically Zirlerberg with winding 18% grade and where bicycling are absolutely prohibited downhill, and for good reason. Even for cares and trucks, several steep run-away tracks exit from curves in the event of brake failure. http://tinyurl.com/jhiu There are commercially available stick-on temperature indicators that record maximum temperatures of the surface to which they're stuck. http://www.tempil.com/Tempilabel.htm Forget about that, it's both temperature and duration that cause dangerous heating and overpressure blow-offs. I've done it as have others with whom I have ridden on steep roads. Unobservant riders might attribute the blow-off to a faulty tire or poor tire mounting but it is heat. ISTR someone checking tandem rims on mountain descents with these things, but I don't recall what the maximum temperature was. After you have ridden a few hundred miles in mountainous terrain and then have a blow-off should be definitive. Jobst Brandt |
#14
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Actually things are worse than you think because rims get much hotter
than suspected. My first introduction to how high rims get was with steam generated from water in the rim on a mild descent with hairpin turns at the end of several straight runs. The experience also showed that on exiting the turn, steam stopped escaping which revealed how fast air cooling reduces rim temperatures. So perhaps for a cheap thrill or two, a cyclist could put a small amount of colored water (to make the steam easier to observe) into their rim prior to a descent? Are you suspecting that the air temp in the tube actually exceeds 100C? In the 0-100C example, the pressure differential didn't appear to be enough to blow a (properly mounted) tire off of a rim (from 100psi to 136psi). I have seen a far greater number of tubes exhibiting snake-bite-type damage (as you'd see on a compression cut) on "blowouts" on steep descents, making me wonder if the material properties of the rubber itself change (for the worse) as temperatures rise. --Mike Jacoubowsky Chain Reaction Bicycles www.ChainReaction.com IMBA, BikesBelong, NBDA member |
#15
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Frank Krygowski writes:
There are commercially available stick-on temperature indicators that record maximum temperatures of the surface to which they're stuck. http://www.tempil.com/Tempilabel.htm Forget about that, it's both temperature and duration that cause dangerous heating and overpressure blow-offs. I've done it as have others with whom I have ridden on steep roads. Unobservant riders might attribute the blow-off to a faulty tire or poor tire mounting but it is heat. As a guy who likes data, I'd still be interested in a "maximum temperature" reading. If someone wanted to log temperature versus time instead, that would be even more interesting, but much more difficult. ISTR someone checking tandem rims on mountain descents with these things, but I don't recall what the maximum temperature was. After you have ridden a few hundred miles in mountainous terrain and then have a blow-off should be definitive. I've done the first, but not the second. I'd hope to find a less scary way of learning about this! OK, nothing lost if you put a few heat indicators on your rear rim, find a steep road (steeper than 12%) in your area and with a tire inflated over 100psi, roll down the hill at between 5 and 8 mph with only the rear brake applied. When the tire blows off, use the front brake to stop and check the highest temperature reached. This will cost you no more than one tube, preferably one that had a few patches. There is no hazard here if you don't have any tight turns. This is an easy test that I have done inadvertently without instrumentation. I'm satisfied that it occurs easily. I have had an opportunity to testify on a tandem case where the rider concocted a story that was immediately apparent to me, because the scenario was based on the belief that brake heating did not cause the tire to bow off. I could prove by the evidence on the bicycle that his story was false and also how the failure actually occurred. Jobst Brandt |
#16
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Mike Jacoubowsky writes:
Actually things are worse than you think because rims get much hotter than suspected. My first introduction to how high rims get was with steam generated from water in the rim on a mild descent with hairpin turns at the end of several straight runs. The experience also showed that on exiting the turn, steam stopped escaping which revealed how fast air cooling reduces rim temperatures. So perhaps for a cheap thrill or two, a cyclist could put a small amount of colored water (to make the steam easier to observe) into their rim prior to a descent? What would colored water do? Are you suspecting that the air temp in the tube actually exceeds 100C? In the 0-100C example, the pressure differential didn't appear to be enough to blow a (properly mounted) tire off of a rim (from 100psi to 136psi). That depends on how long the tube is exposed to how high a temperature. What is proven is that you can blow a tire off the rim with brake heating. I have seen a far greater number of tubes exhibiting snake-bite-type damage (as you'd see on a compression cut) on "blowouts" on steep descents, making me wonder if the material properties of the rubber itself change (for the worse) as temperatures rise. It makes no difference, the tire blows off the rim. That is why you hear a loud bang that produces a long slash in the tube. http://draco.acs.uci.edu/rbfaq/FAQ/8b.4.html Jobst Brandt |
#17
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#18
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wrote in message
Mike Jacoubowsky writes: [...] So perhaps for a cheap thrill or two, a cyclist could put a small amount of colored water (to make the steam easier to observe) into their rim prior to a descent? What would colored water do? Make coloured steam. -- A: Top-posters. Q: What is the most annoying thing on Usenet? |
#19
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Mike Jacoubowsky/Chain Reaction Bicycles wrote:
Are you suspecting that the air temp in the tube actually exceeds 100C? In the 0-100C example, the pressure differential didn't appear to be enough to blow a (properly mounted) tire off of a rim (from 100psi to 136psi). I have seen a far greater number of tubes exhibiting snake-bite-type damage (as you'd see on a compression cut) on "blowouts" on steep descents, making me wonder if the material properties of the rubber itself change (for the worse) as temperatures rise. Checking the Matweb site, http://www.matweb.com/search/Specifi...bassnum=P0RUB1 for properties of vulcanized natural rubber, there are two things that strike me as interesting. One is "Maximum service temperature, Air = 176 deg. F." They don't specify the consequences of exceeding this, and I don't know if they're related to long term degredation or short term failure, but it's interesting. The other is the coefficient of thermal expansion, 125 microinches per inch per degree F. That's pretty high, of course, about ten times as much as aluminum and 20 times steel. I wonder about the change in the interface between the tire and the rim when the dimensions change with temperature. The steel bead wire (assuming that's what you've got) would change the least, by virtue of lowest coefficient and being most insulated. The rim would grow a bit (tightening the fit, I suppose), but the rubber would be trying to grow significantly. Is there a chance this (combined with, say, softening at higher temps) would cause distorion of the bead shape, and cause the bead to lose its grip on the rim? -- --------------------+ Frank Krygowski [To reply, remove rodent and vegetable dot com, replace with cc.ysu dot edu] |
#20
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"DRS" writes:
wrote in message Mike Jacoubowsky writes: [...] So perhaps for a cheap thrill or two, a cyclist could put a small amount of colored water (to make the steam easier to observe) into their rim prior to a descent? What would colored water do? Make coloured steam. Too bad it isnt that easy. |
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