February 2015 Trail Conditions

[mstone]

@Amalitza 108059 wrote:

A bike tire, depending on the tire, will hold up to 110 psi (or more) higher pressure inside the tire than outside. It doesn’t need its own pressure suit, because it basically is a pressure suit.

Worth noting that the real problem in making a space suit isn’t holding in the air, it’s making something that bends in a way that lets people do useful work that’s light enough to be practical. It wouldn’t be technically all that challenging to make a simple bubble for the astronauts to float around in, but you would have just spent ten million dollars to launch a gag for a seinfeld episode.

@dkel 108062 wrote:

Right. This is why Dismal’s example shouldn’t assume constant volume.

The constant volume is close enough for government work. The volume in question is how much space is inside the tire–it’ll expand a little bit as pressure increases, but not all that much. If it was a balloon rather than a tire, then the volume would also be increasing (up until the pop) and the increased volume would somewhat offset the increase in pressure caused by the rising temperature.

[Anonymous]

@dkel 108062 wrote:

Right. This is why Dismal’s example shouldn’t assume constant volume. If volume remains (magically) constant, then the person in the temperate zone of the moon wouldn’t need a pressure suit because the water in their body would (magically) not evaporate away in an attempt to equalize the severe pressure differential. That person would only need a source of air for respiration. It is also why Tim Kelley’s bike tires may not fail, but their pressure (and volume) would increase.

no no no. The temperate zone is only nice from a temperature standpoint. there is still no atmosphere and therefore essentially zero pressure.

PV = nRT. n is the physical number of gas molecules present. Where n=0, P and V (pressure and volume of the gas) will also equal zero, regardless of T, temperature. This is the case in the moon’s atmosphere. There is no gas, anywhere, thus pressure is zero, everywhere.

Our hypothetical bike tire, which has been filled with air (molecules of nitrogen, oxygen, and a few other things), has an n of a real, positive (but constant) number of molecules and as T varies so will P, holding V constant.

It is technically true that V is not exactly constant, but the tire/rim system is relatively rigid and doesn’t change volume much of any significance. Think about whether your tire (not tube, that is different) really changes volume significantly between the time you get the tire seated on the rims (no pressure compared to atmosphere) and a fully inflated at 80or 100psi–there isn’t a change, the tire nor the rim stretches which would be necessary to increase internal volume. And we are talking smaller pressure differences taking it to the moon than just pumping it to full pressure on earth.

Strictly speaking, Tim’s bike tires don’t increase in pressure, but i admit thst’s a bit pedantic of me. The pressure differential between insidemand outside of the tire is 14.7psi greater–so yes, his tire that he pumped to 80psi at sea level on earth, if he finds the spot on the moon with the same temperature will be 94.7 psi. And, again, technically the volume might increase a smidge, but no different than the tire volume changes when he pumps it from 80 to 94.7 here on earth– which is not much.

[dkel]

@mstone 108064 wrote:

The constant volume is close enough for government work. The volume in question is how much space is inside the tire–it’ll expand a little bit as pressure increases, but not all that much. If it was a balloon, then the volume would also be increasing (up until the pop).

I initially thought about that balloon, as I remember seeing that effect in high school science class when the teacher put a balloon in a bell jar (everyone covered their ears as it was about to pop, which was funny to see, since there was no air to transmit the sound to the class). The tire, as was pointed out, can hold a great range of pressure, and can withstand the increase of one atmosphere unless it’s already pressurized to its limit. Even in Dismal’s example with the vast temperature differential, there would be a net pressure gain from the increase of one atmosphere over the drop in pressure due to lower temperature…I think…I’m too lazy to do the math. Someone can prove me wrong if they want.

BTW, I know this whole thing is silly and pointless, but I’m enough of a nerd to find this kind of discussion somewhat fascinating. Besides: bikes!

[DismalScientist]

That’s right. You tell him!!:rolleyes:

[dkel]

@Amalitza: I get what you’re saying. I think our miscommunication was that I was not assuming the tire’s change in volume was negligible, so the rest of the argument seemed ridiculous to me. But the tire’s change in volume is negligible…until the pressure of the tube on the inside of the tire is enough to overcome the resistance of the bead against the rim, at which point the volume of the tube increases drastically and almost instantly through that opening, since the tire no longer contains the tube, and the outside pressure is zero.

Which is what I would like to have happen, because that’s much more exciting than the volume of the tire increasing a negligible amount, and as long as we’re talking about something patently ridiculous ad nauseam, it might as well be exciting!

The real question is: how would the temperature affect the materials in both the tire/tube and the rim? Discuss.

[Anonymous]

I am a bit unclear as to what the moon has to do with whether or not Tim or GB has to worry about temperature changes when inflating their tires. :p What’s relevant there is this:

@DismalScientist 108026 wrote:

Assuming air is an ideal gas (After all, the ideal gas law is PV=nrT), and the tire volume doesn’t increase with the rise in temperature (and pressure), a rise in temperature from 32 F to 72 F will increase pressure by a little more than 8%.

The result is obviously different if one inflates tire with nitrogen as that gas is not ideal.:rolleyes:

Is 8% pressure change when you go outside too much to worry about? yes/no, regardless of what happens on the moon. well, unless you really are taking your bike to the moon of course.

[PotomacCyclist]

WARNING

NERD ALERT

NERD ALERT

NERD ALERT

Sorry. I hadn’t read this thread in… a few hours and saw what happened to the cherished and insightful thread.

I don’t have any physics analysis to add. (I was pretty good at the si-encez and math thing back in school — even skipping ahead two years in math — but I didn’t really enjoy it after a while, so I dropped the physics major in a heartbeat. I haven’t thought too much about equations since then.) However, I do know of a few examples. I can’t say what the causes were in these cases, just that they happened, and will happen again.

Someone at work had a tire explode in the office. This was probably toward the end of the summer, so there wasn’t a great temperature differential between outside air and inside. I think the guy brought in one of the wheels/tires from his bike as he normally did. Then in the early evening, it exploded. I don’t know whether he had been riding on a cut tire, or if he had over-inflated it or the rim was weakened or what.

EDIT – It’s possible that he left the wheel and tire sitting near an A/C unit, but I don’t know if this was the case.

In many of the larger triathlon races, competitors are required to bring their bikes to the transition area the day before and leave them overnight (to avoid a traffic jam on the morning of the race with people trying to bring their bikes and other gear all at the same time). On the following morning, you will often hear some tires popping. Maybe it’s the temperature change overnight. The changes aren’t extreme since I don’t do cold-weather races. Maybe a drop from 75-85F on the day before, down to the 60s, occasionally the 50s. Maybe some people have damaged tires or rims. Or they overinflated the tires. Whatever it is, this is something that happens at larger races. Perhaps not at every race, but it’s not unusual.

I’ve never had it happen to me, but I’ve heard many of these tires popping, either at work or at triathlons.

[dbb]

@dkel 108062 wrote:

It is also why Tim Kelley’s bike tires may not fail, but their pressure (and volume) would increase.

Now that we have resolved the issue of Tim’s bike tires, how long would his training ride on the moon need to be for him to find himself in an anaerobic training zone?

[brendan]

Depending upon the (atmospherically unloaded) tire pressure, and which side of the moon, I think we’d want to take a look at the phase-change curves involved with the particular gas mixture we’d be using too. E.g. “At [earth] atmospheric pressure, liquid nitrogen boils at −195.79 °C (77 K; −320 °F)…”

B

PS – I believe the astronauts had heaters in their space suits…oops wrong, no heaters in the suits…it’s all about the insulation and reflective outer layer…they planned all EVAs for lunar dawn…neat.

[hozn]

@dkel 108071 wrote:

The real question is: how would the temperature affect the materials in both the tire/tube and the rim? Discuss.

I would think that atmospheric temperature are not the most significant temperature changes that bicycle tires experience — especially with rim-brake wheels. I think bike tires popping in the heat has nothing to do with the heat unless the tire was inflated to the very maximum that the tire could withstands (or within a few psi, obviously). So this idea is popular with triathletes. This seems to be the most reasoned answer (which really just echoes the science above):

Unless you pump your tires near to the very maximum that the tire/rim will handle (which is likely more than the max rating molded into the tire sidewall), it won’t be an issue. For instance, take the extreme example of pumping your tire to 120psi at 60 degrees F. If the sun baked your wheels to 150 degrees F. (not bloody likely), your tire pressure would raise to 140psi.

Tires popping in transition can most often be attributed to poor mounting and pinching the tube between bead and rim. Another reason not to change tires/tubes right before the big race, or at least to pump them up and ride them before racking your bike.

I think the avenue of investigation should probably be the rim in this case, if tires are blowing off. GB, what rims are these? And what size is the tire? Maybe there is just an issue with the bead socket on this rim (or rim + tire). Mavic publishes max pressures for their rims, for example; For their XC rims they have the max pressure for a 2.0 tire being 66psi. That seems to be within spec here, but other rims might have lower limits.

Though I like the direction this thread is taking with atmospheric pressure and the moon.

[DismalScientist]

@dbb 108074 wrote:

Now that we have resolved the issue of Tim’s bike tires, how long would his training ride on the moon need to be for him to find himself in an anaerobic training zone?

Given the moon’s lack of atmosphere, I would think he would immediately enter an anaerobic zone.

[PotomacCyclist]

I don’t know about the phase-change curves, but I did hear about spacewalks from three of the people who went up in space. In the past few years, I’ve seen John Glenn, Buzz Aldrin and Gen. Thomas Stafford speak at the Air and Space Museum. Most know who Glenn and Aldrin are, but Stafford isn’t as well known. He has just as distinguished a career, serving as an Air Force pilot and later as pilot of Gemini VI-A on the first vehicle rendezvous in space, pilot of Gemini IX, commander of Apollo 10, and commander of the Apollo-Soyuz Test Project. (The Apollo 10 crew set a human speed record of 24,791 mph during their return from the Moon.)

I think it was in the talk/interview with Sen. Glenn and Gen. Stafford, with Glenn interviewing Stafford, where Stafford described early problems with spacesuits. He said that when the Soviets tried their first spacewalk, the suit inflated quickly, making it nearly impossible for the cosmonaut to get back into the spacecraft. They almost had to leave that person out in space, but they just barely managed to squeeze him back through the door. In one of Stafford’s missions, the door to the spacecraft jammed and the crew spent quite a while trying to get the door closed again.

I don’t remember if he mentioned heaters or not. Or tire pressure. Probably not, because all this talk of inflating tires on or before reaching the Moon is academic. NASA avoided all this by using non-pneumatic tires for their lunar rovers. They were made of wire mesh, a tread, an inner frame and a hub.

[ATTACH=CONFIG]7808[/ATTACH]

http://www.collectspace.com/news/news-120508a.html

http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_lrv.html

[DismalScientist]

@brendan 108075 wrote:

At [earth] atmospheric pressure, liquid nitrogen boils at −195.79 °C (77 K; −320 °F)…”

Did I not say that nitrogen was less than ideal?:rolleyes:

[dkel]

@hozn 108077 wrote:

I would think that atmospheric temperature are not the most significant temperature changes that bicycle tires experience — especially with rim-brake wheels. I think bike tires popping in the heat has nothing to do with the heat unless the tire was inflated to the very maximum that the tire could withstands (or within a few psi, obviously).[/FONT][/COLOR]

My question was not about atmospheric temperature’s effect on the wheel, it was about whether lunar temperature extremes would compromise the materials of the rim and/or tire, causing it to fail under the (now obviously) minor pressure changes the tire would undergo on the moon. C’mon. Get with the program. :rolleyes:

[americancyclo]

WOD clear from Shreve to DCA. MVT had a bit of a headwind.

[Author]

Posts