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Old 07-15-06, 06:54 AM   #1
boozergut
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Blown Tire Tubes and Boyles Law

Hello:

I am trying to figure out a blown tire issue. Recently before commuting to work I checked the tire presure on my touring bike and aired the tires up to the maximum 90 psi before heading to work. It got up to around 100 degrees that day and the front tire was already blown when I got ready to head home.

So I replaced the tube, aired the tires up to just 80 psi and went to work the next day. The air temp was very high and on the way home the front tire tube pushed itself out of the tire bead. I got off the bike and while I was cmntemplating the problem the tube blew again.

Any once else have issues like this or is it just me. Any suggested solutions?

Thanks, Boozer.......
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Old 07-15-06, 07:46 AM   #2
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Boyles law says if you set the tires at 90 psi at 40°F and the temperature went up to 100°, the tire pressure would have increased to about 101 psi. The pressure increase was not the problem unless:

What rims do you have? If they are the old straight wall design, they will not retain a tire reliably at 90 psi either. If they are modern hook-bead rims, 100 or even 120 psi is absolutely no problem and the temerature increase should have had no effect.
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Old 07-15-06, 07:53 AM   #3
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My guess is that you've pinched a little bit of inner tube under the tire bead when you installed the tire. That's easy to do and most of the time that turns out to be the problem. Before you start to inflate your tire, push the bead in toward the centerline of the rim and peek in all of the way around on both sides to be sure you haven't caught the tube under the tire bead.
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Old 07-15-06, 07:53 AM   #4
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Sounds like a tire/rim issue rather than a pressure or tube issue. Your tube should not be able to push your tire off the bead. What size tires are you running. 90 psi is not high for road type tires, but is high for mtb type tires.

With the tire deflated, how easy is it to get the tire off? The tire should stay on by itself and not fall off the rim. I don't know how to tell you to check bead on the rim and the tire, but basically, those two parts are what keep the tire on the rim. Also ensure when you do inflate that the tire is seated correctly all the way around. Pump up 10-20 psi, check seating and continue if good. However if not seated properly you would likely have a bump that you would feel while riding.
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Old 07-15-06, 07:53 AM   #5
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Thanks Hill Rider I think you may be right as I changed the rear wheel a while back and it is not causing me any trouble. Thank you so much for solving my problem.
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Old 07-15-06, 08:19 AM   #6
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The cheap azz ties I bought may be contributing as well, the bead seems pretty flimsy compared to my other tires.
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Old 07-15-06, 06:29 PM   #7
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You're actually interested in Gay-Lussac's Law, not Boyle's Law - they're each part of the set of ideal gas laws, and Boyle's law says that pressure * volume is a constant, while Gay-Lussac's Law says that pressure divided by temperature is a constant. You're looking at the change in pressure (in a constant quantity of gas in an essentially constant volume) as the temperature changes.

As was pointed out, the actual change is relatively small. That's because you need to measure the temperature in degrees Kelvin.

I'm pointing this out mainly so folks realize how relatively little temperature matters. The temperature in degrees Kelvin is degrees Centigrade + 273.15. So going from 15 degrees C (a cool day) to 35 degrees C (a very hot day) does not increase the pressure by 35/15 or 2.33, but increases it by a factor of 308.15/288.15 or 1.07. That's only a 7% increase in pressure and is extremely unlikely to cause any kind of equipment problems .
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Old 07-16-06, 06:48 AM   #8
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Shouldn't the valve just bleed pressure if it gets excessively high in the tube?
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Old 07-16-06, 07:14 AM   #9
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Quote:
Originally Posted by edmcnierney
You're actually interested in Gay-Lussac's Law, not Boyle's Law
Right. Well, it's been a long time since I took High School and College physics so I gave credit to the wrong Researcher. I do know the ideal gas law so my results were correct even if the attribution was wrong.

BTW, the temperature effect can also be calculated using degrees Rankine (F+459.7) for the Celsius-challenged.
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Old 07-16-06, 07:16 AM   #10
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Quote:
Originally Posted by Treefox
Shouldn't the valve just bleed pressure if it gets excessively high in the tube?
It shouldn't. Tube valves are just blockers, not pressure regulators.
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Old 07-16-06, 07:39 AM   #11
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Check the tire size, and I mean measure it. I just went through this with a set of panaracer ruffy tuffy tires.
They are marked 700c x 28 and after blowing off the rim the second time I measured. They are more like 26mm and a 28-32 tube will take them off of the rim every time. Now using 19-26 tubes and problem solved.
It seems that this is how the manufacturer can make a 28mm flat resistant tire as light as these are...don't make them 28 just mark them 28!!
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Old 07-16-06, 08:07 AM   #12
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Quote:
Originally Posted by boozergut
The cheap azz ties I bought may be contributing as well, the bead seems pretty flimsy compared to my other tires.
The bead is probably just rubber, no cord in it. Get tires with a steel or kevlar cord in the bead. The original narrow Michelin Bib TS from the late 70's early 80's had the same problem. They stayed on the rim for about a max of three tube fixes, then were too stretched out to stay on at a high pressure. Thats the unfortunate bit about very cheap tires and very lite (expensive) tires.
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Old 07-16-06, 09:39 AM   #13
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It would have to be a really cheap tire to have no wire or kevlar bead. The problem with the narrow tires from the 70's was that hook bead rims were not in common usage and it is extremely easy to blow a high pressure/narrow tire off of a straight wall rim.
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Old 07-16-06, 09:55 AM   #14
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Quote:
Originally Posted by Retro Grouch
My guess is that you've pinched a little bit of inner tube under the tire bead when you installed the tire. That's easy to do and most of the time that turns out to be the problem. Before you start to inflate your tire, push the bead in toward the centerline of the rim and peek in all of the way around on both sides to be sure you haven't caught the tube under the tire bead.

I've done this twice, (I'm a slow learner).

You can test for this problem by pumping your tires up a lot higher than you will ever normally use them at after you install the new tube, if they don't blow, they are ok.
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Old 07-16-06, 10:06 AM   #15
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Quote:
Originally Posted by bccycleguy
I've done this twice, (I'm a slow learner).

You can test for this problem by pumping your tires up a lot higher than you will ever normally use them at after you install the new tube, if they don't blow, they are ok.
You can do a better test by pushing the tire beads inward and looking all around the rim to be sure the tube is completely inside the tire and none is caught between the tire bead and rim. This is a much safer (and potentially cheaper) test than over inflating the tire and having your installation mistake demonstrated by a LOUD noise.

As to smellygary's comment that the tires may have no wire or kevlar bead, ALL bike tires have a bead of some sort, usually steel wire on cheap ones, even K-Mart's cheapest.

I'm still convinced the problem is an old-style straight wall rim.
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Old 07-16-06, 10:42 AM   #16
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Quote:
Originally Posted by boozergut
Hello:

I am trying to figure out a blown tire issue. Recently before commuting to work I checked the tire presure on my touring bike and aired the tires up to the maximum 90 psi before heading to work. It got up to around 100 degrees that day and the front tire was already blown when I got ready to head home.

So I replaced the tube, aired the tires up to just 80 psi and went to work the next day. The air temp was very high and on the way home the front tire tube pushed itself out of the tire bead. I got off the bike and while I was cmntemplating the problem the tube blew again.

Any once else have issues like this or is it just me. Any suggested solutions?

Thanks, Boozer.......

Just out of curiosity, why in the world would you want that much air pressure in a bike tire. What kind of bike is it? Mountain ,freeroad? For mountain bikes you need about 45-55 pounds of air.
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Old 07-16-06, 10:54 AM   #17
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As he said, it's a touring bike, which probably means road-touring, thus the higher tire pressures being customary.

Another tube issue to watch for, is the thicker rubber at the base of the valve. If it's down in the bed of the rim, it can keep the tire's bead from engaging the rim's hooks. Solution: before inflating the tire, push the valve up into the rim somewhat, so the base of the valve gets out of the way and the tire's beads can get seated on the rim.
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