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Old 09-06-09, 09:23 AM   #1
Riverside_Guy
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Inflation Pressure?

Want to make sure that what I think is, in fact, true. Recommended inflation pressure seems to come from the tires, not the tubes. My 700x38s carry 50-75 and I just read that someone with 700x32s says it's 110 for him.

So, is it all about the tube and not the tube? That essentially all tubes to fit such tires can handle much higher pressures than what the tire may be marked with? That the actual danger lies in the design of the tires sidewall and bead?
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Old 09-06-09, 09:33 AM   #2
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the tube is a balloon, it will inflate and expand until it pops.
if you give the balloon a cotton outer layer, the balloon will only expand to the limits of the cotton sheet and you can keep inflating the balloon until the cotton sheers.
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Old 09-06-09, 10:18 AM   #3
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Your analysis is correct. A bare tube will handle very little pressure. Confined in a tire, it will handle whatever pressure the tire is rated for.
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Old 09-06-09, 10:33 AM   #4
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Quote:
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the tube is a balloon, it will inflate and expand until it pops.
if you give the balloon a cotton outer layer, the balloon will only expand to the limits of the cotton sheet and you can keep inflating the balloon until the cotton sheers.
Unless if it pops at the valve or a stem. I had a Kenda tire that had it rubber stem popped.

I would think that the max safe pressure is the lowest max pressure of the tube, tire, rim, rim tape and pump.

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Old 09-06-09, 10:58 AM   #5
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another problem is if you use a very narrow rim with very wide tire. you can't inflate the tire to maximum rating because the tire will just blow off the rim.
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Old 09-06-09, 11:02 AM   #6
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There are a number of factors that influence what pressure to run. But, the type of tube is not one of them.

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Old 09-06-09, 11:19 AM   #7
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Originally Posted by Riverside_Guy View Post
Want to make sure that what I think is, in fact, true. Recommended inflation pressure seems to come from the tires, not the tubes. My 700x38s carry 50-75 and I just read that someone with 700x32s says it's 110 for him.

So, is it all about the tube and not the tube? That essentially all tubes to fit such tires can handle much higher pressures than what the tire may be marked with? That the actual danger lies in the design of the tires sidewall and bead?
My 700 x 40s are rated up to 85psi I have been inflating them to 90. After reading a recent article in Bicycle Quarterly that says the best pressure is one that gives a sidewall deflection of 15% ( more gives little reduction in rolling resistance for marked increase in ride harshness ) I'm going to be running them at 55 front 60 rear.

If speed is of the essence, then of course, inflate to max tire rating.
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Old 09-06-09, 06:47 PM   #8
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I normally run my tires at about 10% less than the maximum rating on the tire. So, I do 90psi for a 100psi tire, about 60psi for a 65psi tire. That leaves me a little cushion as the tires heat up while I ride. I might start a ride early in the morning, but, by the end of the ride, the pavement can be cooking from the desert sun. The "max minus 10%" works well for storage, too.
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Old 09-06-09, 07:25 PM   #9
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My 700 x 40s.......If speed is of the essence....
These two statements are an oxymoron.
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Old 09-06-09, 08:44 PM   #10
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These two statements are an oxymoron.
I believe you mean contradictory. And you might be surprised how well they roll.
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Old 09-07-09, 07:59 AM   #11
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Thanks for all the feedback folks... one would THINK that tubes themselves COULD also have inflation guides.

I started running mine around 60. Wasn't paying too close attention and eventually they ran around 50. Thought higher pressure would make for a much stiffer ride, especially as how one of my routes is quite bumpy (concrete roadway with a load of frost heaves). Started asking questions... got a few indicating I should go way over the rating. One guy said 70 front, 90 rear. Am a clyde (225) so I thought I probably should be closer to the top end.

Now I'm running 70/70 give or take a few. Oddly enough, I don't get the feeling of a harsher ride. But I think I got a tad faster... one the same stretch of pathway, I found I could average 1-2 more mph (with little or no wind... I'm also finding headwind CAN make a BIG difference). Of course, that COULD also be because I'm getting in better shape, having re-joined the bike community last June.

I may go with 70/80... I have read 2/3 of the weight load on a bike is on the rear wheel.
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Old 09-07-09, 08:07 AM   #12
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If speed is of the essence, then of course, inflate to max tire rating.
If anybody really cares I can post a lot longer answer but that statement isn't entirely true.
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Old 09-07-09, 08:12 AM   #13
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I'd care... would appreciate more info!
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Old 09-07-09, 08:13 AM   #14
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Quote:
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If speed is of the essence, then of course, inflate to max tire rating.
True *if* the road is silky smooth.

On a bumpy road higher pressure leads to more bouncing of the bike.
Energy used to bounce the bike vertically, is energy that does not go into propelling the bike horizontally.
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