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Thread: Tire Pressures

  1. #1
    aL1
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    Tire Pressures

    When I put air in the tires (about every month or 2), I notice that the rear tire pressure is almost always much lower than the front...& this even happened last year with different tubes and tires-

    Today for example, I checked the pressure before I inflated them and got:

    Front= 50 psi
    Rear= 38 psi

    Not too long ago the front was around 40 psi, and the rear 20 psi-

    -I always put 60-65 psi in front and rear when I inflate them
    -I ride almost every day, or at least x3 a week, for a minimum of 10-20miles/week
    -the bike is ridden in south florida
    -the bike is kept indoors, with the air conditioning on once-in-awhile mostly at night
    -tires 26x1.9 Geax Evolutions (and samo-samo with Kenda Kozmic Lites X2)

    So, does it sound like a leak, bad rim, or does this sound normal?? -thanks-
    2009 Hardrock Sport Disc

  2. #2
    T.W.O.
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    Tubes and tire are porous so when you pump them up tiny air molecules escape through these. If you don't want to keep putting air every 2 months you can try "stayfills" Some kind of proprietary gas that is bigger than co2 and air it can't escape through the tubes and tires.

    Only cost $30 to fill 2 tires. Down side it's a bit heavier than air, so don't expect to save weight there. It's not protect you against flats so, once you flat your $30 is gone with the wind. If you have to inflate once every 2 months it may even double or triple the time between pumps.

    Good luck,

  3. #3
    aL1
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    I was just wondering if the amount of air I was losing was to be expected, and within range...yeah, $30 a few times a year would be prohibitive on my wallet, but it's worth noting, thanks-
    2009 Hardrock Sport Disc

  4. #4
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    I don't think it's for recreational use as much as racing. It's important to most people to keep the air exactly where they want and not messing with it. They don't mind paying as long as the product works.

  5. #5
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    See if you can find a tire store or car dealer using nitrogen in their tires. The car dealer I work for promotes nitrogen to all the customers. It has some advantages over regular air.

    95-97% pure nitrogen vs air at 78%.
    "supposedly" the nitrogen molecule is larger so less leaks through porous (sp) tires and tubes.
    little to no moisture within the tube
    less tire expansion with nitrogen vs reg air. Probably not a factor in bikes as they don't get as hot.

    It may be worth a try.

  6. #6
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    Sounds fairly normal to me. I air my tires either every ride (mtb which is typically once a week) or every other day for my road bike (which is the bike I ride to work). I have noticed losses of 20psi on my road bike over a few days, which makes sense since tubes are porous and road tubes are low volume-high pressure.

    I think:

    What you're experiencing is normal
    Try to air your tires more frequently.
    :wq

  7. #7
    weirdo
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    Maybe your neighbors are sneaking in and stealing tire air. Any chance you live in an area with lousy air quality and usually pump up your tires in a place with GOOD air, so they come by and suck out the good stuff from time to time?

    Just kidding. I dont know- I used to check mine as I filled them, but I dont think I ever took note of what pressures they were at before filing, so I couldn`t say. My best guess is a very slow leak in one of the tires (I have had those before) that you can`t see by dunking the tube. You`re probably right to just keep pumping them up when you need to and ignore it in the mean time.

  8. #8
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    It is absolutely....

    normal for bike tires to loose air over time. It's one of the reasons that you should check the pressure every couple of days, not every month or so. This is particularly important for off road riders that tend to run their tires at much lower pressures. Bike tires do not have to meet the same strict standards as automotive tires and don't have the air volume of an automotive tire. So the air loss is more rapid with a bike tire. But even a car tire if not checked regularly will loose air. And even nitrogen filled tires must be checked for pressure regularly.

    The reason that your rear is usually lower than the front, even though you filled it to a higher pressure is the weight distribution when you are riding the bike. Most bikes have a rearward weight bias. So the rear takes allot more pounding as you ride over bumps and obsticals. This can actually force air molecules through the rubber at a higher rate.

    So what you are expiencing is pretty much normal. If you want a real exercise in frustration try running super light latex tubes. They tend to loose air at about twice the rate of a regular butyl rubber tube.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  9. #9
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    This is totally normal. And makes sense if you think about it.

    1) You know that air will slowly leak through the microscopic holes in the tube if it's just sitting there
    2) Logically, as the bike is rolling and the tire is under compression, the tube is flexing and stretching more at the contact patch. This causes those micro "holes" to speed up leakage
    3) around 60% of the rider weight weight is on the rear wheel. Which means the rear tire tube is going to flex around 50% more
    So, if you lost 22 lbs on the rear and 10 on the front, that's not far off off what the numbers would dictate. You're pushing the rear more than the front

    It doesn't sound like you have a problem at all. However for commuting I would go with a better tire like a 1.25 Fatboy. The higher pressure and lower rolling resistance will make the bike feel much faster. They look a bit weird on an MTB frame, but anyone laughing will be left in your dust. Just a thought
    Life is too short to race through it. When life is a blur, you'll miss the magic.

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