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  1. #1
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    From 27.5 to 29er - what changes in tire pressure?

    Hi,
    I've moved from 27.5 to a 29er bike. On the 27.5 I've run 22-24 psi on rear and very rarely had flats. With the new 29er I've already flatted 4 times in last couple of months, riding the same trails.
    If you've had similar experiences - can you provide number of tire pressures you've changed going from 27.5 to 29er (or vice versa)? That would help me decide whether that's the issues or something else*

    Thanks!

    *It could also be the tire choice - I've asked for experience on Spec' Purgatory GRID 2.6 rear on another thread.
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  2. #2
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    I find tire pressure to depend mainly on tire casing width and not very sensitive to wheel diameter. What's the actual casing widths of your 27.5 and 29 tires? Are you tube or tubeless? And, of course, some tires are tougher than others.
    Do the math.

  3. #3
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    On the 27.5 it was Maxxis Minion DHR EXO. On the 29er it's Specialized 2bliss GRID. Both setups not tubeless. I was wondering maybe the wheel diameter has more leverage on the tires.
    --
    If I work to pay for my rides - then why am I at work right now??

  4. #4
    Ride Fast Take Chances :)
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    Tire pressure is a function of volume. more volume = less pressure to support your weight. Generally using the same tire and size on a 29er lets you run lower pressure.
    Sounds like your current tires are not a tough as your old ones.
    Making shit harder than it needs to be isn't awesome, it's just...harder.

  5. #5
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    Quote Originally Posted by amirh1 View Post
    On the 27.5 it was Maxxis Minion DHR EXO. On the 29er it's Specialized 2bliss GRID. Both setups not tubeless. I was wondering maybe the wheel diameter has more leverage on the tires.
    Go tubeless! It's SO worthwhile. Been years since I've suffered a flat, and I ride unreasonably hard (though I tend to float). Oregon lava, Moab (3x in 12 months), whatever -- tubeless is my savior.
    =sParty
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    We get old because we quit riding.

  6. #6
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    Quote Originally Posted by alexbn921 View Post
    Tire pressure is a function of volume. more volume = less pressure to support your weight. ....
    That's actually not true. It's the tension in the sidewalls that supports the wheel. Tension is created by air pressure and is the hoop stress in the tire itself. Hoop stress is determined by the width of the tire casing and the pressure. Somebody somewhere got on this volume thing thinking it sounded smart but it's specious. Tire casing width is the determinant. Of course, all else equal, a wider tire will have more volume, but that's an irrelevant side effect.
    Do the math.

  7. #7
    Barely in control
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    Yeah, the volume myth was started by some mountain biking meat head journalist about ten years ago and it just won't go away. GO AWAY.

    there. That should do it.

  8. #8
    Ride Fast Take Chances :)
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    Quote Originally Posted by Lone Rager View Post
    That's actually not true. It's the tension in the sidewalls that supports the wheel. Tension is created by air pressure and is the hoop stress in the tire itself. Hoop stress is determined by the width of the tire casing and the pressure. Somebody somewhere got on this volume thing thinking it sounded smart but it's specious. Tire casing width is the determinant. Of course, all else equal, a wider tire will have more volume, but that's an irrelevant side effect.

    Do you have any links to information on the subject?

    Are you implying that a 16x2.25 will support the same weight as a 29x.2.25 at the same pressure? Or does the size of the hoop play a roll in the support too.
    Larger tires tend to be wider and taller giving both sidewall support and extra compression travel. In your option what is the correct way to refer to larger volume tires?
    Making shit harder than it needs to be isn't awesome, it's just...harder.

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