Can someone explain the Physics for this...

Ross Gardner
Ross Gardner Posts: 230
edited January 2017 in MTB general
If you don't use your bike, the tyres run flat. If you use your bike regularly, the tyres keep their PSI for longer. What gives?

Comments

  • cooldad
    cooldad Posts: 32,599
    Er um, so they go flat quicker if you use it?

    Makes sense, as tubes/tyres are slightly porous, so more force will force out more air.

    But to be sure, have you actually tested your hypothesis?
    I don't do smileys.

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  • cooldad wrote:
    Er um, so they go flat quicker if you use it?

    Yeah I worded that wrongly. Thanks for helping me correct myself.
    cooldad wrote:
    Makes sense, as tubes/tyres are slightly porous, so more force will force out more air.

    But to be sure, have you actually tested your hypothesis?

    I was being lectured by someone that tyres get flat quicker if you don't use them. I was like... what?
  • cooldad
    cooldad Posts: 32,599
    Ask them if they've ever tested their hypothesis.

    Leaving a bike in a shed for a year or two doesn't count.
    I don't do smileys.

    There is no secret ingredient - Kung Fu Panda

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  • cougie
    cougie Posts: 22,512
    Your mate is wrong. Simple as that.
  • cooldad wrote:
    Ask them if they've ever tested their hypothesis.

    Leaving a bike in a shed for a year or two doesn't count.

    Well he's been riding seriously for twelve years, and I just recently restarted the sport last year. I could tell if he was winding me up.
  • robertpb
    robertpb Posts: 1,866

    Well he's been riding seriously for twelve years.

    Must be because he's so tired he became confused
    Now where's that "Get Out of Crash Free Card"
  • FishFish
    FishFish Posts: 2,152
    If it is kept at the back of the shed as opposed to say by the back door then it will be marginally warmer and thus the tyres will expand slightly more than those of the one left outside thus imitating the effect of lower pressure. The back door bike will be more exposed to the sun and so the tyres will be slightly hardened by polymer cross linking which would proxy a higher retained pressure.

    Also, as Cougie says, the molecules in the shed tyre are described by Bose Einstein Statistics whilst those in the back door bike follow Fermi Dirac stats - surely you knew that?

    Also if you had a Mammoth or other paciderm in the shed then this would press up against the tyre and force air from the valve thus reducing the pressure - surely you knew that?
    ...take your pickelf on your holibobs.... :D

    jeez :roll:
  • Angus Young
    Angus Young Posts: 3,063
    I think that's 'Pachyderm'... but surely you knew that?
    All the gear, no idea and loving the smell of jealousy in the morning.
    Kona Process 134 viewtopic.php?f=10017&t=12994607
  • veronese68
    veronese68 Posts: 27,562
    Do they really go flat quicker or is it a way of trying to convince yourself it's not really that long since you last rode your bike.
  • Angus Young
    Angus Young Posts: 3,063
    Veronese68 wrote:
    Do they really go flat quicker or is it a way of trying to convince yourself it's not really that long since you last rode your bike.

    Lol!
    All the gear, no idea and loving the smell of jealousy in the morning.
    Kona Process 134 viewtopic.php?f=10017&t=12994607
  • fat daddy
    fat daddy Posts: 2,605
    I have a bike permanently sat on the turbo atm

    the back wheel gets a daily workout rotating around 30 miles of distance ... the front wheel get a 1/4 rotation every week or so when I remember

    neither one looses pressure any quicker

    does sitting on the bike but not turning the wheels constitute as using the bike .... in which case if you balance 70kg on your saddle when you park the bike, the tyres will stay up ?
  • philcubed
    philcubed Posts: 260
    Surely as soon as the wheels stop turning, all the air falls to the bottom of the wheel? If the valve is at the bottom all the air will come out through it. Obvious really. :lol:
  • JBA
    JBA Posts: 2,852
    philcubed wrote:
    Surely as soon as the wheels stop turning, all the air falls to the bottom of the wheel? If the valve is at the bottom all the air will come out through it. Obvious really. :lol:

    That can't be right.
    Haven't you ever had a flat tyre? When you do you will notice that all the air goes to the top of the tyre, leaving only the bottom flat.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • FishFish
    FishFish Posts: 2,152
    That is because the air is lighter than air.

    Surely you knew that?
    ...take your pickelf on your holibobs.... :D

    jeez :roll:
  • FishFish
    FishFish Posts: 2,152
    I think that's 'Pachyderm'... but surely you knew that?


    Right - but I was making a trunk call at the time.
    ...take your pickelf on your holibobs.... :D

    jeez :roll:
  • veronese68
    veronese68 Posts: 27,562
    bompington wrote:
    I think you're all just ignoring the elephant in the room. The real issue is that, if you don't use the bike, the air just gets bored and wanders off to find something to do.
  • You know the stare effect, right? When you're cooking, directly observed food gets done quicker than food left without any supervision. This is a similar concept. If you're not around your bike, it's performance drops significantly and as a consequence, tires don't hold air as well as they should.
  • FishFish
    FishFish Posts: 2,152
    ...I think it is tyres - but surely you knew that. Maybe you were tyred from stairing at your cooking.
    ...take your pickelf on your holibobs.... :D

    jeez :roll:
  • I almost felt like letting you know that both tire and tyre are accepted and perfectly correct, but then I realised that there is no way someone with access to the internet doesn't know that.
  • JBA
    JBA Posts: 2,852
    edited January 2017
    I almost felt like letting you know that both tire and tyre are accepted and perfectly correct, but then I realised that there is no way someone with access to the internet doesn't know that.

    Nonsense! The word is 'tyre'.

    You'll try telling us next that your bike is made of something called 'aluminum'. :wink:
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • Where are you guys from?
  • supersonic
    supersonic Posts: 82,708
    It is because the butyl in the tubes start harden when not used, causing microscopic gaps to open up thus releasing air. When the tyre is rolling and in use, the tube stays more supple.

    Maybe. It's witchcraft really.
  • philcubed
    philcubed Posts: 260
    JBA wrote:
    philcubed wrote:
    Surely as soon as the wheels stop turning, all the air falls to the bottom of the wheel? If the valve is at the bottom all the air will come out through it. Obvious really. :lol:

    That can't be right.
    Haven't you ever had a flat tyre? When you do you will notice that all the air goes to the top of the tyre, leaving only the bottom flat.
    FishFish wrote:
    That is because the air is lighter than air.

    Surely you knew that?

    I think we've all been a bit Newtonian about this. I didn't want to get into Quantum Physics as no-one understands it, but the air in the tyre is both lighter than, and heavier than air. It is only when the tyre is observed that this duality is resolved. Physicists know this effect as 'Schrodingers Shed'
  • bompington
    bompington Posts: 7,674
    I almost felt like letting you know that both tire and tyre are accepted and perfectly correct, but then I realised that there is no way someone with access to the internet doesn't know that.
    Wrong - tire is the American spelling, tyre is the correct spelling ;-)