Are you setting the right tyre pressure

sgbman
sgbman Posts: 44
edited March 2015 in Workshop
Hello, I have 2 track pumps, a SKS and JoBlow2.
These both start at 0 bars, and when I want to inflate to 7 bar, i pump up to the 6 on the gauge.
The reason being that normal air pressure is 1 bar and if I want 7 bars I need to increase pressure by a further 6 bars(ie go from 0 to 6 on the gauge).
If this sounds wrong, try this- put your finger over the end and inflate to the one, and you can feel that the pressure build-up is higher than normal air pressure(ie feels like double)

Please could people tell me if my reasoning is correct.

Comments

  • diamonddog
    diamonddog Posts: 3,426
    You are wrong, in your example the pressure in your tyres should be 7 bar gauge pressure not atmospheric pressure.
  • gozzy
    gozzy Posts: 640
    (Assuming you have a car) Do you inflate your car tyre tyres to 1 bar less than the recommended pressure too?
  • Ben6899
    Ben6899 Posts: 9,686
    Manc33, is that you?
    Ben

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  • rafletcher
    rafletcher Posts: 1,235
    sgbman wrote:
    Hello, I have 2 track pumps, a SKS and JoBlow2.
    These both start at 0 bars, and when I want to inflate to 7 bar, i pump up to the 6 on the gauge.
    The reason being that normal air pressure is 1 bar and if I want 7 bars I need to increase pressure by a further 6 bars(ie go from 0 to 6 on the gauge).
    If this sounds wrong, try this- put your finger over the end and inflate to the one, and you can feel that the pressure build-up is higher than normal air pressure(ie feels like double)

    Please could people tell me if my reasoning is correct.

    OK, you're wrong. Happy now? :roll:
  • sgbman
    sgbman Posts: 44
    1 bar =0.986923267 atmosphere, so roughly the same.

    Off wiki:
    Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
    Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."

    So I am wrong, it seems that 7 bar pressure is the same as 8 bar absolute. This is never stated in the manuals. :roll:
  • sgbman wrote:
    1 bar =0.986923267 atmosphere, so roughly the same.

    Off wiki:
    Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
    Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."

    So I am wrong, it seems that 7 bar pressure is the same as 8 bar absolute. This is never stated in the manuals. :roll:

    I think because most people would naturally assume 7 bar means 7 bar above ambient air pressure :)
  • diamonddog
    diamonddog Posts: 3,426
    OP it's pumping tyres not scuba diving so not really that important. If you want to stick to your theory then go ahead it probably won't be life threatening. :evil:
  • desweller
    desweller Posts: 5,175
    sgbman wrote:
    1 bar =0.986923267 atmosphere, so roughly the same.

    Off wiki:
    Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
    Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."

    So I am wrong, it seems that 7 bar pressure is the same as 8 bar absolute. This is never stated in the manuals. :roll:

    You could have figured it out by dint of the fact that the gauge reads zero when it's at atmospheric.
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    On Strava.{/url}
  • Matthewfalle
    Matthewfalle Posts: 17,380
    But how accurate is the gauge in the first place? Is 0 really 0?
    Postby team47b » Sun Jun 28, 2015 11:53 am

    De Sisti wrote:
    This is one of the silliest threads I've come across. :lol:

    Recognition at last Matthew, well done!, a justified honour :D
    smithy21 wrote:

    He's right you know.
  • desweller
    desweller Posts: 5,175
    Oh god no they're crap ha ha ha. 6 probably ain't 6, never mind 0 being 0.
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  • sungod
    sungod Posts: 17,400
    it's a conspiracy!
    my bike - faster than god's and twice as shiny
  • team47b
    team47b Posts: 6,425
    I live virtually ( :D ) at sea level should I put more air or less in my tyres as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation?
    my isetta is a 300cc bike
  • Bozman
    Bozman Posts: 2,518
    Use your thumb to gauge the tyre pressure.
  • Matthewfalle
    Matthewfalle Posts: 17,380
    Does this mean that if I pump up my tyres at my house (sea level) and ride up the nearest really, really, big hill (so now not sea level) I have to let out air/pump up my tyres more to allow for pressure changes?

    Does this also mean that I have to let out air of my tyres as they get hotter as I ride?
    Postby team47b » Sun Jun 28, 2015 11:53 am

    De Sisti wrote:
    This is one of the silliest threads I've come across. :lol:

    Recognition at last Matthew, well done!, a justified honour :D
    smithy21 wrote:

    He's right you know.
  • southdownswolf
    southdownswolf Posts: 1,525
    Is a reading of 6 on the SKS exactly the same as a reading on the JoBlow2 ?
  • sgbman
    sgbman Posts: 44
    Yes 6 on both.
    Tyres always feel rockhard on 6, I weigh 75kg, and use 23mm tyres.
  • veronese68
    veronese68 Posts: 27,836
    Does this mean that if I pump up my tyres at my house (sea level) and ride up the nearest really, really, big hill (so now not sea level) I have to let out air/pump up my tyres more to allow for pressure changes?

    Does this also mean that I have to let out air of my tyres as they get hotter as I ride?
    Give yourself a slow puncture at the bottom of a big climb. As you climb you will lose some air allowing you to maintain a constant pressure. On the descent you need to drag your brakes sufficiently to build up the heat required to maintain air pressure. Obviously this technique stops tyres blowing off rims through overheating as well. It's a win/win situation.
    There is of course an exception to this. If you have disc brakes you will die to death as you will not be able to maintain sufficient pressure through heat build up so will crash. At which point the red hot disc will cut your head off.
  • Good grief there's some right nonsense written on internet forums!

    What about the effects of the sun? You've completely ignored those! If you are riding in bright sunshine, the effect will be to heat the tyre, and so heat the inner tube, and eventually heat the air inside as well. This will, of course, affect the pressure. To be on the safe side, it's best to make sure that you ask about the thermal transfer coefficients of both the tread and the sidewall in your LBS when buying tyres, so that you can best determine how much extra heat will be transferred through to the air inside.

    I avoid the problem by the simple means of painting my tyres white, and making sure I only ride on cloudy days.
  • Matthewfalle
    Matthewfalle Posts: 17,380
    If I pull loads of wheelies will this mean my front tyre is running at a lower pressure than my rear tyre as it's less in contact with the road and getting more air cooling?
    Postby team47b » Sun Jun 28, 2015 11:53 am

    De Sisti wrote:
    This is one of the silliest threads I've come across. :lol:

    Recognition at last Matthew, well done!, a justified honour :D
    smithy21 wrote:

    He's right you know.
  • shmooster
    shmooster Posts: 335
    Pump them up to 11
  • veronese68
    veronese68 Posts: 27,836
    If I pull loads of wheelies will this mean my front tyre is running at a lower pressure than my rear tyre as it's less in contact with the road and getting more air cooling?
    Yes, of course, but this is a good thing because the slight drop in pressure after a short wheelie means you get a slightly more cushioned landing so reducing the stress on other components. Unless you wheelie too far in which case you will lose too much pressure, get a pinch flat from the impact of landing and crash. If this does happen, you'd better hope you haven't got disc brakes because ...
  • Matthewfalle
    Matthewfalle Posts: 17,380
    What if I pump my tyres up over here then fly to Australia and ride my bike there? Will being upside down affect the pressures?
    Postby team47b » Sun Jun 28, 2015 11:53 am

    De Sisti wrote:
    This is one of the silliest threads I've come across. :lol:

    Recognition at last Matthew, well done!, a justified honour :D
    smithy21 wrote:

    He's right you know.