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Tyre Pressure And Altitude

Discussion in 'Tyres' started by Sam1199, Apr 4, 2016.

  1. I've spent the last couple of days puzzling over the counter steer on my ST4.
    I knew it wasn't doing it when I set off.
    First thing I did this morning was check the pressures. I check them a lot nowadays but yup they were down, 29.7 in the front. Under pressure despite checking them 3-4 days ago.

    After checking this out on the web it seems that you lose around 1lb pressure for every 250m you climb.

    We climbed to 1000m on the last day. No wonder I could feel massive counter steer.
    The pressure must have been the equivalent of 26psi.

    I wish I'd had this info before. Instinctively I could feel it in the bike. Next time I go over a pass I'm going to check the tyres before and bung in a few extra PSI.

    Thought you guys might like to know. Please let me know if I'm wrong.
     
  2. I don't think you're wrong.
    PSI ratings for tyres are worked out for sea level barometric pressure, give or take. If you go high enough above sea level, valves that are good enough to seal in air at one atmosphere may not be up to keeping that seal against a more rarefied atmosphere.

    Going up 1000m brings atmospheric pressure down from 15psi to about 13psi. That's like upping your tyre pressure at seal level from 34PSI to 38PSI. You'll lose air pressure in the tyre more quickly than you are accustomed to, as the tyre has been effectively over-inflated. Combine that with valves that are being placed at greater than normal stress due to low barometric pressure and you'll get the situation you described.

    I think. I could be wrong :)
     
  3. I think you are, wrong that is.

    The volume within the tyre doesn't change, significantly, and the mass of the air is fixed, so the pressure is mainly dependent upon temperature.

    Relative to ambient the pressure will increase with height, but the absolute pressure will remain the same, except for that temperature effect.

    Also the OP's assumption is obviously wrong as it would mean that at 3750m atmospheric pressure would be zero, and it isn't.
     
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  4. I see what you are saying but air pressure inside a tyre isn't a closed system. It is affected by the pressure outside.

    The air pressure surrounding the tyre will affect the volume of the tyre in a similar way (if not to the same degree) to the air pressure inside the tyre. This is because the tyre is deformable. If atmospheric pressure outside the tyre was say, doubled, the tyre would appear to be flat, compared to how it looks in normal atmospheric conditions.
     
  5. Indeed. This is all about the relationship between "gauge pressure" and "absolute pressure". Tyre pressure gauges conventionally assume standard atmospheric pressure of 1 bar as a starting point, so a gauge pressure of say 36 psi means 36 psi above atmospheric (which at sea level means about 50 psi absolute). The gauge shows the difference between the pressures inside and outside the tyre.

    If you ride up high mountains, the absolute pressure inside the tyre will remain the same as before (assuming the same temperature), so 50 psi. But instead of the gauge (difference) pressure being 36 psi it will now show a higher figure somewhere between 36 and 50. Tyre valves can easily cope with higher pressures that that.

    If later on the pressure gauge shows a lower figure, that means some of the air has leaked out. A sensible response is to track down the slow leak and fix it.
     
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  6. The tyre pressure is also dependent upon the load placed upon the wheel and whilst the tyre is deformable it isn't particularly elastic.

    I think the temperature drop at 1000m altitude (dry adiabatic lapse rate is 1deg C per 100m) would have more of an effect upon the pressure and performance of the tyre than atmospheric pressure.
     
  7. Temperature will have an effect, yes, but I think you underestimate the effect of ambient air pressure and the elasticity/deformability of modern rubber.

    However, if we don't start discussing this in purely scientific terms, with facts, figures and verified research information, @Pete1950 is going to pop in and give us a right telling off. Time for me to bow out.
     
  8. Corrected for you.
     
  9. I have a bad hip, you know. My legs are not for pulling :p
     
  10. dry adiabatic lapse rate is 1deg C per 100m.. I love it whatever it means.

    This is how it went. I pumped up the tyres. I went up 1000m
    They definitely definitely performed differently. Like they didn't have enough pressure in them.They didn't work in the way they should. they didn't work the way they were working.

    I spent the whole time counter steering, which I wasn't doing before or radically accelerating through the bendin order to counter the counter steer.


    I swear the pressure in the tyre was effectively lower up the mountain.
     
  11. I lent my slide rule to my sister many years ago and never got it back.

    It is an interesting problem with many variables.

    It means that if you take a packet of dry air, raise it 100m, allowing it to freely expand without loosing any energy to it's surroundings then it's temperature will drop by 1 deg C.

    This I do not dispute, the interesting question is Why? I think it was probably a temperature effect.
     
  12. Of course, air temperature will have a smaller effect on tyre dynamics than track temperature. The variance of track temperature won't necessarily mirror air temperature differentials.
     
    • Agree Agree x 1
  13. kind of how fridge works. at altitude does it feel like your riding a zannossi ?.
     
    • Face Palm Face Palm x 1
  14. At 1500m your tyre pressure will have increased by 2-3psi. However the ambient temperature at 1500m will be about 10-15C lower and that will reduce the tyre pressure by about 2-3psi. However this does not take into account various variables that come into play. As you ride up/down a mountain the temperature, pressure and humidity will change due to the weather and time of day as well as the altitude and this will affect your tyre pressures. As you ride you increase the tyre temperature which affects the tyre pressure and the level of grip, in which the road surface and road temperature have a big part to play.

    Personally I would just check/set my tyre pressures at the start of each days riding and accept that many factors will affect the handling of the bike during the day.
     
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  15. I should have checked them again that morning. I'd unknowingly lost 2-3lbs in the previous 2-3 days.
    That made all the difference. Add in this variable and I was about 6lbs down. Hence counter steer in hairpins and faster bends but not really fast bends.
     
  16. What did you get to measure the pressure at 29.7? That is a very specific figure considering that the average tyre pressure gauge can be + or - 3 psi. Then, as has been said, air temperature and road temperature could raise or lower the pressure a few psi. Then there is the weather, a local high pressure or low pressure will affect the tyre pressure.
    The mean pressure at sea level is assumed to be 1013 but you will be hard pressed to find anywhere at sea level that is that exact pressure for any length of time, it will be higher or lower.
    Perhaps the road surface was different at 1000 metres than the road at sea level. I am pretty sure that I could not feel a difference of about 3 psi when I am climbing and descending on different roads in a mountainous area.
     
  17. dont know if its comparable but any fiat owners manual iv read (to many) advise you to put an extra .4bar in your tyres if they are warm. ie f you have traveled any kind of distance before you check them.
     
  18. That's a curious statement @SunEye . I wonder what you could possibly mean by it? Surely you cannot be suggesting that extra air has somehow got into the tyre, thus increasing the absolute pressure?

    Perhaps you mean that the difference between the pressures inside and outside the tyre has increased, while the absolute pressure remains the same.
     
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  19. Apologies if I was not clear in what I said, but you are absolutely correct that I meant that the difference between the internal and external pressures had increased.
     
  20. I love the physics class we all get to enjoy here.
     
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