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Tyre pressure

genkigenki Posts: 305
OK, similar to the air pressure topic - here's one for the scientists... I'm doing the Marmotte on Saturday and have to get my tyre pressure right. I'll be pumping the tyres up at 5:30am and then riding them at different temperatures and elevations, not to mention heating the rims on some alpine descents.

At 5:30am I'll be at 1800m altitude and let's say the temperature is 10C. Later in the day I'll be at 1000m in temperatires as high as 35C and at 2650m when it might be 15C. Braking on the descents will cause the rims to heat, but they're aluminium and I'm not the world's worst descender. Tyres are Vittoria Corsa EVO which say they are OK up to 145psi. The rims are Shimano Dura Ace 7850 CL's and should be fine up to at least 120psi. But if I want to be safe and keep the pressure and temperature changes from putting the pressure to above 120psi, what pressure should I put in the tyres to start with when at 1800m and at 10C?

Posts

  • redddraggonredddraggon Posts: 10,862
    100psi, no need for anymore.
    I like bikes...

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  • dennisndennisn Posts: 10,601
    The average air pressure at sea level is 14.69 pounds per square inch. The average pressure in deep space is 0. This means that if you pumped up your tire, at sea level,
    to 100 psi, then launched it into space, the pressure would increase to about 115 psi.
    I doubt you'll be climbing quite that much vertical so don't worry about your tires. Pump them up to whatever is normal for you and ride. Nothing is going to happen.
  • ScrumpleScrumple Posts: 2,665
    Which way do you screw the dust caps on though? Doesn't it change abroad?
  • dennisndennisn Posts: 10,601
    Scrumple wrote:
    Which way do you screw the dust caps on though? Doesn't it change abroad?

    No, you're thinking of south of the equator and the toilet swirl direction. :wink::wink:
  • ScrumpleScrumple Posts: 2,665
    I didn't think they had toilets in the southern hemisphere
  • genkigenki Posts: 305
    Scrumple wrote:
    Which way do you screw the dust caps on though? Doesn't it change abroad?

    Dust caps will be removed for the day. They rank ahead of trimming finger-nails in the weight savings gains.

    OK, so we've nailed the difference in air pressure due to altitude, though I'm thought it would be be more given how much it appears to affect a half-filled bottle of water for instance. But what about the temperature difference. How much will psi change going from 10C to 30C. I'm thinking about tyres going bang when left in the back of a car in the sun.
  • bigmatbigmat Posts: 5,132
    Just fill them with hot air and you'll be fine. Seriously, I'd drop pressure maybe 5 psi to be on the safe side but can't see that you've got much to worry about. I'd be going 115psi, maybe 120. 100 feels a bit sluggish to me, certainly don't agree that you don't need any more especially on billiard smooth continental roads.
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