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My bicycle tire pressure

  1. Apr 16, 2005 #1
    i have a small hand pump, that i can set on a block of wood and put all of my weight against to get some pretty high pressures. i dont' have a good gauge so i'm not really sure what i can do with it, other than get tires pretty high pressure (if i had to guess, at the least 75psi).

    i recently built up a new bike, with rather skinny road tires (25c wide). i usually ride tires a bit wider, but i wanted to go faster and have less rolling resistance. normally, i can pump up my bike tires so they are hard as a rock, and uncomfortable almost to ride. the new bike though, the tires are still a bit low feeling.

    is it harder to get the same pressure with the same pump on a smaller volume tire? or am i getting the same pressure, but the pressure is less effective since my tire is narrower, meaning i really need a pump that can handle a greater pressure altogether?

    my guess is that i just need higher pressure, and my pump is going to be able to handle the same pressure regardless of hte volume it is filling, but, i thought i would ask you guys and save myself embarrassment somewhere else. thanks
  2. jcsd
  3. Apr 16, 2005 #2


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    The tire pressure needed to support a load is roughly equal to the contact pressure between the tire and road (plus some additional pressure for internal tire stresses). The contact pressure between tire and road is simply the load on the tire divided by the contact patch area. So the smaller the contact patch area, the higher the tire pressure has to be for any given load.

    That's just a rough order of magnitude calculation since there are still some affects from internal stresses within the tire which results in the tire pressure actually being slightly higher than this, but it's a good way of looking at tire pressure.
  4. Apr 16, 2005 #3
    the pump create the pressure difference, so it does not matter what voulume do you want to pressurize. But I am a little bit suspicious about 75 psi- that is rather high. I guess you do not have specifications for you manual(?) pump, but can you check them in the store? And there should be some specifications for the tire pressure, which means that the tire will not immediately blow up above it, but you should be careful with that.
  5. Apr 16, 2005 #4
    these tires are typically run between 90 and 120psi. yeah, anyways, i've got it figured out, thanks
  6. Apr 16, 2005 #5


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    It is harder because the way most pumps work, disconnecting the pump from the tire results in the release of some air. When the tire volume is small, this small amount of air can be a significant fraction of the total air in the tire.
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