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Tyre Pressure Question

Discussion in 'Bicycle Mechanics and Repairs' started by Plax, 21 Apr 2008.

  1. Plax

    Plax Veteran

    Location:
    Wales
    Just a stupid question really.
    On my tyres it says the min - max pressure should be;
    345 - 55 KPa
    50-75 PSI
    3.4 - 5.1 bar

    I've attached an image of the foot pump I have. I take it the black is the reading in PSI and the red is the bar reading?

    I usually pump the tyres up to the 5 on the bottom red reading which I'm presuming is ok.
    What would happen if I pumped the tyres up to 6 bar? Would this affect the handling of the bike, or does it just put strain on the inner tube leading it to burst if, for example, you go over a pot hole?
     
  2. yep thats right
     
  3. Milo

    Milo Veteran

    Location:
    Melksham, Wilts
    You can go a good 30 psi over the recommended psi in my experience.
    I run at about 110 with max recommended being 90
     
  4. Tim Bennet.

    Tim Bennet. Entirely Average Member

    Location:
    S of Kendal
    Yes you are right in thinking that the kg/cm2 marked on your pump are equivalent to 'Bars', and also incidentally to roughly 10 of the new SI units Kilo Pascals (KPa).

    The manufacturer has calculated that his tyre only stays stable on the rim up to 5.1 Bar. If you overinflate it to 6 Bar, then you increase the risk of this happening. Some people consider the manufacturer's upper limit to be conservative and regularly over inflate, but I've never found the need, nor would I want to court problems: I would rather the tyre stayed on the rim. Seems the best place for it.

    Is there some specific reason you want to increase the pressure?
     
  5. OP
    OP
    Plax

    Plax Veteran

    Location:
    Wales
    No not really, I'm just wondering in case I get a bit carried away and go over the 5 bar mark or forget and think it is 6 and not 5.

    Well I'm mightily glad to see I'm not completely stupid anyway ;)
     
  6. Jacomus-rides-Gen

    Jacomus-rides-Gen New Member

    Location:
    Guildford / London
    Well, going over pressure will reduce grip, and give you a harsh ride, and can increase rolling resistance.
     
  7. walker

    walker New Member

    Location:
    Bromley, Kent
    Remember that a rolling tyre produces heat. And heated air particles expand. if the air in the tube is over the recomended PSI then the air has no room to expand thus leading to a blow out. not the sort of thing you want on a decent.
    try and go about 5-10 psi below the recommended in the summer
     
  8. walker

    walker New Member

    Location:
    Bromley, Kent
    How does it increase rolling resistance? Surely the more pumped up the tyre the less contact to the road equalling less rolling resistance? I might not be right.
     
  9. Jacomus-rides-Gen

    Jacomus-rides-Gen New Member

    Location:
    Guildford / London
    I can't remember :wacko:

    I think Sir Sheldon might have the answer, but AFAIR it is to do with the rubber reaching a point where instead of conforming around the small imperfections in the road surface, it skitters across them, which increases the rolling resistance by increasing the friction between the tyre and the road.

    Happy to be prooved wrong though, as I can't remember where I read it!
     
  10. Brock

    Brock Senior Member

    Location:
    Kent
    That theory works for me. I've always assumed harder tyre = faster rolling, but that's presumably only true if you're talking about rolling on a glass flat surface. Small imperfections in the surface will cause a very hard tyre to bump up over them, wasting energy, whereas a sensible pressure would allow the tyre to roll over and absorb the surface profile.
     
  11. hubgearfreak

    hubgearfreak Über Member

  12. Disgruntled Goat

    Disgruntled Goat New Member

    Max pressure doesn't necessaryily mean optimum pressure