Bad rim/tyre combinations + tube disrecommendation

Discussion in 'Cycling Archive' started by Phil W Lee, Sep 11, 2015.

  1. Phil W Lee

    Phil W Lee Guest

    I got stranded today at the doctors, when I had a blowout on my
    wheelchair, rather loudly - both receptionists dived under the desk
    :)
    I discovered, after being recovered home (using a horribly
    uncomfortable borrowed chair, which only had small wheels so I
    couldn't even self-propel it) and examining the damage to my own, that
    the grotty cheap sods that supplied the wheels had fitted tubes that
    were not capable of withstanding the pressure the tyres themselves
    were rated for - the (shrader) valve blew right off, and when I
    checked the other one, it was on the point of doing so.
    I immediately ordered, online, 3 inner tubes (two for immediate use
    and a spare) of the correct size (for a 25-540 tyre, so not common
    enough to be held in stock by any LBS).
    So now I'll be able to replace the crappy ones with decent ones
    (Schwalbe AV9A). The CST originals have the valve fitted
    in a rubber sleeve, which swells under the high pressure and cuts
    itself on the valve hole in the rim. The Schwalbe ones have valves in
    a full length metal cylinder, which won't suffer the same fate.
    Looking on the bright side, I've gained a couple of bungees :)

    Then, in the course of checking all my wheels and tyres, I discovered
    a new worst combination of rim and tyre for removal and replacement.
    Sun CR20 540 (24") rims with Marathon Plus 38-540. The rims have
    almost no well at all in the middle, and of course Marathon Plus are
    notoriously inflexible. Even removing them is hard work, using
    levers. Fitting without levering is only just possible, and with my
    arthritic hands I need a bead lubricant. Toughest ones I've ever done
    on what is, essentially, a bicycle wheel.
    And I won't touch CST tubes again, either - any 1" wide tube should be
    capable of taking over 100psi without trouble..
    The same Sun CR20 rim profile is available in 559 for MTBs (but
    without the slots for the hand ring), so it's worth bearing in mind
    that it's a complete B45t4rd to fit tight tyres onto!
     
    Phil W Lee, Sep 11, 2015
    #1
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  2. Phil W Lee

    Rob Morley Guest

    I wonder if it would be worth smoothing the edge of the valve hole, or
    rounding it over with a hammer and an appropriately shaped implement.
     
    Rob Morley, Sep 12, 2015
    #2
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  3. Phil W Lee

    Phil W Lee Guest

    I did check for burrs on both inner and outer walls - nothing
    obviously amiss. I think it may be the slight movement that is
    inevitable when using a hand pump to inflate the tyres that rubs the
    rubber valve body against the metal. If you only ever use a track
    pump or airline, it may not be a problem. I'd used my Zefal HPx on
    both (it's the best one I can carry on the chair).
    It'll be fine with the Schwalbe tubes though, so no worries, other
    than ParcelFarce playing knock down ginger when they fail to deliver
    them (SOP for most parcels companies if you can't sprint to the door
    faster than they can sprint away from it).
    If tubes with all metal shrader valve bodies stop being available,
    I'll have to modify the rims with sleeves or grommets to take presta
    valves. I might get some anyway, just in case (and I prefer presta,
    as you don't let so much air out when you disconnect the pump).

    The Sun CR20 rims lacking any significant well is a problem though -
    it was tough even with a pair of 25-540 RightRuns, which may give an
    idea of how painful it was with Marathon Pluses (I have two sets of
    wheels, one for smooth surfaces, with the RightRun tyres, and the
    other with 37-540 Marathon Plus tyres, for more challenging surfaces.
    In an emergency, I can put the hideous original "Duro" grey tyres back
    on, although they are even harder to wheel than the Marathons, but
    lacking any of the other benefits.

    It was a pain that my own car is off the road, and I'd had a lift to
    the doctor's, as if I'd been in my own car, I could have just swapped
    to the wheels with Marathons fitted.

    The huge bang did cause a bit of alarm in the Doctor's surgery!
     
    Phil W Lee, Sep 13, 2015
    #3
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