Tyre Size Madness

Discussion in 'UK and Europe' started by ChrisW, Aug 21, 2003.

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  1. ChrisW

    ChrisW New Member

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    Halfway through my morning commute yesterday I heard an ominous hiss coming from my front wheel. "Oh bother!", I thought, but I had a spare tube so I didn't anticipate much of a delay.

    It turned out that the tyre casing had split, so I stuck a big patch over the inside of the hole and went slowly and carefully on my way. But I needed a replacement tyre for the journey home. The old tyre (Schwalbe Stelvio) was clearly marked "20 x 1 1/8". The closest I could find was 20 x 1 3/8, a reasonable substitute as the old tyre was arguably rather too narrow. It turned out that the new tyre was so large that I could put my wheel, complete with the old tyre, inside it.

    What is it with bike tyre sizes? How can 20 inches vary by 10% from one to another? Admittedly the ETRTO sizes seemed to reflect the difference, but how can two different ETRTO sizes be the same number of inches? (In any case, my experience is that if you ask for an ETRTO size in most bike shops they translate it to something else.)

    The rim of a bike wheel isn't really that complicated. As far as I can see it has three dimensions that matter for tyre sizing: the external diameter, the depth of the flanges and the width across the flanges. The first is critical, the second may be important but doesn't vary much, and the third allows for a certain amount of variation in tyre width. But there are three different sizing standards and they still sell you something hopelessly wrong.

    I've had the same experience with tubes: on getting out the spare tube for a roadside repair (rain, dark etc) I found that the size *recommended on Schwalbe's web site* had about 3 inches to spare. Maybe they hoped the wheel would grow into it?

    Incidentally, Schwalbe Stelvios are apparently made of cheese. Kevlar-reinforced cheese, but still cheese. The tyre that split had less than 2000 miles on it. I've already had to replace the Stelvio on the back wheel because I was getting pinch flats at the rate of one every day, whatever pressure I pumped it to.

    (In the end I put three patches on the inside of the casing and it held out till I reached home.)

    Chris Walker
     
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  2. ChrisW wrote:

    > What is it with bike tyre sizes? How can 20 inches vary by 10% from one to another? Admittedly the
    > ETRTO sizes seemed to reflect the difference, but how can two different ETRTO sizes be the same
    > number of inches? (In any case, my experience is that if you ask for an ETRTO size in most bike
    > shops they translate it to something else.)

    Only two is not bad. There's at least seven different "Twenty-six inch" sizes... AASHTA -
    http://www.sheldonbrown.com/tire_sizing.html

    /Always/ check the tyre for an ISO number, especially if it's an unusual size...

    Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
    ===========================================================
    Editor - British Human Power Club Newsletter
    http://www.bhpc.org.uk/
    ===========================================================
     
  3. Pete Biggs

    Pete Biggs Guest

    ChrisW wrote: ............
    > It turned out that the tyre casing had split, so I stuck a big patch over the inside of the hole
    > and went slowly and carefully on my way. But I needed a replacement tyre for the journey home. The
    > old tyre (Schwalbe Stelvio) was clearly marked "20 x 1 1/8". The closest I could find was 20 x 1
    > 3/8, a reasonable substitute as the old tyre was arguably rather too narrow. It turned out that
    > the new tyre was so large that I could put my wheel, complete with the old tyre, inside it.

    > What is it with bike tyre sizes? How can 20 inches vary by 10% from one to another? Admittedly the
    > ETRTO sizes seemed to reflect the difference, but how can two different ETRTO sizes be the same
    > number of inches?

    I'm not clear what you've got. Do the old and new tyres both have the same ETRTO (aka ISO)? It must
    match the rim. Numbers will be marked on both tyre and rim.

    If numbers match, does the tyre actually fit the rim or not? Sometimes they can seem too big at
    first but actually work ok.

    According to http://www.sheldonbrown.com/tire_sizing.html, there are two different ISO sizes for
    20": 451 and 419mm - these are for different rim sizes. There's no excuse for supplying the wrong
    size if the bike shop can see your wheel or old tyre.

    I agree the inch standards are daft. It's the same with 26" with modern mountain bikes compared to
    old British roadsters. They're both called 26" yet they are two different incompatible sizes. Always
    check the ISO number when in doubt.

    /snip I know what you mean about the tubes but any excess length isn't so much of a problem when you
    don't blow it up too much while fitting and/or once it's all fitted.

    ~PB
     
  4. Ian

    Ian Guest

  5. Just Zis Guy

    Just Zis Guy Guest

  6. Ian

    Ian Guest

    Just zis Guy, you know? must be edykated coz e writed:

    > "Ian" <[email protected]> wrote in message news:BB6A8710.D8D1%[email protected]...
    >
    >> Actually 406 and 451 are two most common bead diameters on 20" tyres.
    >
    >
    > And even Schwalbe get confused - I bought two tubes lately in boxes marked
    > 28/406 but the tubes are definitely 451...

    The 406 tubes are a little fatter than the 451 I have notices, using 451 tubes should be a bit like
    those wrap over tubes from Halfords.

    --
    Ian

    http://www.catrike.co.uk
     
  7. Chris French

    Chris French Guest

    In message <[email protected]>, ChrisW <[email protected]> writes
    >
    >What is it with bike tyre sizes? How can 20 inches vary by 10% from one to another? Admittedly the
    >ETRTO sizes seemed to reflect the difference, but how can two different ETRTO sizes be the same
    >number of inches?

    Presumably because the 'inch' sizes have never been defined in one single standard, and different
    countries/regions seem to have developed there own different tyre sizes - but then given them the
    same inch sizes (hence the various '26 inch' sizes - MTB and British roadster sizes are very
    different. I suspect the size varies so much because some measured the diameter of the tyre, some
    measured the diameter of one part of the rim or another and some maybe just picked a nearby
    number ......

    If buying a less common size in particular I always check the ETRTO sizes

    >The rim of a bike wheel isn't really that complicated. As far as I can see it has three dimensions
    >that matter for tyre sizing: the external diameter, the depth of the flanges and the width across
    >the flanges.

    >The first is critical, the second may be important but doesn't vary much, and the third allows for
    >a certain amount of variation in tyre width.

    The one that is really critical you don't mention, and that is bead seat diameter (which is what the
    first number - on the ETRTO size refers to) this is the bit inside the rim where the tyre sits - the
    rim above that can varying as bit in size without problem.

    >But there are three different sizing standards and they still sell you something hopelessly wrong.
    >
    AFAICS there is only one reliable 'standard'
    --
    Chris French, Leeds
     
  8. Just zis Guy, you know? wrote:

    > And even Schwalbe get confused - I bought two tubes lately in boxes marked 28/406 but the tubes
    > are definitely 451...

    The ones I have - some Schwalbe and some Conti - are marked 28/406-451, indicating a dualness of
    purpose which is less than ideal for those running
    406s. Zach Kaplan says there are some 19-400 (Cheng Shin?) tubes Out There which are the bestest
    for performance 406 tyres, but I have not yet been able to find any over here, chiz. Perhaps
    I will detour via Zach's place on the way home from Battle Mountain...

    Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
    ===========================================================
    Editor - British Human Power Club Newsletter
    http://www.bhpc.org.uk/
    ===========================================================
     
  9. Pattledom

    Pattledom Guest

    ChrisW wrote:
    >
    > What is it with bike tyre sizes? How can 20 inches vary by 10% from one to another?

    It's because, traditionally, inch sizes were measured over the tyre *tread* and not the rim.
    Therefore tyres with the same tread diameter but different sections are not interchangeable. It's
    worse than that though: the system used for modern inch sizes (like those on MTBs) the stated
    diameter doesn't refer to any actual part of the tyre or rim. On a 26" MTB tyre it means "This
    tyre fits a rim that also would take a tyre with a 26" tread diameter if it just happened to be a
    2" section".

    For even more horrible details, see http://www.users.globalnet.co.uk/~pattle/nacc/arc0307.htm

    > Admittedly the ETRTO sizes seemed to reflect the difference,

    It will... the ISO/ETRTO is the single useful bit of information on the tyre.

    > but how can two different ETRTO sizes be the same number of inches? (In any case, my experience is
    > that if you ask for an ETRTO size in most bike shops they translate it to something else.)

    Let them... just make sure that you check the ISO/ETRTO size of the tyre they give you.

    --
    Andrew Pattle
     
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