Tyre Size Madness

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



--
--------------------------
Posted via cyclingforums.com
http://www.cyclingforums.com

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

Pete Biggs must be edykated coz e writed:

ChrisW wrote:

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

Actually 406 and 451 are two most common bead diameters on 20" tyres.

--
Ian

http://www.catrike.co.uk

"Ian" wrote in message
...

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...

--
Guy
===

WARNING: may contain traces of irony. Contents may settle after posting.
http://www.chapmancentral.com

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

"Ian" wrote in message
...

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

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/
================================================== =========

In message , ChrisW
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