# Wheel Size

Quoting Roger Thorpe <[email protected]>:
>David Damerell wrote:
>>Quoting Naqerj <[email protected]>:
>>>Not really, as the 700 in 700C doesn't refer to real millimetres but
>>>twenty-fifths of an inch.

>>Why do you say that, please?

>could this be the "Metric Inch" rearing it's head
>http://en.wikipedia.org/wiki/Metric_inch

Only the other way around. I've never heard of 1/25" being used as an
erzatz millimetre.
--
David Damerell <[email protected]> Oil is for sissies
Today is Monday, June.

David Damerell wrote:
> Quoting Naqerj <[email protected]>:
>> Not really, as the 700 in 700C doesn't refer to real millimetres but
>> twenty-fifths of an inch.

>
> Why do you say that, please?

Because it's true. Tyres used to all be marked in inches but, because
the French wanted a system that looked as if it was metric, but without
actually having a completely different set of sizes, they came up with
this wheeze of multiplying the inch sizes by 25. It all bears out in
the actual sizes. 650A is 26x1-3/8, 650B is 26x1-1/2, 650C is 26x1-3/4,
700C is 28x1-3/4, and so on.

The sums all work too. Take the traditional French Roadster size: 650B.
Under my 25ths of an inch rule, that's the same as 26x1-1/2 so we can
calculate the bead seat diameter as 26" - 2 * 1-1/2" = 23". Convert
that to real millimeters and it comes to 584.2. And the ETRTO size for
650B Standard is ... 40-584 ... as is a 'British' 26x1-1/2.

--
Andrew

Roger Thorpe wrote:

> The inch is now (as I understand it) defined as exactly 25.4 mm

Agreed. NPL provides a nice potted history,
covering the period back to 1959 :

> On what basis is one inch exactly equal to 25.4 mm?
> Has the imperial inch been adjusted to give this
> exact fit and if so when? (FAQ - Length)
>
> The (international) inch has been exactly 25.4 mm
> since July 1959. At this point in time the (international)
> yard was redefined as 0.9144 metre - until this time the
> ratio between the US yard and the metre was different to
> the ratio between the UK yard and the metre. For more
> information, see Engineering Metrology by K J Hume (2 ed)
> Macdonald London 1967. The American inch changed by
> 2 millionths of an inch and the UK inch by 1.7 millionths
> of an inch. The international inch falls mid way between
> the old UK and US inch.
>
> References:
>
> * The Yard Unit of Length Nature Vol. 200 No 4908 pp 730-732 23 Nov 1963
> * The United Kingdom standards of the yard in terms of the metre (British Applied Journal of Physics)

Roger Thorpe wrote:

> I know that it doesn't matter at all and I know that the fact that I
> checked it makes me look rather "sad", but ..
> 1 inch = 25.4 mm. you seem to have 25.54mm.
> sorry to spoil things

OK, so I'm even sadder, but this has bugged me all night and

I was brought up using Imperial units and, later, the CGS system;
when I was forced to switch from CGS to MKS, I must have mentally
carried across the ".54" of "2.54" (cm) to make the incorrect
"25.54" (mm). All is now clear, after a good night's sleep !

** Phil.

On May 29, 2:10 pm, Colin MacDonald <[email protected]> wrote:
> On 28 May, 17:10, gargoyle47 <[email protected]> wrote:
>
> > After cleaning the tyres I have deteced:  700 x 38C   28  x  1 5/8  x
> > 1 1/2
> > Performing the circumference test using ground measurements, I get
> > 86" / 3.14 = 27.39".
> > Confused!

>
> OK, a 700x38 tyre will have a circumference of 218cm (as near as makes
> no difference).  Doing the maths gives a diameter of 27.333" so try
> that and see what happens.
>
> I'd be somewhat surprised if the computer was actually looking for an
> diameter in inches.  You say that the lowest setting is 16; what's the
> highest?
>
> Colin

The highest is 28, which is what I've selected.

When I can find time I shall cycle a route that I have clocked by car
and see if a one mile reading matches. I know it's not exactly an
accurate method, but it's good enough for me.

Thanks for everyone for feedback.

On Fri, 30 May 2008 01:27:59 -0700 (PDT)
gargoyle47 <[email protected]> wrote:

> When I can find time I shall cycle a route that I have clocked by car
> and see if a one mile reading matches. I know it's not exactly an
> accurate method, but it's good enough for me.
>

Ask a friendly traffic policeman if there's a measured mile nearby
that you could use. Or try your local cycle club if you fancy 10 miles

Quoting Naqerj <[email protected]>:
>David Damerell wrote:
>>Quoting Naqerj <[email protected]>:
>>>Not really, as the 700 in 700C doesn't refer to real millimetres but
>>>twenty-fifths of an inch.

>>Why do you say that, please?

>Because it's true. Tyres used to all be marked in inches but, because
>the French wanted a system that looked as if it was metric, but without
>actually having a completely different set of sizes, they came up with
>this wheeze of multiplying the inch sizes by 25. It all bears out in
>the actual sizes.

Fascinating. So it does.

>The sums all work too. Take the traditional French Roadster size: 650B.

I did, notwithstanding the need to persuade Gilles Berthoud in Paris to
send me some parts.
--
David Damerell <[email protected]> Kill the tomato!
Today is Tuesday, June.

"David Damerell" <[email protected]> a écrit:

> I did, notwithstanding the need to persuade Gilles Berthoud in
> Paris to send me some parts.

What was Gilles doing in Paris? Up from Burgundy for the *Mondiale*?

James Thomson

Quoting James Thomson <[email protected]>:
>"David Damerell" <[email protected]> a écrit:
>>I did, notwithstanding the need to persuade Gilles Berthoud in
>>Paris to send me some parts.

>What was Gilles doing in Paris?

I mean his bike shop which apparently I think is in Paris for no reason
at all.
--
David Damerell <[email protected]> Kill the tomato!
Today is Tuesday, June.

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