Wheel Size

Roger Thorpe

Philip TAYLOR wrote:

> 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.
>
>
> Hmmm, having checked, it seems a non-uncommon (mis)conception :
>
>
> from http://www.jknowles.com.au/shop/pages.php?pageid=6
>
> "And if a measurement has full inches before the Imperial
> Fraction you must add 25.54mm for each inch - 1 Inch is 25.54mm. "
>
>
> from http://www.stitchdelight.net/sd064.html
>
> "W: 1.00" / 25.54 mm. "
>
>
> from http://www.apug.org/forums/forum127/38278-old-lens.html
>
> "Divide or multiply by 25.54 (25.54 mm = 1 inch)"
>
> I wonder where we all got this idea from : most authoritative
> sites seem to agree that the official figure is 25.3995mm !
>
> ** Phil.
As usual these things are more complicated than they fist seem.
Wikipedia is good on this.
The inch is now (as I understand it) defined as exactly 25.4 mm
But then there's the Metric Inch and the Swedish Working Inch, the
Pyramid Inch.......
For me it's the width of my thumb.
Roger Thorpe

Philip TAYLOR

Some more good stuff here :

http://groups.google.com/group/uk.t...h%2F2001-09%3F&

As I was (in my callow youth) a radio amateur,
I suspect I used some of the sources that are
cited there ...

** Phil.

David Damerell

Quoting Naqerj <pattle@globalnet.co.invalid>:
>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?
--
David Damerell <damerell@chiark.greenend.org.uk> Oil is for sissies
Today is Monday, June.

Roger Thorpe

David Damerell wrote:
> Quoting Naqerj <pattle@globalnet.co.invalid>:
>
>>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
Roger Thorpe

Colin MacDonald

On 28 May, 17:10, gargoyle47 <gjdcat...@yahoo.co.uk> 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

David Damerell

Quoting Roger Thorpe <myinitial.mysrname@warwick.ac.uk>:
>David Damerell wrote:
>>Quoting Naqerj <pattle@globalnet.co.invalid>:
>>>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 <damerell@chiark.greenend.org.uk> Oil is for sissies
Today is Monday, June.

Naqerj

David Damerell wrote:
> Quoting Naqerj <pattle@globalnet.co.invalid>:
>> 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

Philip TAYLOR

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)

Philip TAYLOR

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
at about 06:00 this morning the answer suddenly became clear.

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.

gargoyle47

On May 29, 2:10*pm, Colin MacDonald <colin_mac2...@yahoo.co.uk> wrote:
> On 28 May, 17:10, gargoyle47 <gjdcat...@yahoo.co.uk> 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.

Rob Morley

On Fri, 30 May 2008 01:27:59 -0700 (PDT)
gargoyle47 <gjdcatman@yahoo.co.uk> 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
instead. :-)

David Damerell

Quoting Naqerj <pattle@globalnet.co.invalid>:
>David Damerell wrote:
>>Quoting Naqerj <pattle@globalnet.co.invalid>:
>>>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 <damerell@chiark.greenend.org.uk> Kill the tomato!
Today is Tuesday, June.

James Thomson

"David Damerell" <damerell@chiark.greenend.org.uk> 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