Wheel Size

gargoyle47

This has probably been asked a thousand times. . .

For my (very old) cycle meter I need to enter the actual wheel size.

How do I measure the wheel size:-
1. inner rim to inner rim?
2. outer rim to outer rim?
3. outer surface of tyre to outer surface of tyre?

When measuring the wheels with option 1 I get about 24", option 2
about 25+ and option 3 about 27, so I'm confused and need some
guidance please.

My bike is about 17 years old and I can't remember what size it is - I
think it is a 26" wheel.
Unfortunately the tyres have very dirty and smudged edges so I can't
even read what is printed on the tyre walls.

Colin MacDonald

On 28 May, 14:18, gargoyle47 <gjdcat...@yahoo.co.uk> wrote:
> This has probably been asked a thousand times. . .
>
> For my (very old) cycle meter I need to enter the actual wheel size.
>
> How do I measure the wheel size:-
> 1. inner rim to inner rim?
> 2. outer rim to outer rim?
> 3. outer surface of tyre to outer surface of tyre?
>
> When measuring the wheels with option 1 I get about 24", option 2
> about 25+ and option 3 about 27, so I'm confused and need some
> guidance please.
>
> My bike is about 17 years old and I can't remember what size it is - I
> think it is a 26" wheel.
> Unfortunately the tyres have very dirty and smudged edges so I can't
> even read what is printed on the tyre walls.

You should be able to find all you need here:
http://sheldonbrown.com/cyclecomputer-calibration.html

The quick answer, though, is 3. outer surface. Check if you need the
wheel circumference or diameter; normally it would be circumference,
but with old technology it's hard to say.

Colin

gargoyle47

On May 28, 2:31*pm, Colin MacDonald <colin_mac2...@yahoo.co.uk> wrote:
> On 28 May, 14:18, gargoyle47 <gjdcat...@yahoo.co.uk> wrote:
>
>
>
>
>
> > This has probably been asked a thousand times. . .
>
> > For my (very old) cycle meter I need to enter the actual wheel size.
>
> > How do I measure the wheel size:-
> > 1. inner rim to inner rim?
> > 2. outer rim to outer rim?
> > 3. outer surface of tyre to outer surface of tyre?
>
> > When measuring the wheels with option 1 I get about 24", option 2
> > about 25+ and option 3 about 27, so I'm confused and need some
> > guidance please.
>
> > My bike is about 17 years old and I can't remember what size it is - I
> > think it is a 26" wheel.
> > Unfortunately the tyres have very dirty and smudged edges so I can't
> > even read what is printed on the tyre walls.
>
> You should be able to find all you need here:http://sheldonbrown.com/cyclecomputer-calibration.html
>
> The quick answer, though, is 3. outer surface. *Check if you need the
> wheel circumference or diameter; normally it would be circumference,
> but with old technology it's hard to say.
>
> Colin- Hide quoted text -
>
> - Show quoted text -

Thanks for the info.
It doesn't say circumference or diameter, so I assume the latter.
The lowest value available for setting is 16.

Rob Morley

On Wed, 28 May 2008 06:18:06 -0700 (PDT)
gargoyle47 <gjdcatman@yahoo.co.uk> wrote:

> This has probably been asked a thousand times. . .
>
> For my (very old) cycle meter I need to enter the actual wheel size.
>
> How do I measure the wheel size:-
> 1. inner rim to inner rim?
> 2. outer rim to outer rim?
> 3. outer surface of tyre to outer surface of tyre?
>
> When measuring the wheels with option 1 I get about 24", option 2
> about 25+ and option 3 about 27, so I'm confused and need some
> guidance please.
>
> My bike is about 17 years old and I can't remember what size it is - I
> think it is a 26" wheel.
> Unfortunately the tyres have very dirty and smudged edges so I can't
> even read what is printed on the tyre walls.

Find a piece of flat ground next to a wall, where you can wheel the
bike along while sitting on it and using the wall for support (or get
someone to steady the bike while you sit on it). Mark a line (chalk, bit
of string ...) under the front wheel. Place the tyre valve directly over
the line. Sit on the bike and roll it forward one revolution of the
wheel, then dismount and make another line directly under the tyre
valve. Measure between the lines. If the computer wants the wheel
circumference you're done, if it wants the diameter just divide the
measured distance by 3.14. This is the best way of measuring the
effective wheel size[1], as it accounts for tyre width, tyre pressure
and rider weight.

[1] For greater accuracy wheel the bike several revolutions and divide
the resulting distance by the number of revolutions.

gargoyle47

On May 28, 3:05*pm, Rob Morley <nos...@ntlworld.com> wrote:
> On Wed, 28 May 2008 06:18:06 -0700 (PDT)
>
>
>
>
>
> gargoyle47 <gjdcat...@yahoo.co.uk> wrote:
> > This has probably been asked a thousand times. . .
>
> > For my (very old) cycle meter I need to enter the actual wheel size.
>
> > How do I measure the wheel size:-
> > 1. inner rim to inner rim?
> > 2. outer rim to outer rim?
> > 3. outer surface of tyre to outer surface of tyre?
>
> > When measuring the wheels with option 1 I get about 24", option 2
> > about 25+ and option 3 about 27, so I'm confused and need some
> > guidance please.
>
> > My bike is about 17 years old and I can't remember what size it is - I
> > think it is a 26" wheel.
> > Unfortunately the tyres have very dirty and smudged edges so I can't
> > even read what is printed on the tyre walls.
>
> Find a piece of flat ground next to a wall, where you can wheel the
> bike along while sitting on it and using the wall for support (or get
> someone to steady the bike while you sit on it). Mark a line (chalk, bit
> of string ...) under the front wheel. Place the tyre valve directly over
> the line. Sit on the bike and roll it forward one revolution of the
> wheel, then dismount and make another line directly under the tyre
> valve. *Measure between the lines. *If the computer wants the wheel
> circumference you're done, if it wants the diameter just divide the
> measured distance by 3.14. This is the best way of measuring the
> effective wheel size[1], as it accounts for tyre width, tyre pressure
> and rider weight.
>
> [1] For greater accuracy wheel the bike several revolutions and divide
> the resulting distance by the number of revolutions.- Hide quoted text -
>
> - Show quoted text -

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!

leandr42@googlemail.com

On May 28, 5:10*pm, 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!

Short version is that there are lots of very similar wheel sizes - you
can (or could) get 27" and 29" and the difference between them is a
lot less than 2", some sizes are names rather than measurements. Read
more about it here:

http://www.sheldonbrown.com/tire-sizing.html

Sounds like you've got modern 700C, which is very similar to the old
27". Go with your actual measurement and enter as close to it as you
can on your cycle computer.

Philip TAYLOR

gargoyle47 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!

Your ground measurement is unexpectedly accurate : 700mm =
27,407987470634299138606108065779 inches !

** Phil.

David Damerell

Quoting <leandr42@googlemail.com>:
>Short version is that there are lots of very similar wheel sizes - you
>can (or could) get 27" and 29" and the difference between them is a
>lot less than 2", some sizes are names rather than measurements.

That's a particularly good one because 29" is what the mountain bikers
call 700C - ie, smaller rims than 27" and very likely smaller wheel
circumference.
--
David Damerell <damerell@chiark.greenend.org.uk> Oil is for sissies
Today is Olethros, June - a weekend.

Naqerj

Philip TAYLOR wrote:
> gargoyle47 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!
>
> Your ground measurement is unexpectedly accurate : 700mm =
> 27,407987470634299138606108065779 inches !
>

Not really, as the 700 in 700C doesn't refer to real millimetres but
twenty-fifths of an inch.

Mind you, if you take the rim diameter (622mm) add twice the section
(38) that gives 698mm, or 27.48"... here, how come my 698mm is bigger
than your 700mm?

--
Andrew

Paul Luton

gargoyle47 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!

What the markings mean is that IF you fitted a 1 5/8" tyre to your wheel
it would then be 28" in diameter but you actually fitted a 1 1/2" tyre.
Theoretically that would make it 27.75" but tyre sizes are a bit
approximate - go with the measured diameter.

--
CTC Right to Ride Rep. for Richmond upon Thames

Naqerj

Paul Luton wrote:
> gargoyle47 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!
>
> What the markings mean is that IF you fitted a 1 5/8" tyre to your wheel
> it would then be 28" in diameter but you actually fitted a 1 1/2" tyre.

Not even that... a 700C tyre is actually equivalent to 28x1-3/4 [2]
rather than 28x1-5/8. (28 - 2 * 1.75) * 25.4 = 622.3 - near enough to
the ETRTO size of 622. The 1-5/8 on the side wall is a red herring. It
just goes to show that the only size on the tyre worth taking any notice
of is the ETRTO one.


[1] I do.
[2] Which turns out to be rather handy if you happen to have a 1920s
tandem [1] or a pre-war Post Office bike.
--
Andrew

Rob Morley

On Wed, 28 May 2008 09:10:25 -0700 (PDT)
gargoyle47 <gjdcatman@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!

There are two sorts of tyre size - the nominal one as reported above
and the actual one which is usually referred to as the ISO or ETRTO
size. The former refers to what the overall size would have been with a
tyre fitted that was historically standard for that size of rim, the
latter is a size in millimetres that reflects the actual diameter of the
rim and actual width of the tyre casing (although there's sometimes a
degree of inaccuracy in the width - some manufacturers like to pretend
their tyres are a bit wider than they actually are so they quote the
width of the tread instead of the casing).

Anyway, the actual rim diameter of a 700C tyre is 622mm, so the overall
diameter of your 700 x 38C is 622 + 2 x 38 = 698mm. Convert to inches:
698 / 25.4 = 27.5 - slightly more than your measured result. Most
likely the tyre casing is pretty much 38mm wide, the tread thickness
makes it a bit bigger then your weight compresses it so it ends up a
bit smaller.

Simple, isn't it? :-)

Roger Thorpe

Philip TAYLOR wrote:
> gargoyle47 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!
>
>
> Your ground measurement is unexpectedly accurate : 700mm =
> 27,407987470634299138606108065779 inches !
>
> ** Phil.
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

Roger Thorpe

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.

D@mn : you're quite right ! Can't think where
I got 25.54 from (apart from a failing and
near-senile mind !).

** Phil.