Metrics Units for wheel circumference and diameter

I ride 406's and I've heard of 700c. They ain't centimeters.

"Rik O'Shea" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Is there any standard or industry preference for specific metric units
for expressing wheel circumference and diameter i.e.
is [mm] preferred over [cm] ?

(Rik O'Shea) said:

Is there any standard or industry preference for specific metric units
for expressing wheel circumference and diameter i.e.
is [mm] preferred over [cm] ?

The World is a metric place, USA excepted. Few things on modern
bicycles, such as the pedal threadings, aren't metric. Tire sizes are
metric and the majority of bike computers seem to want metric
measurements too.

Rik O'Shea said:

Is there any standard or industry preference for specific metric units
for expressing wheel circumference and diameter i.e.
is [mm] preferred over [cm] ?

Depends on the context. Generally, milllimeters would be
preferred...centimeters are too coarse.

However, many cyclecomputers use centimeters because it's one less digit
for the display and processor to have to deal with.

The "industry" is notoriously sloppy and inconsistent in dealing with
wheel/tire sizes. Indeed, most tires are marked prominently with
obsolete and often incorrect size designations. German manufacturers
are the worst offenders at this, despite their reputation for numerical
precision.

See: http://sheldonbrown.com/tire-sizing

Sheldon "ISO/E.T.R.T.O." Brown
+---------------------------------------------+
| The nice thing about standards is that |
| there are so many of them to choose from. |
| --Andrew S. Tanenbaum |
+---------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

Rik O'Shea said:

Is there any standard or industry preference for specific metric units
for expressing wheel circumference and diameter i.e.
is [mm] preferred over [cm] ?

ERD and spoke length is almost always quoted in mm.

The centimetre has an unfortunate history. "Metric" units used to be
cgs (centimetre, gram, second) and this is what I was taught in school
in the 1970s...except that in the UK we had to learn imperial measures
on the quiet from our parents, because they were Frowned Upon.

SI (Systeme Internationale) units are metre, kilogram, second and are
now used in all scientific circles and have pretty much displaced the
cgs system. UK schools, I understand, have gone back to teaching both
metric and imperial measurements with an emphasis on metric. All UK
road signs, for instance, are still in miles although you occasionally
see the odd "200 metres" sign at roadworks. The difference between a
metre and a yard is unimportant when you're driving at 70mph so no-one
cares about the inconsistency.

Metric units may be easier to work with but they're based on a couple of
miscalculations. A metre was supposed to be one ten-millionth of the
distance from the North Pole to the equator, measured on what was then
the prime meridian through Paris [1]. It's not quite right. Likewise,
a kilogram was supposed to be the mass of 1 litre cubic decimetre - of
pure water. An error was made in constructing the standard kilogram
and, again, it's not quite right. A litre is now *defined* as 1 dm^3,
where it used to be the volume of 1 kg of water. A kilogram is the mass
of that "standard kilogram" - a lump of platinum-iridium alloy.

Reading this, defining the a yard as the distance from Henry VIII's nose
to the tip of his outstretched middle finger seems quite reasonable.

[1] the French have never quite forgiven us for changing 0 degrees
longitude to Greenwich, London.

Sheldon Brown said:

...
The "industry" is notoriously sloppy and inconsistent in dealing with
wheel/tire sizes. Indeed, most tires are marked prominently with
obsolete and often incorrect size designations. German manufacturers
are the worst offenders at this, despite their reputation for numerical
precision....

E.g., Continental, who marks their 28-406 Gran Prix tire as "20x1-1/8"
implying that it is an ISO 451-mm tire. 🙁

--
Tom Sherman - Earth

Zog The Undeniable said:

A litre is now *defined* as 1 dm^3,
where it used to be the volume of 1 kg of water.

The history is a little more complicated.
The 1964 redefinition of the litre as a cubic decimetre
was a return to the original definition of 1793.

Tom Ace