Cycling units

[border-rider]

At junior school everyone was given a 200mm long piece of straight wood .  One half was Red and the other half was Yellow.  I am still waiting for an opportunity to use decimetres in the grown up world.

1 litre is a cubic decimetre

Try it next time you fancy a large beer in Germany

[Ham]

My degree is in engineering and all of the structural and civil engineers I've worked with (England and Oz) use metric, running screaming from working in imperial units (All that 6/66 ths of stuff must be the work of the devil).

That's a very different type of engineering from the sort that's used to make the things that make the things, I'm sure you will agree. The occasions when you need to work to tight tolerances (vertical, direction) are normally done by dead reckoning rather than measurement I would have thought. I doubt that you ever try to measure even down 0.1 of a mm but I may be wrong.

[border-rider]

I would say so

We use vernier micrometers to specify the tolerances on the things we make to a fraction of a mm, and laser interferometers for the fine detail.

[clarion]

My dad wasn't quite an engineer, but a metallurgist, and, even in the 60s, worked in metric to incredibly fine tolerances.  For Rolls-Royce.

[LittleWheelsandBig]

I doubt that you ever try to measure even down 0.1 of a mm but I may be wrong.

Measurements in millimetres for roadbuilding, so not quite dead reckoning.  Surveyors like things to be right and get very upset when their computer models show negative measurements.

Specs and measurements in microns for things like coating thickness on steel (ultrasonic usually).  That accurate enough for you?

[tom_e]

We do our design stuff down to 0.1mm, using digital callipers to measure easily.

The strange bit is the circuit board layout stuff, which is always traditionally done in thou - thousands of an inch.  Even then I've never seen anyone suggest using fractions rather than decimals.  And we do board thickness in mm.

Done work on optical waveguides with scales 100um down to submicron - didn't occur to anyone that we needed any other units or fractions there either.

[Ham]

I'm not staging a last-ditch defence of Imperial, just pointing out that it had its uses in its time (which may not be fully passed) given the modern materials and techniques, for the most part the difference is redundant and you can more easily use metric as you can imperial. But think for a minute on the pattern makers - the people that make the tools that make the tools. And, listen to what you say.

For example "I use a vernier scale to measure down to 0.1 of a mm" That's a device (in its mechanical form) that takes a standard unit (inch or cm) and divides the sub-division down. So, 0.9 of a cm is divided into 10. How do you think that was done? If you are using a screw micrometer, how was the screw pitch set up on the first machine? Ever seen a dividing head on a lathe?

Sure, metric is nice and easy, but fractions and Imperial is a good old friend that deserves better treatment in retirement than to be discarded. (note, you obviously can use fractions on metric too)

[tom_e]

I agree imperial base units have served for a long time, they're still in quite a few machines, and fractions are indeed a perfectly useful concept.  But decimal numbers are vastly easier to work with, and to express a measurement after it has been made.

An interferometer for measuring distance may have an internal measurement in phase angle rather than the fraction in your vernier.  That doesn't mean the result has to include fractions or phases in the distance.  You don't need to honour the concept or mechanism of your measurement by writing the result in a funny way.

[matthew]

The other day I bought a 17mm ½" socket so that I could change a tap washer.

you sure?

1/2" ~= 12.75mm
17mm ~= 11/16"

[clarion]

Yes.  Sounds right.  Most common drive sizes are 1/2" & 1/4", and you can get metric sockets for them.  My smallest for the larger drive is 10mm.  Largest, I couldn't tell you.  Something the size of which I don't have on my bike

[Jurek]

Yes.  Sounds right.  Most common drive sizes are 1/2" & 1/4", and you can get metric sockets for them.  

In all my years of spannering I've never come across socket drivers in anything other than imperial, including 3/8ths and 1" for the beefier (usually air driven) stuff.
One of my (German made) socket sets has a 1/2" driver for metric, imperial AF and (you'll like this, Clarion  ) Whitworth sockets.

I did once buy 2m of 2x4 from a builders' merchant. Transitional days, eh ?

Any standard aluminium section I have ever bought in the UK has always been specced in imperial, and sold in either 4 or 5m lengths  

[bobb]

Upholstery fabric is always sold in length by the metre, but the width in inches. 36" being the standard IIRC.

When I worked for a company that manufactured hydraulic hoses, the diameter was always specified in inches (even by our continental cousins) but always sold by the metre.

Car wheel/tyre combos are another mish mash of imperial and metric units.

[Panoramix]

I don't think I've ever heard of gear centimetres

In France we compare gears by giving in metres the distance you cycle with on pedal revolution which is:

 PI * wheel diameter in meter * chainring teeth / sprockets teeth

[LittleWheelsandBig]

I don't think I've ever heard of gear centimetres

In France we compare gears by giving in metres the distance you cycle with on pedal revolution which is:

 PI * wheel diameter in meter * chainring teeth / sprockets teeth

Often known as development.

[bobb]

Or metres gain.

What units do men in France measure their penises in?

[Panoramix]

I don't think I've ever heard of gear centimetres

In France we compare gears by giving in metres the distance you cycle with on pedal revolution which is:

 PI * wheel diameter in meter * chainring teeth / sprockets teeth

Often known as development.

Two words to describe the same thing. For a 700 wheel there is a factor of PI * 0.7 / 27 to go from one to the other. Development is just the translation of the French word développement but it is the same concept unless you ride a penny farthing and can't do maths.

[Panoramix]

Or metres gain.

What units do men in France measure their penises in?

Metres because we struggle with big numbers.

[bobb]

Very good 

[Panoramix]

That's interesting: is that when measuring the detailed stuff (like lintels, or rivets) or the overall plans - height of a building, size of a storage tank ... ?

[There are probably better examples than the above, so cut me some slack!]

All dimensions in mm or m, as applicable.

So are '2-by-4' planks (and the like) now specified in mm? (I've probably watched too many old TV shows ... )

Yup!

a 2 by 4 is a 47x97 but on a French building site it is a " 5 10" and an English or Welsh one a "2 by 4".

Timber sizes have been metricated in the 60s, in theory everybody in Europe use the same sizes. In practice people in the design office give sizes in millimeters and on site they use the old denomination which may or may not reflect the timber section they have in their hand.

For some reason the germans use odd sizes for glulam beams.

[citoyen]

What units do men in France measure their penises in?

I was going to say millimetres but Panoramix beat me to the gag.

d.

[hellymedic]

The other day I bought a 17mm ½" socket so that I could change a tap washer.

you sure?

1/2" ~= 12.75mm
17mm ~= 11/16"

Pedant alert! 1 inch = 25.4mm
1/2 inch - 12.7mm,, not 12.75
11/16 is nearer 17.5 mm (17.4625 mm) than 17mm.

I don't want you to round my spanners or nuts, thanks!

[Polar Bear]

The other day I bought a 17mm ½" socket so that I could change a tap washer.

you sure?

1/2" ~= 12.75mm
17mm ~= 11/16"

Pedant alert! 1 inch = 25.4mm
1/2 inch - 12.7mm,, not 12.75
11/16 is nearer 17.5 mm (17.4625 mm) than 17mm.

I don't want you to round my spanners or nuts, thanks!

I think Wow meant a 1/2 inch drive, 17mm jaw.

[LittleWheelsandBig]

Bonus points: Do you know who defined the exact conversion between millimetres and inches?

[Kim]

For example "I use a vernier scale to measure down to 0.1 of a mm" That's a device (in its mechanical form) that takes a standard unit (inch or cm) and divides the sub-division down. So, 0.9 of a cm is divided into 10. How do you think that was done? If you are using a screw micrometer, how was the screw pitch set up on the first machine? Ever seen a dividing head on a lathe?

Sure, metric is nice and easy, but fractions and Imperial is a good old friend that deserves better treatment in retirement than to be discarded. (note, you obviously can use fractions on metric too)

Perfectly reasonable when you put it like that, and not all that different from the preference for powers of two (and subsequently hexadecimal or, less fashionably, octal notation) in all things digital electronics.  At a high level of abstraction there's nothing wrong with decimal, but when you're actually dealing with the hardware, it rapidly gets messy.

I suppose I shouldn't be so scathing of all those arsebackwards fractional-inch-based units, when I can (and infrequently do) convert between hex and decimal in my head...

[andrew_s]

In France we compare gears by giving in metres the distance you cycle with on pedal revolution which is:
 PI * wheel diameter in meter * chainring teeth / sprockets teeth

Conversion factor: 1 metre = 12.5 inches
(just to confuse anyone who thought that 1 metre = 39.4 inches)