"Steve Walker" wrote in message
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One thing about ISO sizes bewilders me: why do they specify the height of
the tyre (OD - ID) as a percentage of the width? Seems an odd way to do
it. Surely the three measurements that need to be specified explicitly
a inside diameter (ie diameter of wheel and of bead), outside diameter
(ie of tread) and width. And preferably specify all three in the same
bloody units - either all in mm for Europe (and *maybe* UK) or else all
in inches (for US) - not some half-arsed mixture of the two.

Similar to car tyres then? 205/55R16 has a sidewall height of 55% of 205mm
on a 16" rim. If the second figure is missing, such as 185R13, then the
height is 80% of the first figure.

Exactly. I wonder why the notation isn't (to use your example) 205/113R406,
where 113 = 55% of 205 and 406 is 16" in millimetres. That would use the
same units throughout and would give three independent measurements rather
than having the height being a function of the width.

I could understand it better if the height was a function of the inside
diameter so the OD and ID were (by default = 80%) related by some common
factor. But to relate the height to the width seems weird: why should a tyre
have a greater height just because it's a greater width?

Are all wheel rims the same width, or is it assumed that the width of the
rim/bead will be related by some fixed proportion to the tyre width?