On 18/10/2018 21:35, NY wrote:
"Steve Walker" wrote in message
news
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?

The width of the rim is also in inches and apparently, a 205/60R15 tyre
is measured on a 6J rim, but is approved for 5.5J to 7.5J - check
manufacturer's info for each make.

The machined face of the wheel for mounting to the hub has an offset
measured from the centreline of the wheel, +ve (toward the outside) or
-ve (toward the inside), in mm.

That all makes far more sense than tyre load indexes (65 = 290kg to 124
= 1600kg, but not in even steps) or speed ratings (A1 to A8 cover very
low speed ratings, B = 31mph, up to Y = 186mph, with the letters
in-between covering the range in-between - mostly, but not completely,
in alphabetical order!)

SteveW