BSD on ISO/ERTO?

Duncan Smith wrote:
I'm a little confused by the ERTO system, look at Sheldon's table:
http://www.sheldonbrown.com/tire-sizing.html#fraction

The first number's the tyre width, OK. Next number is the bead seat
diameter. So how come the following 16" wheels have different BSDs

16 x 1 3/8 349 mm Older Moulton, Brompton & other folders, Recumbent
front, juvenile
16 x 1 3/8 337 mm Mystery tire
16 x 1 3/8 335 mm Polish juvenile
16 x 1 3/4 317 mm Schwinn Juvenile

or how come the 2" difference in these two tyres is only represented
by a 6mm difference in the BSD?

355 mm 18 x 1.5- x 2.125
349 mm 16 x 1 3/8

Or put another way, how can you tell the diameter of a tyre by its ISO
designation, as different 16" wheels can all have different BSDs?

A so-called 16 or 18" tyre isn't necessarily actually 16 or 18 inches in any
way. These sizes originally referred to tyre outer diameters, but later
came to be associated with bead seat diameters, and confusingly, various
different ones. It's quite a mess, so it's important to check the ISO when
in doubt.

(There is a page on the Sheldon Brown site that explains more and better.
Keep reading it).

A tyre that is 2 inches wide, for example, on a typical wheel rim for the
tyre, is also about 2 inches deep. So you can work out the approximate
outer diameter once you know the ISO and width. But you don't normally need
to know the outer diameter. (Note that a wide rim makes a tyre shallower as
well as wider, and vice versa).

All you usually need to know is the BSD of your rim, and the range of
widths/depths that the rim and the bike can take.

~PB

A tyre that is 2 inches wide, for example, on a typical wheel rim for the
tyre, is also about 2 inches deep. *So you can work out the approximate
outer diameter once you know the ISO and width. *But you don't normally need
to know the outer diameter. *(Note that a wide rim makes a tyre shallower as
well as wider, and vice versa).

All you usually need to know is the BSD of your rim, and the range of
widths/depths that the rim and the bike can take.


Thanks Pete,

That makes a little more sense already. Bikefix sent me some MP+
35-355s instead of the 35-349s I'd asked for. Couldn't quite see how
the 2" diff was made up for, especially since they're the same width.
I'll have a closer read of the Sheldon pages tonight.

Many thanks,

Duncan