On Sun, 21 Oct 2018 10:43:06 +0100
"NY" wrote:

"Rob Morley" wrote in message
news:20181020233448.60296533@Mars...
On Thu, 18 Oct 2018 12:43:41 +0100
"Jim GM4DHJ ..." wrote:

[...]

English "28 inch" or German "28 inch"? There's half an inch
difference in the diameter of those.

So does Germany actually use "28" in the specification of its wheel
diameter?

You'll still see their traditional "28 x 1-5/8 x 1-3/8" marked on the
side of many tyres.

It must be one of the few measurements in the imperial
system that has survived Europe's use of the SI metric system.

Some Germans still use Fuß, Zoll and Pfund as we use foot, inch and
pound.

Are
there any other cases where items are sold in imperial units (as
opposed to being an integer number of inches which is translated into
metric *)? If there is 1/2 inch difference in diameter of English and
German 28" wheels, which one is the true diameter that you would
measure? Is one the external diameter of the rim and the other the
diameter of the tyre bead which is slightly smaller and has to be
levered over the rin?

The ISO/ETRTO measurement is the actual millimetre diameter of the
bead/seat, which is the same for the tyre and the rim, although there
are variations in manufacturing tolerances which lead to some tyre/rim
combinations being looser or tighter than normal. The traditional tyre
sizes are based on effective diameter: a fat tyre on a smaller rim has
the same diameter as a skinny tyre on a larger rim, hence you get two
English 26 inch tyre sizes, one with a width of 1-1/4 inch and the
other of 1-3/8, with a corresponding 1/4 inch difference in rim size
which seems pretty reasonable. But there is some strangeness - English
28 inch (28 x 1-1/2 as fitted to vintage "police bikes") is 635mm, a bit
bigger than English 27 inch at 630mm but the German "28 inch" (commonly
called 700C) is 622mm, actually smaller than 27 inch.

Incidentally, has anyone actually had to use tyre levers for fitting
a bicycle tyre? I always find that I just tuck the bead in at one
side, then ease it in by moving my thumbs towards the opposite side
and then pull it away from the rim at the very opposite until it pops
into place; and vice versa for removing. You'd think that a lever
would be needed at least to make the bead pop out from the rim when
removing, but I've never found one necessary. Am I unusual?

Wider tyres on wider rims tend to be easy to fit and remove without
levers, although some cheap tyres can have very thick inflexible beads
that make it harder to get them into the well of the rim, in order to
get that bit of slack to pop the last bit over the edge. Conversely some
wide but very light and flexible tyres (e.g. racing mountain bikes
tyres with Kevlar beads) can be fiddly to fit because they're so floppy
they won't stay on until they're held in shape by the partially
inflated inner tube. Narrower tyre/rim combinations are more likely to
be too tight to fit without levers; I have such a combination on one
of my bikes, and the usual method of strapping the bead into the well
of the rim just doesn't work. That one needs the application of my
workshop-quality steel levers, and it's still not easy. You have to be
very careful not to pinch the inner tube when you're using levers, but
it shouldn't normally be necessary when fitting a tyre - when removing
a tyre it may be quicker and easier to user levers, and if you're going
to bin the tube you don't care about pinching it anyway.