Dear All,
Apparently, engineers in the 1970s saw an opportunity, through
metrication, to rationalise some of the original design faults in tyre
design and construction. Here is a quote from: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46
Tires and wheels that have a rim diameter expressed in millimeters
(190/65R390, as well as, 365 and 415) are called millimetric sizes.
Michelin initiated millimetric sizes for their TRX tires that saw
limited use on many different car models in the late 1970s and 1980s.
Michelin PAX System run flat tires have been introduced as an
integrated wheel/tire system on a very limited basis as Original
Equipment (O.E.) in North America. An example PAX System size of
235/710R460A 104T expresses tire and wheel dimensions in millimeters
(235 mm Section Width, tire Overall Diameter of 710 mm and a 460A mm
rim diameter, with the "A" in 460A signifying these tires feature
"asymmetric" beads in which the outside bead (450 mm) and inside bead
(470 mm) are actually different diameters.
All of these "unique" tire/wheel diameters were developed specifically
because the tire and wheel design or intended vehicle use required
them to be different than conventional tires and wheels. All of these
tires and wheels feature bead profiles that have a different shape
than traditional "inch rim" sizes.
Although the millimetric sizes were later withdrawn from the market,
they left their legacy in the branding on the side of the tyre. When
you see something like:
225/50R16
You know that the tyre is 225 millimetres wide; 50 % of this width
(113 mm high from rime to outside diameter); it is a radial tyre; and
it is 16 inches (exactly 406.4 mm) in diameter from rim to rim.
By doing this only 2 of the 3 measurements are metric and not hidden
from the public; the third is also metric and hidden from the public.
Clearly the opportunity was lost to rethink the design of wheels and
tyres to allow for the increased speeds that have occurred since Henry
Ford's days as chief engineer. For example, asymmetric beads on the
rims could be useful.
Cheers,
Pat Naughtin
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
Metric system consultant, writer, and speaker, Pat Naughtin, has
helped thousands of people and hundreds of companies upgrade to the
modern metric system smoothly, quickly, and so economically that they
now save thousands each year when buying, processing, or selling for
their businesses. Pat provides services and resources for many
different trades, crafts, and professions for commercial, industrial
and government metrication leaders in Asia, Europe, and in the USA.
Pat's clients include the Australian Government, Google, NASA, NIST,
and the metric associations of Canada, the UK, and the USA. See http://www.metricationmatters.com
for more metrication information, contact Pat at [email protected]
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