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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 |
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#2
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BSD on ISO/ERTO?
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 |
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