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Old 10-03-06, 11:27 AM   #1
JohnBrooking
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Newbie tire sizing question

I tried Googling this, but I don't see it addressed even on Sheldon Brown's pages. I've got a very basic question trying to interpret the labels on my tubes and tires. I realize (from reading Brown's pages) this is a very non-standardized area and very dependent on the manufacturer, but my question is general enough that I hope it will be easily answered here.

My mountain bike tire says "26 x 38/32". Obviously the 26 is the diameter, but what is the "38/32"? It would be a non-standard fraction, but maybe it's just an easier way of writing "1 6/32" as the width? Or is it not a fraction, but maybe a range or something? But the larger number is listed first, which makes it seem odd for a range. And a range of what units?

Similarly, two tubes that I have for a 700C wheel are marked "700 x 28/32C" and "700 x 35/43C". Again, fraction, range, or something else? 43rds? Plus I wouldn't expect imperial fractions mixed with metric measurements. At least interpreting them as a range of millimeters seems a little more believable here.

I always seem to get stuck on the simple things...
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Old 10-03-06, 01:26 PM   #2
Sheldon Brown
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Quote:
Originally Posted by JohnBrooking
I tried Googling this, but I don't see it addressed even on Sheldon Brown's pages. I've got a very basic question trying to interpret the labels on my tubes and tires. I realize (from reading Brown's pages) this is a very non-standardized area and very dependent on the manufacturer, but my question is general enough that I hope it will be easily answered here.

My mountain bike tire says "26 x 38/32".
Tire or tube? That doesn't make any sense to me if it's the tire, but tubes are commonly sold to fit a range of different widths. They are just stretchy rubber baloons, so sizing is not at all critical.

Sheldon "Licorice Bagel" Brown

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Originally Posted by JohnBrooking
Obviously the 26 is the diameter, but what is the "38/32"? It would be a non-standard fraction, but maybe it's just an easier way of writing "1 6/32" as the width? Or is it not a fraction, but maybe a range or something? But the larger number is listed first, which makes it seem odd for a range. And a range of what units?

Similarly, two tubes that I have for a 700C wheel are marked "700 x 28/32C" and "700 x 35/43C". Again, fraction, range, or something else? 43rds? Plus I wouldn't expect imperial fractions mixed with metric measurements. At least interpreting them as a range of millimeters seems a little more believable here.

I always seem to get stuck on the simple things...
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Old 10-03-06, 01:51 PM   #3
moxfyre
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Quote:
Originally Posted by JohnBrooking
I tried Googling this, but I don't see it addressed even on Sheldon Brown's pages. I've got a very basic question trying to interpret the labels on my tubes and tires. I realize (from reading Brown's pages) this is a very non-standardized area and very dependent on the manufacturer, but my question is general enough that I hope it will be easily answered here.

My mountain bike tire says "26 x 38/32". Obviously the 26 is the diameter, but what is the "38/32"? It would be a non-standard fraction, but maybe it's just an easier way of writing "1 6/32" as the width? Or is it not a fraction, but maybe a range or something? But the larger number is listed first, which makes it seem odd for a range. And a range of what units?
Is that a "WTB" brand tire? They seem to label their tires in a bizarre way... for example my 700C knobby tire says 700x32/30. The width of the tire is about 32mm. I think the first number is supposed to indicate the width in mm, but I've no idea about the second one, which always seem to be slightly smaller.

Quote:
Originally Posted by JohnBrooking
Similarly, two tubes that I have for a 700C wheel are marked "700 x 28/32C" and "700 x 35/43C". Again, fraction, range, or something else? 43rds?
In that case, it's definitely a range. The first tube is recommended for use in tires of width 28-32mm, the second for tires of width 35-43mm. Typical road tubes are 18-26mm (aka "Small"), 28-32mm (aka "Medium"), and 35+mm (aka "Large"). You can generally use a tire that's off the recommended range by a few mm without problems.

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Originally Posted by JohnBrooking
Plus I wouldn't expect imperial fractions mixed with metric measurements. At least interpreting them as a range of millimeters seems a little more believable here.
In this case... the mm interpretation is likely to be correct. But bike parts are NOTORIOUS for mixing metric and imperial measurements! Italian bike threads always use mm for the diameter of the thread, but threads-per-inch for the pitch And many dimensions are based on older fractional measurements, but now expressed in mm... for example 25.4 mm handlebars/stem clamps (= 1 inch), 28.6 mm seat tubes (= 1-1/8 inch). This is because the British bike standards mostly won out over the French and Italian standards, yet the metric system was adopted for most of these dimension. Of course, other things are still customarily referred to in inches... 1" threaded fork, 1/2" chain pitch, 26" tire, 1/2" pedal threads, etc. It's a mess Fortunately, most of the SMALL fasteners on a modern bike are consistently metric, with M4, M5, M6, M8 bolts being very common.
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Old 10-04-06, 06:52 AM   #4
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Good call, moxfyre! The MTB was at home, so I couldn't verify until this morning, but you're right, it's a WTB Slickasaurus. In addition to the "26 x 38/32" marking, it also says just "38/32 (Front and Back)" after the model name.

Sheldon, your tire sizing page mentions WTB's Global Measuring System, is that maybe what this is? By that interpretation, the 38mm would be the casing width, and the 32mm the tread width. But would the casing be larger than the tread? (I guess I'm not 100% sure what you mean by these terms.) Yes, I'm sure the 38 is first number.

BTW, the WTB page linked to on the tire sizing page no longer exists at the WTB site (which now seems to be a very Flash-heavy site with a lot of "not available" resources).
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