Papa Tom

I'm sorry, but after fifty years of riding and repairing bikes with wheel and tire sizes of 16", 20", 24", 26", and 27", I have never taken the time to understand the metric measurements of rims. I have a rim that's a 559 X 27mm and one that is 622 X 17mm. To this old dog, they appear to be 26" rims. What's the real story here?

tyrion

Let's ask Sheldon:

Tire Sizing Systems

Short answer: 559mm "is" 26 incher, 622mm "is" 700c.

CliffordK

559 is a 26" MTB size. However, there are at least a half dozen other rim sizes also marketed as 26". So, say get an old Raleigh with 26" wheels, and the modern MTB tires won't fit on it. Thus, it is handy to go by the BSD.

622 is the BSD of the 700c rims. They aren't much bigger than 26" tires if one uses a FAT tire on the 26" wheels, and a skinny tire on the 700c wheels. However, there are a variety of tires available for 700c (622) rims from your 23mm or 25mm wide "racing" tires to the 29" MTB tires.

Note that 27" wheels have a BSD of 630mm, while 29" wheels have the 622mm BSD.

Papa Tom

Thank you, guys. Now I know everything. (Thumbs Up Emoji, anybody?)

Iride01 

But only if you promise henceforth to only go by ISO/ETRTO sizing. I haven't seen a tire without it for a long time, though I have to get out some strong bifocals sometimes to find and read that very tiny print cast into the sidewalls.

Ghrumpy

Not sure if this helps or not, but the metric measurements are basically just an overlay on the original inch sizes. Problem is, the inch sizes referred to an outside diameter of a wheel and tire combination, and that system just isn't used any more for today's most commonly used rim sizes. Tire size is totally independent of rim diameter nowadays, and has been for some time.

But to illustrate how the old system worked, for example, a 28x1¾" tire is a 1¾" section height tire in a 28" diameter wheel. The rim, therefore, would be 28 - (2x1¾) = 24½". And hey, wouldn't you know it, 24½" = 622mm. When the French adopted the system, they metricated it, and used letters instead of numbers to designate tire section widths. So the "original" 700C was the 28x1¾", and all we have left is the rim size.
Same with 559, as you might expect. It is actually a somewhat later addition to the 26" wheel family. Since 559mm = 22", the "original" tire size would have been 2".

As far as the rim widths go, ETRTO/ISO rim designations refer to a) the bead seat diameter (BSD,) and b) the inside width of the rim channel. Those are the the important dimensions when considering tire fitment. Rims may also have a "c" suffix, which indicates it is a "crochet" rim rather than a straight-sided rim. (There are also the prefixes "SS" straight-sided and "HB" hook-bead, but they are pretty rare these days.)

Tires, on the other hand, are ETRTO/ISO sized by section width and BSD, so you'll see 25-622 for what is aka 700C x 25mm tire. Clincher ("crochet") tires work best on rims whose channel width is within a range of about .5 to .75 of the tire section width. (For straight-sided rims the optimal range is about .6 to .8.) Put the other way, the optimal tire size for a given rim channel width will be about 1.5 to 2x the rim width.
So assuming your 622 x 17 rim is a "crochet" style, the optimal ETRTO/ISO tire size range will be roughly 25-622 to 35-622.
For your 559 x 27 rim, you'll be looking for tires about 40-559 to 54-559 (26 x 1.6" to 26 x 2.125" in "old money".)

Papa Tom

Wow! That is a really serious explanation, Ghrumpy! And now I'm ashamed to reveal why I needed to know this stuff, so I won't.

Anyway, I have ridden the dickens out of a 1996 GT Outpost for more than twenty years and it uses 26" tires. I've got about another decade of bicycling in me and I hope to do it all on this same bike, so I hope I can always get good old 26'ers". The rest of those numbers are more trouble than I want to deal with!

JanMM

26"/559 was the standard MTB wheel/tire for a long time but since the introduction of 29" and 27.5" sizes it seems as if there is somewhat less selection of 559 tires. (My 3 recumbents all use at least one 26"/559 tire).
Judging that since its not a problem to find 27" wheels and tires - an 'obsolete' size - there should be no lack of 26" tires far into the future.

CliffordK

The following dozen or so sizes are pretty much current production, and will likely endure for quite some time in the future. 27"/630 BSD is obsolete, but with a lot of bikes out there, tires will remain available for quite some time.

12 1/2", 203 BSD
16", 305 BSD
18" BMX, 355 BSD (maybe will continue).
20" BMX, 406 BSD
20" narrow, 451 BSD
22" BMX, 457 BSD (maybe will continue)
24" MTB, 507 BSD
24" Road, 520 BSD
24" (600A) Wheelchair, 540 BSD
26" MTB, 559 BSD
26" (650C) Road, 571 BSD
26"/27'5" (650B), 584 BSD
28", 29" (700c), 622 BSD
32", 686 BSD
36", 787 BSD

18", 22", 32", and 36" may remain more on the fringe low production.

We'll likely continue to see a mix of English and BSD/ISO measurements used when referring to tire sizes.

hokiefyd 

I think this makes the current trend of "fatter" tires all the more fascinating, as we're just coming back around to Where It All Started. As noted, the archetypal "700C" tire, by the definition of the specification, is a 39mm tire. 622mm wheel + (39mm * 2) = 700mm. That "C" has stuck, in some way, to pretty much every tire that fits a 622mm rim. The only reason the "C" is there at all is because it was the letter associated with the 622mm BSD rim that's now in common use -- it no longer means what it used to (a code indicating the approximate width of the tire). Many consider a 40-622 to be a "balloon" tire, but that's the tire size that popularized the 622mm rim.

As far as 700mm tires in particular, Wikipedia has a nice reference.

700A = 642 BSD rim (and, thus, 29mm wide tires to keep an overall 700mm diameter)
700B = 635 BSD rim (and, thus, 33mm wide tires to keep an overall 700mm diameter)
700C = 622 BSD rim (and, thus, 39mm wide tires to keep an overall 700mm diameter)

(But, "700C" has since come to indicate any tire that fits a 622mm rim. I think most veloheads agree that the ISO nomenclature, such as "35-622", is the most accurate way to refer to wheels and tires.)

Iride01 

I can't say there might not be some basis to what you are saying, but I don't quite think that's how the original sizes worked. I think it's more how someone later tried to make them work to determine rim size similar to what you can do with ISO/ETRTO to figure outside tire diameter/circumference. Getting your tires outer circumference of diameter is much more accurate results with ETRTO than trying to figure out a rim size from the inch size name of the tire. In fact, the big number on the side of your tire isn't even the true outer diameter in many cases.

You pulled an inch size that just happened to come out. Doesn't work for many others such as:

• 29 x 2.00 ................29 - 4 = 25 or 635mm which is a valid rim but not for the 29 x 2.00 tire on the 29'er bike in my garage. It's a 622mm rim.
• 27 x 1 1/4 ...............27 - 2.5 = 24.5 or 622.5mm which is close to a 700C rim of 622, but fails because this tire uses a 630mm rim.

My main point though is not so much to dispute your claim as much as it is to again demonstrate why the ISO/ETRTO sizing scheme is much more useful and will, so far, never fail for fitting a tire to a rim.

For what it is, the inch sizes still show up more as a term that can be considered marketing or user jargon. It will always leave someone getting the wrong size tire for their rim.

Ghrumpy

Basically correct, but the French didn't really try to make their wheels work out to 700mm in diameter, that was just a nice round number close enough to 28". They were making tires to sell in countries that used Imperial units, after all, so the overall diameters had to work out accurately. So to get back to the original British sizes, you really need to use a wheel diameter of 711mm. More accurate to start with the Inch sizes and work towards the French/ISO than the other way around.

The interesting thing is that even though "700C" has come to mean a rim size, the designation itself gives you very little accurate to go on. As already noted, the wheels weren't really 700mm, they were 711mm. And the "C" indicates something relative; could mean anything bigger or smaller than B. And the increment of difference could be anything. There's literally no way to deduce this from the designation itself. It's a wonder it has lasted as long as it did, giving so little actual dimensional information.

So the ETRTO/ISO designations are in a way a return to the original British system of absolute sizes. In the British system, the nominal size was the overall diameter and tire section. You could easily calculate the rim diameter with that information. The ETRTO/ISO uses the tire section and rim diameter, from which you can easily calculate the overall diameter, should you be so inclined.

Here's the full range of 28" wheels in their original British rim and tire sizes, with French and ISO equivalents:
28 x 1¼, uses 25½" rim; French 700, ISO 32-647 (obsolete)
28 x 1⅜, uses 25¼" rim; French 700A, ISO 35-642 (mostly obsolete)
28 x 1½, uses 25" rim; French 700B, ISO 38-635 (still used on "Omafiets" and some roadsters)
28 x 1¾, uses 24½" rim; French 700C, ISO 45-622 (still used everywhere on road bikes, hybrids, and 29er MTBs.)

prathmann

The 16" 349 BSD size is used on many current folding bikes such as Brompton and some Bike Friday models. Should be readily available for the foreseeable future.

Ghrumpy

It doesn't work that way now, but that is exactly how the original system worked. I didn't "pull an inch size that just happened to come out." I used the original 19th century clincher tire sizing system's sizes as I outlined in the table above. Overall diameter was what they wanted to prioritize, and they worked back to rim diameters from that. But we've long since dropped overall diameter as a primary consideration.

So yeah, you're right, that's NOT how the current "system" works. The ETRTO system works as you say from tire section and rim diameter. Overall diameter is a secondary consideration (though still important for things like frame clearance and toe overlap.)

(By the way, the 27" 630mm BSD rim was not part of the original system, which used 2" increments in overall diameter from 28" on down to 12". The 27" wheel was introduced in the 1930s, when the previous system had already lost most of its relevance as far as OD of wheels was concerned.)

Iride01 

le mans

I collected another junker recently what i believe has 650 wheels even though the rims are marked 26"

it's inbetween what you'd commonly find on a 26" MTB and a 700c wheel size. i've come across a few on italian, english and jap bikes

Iride01 

If there is a question buried in there, then look at the existing tires closely. There should be a ISO size on them. It'll be among some of the very tiny lettering they mold into the tire. Smaller than even the lettering they use tell you the pressure rating. But it's there. Then you'll know for certain if the rim is 571mm, 597mm, 590mm or 584mm. No guessing required as long as you have a tire that fits correctly on the rim.

ThermionicScott 

If it's old enough, it may not have an ISO/ETRTO marking. A fractional tire size such as "26x1-3/8" or "26x1-1/2" would be a major clue as to which particular wheels you have.

le mans

Those gumballs have rotted away, so can't read a thing on those tyres, the thing i'm wrapped about this pick was it had a shimano 3 speed, excellent parts bike.. i was missing the anti rotation washer for a project i'm doing, and has the later gear linkage set up and solid wire cable.

i'll check out my other odd ball 26" tyres, they're black so should be able to read them

Ghrumpy

Yes all those rim BSDs are for 26"/650 wheels and tires. Could be any of them, all were used fairly commonly until maybe 30 years ago. Then 571 (650C) became a racing rim size (because it matches to the 26" tubular size), 590 (650A) and 597 (650) roadster sizes fell out of fashion, and recently 584 (650B) has seen two renaissances. Resurrected as a road tire when randonneuring came back, and then as the 27.5er mountain bike size. Tires be crazy.

The ETRTO size scheme came out in the 1960s, so tires made before then wouldn't be marked as such. They'd have the inch-fractional or French sizes on them.

ThermionicScott 

I was thinking that the rims themselves ought to say more than just "26". Like this:

le mans

what i can make out on this bike, on the rim - UKI26X1.3/8 W/0

ThermionicScott 

Boom, you've got 650A (ISO 590mm) rims.

Asi

ETRTO sizes totally make sense! - measure what it matters (and actually measure it!)

You can find the correct ETRTO size without any prints on the tyre or the rim.. just get some accurate calipers and measure the diameter of the seating bead of the tyre (or the groove in the rim) - there is a full page of the exact location for the point of measure.

Any other system (imperial, french, etc where it's not measured the actual bead seat diameter, radius, circumference or other measurable lenghts related to the bead seat) in absence of any markings it's irrelevant. I see them and treat them as a pure name and not an actual value.. so 700C is equal to 622mm seat diameter (regardless of what 700C stands for, for me it's just "700C".. could have said tyre size "A", size "B" and size "C".. it's all the same. just look in a conversion table to see the actual measurable items like bead seat diameter)

It's really dumb to name something by some non-measurable dimensions. Like screw #2 , or drillbit D, or drillbit #7 or pipe 1" (here the inside is 1", but the outside is another can of worms.. water pipe, gas pipe, steel or plastic, or copper, tapered thread, or straight thread - they all have different OD's and different styles and dimensions of threads ). I get that some of you might have learnt by heart every letter for every size dirllbit, and every letter of the tap, and combinations of number of screws that fit etc.. but it a whole lot more clear to state key measurable dimensions like actual diameter, actual thread pitch, etc. Here in Europe there is no "D" drillbit, but you can buy a 6.25mm.

The whole imperial system is kind of messed up and intentionally confusing. Not naming standard stuff by something measurable is one thing, and the other ugly part is the arbitrary denomination from one unit to the next. 12inches to a foot, 3 feet to a yard, 1760yards to a mile - totally fu**ed up.
While the odd denominations are workable even in the imperial system, not stating a measurable dimension is a fundamental flaw that is not correctable. A #5 drill will have nothing to do with the number 5, or gages for sheetmetal, or gauges for wire diameter, or gauges for firearms, etc.. just measure the damn thing and state it's actual thickness/diameter/etc in a unit of length of whatever system (SI preferable)!

Excuse my rant about this topic - I'm a huge advocate for SI units and for good reasons. (working in engineering and R&D - there is no place for confusion here and all work in SI - even Americans)

fietsbob

Any how French 600 a/b/c 650 a/b/c and 700a/b/c scheme,

each category try to have a similar nominal outside size with a range of
fatter tire smaller rim and, bigger rim skinnier tire..

Of course the ETRTO system is considerably clearer ..





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