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700 x 38B Tire on 622 x 18 Rim?

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700 x 38B Tire on 622 x 18 Rim?

Old 03-14-19, 11:27 AM
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raywood
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700 x 38B Tire on 622 x 18 Rim?

I have a worn-out 700 x 32 tire on a rim whose label says ETRTO: 622 x 18. The replacement that I bought says 700 x 38B. It also says 40-635.

I knew I was getting a wider tire. That part was intended. I also knew I was getting a great price. So I went for it. It was only later, when it arrived, that it occurred to me that the letter B might have some significance. Seriously, I thought I was supposed to watch out for HB, which would have been wrong. Somewhere, I got the impression that letters like B and C were outdated. But the bead on this B tire, the way it curves out, does not look familiar. I'm also not recognizing the 40-635. I don't recall seeing that in the sales literature.

So, the question: did I buy the wrong tire and, if so, the more important question: why is it wrong? What did I buy? And what will happen if I mount it on this rim -- assuming it will fit correctly, regarding which I'm not entirely sure whether misfit would be obvious to me.
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Old 03-14-19, 12:09 PM
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If it's really a 635mm bead seat diameter, you'll definitely notice that the bead is very loose. Probably just holding the tire up to the rim will show that it's not going to fit.
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Old 03-14-19, 12:13 PM
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Originally Posted by raywood View Post
The replacement that I bought says 700 x 38B. It also says 40-635.
That's an uncommon size. The "40-635" is the ETRTO spec. You want "40-622" (622 is the critical number indicating the diameter of the rim) or 700x38C.

Last edited by tyrion; 03-14-19 at 12:20 PM.
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Old 03-14-19, 12:40 PM
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Originally Posted by raywood View Post
did I buy the wrong tire and, if so, the more important question: why is it wrong? What did I buy? And what will happen if I mount it on this rim -- assuming it will fit correctly, regarding which I'm not entirely sure whether misfit would be obvious to me.
The 40-635 means that your tire has a nominal inflated width of 40mm and that it's designed to seat on a rim with a bead seat diameter of 635mm. "Bead seat diameter" is the diameter of the shelf within the rim that the rim bead sits on.

700c means that the tire bead sits at a diameter of 622mm. A 700b tire has a bead designed to sit at a diameter of 635mm. By convention, when describing tire sizes, the letter is sometimes stuck after the nominal inflated tire width: a 700x25c tire is a 700c tire which nominally inflates to 25mm wide, and could more precisely be described as 25-622.

Bead seat diameter between tire and rim must match very precisely. Your 700b tire will sit extremely loosely on a 700c rim, and will not be anywhere near usable.

I've never heard of someone accidentally buying a 700b tire when they needed a 700c tire before!
700c is by far the dominant standard today, used on nearly all road bikes and loads of mountain bikes (in mountain bikes it's usually called "29er" instead of "700c"). But 700b is very obscure, it's hard to find 700b parts nowadays.

Last edited by HTupolev; 03-14-19 at 12:43 PM.
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Old 03-14-19, 01:22 PM
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That's a very old/obscure size.
See-
https://www.sheldonbrown.com/english-3.html#roadster
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Old 03-14-19, 05:24 PM
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Originally Posted by HTupolev View Post
I've never heard of someone accidentally buying a 700b tire when they needed a 700c tire before!
This seems to be a misreading. As far as I can tell, it's not a 700b tire. That's not the confusing part. The confusing part is that, in the Amazon ad I responded to, they described it as (to quote them exactly, this time) "Size: 700cm x 38B-Inch." Surely the "38B-Inch" part is confused. A user Q&A seems to have guessed that they meant 38mm, and that's what I wanted.

In the ad, I'm not seeing any mention of the 40-635 size, nor of any other rim size.

There is a separate problem: the tire is deformed. I didn't want to raise multiple issues, here; I was just curious about the tire size information. It seems I may have gotten such a great price ($28) because they were hoping to unload one weird tire. As soon as I bought it, I saw that the merchant's price went to $47, where it remains.
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Old 03-14-19, 06:15 PM
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Originally Posted by raywood View Post
This seems to be a misreading. As far as I can tell, it's not a 700b tire. That's not the confusing part. The confusing part is that, in the Amazon ad I responded to, they described it as (to quote them exactly, this time) "Size: 700cm x 38B-Inch." Surely the "38B-Inch" part is confused. A user Q&A seems to have guessed that they meant 38mm, and that's what I wanted.
"700cm" would also be confused, because bicycle wheels are generally not seven meters across.
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Old 03-14-19, 06:25 PM
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Originally Posted by raywood View Post
the Amazon ad I responded to, they described it as (to quote them exactly, this time) "Size: 700cm x 38B-Inch." Surely the "38B-Inch" part is confused. A user Q&A seems to have guessed that they meant 38mm, and that's what I wanted.

That confusing and inaccurate ("700cm") ad should be grounds for getting a full refund.
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Old 03-14-19, 07:05 PM
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Originally Posted by raywood View Post
I have a worn-out 700 x 32 tire on a rim whose label says ETRTO: 622 x 18. The replacement that I bought says 700 x 38B. It also says 40-635.

I knew I was getting a wider tire. That part was intended. I also knew I was getting a great price. So I went for it. It was only later, when it arrived, that it occurred to me that the letter B might have some significance. Seriously, I thought I was supposed to watch out for HB, which would have been wrong. Somewhere, I got the impression that letters like B and C were outdated. But the bead on this B tire, the way it curves out, does not look familiar. I'm also not recognizing the 40-635. I don't recall seeing that in the sales literature.

So, the question: did I buy the wrong tire and, if so, the more important question: why is it wrong? What did I buy? And what will happen if I mount it on this rim -- assuming it will fit correctly, regarding which I'm not entirely sure whether misfit would be obvious to me.
40-635 appears to indicate that the tire you bought is 13mm (1/2 inch) larger in diameter than your 700c rim (622mm).

Since you already have the tire, mount it on the rim and inflate to 10 psi to see what happens. If the tire is indeed intended for a 635mm rim then it seems to me that the problem should be immediately apparent. Post some pictures.
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Old 03-14-19, 08:29 PM
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Yep, you definitely bought a tire for the mostly-dead 28"x1-1/2" standard, rather than 700C. Check the bottom of the list here: https://www.schwalbetires.com/bike_t...s/marathon_420
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Old 03-14-19, 10:53 PM
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Thank you all for the information and support. I have put in a refund request.
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Old 03-15-19, 08:13 PM
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A 28 dollar tire on amazon is usually a 14 dollar tire at your better online bike parts retailers.
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