27 and 700c breaks line up,supreme confusion

[Sam Perez]

Hi all I'm working on several projects at the moment, an old steel bike had 27 rims /tires  in the late 2000's I laced the old campy 5s hub to 700c for more tire options. Just now I put an old 27 rim to see the difference since I was told 27 is 650b. To my astonishment the brakes lined up exactly with the 700c rim. On line there was conflicting information.  With a 700c I fit a 35c tire I was hoping to transfer my hetre tires to this frame thinking the smaller diamiter would afford me a wider spot on the chain stays... Thoughts / advice.

[lconley]

27" rims are 630 BSD

700C (29") rims are 622 BSD

That is a diameter difference of 8mm which is a 4mm difference in radius - so the brake pad position is just slightly more than 1/8 of an inch off. The 27" is larger in diameter than a 700C (29").

650B (27.5") is 584 BSD

So a 27.5" rim is considerably smaller than a 27" rim.

When talking about rim diameters, using the BSD (Bead Seat Diameter) is the only reference that lets you compare relative diameters.

There are more than one BSD for 26" rims.

Ditto for 20" rims.

Laing

Delray Beach FL

[Sam Perez]

Oh wow the hetres will definitely not fit on the 27 rim they are a lot smaller. Thank you for clearing that up for me. 

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[Patrick Moore]

Almost. As Laing said, the nominal rim  bead seat diameters discussed are 584 mm, 622 mm, and 630 mm. The designations 650B and 700C (none for 27" rims as the former are a French convention and the latter English or American) correspond in fact to 584 and 622 bsd's. The 650 and 700 numbers refer to nominal overall rim diameter, the  "B" and "C" (there are others) to what used to be tire widths; Sheldon does not specify the widths or the ranges but perhaps someone can fill this in.

27.5" and 29", like 26", refer to de facto, rough, overall wheel diameters when shod with "fat" tires, conventionally 2" or so. Thus a 27.5" wheel has a 650B rim with a fattish tire, and a 29er has a 700 "C" rim with a fatter tire than the original "C" width (but see immediately earlier). And of course 26" wheels are 559 bsd rims with 50-55 mm tires. You can find 650"B" tires as narrow, I think, as 32 mm, 700C tires as fat at least as the almost 62mm ones on one of my bikes, and "26" tires as narrow as an actual 22 mm on narrow 559 rims.

Laing, you probably know this.

Sheldon said it long before I did (and I cribbed from him just now):https://sheldonbrown.com/tire-sizing.html#french

In particular: 

French sizes:

In the French system, the first number is the nominal outside diameter in mm, followed by a letter code for the width: "A" is narrow, "D" is wide. The letter codes no longer correspond to the tire width, since narrow tires are often made for rim sizes that originally took wide tires; for example, 700 C was originally a wide size, but now is available in very narrow widths, with actual outside diameters as small as 660 mm.

(there are others of course). 584 rims build up into 650B wheels. 

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Patrick Moore

Alburquerque, Nuevo Mexico, Etats Unis d'Amerique, Orbis Terrarum

[Patrick Moore]

Followup: I think (I read) that the original purpose of the French system was meant to provide wheels of roughly 26 to 27" in diameter with different tire widths and thus rims of different diameter a la Jan Heine's idea. Thus a 650B 42 mm tires is about the same width as a 700C 23 mm tire (and, just so you know, but not int their system, a 559 rim with a 60-65 mm tire).

[lconley]

It is somewhat easier to understand if you realize that the 650 and 700 were the outside diameter of the tire, not the rim in the French system, but yes, Sheldon has said it all before. 

I even have a couple bikes (Guv'nor and Flying Pigeon) that are 635 BSD and are called 28 x 1-1/2 or 700 x 35B or 40-635.

Not sure what the BSD was for the 36x2.25 tires on my long gone Coker Monster Cruiser.

When I first started riding, clinchers were 27" and sew-ups were 700C. I switched my 73 Paramount P-15 back and forth between 27" clinchers and 700C sew-ups several times until I got Campagnolo side-pulls to replace to 610 front, 750 rear Weinmann center-pulls. The Campagnolo side-pulls could only reach 27" rims, not 700Cs, and then only with a rear drop bolt and narrow rims. I laced up a set of NOS 27x1-1/4 Weinmann Concave rims up a few years ago, and they would not fit the rear brake - the extra width of the 1-1/4 rim over a 1-1/8 rim caused the pads to spread wider and (being single pivot) up, and that was just enough to put the pads into the tire.

Laing

On Monday, October 26, 2020 at 3:20:25 PM UTC-4, Patrick Moore wrote:

Almost. As Laing said, the nominal rim  bead seat diameters discussed are 584 mm, 622 mm, and 630 mm. The designations 650B and 700C (none for 27" rims as the former are a French convention and the latter English or American) correspond in fact to 584 and 622 bsd's. The 650 and 700 numbers refer to nominal overall rim diameter, the  "B" and "C" (there are others) to what used to be tire widths; Sheldon does not specify the widths or the ranges but perhaps someone can fill this in.

27.5" and 29", like 26", refer to de facto, rough, overall wheel diameters when shod with "fat" tires, conventionally 2" or so. Thus a 27.5" wheel has a 650B rim with a fattish tire, and a 29er has a 700 "C" rim with a fatter tire than the original "C" width (but see immediately earlier). And of course 26" wheels are 559 bsd rims with 50-55 mm tires. You can find 650"B" tires as narrow, I think, as 32 mm, 700C tires as fat at least as the almost 62mm ones on one of my bikes, and "26" tires as narrow as an actual 22 mm on narrow 559 rims.

Laing, you probably know this.

Sheldon said it long before I did (and I cribbed from him just now):https://sheldonbrown.com/tire-sizing.html#french

In particular: 

French sizes:

In the French system, the first number is the nominal outside diameter in mm, followed by a letter code for the width: "A" is narrow, "D" is wide. The letter codes no longer correspond to the tire width, since narrow tires are often made for rim sizes that originally took wide tires; for example, 700 C was originally a wide size, but now is available in very narrow widths, with actual outside diameters as small as 660 mm.

(there are others of course). 584 rims build up into 650B wheels. 

On Mon, Oct 26, 2020 at 11:27 AM lconley <lco...@brph.com> wrote:

27" rims are 630 BSD

700C (29") rims are 622 BSD

That is a diameter difference of 8mm which is a 4mm difference in radius - so the brake pad position is just slightly more than 1/8 of an inch off. The 27" is larger in diameter than a 700C (29").

650B (27.5") is 584 BSD

So a 27.5" rim is considerably smaller than a 27" rim.

When talking about rim diameters, using the BSD (Bead Seat Diameter) is the only reference that lets you compare relative diameters.

There are more than one BSD for 26" rims.

Ditto for 20" rims.

Laing

Delray Beach FL

On Monday, October 26, 2020 at 1:16:13 PM UTC-4, Sam Perez wrote:

Hi all I'm working on several projects at the moment, an old steel bike had 27 rims /tires  in the late 2000's I laced the old campy 5s hub to 700c for more tire options. Just now I put an old 27 rim to see the difference since I was told 27 is 650b. To my astonishment the brakes lined up exactly with the 700c rim. On line there was conflicting information.  With a 700c I fit a 35c tire I was hoping to transfer my hetre tires to this frame thinking the smaller diamiter would afford me a wider spot on the chain stays... Thoughts / advice.

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[Patrick Moore]

You are right about that.

On Mon, Oct 26, 2020 at 4:00 PM lconley <lco...@brph.com> wrote:

... 650 and 700 were the outside diameter of the tire ...