Tire Sizes

Making tire molds continues to be an inexact art. For decades, riders have complained that many tires, especially wider ones, do not run “true to size.” In most cases, tires were narrower than their stated width, which caused Sheldon Brown to suspect a plot by the tire makers: To make their tires appear lighter, tire manufacturers overstated the width, so that their 25 mm tires (labeled “28 mm”) were compared to the competition’s 28 mm tires, and so forth.

I don’t know whether this is true, but I suspect that the real reasons for the inaccurate sizes are less sinister: When tire makers design a mold, they don’t know exactly how wide the tires will be. This is especially true for relatively wide tires, and even more so for tires with supple casings. Simply changing the casing from the original Grand Bois Cypres 650B x 32 mm to the more supple (and much faster) “green label” casing increased the width of the tires by almost 2 mm – even though the mold was unchanged. In fact, because most tire makers have limited experience with wide high-performance tires, these tires often run wider than initially planned. This holds true for many Grand Bois tires, but also Challenge’s Paris-Roubaix and Michelin’s Pro Race models in the widest sizes.

We have decided to call our Grand Bois tires by the actual size, no matter what is written on the sidewalls. So we now offer:

700C x 23 mm (same as label)
700C x 26 mm (same as label)
700C x 29 mm (labeled as 28 mm)
700C x 32 mm (labeled as 30 mm)
650B x 32 mm (same as label)
650B x 38 mm (labeled as 36 mm)
650B x 42 mm (same as label)

Of course, even these measurements depend on the width of your rim. If you use wide rims, your tires can run a millimeter or so wider. Also remember that high-performance tires tend to “grow” a bit over the first few weeks of being mounted. In the end, a few millimeters should not matter much, as long as your bike had adequate clearances.

We are happy to offer Grand Bois tires in enough sizes now that most cyclists can enjoy their superior performance and comfort.

About Jan Heine, Editor, Bicycle Quarterly

Spirited rides that zig-zag across mountain ranges. Bicycle Quarterly magazine and its sister company, Compass Cycles, that turns our research into the high-performance components we need for our adventures.
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10 Responses to Tire Sizes

  1. Vlad says:

    Interesting. I used Rivendell Jack Brown tires labelled 700×33.3333. I replaced them with Grand Bois tires labelled as 700×30. If the GB tires’ true width is 32, then Jack Browns must be way wider than 33.3333 because the perceived visible difference in width between these two tires when mounted on the same rim is significantly greater than 1.3333 mm. Jack Browns end about 3mm from the edges of my front fender and the GBs end 7-8mm from the edges of the front fender.

  2. Harald says:

    “these measurements depend on the width of your rim. If you use wide rims, you tires can run a millimeter or so wider. ”

    I think this is crucial to point out. The ETRTO specify which tire width should be measured on which rim width. http://tandem-fahren.de/Technik/Reifentips/index.html#R%C3%A4tsel_Reifenbreite (sorry, German only) So I wonder if your GB widths were measured on rims of the appropriate width?

    • We measured the 700C tires on a Mavic MA-2 rim, the 650B tires on a Grand Bois 650B rim. While it would be ideal to measure each tire on the ETRTO-specified rim width, there are so many factors that it never will be possible to give exactly accurate tire widths.

  3. I have the 650B x 42 mm Grand Bois Hetres on one of my bicycles, and these fantastic tires make me wish that all my bikes were 650B. For whatever reason, I have seen them described as 40mm on some websites, 42mm on others.

  4. superfreak says:

    how bout some 559 tires (im against 650b). would like to try a GB 559×54. this tire would make 650b obsolete.
    thx superfreak

    • We are thinking about this. At Compass Bicycles and Grand Bois, we mostly make parts we ride ourselves. Grand Bois uses 700C tires for sizes up to 32 mm, and 650B beyond. So a 26″ tire has been a low priority in the past. But as you mention, in a very wide tire, you could get great handling and even more air volume, plus the ease of procuring 26″ spares if you are traveling far from home. The downside would be aesthetics. For many of us, a 26″-wheeled bike looks decidedly “small-wheeled.”

  5. Tom says:

    I am one who would like to see these very nice Grand Bois in 26×1.75”. 650B is nice, but a bit of a boutique tire size in the US, although it is making a comeback. Most of my bikes are 700c, and I have the Grand Bois tires on a couple, but I would love to try them on my 26” all terrain/expedition touring bike. All kinds of tires are available in 26”, but wide, high performance tires are few and far between.

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