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  1. #1
    I like to ride my bicycle
    Reputation: JohnŠ's Avatar
    Join Date
    Jan 2008

    Rim diameter (not what you might expect!)

    Something has been bugging me since I've been learning a bit about 650b: rim diameters. I have always believed that a 26inch bike has wheels with a diameter of 26 inches; and 29ers have 28 inch rims, but are "known" as 29ers. Exactly why I'm not sure.

    Enter 650b. I have been told by people who should know that 650b rims measure 27.5 inches. So if all the above is correct, 650b rims are only half an inch smaller in diameter than 28 inch "29er" rims. Which isn't much.

    So I went out into the shed to measure a couple of wheels. We have one 29er out there and a bunch of 26ers. I was a bit surprised to find that 26 inches is the OVERALL diameter, wheel + tyre, on a 26er. Rims measure 22.5 inches. On the 29er, once the overall diameter is approx. 29 inches, rims measure 25 inches.

    Am I the only one surprised? For as long as I've known anything about bikes I have believed that the wheel size (by which the bike is known) is actually the rim diameter. But it seems to be the outside diameter of the tyre.

    So, next question: what's the diameter of a 650b rim? Not 27.5inches, I'm guessing!

    Last edited by JohnŠ; 11-19-2008 at 04:53 AM.

  2. #2
    TNC is offline
    noMAD man
    Reputation: TNC's Avatar
    Join Date
    Jan 2004
    Yeah, this has been an oddball deal from the beginning. Motorcycles use the actual size of the rim to measure wheel standards, but bikes started off with the not-so-great idea of using the alleged diameter of the wheel/tire combo. The difference between a 26'er wheel with a Tom slick 1.0 or a Michelin 2.8 DH32 is massive. I'm sure things were somewhat more standardized back in the days when bicycles started using this method. A lot has changed. As long as one knows this, it's probably not a big deal.

    On the other hand, I still love the fuzzy concept that many people have when they're asked about the size of their bike. The typical bike store conversation with a prospective customer:

    LBS Guy: So...what size bike do you currently ride?

    Customer: Oh, it's a 26" bike.

    LBS Guy: Damn! don't look like an NBA player.

    Hey...what with all the other fuzzy math and concepts in the bicycle industry, can you blame the customer?...LOL!

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