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TIL that 27" tubes can work on a 29" rim. (pawb.social)
submitted 5 months ago by to c/bikewrench@lemmy.world
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When my girlfriend suggested we take the tubes off an old 27" bike to fix the flats on my 29", I thought she was being crazy and overly cheap. No way it would work. Surely I'd have to go out and buy some 29" tubes.

But fuck me sideways, it did work, and I'm currently riding just fine with 27" tubes in my 29" tires. Wasn't even that hard to get them on.

So I guess that's just a PSA for everybody ... apparently, tube sizes aren't really rules, they're more like guidelines.

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[–] 7 points 5 months ago (4 children)

Technically, 29" tyres are just extra-fat 28" tires, the rim diametre is 622 mm for both tyres. Surprisingly, 27" tyres have a larger rim, 630 mm, than 29" tyres.
The other parameter limiting the use of the tube is the width of the tyre. As 27" tyres have a maximum size of 35-630, 29" tyres start at 50-622. Thus, the tube for 27"/28" tyres will be somewhat overly inflated (according to it's specification) in a 29" tyre.

https://www.schwalbe.com/media/29/9b/36/1711611433/faq_reifengroessen_28032024.pdf

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  • [–] 4 points 5 months ago (1 child)

    What's really crazy...

    27" → 630 mm (old road standard)

    29" / 700c / 28" → 622 mm (mainstream)

    27.5" / 650B → 584 mm (had one of these, can also fit a 26" tires in some cases, which are typically 559 mm)

    Now they're doing 32" with 686mm beads.

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  • [–] 2 points 5 months ago* (1 child)

    Wtf how did that happen?

    Is it like 5.56, 5.7, .22, and .223, all being 5.7mm diameter?

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  • [–] 5 points 5 months ago* (last edited 5 months ago)

    I think this bit of insanity is due to 27 referring to the outer diameter of the tire rather than the rim diameter. "27" appears to be a road bike standard as tire sizes are around 1 inch on that Schwalbe chart. If you have a 630mm rim and a 1.1 inch tire, 630 / 25.4 + (1.1 * 2) = 27.0 for the outer diameter.

    On the other hand, "29 inch" is a mountain bike standard with a tire size ranging from 2 to 2.4 inches or so. For a 622mm rim and 2.2 inch tire you get 622 / 25.4 + (2.2 * 2) = 28.8 outer diameter.

    If the OP has a "29" inch tire, that's a mountain bike. And I suspect their "27" inch tire bike is also a mountain bike, which is actually a "27.5" size which is a mountain bike standard, with a 584mm rim diameter. If that's the case they would have had to stretch the tube a bit to get it on the 622mm rim of the 29er, but at least that tube would be intended for a mountain bike sized tire, say 2.4 inches. Using a tube intended for a 1 inch road tire in a 2.4 inch 29er tire would be more of a stretch.

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