Just a quick one, is it OK to use a 26″ tube in a 650b tyre?
I’ve had a 29″ tube in a 26″ tyre before and it was OK, but is it OK to put a smaller tube in a bigger tyre or will it stretch too much?
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Hoping not to need it at all. Borrowing a 650b bike but all mine bikes, and hence spare tubes, are 26″. Will only need to do this if I puncture one of the ones already in it.
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Cheers.

Be fine.

It’s the same except for the label*, it’ll be fine

* 😈

it’ll prob be fine. I know someone who stretched a 26 tube around a 29 rim as temp fix to get him home after a puncture. He was still using it weeks later

As if you even need to ask.

Thanks guys, thought it would be OK but best to check.
I will be racing so if I do need to use it will be inflated with a CO2 canister rather than a pump, I’ll go easy and make sure I don’t overinflate.

I’ll go easy and make sure I don’t overinflate

with a spare 26″ tube, without the constraints of being inside a tyre, inflate it a bit and see how big the outside diameter is…

Just make sure you use 27.5″ specific CO2