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I was wondering how precise replacement inner tubes need to be. I have an old bike that I just use for recreational riding and was told that I can use a 700 x 28-38c to replace a 27 in. x 1/4. A quick conversion of 700 mm to inches gives a little more that 27.5 inches. Could anyone tell me whether that's likely to cause problems?
 

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Banned Sock Puppet
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That should be 27 in. x 1 and 1/4 in.
1 1/4 in. = roughly 32mm. The 700 and 27 in. numbers are not really 700mm or 27 in. They are round numbers that are used for marketing purposes and are easy for people to remember. In fact, your actual rim diameter is 622mm or about 24.5 in.

Bike tire sizing can be very confusing. Without giving you enough info on bike tire sizing to make your head spin, your tube in question will work.
 

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Banned Sock Puppet
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Yes very.
And if you've ever inflated a tube (outside of a tire) you'll know they're very forgiving. A difference in wheel diameter of 1/2" doesn't mean anything.


This may be true. But inserting a tube that is too large can be a problem.

Just for sh!ts and giggles, I tried the experiment in the video once. My tube got way larger than this one before it burst. Cleaning up all the small pieces of butyl rubber was fun. 🙄
 

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I was a course Marshall once during a race when a guy flatted and dropped out. I gave him a spare tube from my greenway bike (38c tire) and he was able to stuff it in his 25c tire and ride back.
 

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Banned Sock Puppet
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A 1/2" in diameter you wouldn't even notice.

Now width... yes that could be a problem. Stuffing a 1.5" wide tube in a 1" tire.
True. The tube can expand inward as well as outward.

I was a course Marshall once during a race when a guy flatted and dropped out. I gave him a spare tube from my greenway bike (38c tire) and he was able to stuff it in his 25c tire and ride back.
He was lucky. I don't see how you can do that without causing multiple folds in the tube which could cause flats. I've seen it happen.
 

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Russian Troll Farmer
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Actually, MTB guys call 700c a "29er", so that number really means nothing. But, you can put a larger or smaller tube inside. Oversized tubes will fold-over in places, and that could cause a lumpy ride. Georgina Terry claims that you don't need to carry 2 tubes for her 'funny bikes' because you can just fold over the 700c tube for the rear tire...

Really small tubes (say, a 19mm tube inside a 50mm tire) are likely to explode before filling all the space, but slightly undersize tubes usually work well. But don't try this if the tube has a patch on it...
 

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Banned Sock Puppet
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Actually, MTB guys call 700c a "29er", so that number really means nothing.
Exactly. These sizes are nothing more than marketing.

But, you can put a larger or smaller tube inside. Oversized tubes will fold-over in places, and that could cause a lumpy ride.
Or a flat. Tube folds inside a tire will eventually pinch it. I've seen it happen.
 
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