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I recently got a puncture and when I went to replace the inner tube I discovered that I'd bought the wrong size tube. My tyre is 700 x 40c, the tube is 700 x 28-35c.

What is likely to happen if I use this tube? Will the tube explode when I attempt to inflate it?

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In my experience smaller ones go in fine and last as per normal (not that I have pushed this to extremes with a 700x20 in a 700x40 but I only have 26x1.5 MTB tubes and they work fine in 26x2.00 tyres).

Allegedly putting a smaller tube in the tyre becomes a problem if and when you repair it, the patch doesn't necessarily stretch with the inner tube and it can come off.

  • Agreed, no issues with doing this. – zenbike Jul 5 '11 at 14:18
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    A patch just came off my 700x18-23 inner tube in a 700x28 tire due to stretch, so I can confirm this. – Sander Heinsalu Jan 13 '16 at 7:06
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You may be perfectly OK, or may experience several possible problems:

  • The area near the valve on the tube is the stiffest part, and does not expand as easily as the rest when over-inflated. This may result in a slight depression in the tire near the valve that you will feel on every revolution (especially if you start thinking about it). Not a significant reliability issue, but it can be a comfort issue.
  • As the tube expands in its width from over-inflation, it expands even more in its overall circumference. The tube can, before it gets "locked in" to the sidewalls of the tire, expand enough that a portion of the tube telescopes on itself, in the short term creating a lump, and in the only slightly longer term creating a stress point that will result in a fairly sudden deflation of the tire.
  • And obviously, the tube is being placed under more stress than it's designed for, and it's apt to simply fail, especially near the valve.

I wouldn't hesitate to use the (slightly) wrong size tube for a relatively brief period, as an emergency measure, but I wouldn't regard such a repair as trustworthy over the long haul.

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Yes, you can, as long as you don't go more than one size smaller, or the differential is not too extreme. A 1.25 in a 1.5 is fine. A 1.25 in a 2.25 is likely to have issues with flats.

Your LBS will have tubes. Are they that bad that you won't even buy tubes from them?

  • My LBS is a Halfords, they mostly sell car parts and accessories with a small cycling section. They didn't have the right size. My other LBS isn't all that local and other than Saturday is only open when I'm at work. – Tom77 Jul 5 '11 at 15:11
  • That sucks. But most people really only have the weekend to do what's important. And shopping online for small parts in particular will eventually deprive you of the resource you need when you most need it. – zenbike Jul 5 '11 at 17:55
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Adding to the thoughts above: I had a puncture yesterday on my ride home and used way smaller tube (had the wrong size with me and got another from a fellow bicycle commuter; both tubes were for racing tyre size where mine is 700x33c). First time I tried the tube immediately got damaged and deflated completely - even before I had the wheel back in the frame. The second time it lasted about 100m. So, probably fine to go just one size down but not too much.

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