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ok to use an undersize tube?

Old 05-26-19, 10:19 AM
  #1  
rojeho
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ok to use an undersize tube?

First road flat on my new bike and discovered that I transferred the saddle pack but didn't check it first. Old bike used a 19-26 tube, new bike has 700x28. Now that I'm back home, should I replace it with a larger tube or leave it until the next time I need to change it?
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Old 05-26-19, 10:23 AM
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u235
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I used a 28 in my 42 for months with no problem. I'm sure there are stories of yes and no for various reasons.
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Old 05-26-19, 10:26 AM
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Tubes have amazing stretch. I used 2.75 in downhill bike tubes in my 4.0 fat bike tires to cut a huge amount of weight. 3 years with no flats, mind you I used it mostly on snow and some single track dirt/gravel.

Last edited by bikeaddiction1; 05-26-19 at 09:46 PM. Reason: typo
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Old 05-26-19, 12:41 PM
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If its a 19-26mm tube in a 28mm tire I wouldn't be too concerned about it. If its the rear tire I'd get around to replacing it if I had nothing else to do, but if in the front I wouldn't worry about it. You can do the math and figure out the difference in volume (just the areas of a 23mm and 28mm circles) and see if you're comfortable with that much expansion.
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Old 05-26-19, 02:32 PM
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That little bit of size difference wouldn't blip my radar. Given the choice, I'd rather use an inner tube that's a size small than one that's a size too large because it stuffs into the tire with less potential for pinching.
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Old 05-26-19, 03:16 PM
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Shouldn't be an issue.
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Old 05-26-19, 03:27 PM
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You never said what caused the flat.
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Old 05-26-19, 03:34 PM
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Originally Posted by rojeho View Post
Now that I'm back home, should I replace it with a larger tube or leave it until the next time I need to change it?
I think you'll be fine and agree that a tad undersized is better than a skosh oversized.

I ride on 650b's (ISO 584mm) and I use 26" tubes (ISO 559mm) in them, as recommended by the tire maker. They go on tight and work well.

A Sheldon Brown tire-sizing article here. God bless Sheldon.
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Old 05-26-19, 03:35 PM
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Originally Posted by Retro Grouch View Post
That little bit of size difference wouldn't blip my radar. Given the choice, I'd rather use an inner tube that's a size small than one that's a size too large because it stuffs into the tire with less potential for pinching.
same
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Old 05-26-19, 03:37 PM
  #10  
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Undersized is better than oversized in my books. Have used them on several occasions with no problems at all.
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Old 05-26-19, 03:41 PM
  #11  
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As people have said it will work fine.

However, my experience is that undersized tubes tend to have fast leaks, and oversized tubes tend to have slow leaks.

So, if you're on the road, pump it up, and hoping to baby it home, then go oversized (or with the maximum recommended tube size).
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Old 05-26-19, 03:43 PM
  #12  
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I am someone who has used 23mm-meant tubes in 28 and 32mm tyres for touring and commuting. Oversized tubes, such as 32mm in 23mm tyres just won't work because they don't fill in properly because the volume of rubber is greater than the volume inside the tyre.

I have inflated tubes in my workshop well over the tyre size on numerous occasions when checking (a) for punctures and (b) after they have been repaired; in no case has there been a problem. I have experimented blowing them up deliberately and they do go to a size of, oh, around 60+mm before even being risky,
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Old 05-26-19, 07:49 PM
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In my world it's very common to use smaller tubes as spares to save weight, especially in the MTB and fat tire world. For example, I carry a 26x2.5-3 inch tube as a spare for my 26x4.8 tires. Only had to use it once and it's still working. Kind of a pain overcoming the resistance when inflating a smaller tube to fill the large space. Huge weight/space/cost savings over the normal 4.8 inch tube.

Last edited by Dr.Lou; 05-26-19 at 07:59 PM.
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Old 05-26-19, 08:15 PM
  #14  
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A coworker asked me to change the tires on her bike. New tires were 700 x 38, same as old ones. Front one was OK, but the rear wheel had a 27.5 x 1.75/2 tube. I told her it would be better to replace that tube for a proper sized one, but she said if it had no problems before, why should it now? 3 months on and no problems so far.
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Old 05-27-19, 01:47 AM
  #15  
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On fatter tyres it can give problems around the valve, where the tube can't stretch as much.

I have a 24x1.5-1.9 tube in a 24x2.35 tyre, and I have to whack the pressure right up to get the sidewall filled out at the valve.

That said, I have 2.1 tubes in 2.35 tyres (this time 26s) with 2in wide rims, and didn't notice the same problem there.
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Old 05-27-19, 08:39 AM
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I think there are reasons for matching tire size and tube size . I also think you can get away with a narrow tube(700 x 23) in a wider tire 700 x 28 without much problem. In theory , the rubber would not be as thick if inflated beyond its stated application . That said , I really don't worry about the tube being too narrow. I care more about rim tape and tire seating when changing them. My Mondia has 27" x 1 1/4" tires that measure 1.390" when inflated and I am using 700 x 23-28 tubes with many miles and no problems. Joe joesvintageroadbikes.wordpress
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Old 05-27-19, 09:25 AM
  #17  
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No guarantees it will work. I recently used a 23-25 tube in my 28 tire and it caused a very noticeable low spot in the tire when fully inflated. The tube was not stretching fully/consistently at the valve. I could see the low spot with my naked eyes and feel the thump while rolling. I believe it was a Giant branded tube.
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Old 05-27-19, 10:44 AM
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rojeho
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Originally Posted by Rick View Post
You never said what caused the flat.
Small piece of either glass or hard stone in the center of the front tire. Riding on a relatively clean asphalt path. I didn't see it but immediately heard the pfft pfft pfft with every revolution.

Thanks everyone for the helpful comments.
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Old 05-27-19, 11:56 AM
  #19  
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Originally Posted by Riveting View Post
No guarantees it will work. I recently used a 23-25 tube in my 28 tire and it caused a very noticeable low spot in the tire when fully inflated. The tube was not stretching fully/consistently at the valve. I could see the low spot with my naked eyes and feel the thump while rolling. I believe it was a Giant branded tube.
Try this:
1. Deflate your tire.
2. Push your valve stem straight down into the tire.
3. Reinflate your tire.

Bet that fixes your noticeable flat spot adjacent to the valve stem problem.
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