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The simple answer is No, you can’t use a smaller inner tube in your bike tire.
The best answer is to use a tube that’s the correct size and I’ve written a handy guide here to my recommended inner tubes for a range of different wheel sizes. It’s as simple as picking your tube size from the list and clicking the link.
There is, however, a little wiggle room and we’ll take a look at those details in a moment. I’ll also give you an idea of the problems you can have if you use a tube that’s not right.
First, let’s take a look at how bike tires are measured as this will explain why you need the correct size.
How are bike tires (and tubes) measured?
You’ll normally find the size of the bike tire embossed or printed on the sidewall (the smooth rubber strip that sits between the tread and the wheel rim). There will be a variety of numbers and words there but the ones that we’re interested in will say something like “700x23c” or “27.5×2.1”.
The first number (in this case 700 or 27.5) is the diameter of the tire. This is measured in either millimeters (i.e. 700mm) or inches (i.e. 27.5”). The second number is the width of the tire, again in millimeters or inches (so, 23mm or 2.1”). If you see a letter ‘c’ you can safely ignore it. It’s a reference to a long dead French tire classification system that many manufacturers still continue to put on the side of tires to this day.
Once you have this information you can go ahead and buy a replacement tire (look for one that says it is “700×23” or 27.5×2.1” in these examples).
Inner tubes are a little trickier and I’ll explain why. Imagine that your inner tube is like a kid’s party balloon. The diameter of the tube (700mm/27.5”) will stay the same, but the width (23mm/2.1”) will vary according to the amount of air you inflate it with.
For this reason, inner tubes are listed as having a fixed diameter with a range of widths. In our examples, you would be looking for a tube that said either “700×20-25” or “27.5 x 2.0-2.3”.
Can I use a bike tube that’s slightly the wrong size?
If we continue with these examples you can see that you could choose a tube that either had a different diameter (bigger or smaller) or a width range that was either bigger or smaller than your required width.
With a diameter that is too big (e.g. a 29” tube for a 27.5” wheel), the tube will be too loose on the wheel rim and you may end up with bunching in the tube rubber. This can lead to punctures.
If the diameter is too small (e.g. 24” for a 27.5” wheel) it is unlikely that the tube will stretch around the wheel rim. Even if you can force it on it may have weakened the rubber and this can also lead to punctures.
Looking at the tube widths, there’s a little wiggle room. However, I would stress that this is only for emergencies in order to get you home (or to the nearest bike shop) and is not a long-term solution. Let’s take a look at an example.
If you have a tire that is 700x23mm you would ideally use a tube that is 700 x 20-25 (20 being less than 23 and 25 being more than 23). As we’ve seen, the diameter (700) is essential for the tube to fit on at all. What about the tube width? Well, if you used a tube that was too small (say, 700 x 18-20) then you would need to over-inflate the tube to get the required fit inside the tire. The tube might be able to take this pressure however the rubber will be over-stretched and fragile. Hit the first bump in the road and you might find that the tube pops. If this tube is your only option to get home, then ride carefully. With a tube that’s too big (say, 700 x 25-28) you may find that you can’t inflate the tube sufficiently and so the tire will stay too soft. Again, bumps in the road can lead to punctures, so you must ride very carefully.
To sum up:
- If the tube diameter is too small, then it may not fit on your wheel rim
- If the tube width is too small then you may have to over-inflate the tube to fill the bike tire and this will weaken the tube rubber, with the possibility of punctures
Conclusion
Bike tubes aren’t expensive and it’s easy to track down the right size and purchase. Using a tube that’s the wrong size can lead to punctures and, according to Sod’s Law, these will always happen when you’re far away from home or a bike shop.
Be safe:
- Buy and use the correct size tube
- Keep a spare tube tucked in your pocket or saddle bag whenever you go out for a ride.
Have fun!
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