Let's talk about buying inner tubes, because when They sold you the bike there wasn't any brochure explaining how to select inner tubes. How could they sell you a product that assumes you possess implicit knowledge? Who do they think they are, Microsoft?
These are the factors you need to know about your bike in order to purchase inner tubes:
- Valve type
- wheel size
- tire width
So, valve type. There are two valve types, as shown in the picture below. You have a Schrader valve (like a car tire) or a Presta valve. Just look at your bike and you'll be able to tell which. You don't have to choose which; the holes drilled into your rims have decided for you.
Now you should know: Valve Type, Presta or Shrader? If you have Presta valves on a bike, while you're at the LBS buying your inner tubes you might want to pick up a little $2 gizmo that adapts your Presta valve and lets you use a gas station air pump on your fancy-schmancy tires. Once you buy it, you'll never need it. If you don't buy it, you may rue the decision.
Wheel Size. So there are both wheels and tires; wheels are the metal circles with spokes; tires are the rubber rings that go around the wheel. Your wheel size is probably 700, or else it's 26". Or it's 650, or 29". Unless it's 24". At one time, your wheel size actually reflected the diameter of the metal circle with spokes, but now there's a lot of marketing going on. Argghhhh, marketing, scourge of truth and understanding.
Here's a marketing perversion: those labels with the numbers? Those aren't really numbers as we usually know them, they're only labels - just like a dress is a Size4. There are different types of number scales - nominal, ordinal, interval, ratio (don't get me started) - and these are nominal numbers, meaning they're names of categories. Just like Diabetes is Type1 and Type2. Is Type2 more than Type1? Is a Size Eight skirt twice the size of a Size Four skirt? No, they're just names which signify a different category. Don't let the marketing and the hype of pseudo-precision lull you into thinking that 700 is a number, it's just a label like: Fireball XL-5.
Also, your wheel is designed for a certain width of tire, and (sometimes) there's a letter than designates a width category: 650B, 700C. Look at the tires on your wheels, and look for a wheel size. (more later)
Now you should know: Wheel Size: 26, 27, 650b, 700c, 29 ?
If you want to read more and realize that the confusion isn't the consumer's fault, read Sheldon Brown on tire sizing.
Tire Size. You're buying an inner tube, to go inside a tire, that sits on the wheel. Having discussed the wheel size, let's look at the tire. The tire you have on the bike should have Secret Codes written on it. They probably look like this:
And so, the tire sizes shown are 26x20, and 700x23c. Sheldon Brown has identified a bit of wisdom:
Brown's Law Of Tire Sizing: If two tires are marked with sizes that are mathematically equal, but one is expressed as a decimal and the other as a fraction, these two tires will not be interchangeable.Now you know: Valve Type, Wheel Size (26 or 27 or 650b or 700c or 29), Tire Width( 2 inches or 23 or 45) You go to your store, you say: I need two inner tubes, please: Shrader, 26" wheels, 2" wide tires.
In other words: 27.5 is not the same as 27 1/2 these are text labels as much as numbers. crazy
And here's where the marketing runs into reality: the shop looks at their stock on hand and gives you something close because they don't have the exact right thing. Maybe they give you a tube that's Shrader-26 but 2.5 inch width and says, "this will work fine". And they're probably quite right. This is why you don't buy a tube based on the one that's already in there.
But wait there's more! On the tire width picture above, there were other numbers printed for tire sizes: one was 50-559 and the other was 23-622. This is an ISO standard that used to be called ETRTO. This ISO standard should replace some ambiguity, eventually. For now, you probably want to know all the numbers on your tire. Take a picture with your cellphone, bring it to the shop.
This is an example of really good labelling on an inner tube package. With a clear picture it identifies the type of valve. It also provides yet another number: 42mm is the length of the valve stem. Then it gives the range of tire sizes this tube is compatible with, in both ISA/ETRTO, inches, and pseudo-metric parameters (labels):
And that's what I know about buying inner tubes. In the end,you have to get valve type and wheel size correct, you can't fudge these; but there is a small range of accomodation on the tire width.
Three more things, in the realm of opinions:
- Support your Local Bike Shop (LBS). That doesn't mean Performance or Trek or even REI. Support a local, indy one-off. (Pittsburgh: Ambridge Bike Shop, Thick, Dirty Harry's, Big Bang, Love Bikes, Iron City Bikes) Meet the people. Learn their names. Tip the mechanic that helps you! Build a relationship.
- Never. ever. never use a brick-and-mortart LBS as the try-on room for your internet shopping, or I hope people will let the air out of your tires on cold rainy night rides in the middle of a scary forest and ride away with your cellphone and GPS. Seriously. Ain't nobody got time for that.
- Never buy Slime or Goop tubes. If you want to avoid flat tires, buy quality, flat-resistant tires: Schwalbe, Continental, Ribmo's. Tell your LBS folks: I want tires that won't get flats. Then just keep them inflated until they wear out. It'll cost more, and you'll love it.