What is the purpose of bicycle inner tubes?
An inner tube is basically a balloon with a valve. A standard tyre is not airtight, so bicycle inner tubes keep the air in your tyre. Currently the pneumatic system is the most energy efficient way to encase a wheel.
Quite a lot of bikes are now coming with tubeless tyres. The actual tyre is so well sealed around the rim and valve hole, that it can hold air at a high pressure. Even so, it involves different tyres, rims or rim conversions and a special pump, so many bikes still run inner tubes. Inner tubes come in different sizes and materials and it is important to select the correct one for your particular bike.
What are bicycle inner tubes made of?
Latex inner tubes – Latex is a natural rubber that is lighter weight than butyl and more expensive. The tubes are very elastic and lightweight and they are used to reduce rolling resistance. However, they lose air quicker than butyl tubes and usually can’t be repaired once they get a flat.
Lightweight butyl inner tubes – They are elastic, more airtight than latex and offer overall better ride experience than standard tubes. However, they are more expensive than standard tubes and prone to pinch while replacing them.
Standard inner tubes– They are usually made out of a mix of butyl and other forms of synthetic rubber. They are least expensive and most durable. However, they are less flexible and heavier which means that they have increased rolling mass.
Self-sealing or “autofix” tubes – These have a liquid sealant inside. As you cycle the sealant is spread around the inside of the tube and if you have a puncture the sealant flows into the hole, dries when it hits the air and blocks the puncture. They are designed to potentially seal very small punctures and if the hole is more than 0.2mm then it is unlikely to seal because it is too big. However, the sealant can harden over time and block the valve, and if you have a larger puncture then the sealant can leak out and if there is a blowout, then the sealant is fired around the wheel and frame and can be difficult to remove. An advantage is that it slows down the rate that the tube loses air.
What are bicycle inner tube valves?
A valve allows the air to enter, but then mechanically blocks and stops the air escaping. Bicycle inner tubes come with three types of valve:
It is narrow and comes in different lengths. Shorter valves (normally 40mm) are used on standard rims and longer ones on deeper section rims. Presta valves are also referred to as French valves or Sclaverand valves so abbreviated to “FV” or “SV”
It is wider than presta and more common as it is also used on motorbikes and cars. Schrader valves are sometimes called American or Auto valves so the abbreviation is “AV”
Also called “English” or “Woods”. They are not very common these days. The valve mechanism used to just be a tiny rubber hose that allowed air in, but because it is elastic, closed to stop air escaping. The rubber easily perished and it was possible to buy new rubber hose. It was replaced by an “easy pump” system, more akin to Schrader valves. This valve has a diameter of a Schrader valve but uses the same pump adapter as a presta. Tubes with this kind of valve can be replaced by both presta and schrader.
If you currently have a Dunlop valve it can be difficult to buy an inner tube. In this case you can buy a tube with either of the other two more accessible valves.
Normally you would replace it with what you had before. The only reason you can’t fit a different valve is because of the hole in the rim. Schrader valves have a greater diameter, so won’t fit through the hole for a presta valve. Don’t be tempted to drill it out as it may compromise the integrity of the rim. Presta valves are narrower than Schrader valves so can be fitted in a larger diameter hole. It is a good idea to fit a valve collar or valve hole adapter.
What size bicycle inner tube do I need?
To find what size inner tube you need, read the measurements written on the side of the tyre, rather than actually measuring anything. There are various bits of information written on the tyre, but you are looking for 2 numbers separated either by a dash “-” or a multiplication sign “x”. The two measurements are the width and diameter.
If you have a dash. The first figure is the width of the tyre in millimetres. This is not an exact measurement. It represents the width of an inflated tyre mounted on a rim of standard width for that tyre size. The next figure will be 3 digits and this is an exact measurement. It is the diameter of the inside of the wheel rim that that tyre will fit.
If you have a multiplication symbol. The first figure is the tyre diameter. If it has 2 figures then it is in inches and if it has 3 then it is in millimetres. The second figure is the width. If it is expressed with a decimal point or a fraction then it is imperial and if it is 2 figures then it is metric.
There are two measurements, the diameter and the width. Because tyre and tube sizing was developed at a similar time, all over the world, measurements were taken using different standards. Sometimes the measurements on one tyre are expressed in different units, but the main thing is, they are actually the same size.
Measurement system of bicycle inner tubes
The ISO (The International Organisation for Standardisation), formerly known as the ETRTO (European Tyre and Rim Technical Organisation) has created a system that standardised the measurements, but they are still not the measurements that most people recognise!
This system uses 2 measurements, expressed in millimetres, separated by a dash. The first represents the width of the tyre and the second its diameter. If you read this number and take the information to your local bike shop they will know what size tyre you need.
The metric system
This also uses millimetres, but the diameter is written first, followed by a multiplication symbol and then the width.
The imperial system
This uses inches and is expressed in the same way as the metric system. The diameter is written first and the second is the width of the tyre. For smaller increments, modern sizes use a decimal point and for older ones a fraction is used.
The French system
This system uses the metric system for the diameter with the addition of a, b, c or d. This is left over from an old system that designated tyre sizes using the letters a, b and c. Nowadays, many of the sizes are obsolete. 700c is used a lot and refers to modern road and hybrid bikes. If you find a letter after the diameter, and you know it’s not a common size, you may want to refer to an online sizing chart for reference.
Looking for your tyre size
Here is an example: 700x28c and 28-622 written on the same tyre. The width is clearly the same, 28mm. The actual diameter of the inside of the rim is 622mm. A tyre on that rim measures approximately 700mm. Historically the diameter was measured with the tyre on the wheel and the width expressed by a letter, a, b, c or d, a being the narrowest. Therefore 700b was a different rim size to 700c, but the overall diameter was the same. 700c has a rim diameter of 622mm whereas 700b has a rim size of 635mm. When international standards were introduced, some of the sizes became obsolete and the standard that stayed was the 700c; a rim and tyre diameter of 622mm. The c now no longer represents a tyre width, and tyre manufacturers continually make tyres with different widths. The c is sometimes written at the end of the sizing e.g. 700 x 28c. The c is now obsolete. If the c was anywhere it should be after the 700 to show it is a 622mm diameter wheel. e.g. 700cx28.
Looking for your bicycle inner tubes size
In terms of inner tubes, you need to buy the size that matches the size on your tyre. Modern tubes are very stretchy so you can buy tubes that fit a variety of widths. It is important, however, to buy the matching diameter. Because the tube is manufactured in a circular tube, it will stretch less to accommodate different diameters. There are examples of inner tube manufacturers who make tubes to fit a number of different diameters. As long as your size is on the box, the tube will fit.
What happens if I get the wrong size of bicycle inner tubes?
Too small an inner tube might overstretch and split at the seam. Too big an inner tube might get pinched between the rim and the tyre. To make sure that you get the correct size the range of tyre diameters has been printed on the inner tube’s box. It is shown either as a list or as a range. In this example we have a tyre with the measurements:
Here is an inner tube showing the ranges it will fit into:
20-622 and 25-630 (ISO)
27×3/4 and 27×1.00 (Imperial)
700x20c and 700x25c (Metric)
Because the bicycle inner tube’s measurements are below the tyre width it means that this tube is slightly too small.
Let’s have a look at this inner tube. The measurements written on the box are shown here:
- 28-622 28×1.10 700x28c
- 30-622 28×1.20 700x30c
- 32-622 28×1.25 700x32c
- 37-622 28×1.40 700x35c
- 40-622 28×1.50 700x38c
- 42-622 28×1.60 700x40c
- 47-622 28×1.75 700x45c
- 32-630 27×11/4
- 40-635 28×11/2 700x38b
Here you can see that the measurements on the box matches the width of the tyre. So this inner tube is the correct size.
More about diameter
We are going to use the ISO 622 as an example as this is a very common size. 700 or 28. The diameter is very easy to find. With modern tubes stretching so well you may find a box that says it will do 28”, 27.5” and 29”. Although the ISO is slightly different between them (622, 584 and 622 respectively), they do fit the variety of tyre sizes as listed on the box.
More about width
Let’s use our 700 example. Many hybrids use this size of tyre. The width written on the tyre will vary and could be any of the following:
We will need to fit a 700c inner tube and the tube has been made to fit that diameter, but because the tube is stretchy you can use the same inner tube in any of the above tyres. When buying an inner tube the box will either express it as a range, e.g. 700 x 28 – 45mm or will list each size.
Road bikes also use 700c wheels, but they are much narrower. The 2 common widths are 23mm and 25mm. Most road bike inner tubes will do either of these sizes.
Gravel bikes normally have quite wide tyres. 700 x 30 – 40mm.
So while the diameter of the tyre on a narrow road bike is the same as a hybrid, the width is often very different, 700×23 as opposed to 700×45 and the tubes at these extremes will probably not be interchangeable.
If you fit the narrow tube into the wide tyre, although the rubber will stretch, that ability is limited and all that volume of air will probably over stretch the rubber and cause the tube to fail on one of the weak points such as the seam. If you put a wide tube into a narrow tyre, you will probably pinch the inner tube and cause a puncture by just trying to get it into the tyre.
Overview of bicycle inner tubes
So, although we can only ever see the valve, we know that the humble inner tube is doing its job in making our ride comfortable and efficient.