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About Bicycle Inner Tubes

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Sheldon Brown photo John Allen photo
by Sheldon "Expansive" Brown
and John "Totally Toroidal" Allen
and with contributions by Aaron Goss
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This article gives general information about inner tubes. Other articles on this site cover repair of flat tires, and tire/inner tube sizes. There are links at the end of this article to other information about inner tubes.

What's inside your tire?

[This section by Sheldon Brown]

People usually think that tires are made of rubber. This is understandable, because rubber is all that you can see, but it's a serious oversimplification.

A tire is actually made up of three parts:

  1. The beads are two hoops of strong steel cable (or, sometimes Kevlar ®.)
  2. The cords, cloth forming the body of the tire, woven between the two beads. Most modern tires use nylon cords.
  3. The rubber, which covers all the other parts. The rubber on the part that contacts the road is thicker, and is called the tread.

A bicycle tire is not airtight by itself, so it uses an inner tube, which is basically a doughnut-shaped rubber balloon, with a valve for inflation. The only requirement for an inner tube is that it not leak. Being of rubber, it has no rigid structure. If an inner tube is inflated outside of a tire, it will expand to 2 or 3 times its nominal size, if it doesn't pop first. Without being surrounded by a tire, an inner tube can't withstand any significant air pressure.

(Tubeless tires are beginning to appear on bicycles. They require the tire and rim to be air-tight, adding complications -- especially on a spoked wheel. Topic for a future article...)

Dimensions

[This section by John Allen]

Several dimensions of an inner tube are important.

Diameter

Bicycle tires and inner tubes are sold in a variety of diameters. Because inner tubes stretch, the fit can be inexact. For example, the same inner tubes are used with tires in the E.T.R.T.O. 630mm (English "27 inch") and 622mm (French, "700C") sizes.

There are limits, though! An inner tube with too large a diameter will fold over inside the tire. It will give a bumpy ride and can possibly be damaged. An inner tube with too small a diameter will stretch over the rim like a rubber band and make tire installation difficult.

Sometimes, sizes of inner tubes are not close enough, even when they have the same markings. For example, there are two sizes called 24 x 1 1/8, but the rim sizes are 20mm different, 520mm and 540mm. The larger inner tube will fold over inside the smaller tire, but the smaller inner tube might be made to fit the larger tire with some coaxing. Diameters may be compared quickly by extending inner tubes between the two hands, as shown in the photo below.

Comparing inner-tube diameters
Checking inner-tube sizes

Width

Inner tubes are made to fit different tire widths. Again because inner tubes stretch, many are marked for a range of widths.

You can check before installing the tire on the rim. When inflated so it just holds its shape, the inner tube should fit inside the tire, leaving a gap about as wide as the inside of the rim.

Once the tire is installed, if the inner tube is too fat, it sticks out from under the tire, preventing the tire from seating on the rim, or it crumples inside. If the tube is too skinny, it has to stretch too much to fill the tire, and it may tear or split.

In an emergency, you could use a tube which is too long, folding it over, and live with the bumpy ride to the next bike shop. Avoid using a tube which is too skinny, especially on the front wheel. A sudden front-tire deflation or blowout often makes the bicycle impossible to control.

Wall thickness

Most inner tubes have a wall thickness of about 1mm, enough so air won't quickly seep through the rubber, and so rough spots inside the tire and rim will not puncture it.

Lightweight inner tubes are thinner, more like medical gloves, and require more careful treatment. Some must be pumped up before every ride.

In some regions, notably the Southwestern U.S., "goat-head" (tribulus terrestris) thorns are so common that thorn-proof inner tubes are a desirable option. They are thickened under the tread to help prevent flat tires. These tubes are not only heavier but also increase rolling resistance. They make your wheels heavy and sluggish, and, if incorrectly installed, they can actually cause flats! They can make sense in areas with "goat-head" thorns. Sealant, a liquid inserted into the inner tube which closes small holes, and aftermarket tire liners, such as the well-known Mr. Tuffy, are other options.

Valve dimensions

-- will be covered in the next section.

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Valves

[This section by Sheldon Brown and John Allen with assistance from Aaron Goss]

Three types of valves are in common use for bicycle tires:

Schrader Woods
Schrader valve Woods valve
Presta-closed Presta-open Presta-adaptor
Presta valve
closed
Presta valve
open
Presta valve
with Schrader adapter

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Valve caps

Bicycle inner tubes usually come with plastic valve caps. These keep the valve clean, and that is all. A Presta valve cap will work on a Woods Valve, but a Woods valve cap is too short for a Presta valve. Metal Schrader valve caps, available at an auto-parts store or better bike shop, have a rubber seal to prevent leakage even if the valve itself is leaky. Some include a special wrench to unscrew the valve core.

Best kind of Schrader valve cap, with seal and wrench (larger than life-size!)
Schrader valve cap

Valve stem length, valve extenders

Presta valve stems of different lengths are available, for rims of different depths. It is best for a valve stem to be only just long enough, to reduce the risk of its being bent or broken. There are also valve-stem extenders, of three kinds:

Unless an extender has its own valve core, a pressure reading is possible only with a pump which has a built-in pressure gauge. Like a Presta to Schrader adapter, an extender which screws onto the valve-cap threads must have a rubber O-ring seal, or it must be sealed with silicone caulking compound or the like, else it will leak during pumping.

Rim hole size

The Presta valve hole is 7mm in diameter, and the Schrader valve hole, 8.5 to 9mm. Presta inner tubes may be used in rims that are drilled for Schrader or Woods valves, though it is advisable to install a rubber grommet in the valve hole. A Schrader or Woods valve won't fit through the valve hole of a rim drilled for Presta. Presta-drilled rims (except the very narrowest) may be reamed, or drilled to accommodate Schrader valves, using an 8.5mm (0.326 inch), 21/64" (0.328 inch) or Q (letter size designation, 0.332 inch) drill bit. Some rubber-coated Schrader valves may require a slightly larger hole.

Valve attachment

Most inner tubes have the metal of the valve stem bonded to the rubber, with a thicker rubber patch for reinforcement. Some valves, mostly older Presta ones, clamp to the inner tube with a mushroomed bottom end of the valve stem, nut and washer. The washer may be too wide to fit in the bottom of the rim. In this case, you need to build up the area inside the rim either side of the valve hole so the inner tube doesn't sag and tear. Silicone rubber caulking compound works for this.

Ditch the jam nut

Some inner tubes have a valve stem threaded all the way down to the rim, and a knurled jam nut to clamp the valve stem to the rim. I (John) know that Sheldon's photos show this nut in place on the Presta and Woods valves, but it has only one use -- to make installation of the pump head slightly easier when the tire is uninflated. The valve stem is more likely to bend, break or pull loose from the inner tube if clamped down. It is easy enough to pinch the tire between finger and thumb to keep the valve stem extended when installing the pump head.

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Quality and reliability

[This section by John Allen]

A blowout or sudden deflation can result in a serious crash. Inner tubes are not expensive, and it makes sense to use only high-quality ones. I have some inner tubes which are 30 or 40 years old and still just fine, pliable and strong. I know their age, because they are the original ones from bicycles which people bought and used briefly, then set aside. On the other hand, in recent years I've had, or seen, inner tubes which never should have gotten out of the factory:

All of these reports, though, are of one or a few failures. A large sample of product sales and failure numbers is needed to draw conclusions about reliability -- yet there is no systematic recording of inner-tube failures, except when they are reported back to manufacturers. Manufacturers usually don't release the data unless forced to by a court of law.

Here are some things you can do though to protect yourself:

The lesson for a cyclist or bike shop owner is that you need to look after your own safety, or the quality of the product you sell. Quality control in manufacture is a task for the industry and government. The task lamentably is not being addressed as it should be, even though with bicycle inner tubes, it is safety-critical.

Tube tricks

[This section by John Allen]

Rolling on a flat tire...

Because a tire's outer diameter is greater than the rim's, a flat tire "creeps" forward (!) as the bicycle is rolled, and pulls the inner tube with it -- except the valve, caught in the valve hole. With a double-wall rim or a valve stem secured by a jam nut, the tube will only bunch up behind the valve. With a single-wall rim, the valve stem increasingly leans backward. You need only roll a bicycle a few feet for this to happen. Assuming that the inner tube is still intact, you may be able to rearrange the inner tube by rolling the bicycle backwards before inflating the tire. Once done, push the tire sidewalls in all around and take a look to check that the tube is intact and seated properly inside the tire.

Deflating and packing an inner tube

So, after replacing an inner tube during a ride, you want to take it home to patch it (or maybe just to avoid being a litterbug). You need to flatten it completely and roll it up. Here's how.

  1. Open the valve to let air out. With a Schrader tube, you use a valve cap with a wrench to loosen the valve core, as already described. A bobby pin is reported also to work. With a Presta or Woods valve, loosen the knurled nut.
  2. Roll up the inner tube tightly starting directly opposite the valve, to push all of the air out through the valve.
  3. With the inner tube rolled up tightly, close the valve, and replace the valve cap.
  4. Unroll the inner tube and roll it up again starting at the valve. This way, it will protect the valve.
  5. Wrap an old toe strap, or a rubber band -- which might have come with the replacement inner tube -- around the rolled-up inner tube to hold it together.

Modifying a Schrader valve

Suggested by Aaron Goss...It is possible to cut the spring off a long-type Schrader valve core as shown in the photo below, and to remove the pusher pin from the pump hose. Air pressure will then close the valve and prevent leakage as the hose is unscrewed. The seal inside the valve is not as good without the spring: a metal valve cap with a rubber seal should be used. To inflate a tire with a modified or unmofidied Schrader valve, you must carry two pump hoses, one with and another without the pusher pin. Is it worth all this trouble to make a retro pump work better? Decide for yourself!

Cutting a Schrader valve core
Cutting a Schrader valve core

Uses for old inner tubes

An old inner tube can be cut crosswise to make rubber bands, for example, to hold together a flattened, rolled-up inner tube. If you need the rubber band to be longer, cut the tube diagonally.

To make a rubber grip for a length of pipe: Cut an inner tube crosswise at one place near the valve. Place a length of metal pipe -- larger than the inner tube's uninflated cross-section -- inside a length of larger-diameter pipe which is closed at one end. Stretch one end of the straightened inner tube over the end of the larger pipe, and hold it in place with a hose clamp. Clamp the other end off. Pump in air, inflating the inner tube. Stand the assembly up so the smaller pipe slides down inside the inner tube, then let out the air. Trim off the ends of the inner tube, leaving the smaller pipe with a rubber, cushioned grip surface.

Old inner tubes may be used like bungee cords. MIT Professor Dr. David Gordon Wilson, author of the book Bicycling Science, could have afforded any kind of bike rack for his car, but he made one with a wooden frame, using numerous old bicycle inner tubes to secure the bicycles. The bike rack worked fine.

Inner tubes may be used as springs for trampolines, catapults, slingshots and the like. I leave the details up to your imagination.

Weaving: Intact inner tubes can be woven into a fabric and stretched inside a frame, looping over the rods at the four edges to make a resilient platform -- to return a thrown baseball, etc. There are even examples of clothing made by weaving together strips of old inner tubes, but I imagine that it is rather clammy!

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Links

Our page on tires

British English version! Our page on tyres

Tires from Harris Cyclery

Our page about flats and how to repair them

Our page about tools to work on tires

Tire sizing systems

Tire rotation

Jobst Brandt on Presta vs. Schrader valves

Jobst Brandt on flats (go to "Flats" subheading -- several articles)

German Wikipedia page about tire valves -- nice illustrations (in German)

German commercial page about tire valves (in German)

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Last Updated: by John Allen