In choosing the correct tube for your bike, there are a number of factors to consider -

 

1. Wheel diameter
Select the correct wheel diameter for your bike. This is usually measured in inches, and the current MTB standards are 26", 27.5" (aka 650b) or 29". This will be written on the side of your tyre. e.g. 29x2.25 is a 29" wheel.

Different wheel diameters, left to right - 26", 27.5" (650b), 29".

 

2. Tube width
The width of your tyre will determine the size of the tube you install. Tyres for mountain bikes range from 1" (Road/commuting tyres) to 2.8" (Downhill racing tyres).

If you select a tube that is too small for the tyre you are running, it will expand and may be under unnecessary tension - this might cause the tube to pinch flat more easily.

If you select a tube that is too large in diameter for the tyre you are running, it may be difficult to install without pinching the tube.

Tubes generally have a recommended width range. For example a 26x1.75/2.1" tube is designed to work on a tyre between the width of 1.75" and 2.1". If there is only a specific width mentioned, it is safe to install on any tyre that is +/- 10% of the width. For example, a 26x2.25 tube will be fine on a 26x2.0" or a 26x2.4" tyre.
 

3. Valve types
Valves (which allow you to inflate and deflate the tube) on bicycle tubes are usually either Presta or Schrader.

Presta Valve (also known as French Valve or F/V) are the most common on mountain bikes. They generally have a long and thin stem that only requires a small hole in the rim.  This is a benefit for lightweight rims, as the rim strength can be compromised by the larger hole required by Schrader tubes.

Schrader Valve (Also known as Car Valve, American Valve or A/V) are common on entry level mountain bikes.  As most mountain bike wheels are drilled for Presta valves only, drilling out the rim is usually required to accept Schrader valves. With the growing popularity of tubeless setups this is now even less common (as tubeless valves are Presta valves, and a larger hole will result in a less reliable seal).

 

4. Valve Stem Length

As some rims are deeper (see example below), sometimes a longer valve stem is required to penetrate the rim fully.

Presta valves are often available in 40, 48 and 60mm lengths. If you have a deep-dish rim you will need to be sure the valve you are purchasing is long enough to attach your pump properly.

The valve stem length will often be mentioned after the valve type if it is a Presta. Eg 26x2.125 FV 48mm. This means the tube has a 48mm stem.  Most tubes are 40mm or 48mm, however unless you have a deep dish rim,  this is not usually something to worry about.

This diagram shows the cross section of two different rims from the same manufacturer. A 40mm Presta valve stem may not be long enough to work with most pumps on the rim on the right.

 

5. Tube Thickness

As a thicker-walled tube is less likely to result in a pinch-flat when the tube is under high load, there are a number of thickness tubes on the market.  However a thicker-walled tube weighs more.

Below is a table comparing Maxxis 26" tubes, as you can see the weights of a tube for the same size wheel varies significantly.

Model    Thickness    Weight
Flyweight    0.45mm    93g
Ultralight    0.6mm    123g
Welterweight    0.9mm    180g
Freeride    1.2mm    295g
Downhill    1.5mm    430g

 
The tube thickness is not always advertised and most tube manufacturers only make a "middle weight" tube. However it is something to keep in mind, as it will have a dramatic effect on flat-resistance, and rotating mass on your wheels (and therefore acceleration of your bike).

 

A regular tube (Left) and a Thornproof tube (Right).


Thornproof tubes
Although popular in K-mart, thornproof tubes are not often used for mountain bikes off road.

A thornproof tube uses a very thick sidewall on the outside of the tube, designed so that a thorn won't penetrate fully.

Although thorns/prickles/bindis are common in grass, for most places in Australia, thorns are not common on the trails. Instead, most off-road flat tyres are caused by a "pinch flat", where the tube is squashed between the ground and the rim.

As thornproof tubes are heavy, they are not a popular choice.

If prickles are an issue on your local trails,  I would strongly recommend switching to a tubeless setup as it will be much lighter and more effective than a thornproof tube.

 

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