Inner tube
An inner tube is a ring shaped balloon designed to fit inside a tyre and hold air at an appropriate pressure in order to maintain ride comfort and performance. They are used by a range of vehicles from aircraft to trucks, to bicycles and are generally made from butyl rubber or latex.
Inner tubes are usually designed for a very specific size of tyre and are measured against both the diameter of the wheel and the diameter of its rim. Larger, heavier vehicles generally tend to have larger tyres and therefore larger inner tubes. They also tend to inflate to a lower pressure than smaller tyres.
The pressure inside the tube is maintained by periodic inflation through a valve on the side of the tube. The valves come in various types but are generally either Schrader, Presta, or Woods valves. The valve is attached to a pump or air compressor to inflate to the desired pressure.
Inner tube materials
Inner tubes tend to be manufactured in two basic materials, butyl rubber and latex. Butyl rubber is a highly impermeable synthetic rubber compound with similar elasticity to latex which makes it ideal for inner tubes. Halobutyl is also a major component of tubeless rubber tyres.
Latex in its raw form is found naturally as a milky fluid in various types of plant. It is extracted on a large scale from trees on latex plantations and is used to create natural rubber. This natural rubber has very similar properties to butyl rubber however it is slightly more flexible, and slightly more permeable. The greater flexibility allegedly gives better ride performance however the greater permeability also means the tube requires more frequent inflation.
The vast majority of inner tubes are made from Butyl rubber.
Valve types
There are three main valve types, Schrader, Presta, and Woods.
Schrader valves are used on both tubed and tubeless tyres as well as a huge range of other applications. The valve contains a core which can be removed by a core removal tool and requires a cap to protect the inner core mechanism from containments. This valve type is thicker than the presta valve which limits its application to wider rims It also has a thread enabling the application of threaded pump adaptors.
Presta valves are thinner than Schrader valves (usually 6mm rather than 8mm). This allows them to be used on thinner wheel rims such as road bicycles where the valve hole is the weakest part of the wheel. They have a screw top mechanism which must be unscrewed to allow air to flow in or out of the tube.
Woods valves or Dunlop valves were once common in the UK but are still used throughout the world. They are a similar width to Schrader valves but have a thinner head allowing a presta valve pump to be connected. They come with a removable core similar to the Schrader valve which requires no special tools for removal.
Sizes
The size of an inner tube is based on the diameter of the wheel it sits in side and the width of the wheels rim. These two figures vary depending on the use of the wheel.
Diameter
Tube diameter is driven by the diameter of the wheel rim which is in turn driven by the requirement of the vehicle. Large diameter wheels are more stable at higher speeds but have less friction on the road surface. Made of the same materials they are also heavier. Large diameter wheels are also used on all terrain vehicles to allow them to drive over larger obstacles. Smaller wheels are lighter and have a greater surface area as a ratio of the wheel size applied to the ground surface. In bicycles smaller wheels are used on slower bikes such as BMX's and mountain bikes, whilst road bikes have larger wheels for increased stablity at high speed.
Width
Tube width like diameter is driven by the width of the wheel rim which is again driven by the requirement of the vehicle. Off road vehicles and those which need to be driven on low friction surfaces tend to have wider wheel rims and therefore require wider tyres and inner tubes. Thinner wheel rims such as those on road bicycles have much less friction on the road surface and therefore require much less effort to reach speed.
Weight vs Durability
In all inner tube applications there is a trade off between the weight of the inner tube and its durability. Some inner tube applications require a greater degree of durability whilst others are looking for optimum performance. Tractor inner tubes for example will be looking for a high degree of durability where as professional racing bikes will be looking for minimum weight.
In bicycle inner tubes there is a range of different weights, again dependant on the application. Downhill mountain bikers use very thick rubber tubes because of the terrain they cover and the stress they put on the tubes and tyres and can weigh in excess of 400 grams. Commuters and infrequent cyclists will be looking for a middle ground where they have a good level of durability with minimal additional weight (usually between 250-150 grams). Professional road bikers on the other hand will look for very light inner tubes, to maximise race performance which can weigh less than 50 grams.
Tyre pressure - Efficiency to comfort trade off
Inner tube pressures again vary greatly across the range of applications. Lower pressure tubes tend to provide a more comfortable ride but reduce fuel/energy efficiency because they increase the friction on the ground surface.
In some applications such as tractors and heavy machinery very low pressure is necessary to overcome obstacles. In bicycles mountain bikers run with very low tyre pressures (sometimes as low as 10-15 psi) to maximise tyre contact with the ground to give maximum grip and help absorb some of the ground uneven surface.
Higher pressures are generally used on road based vehicles where the surface is smooth and the friction level is already sufficient for reasonable grip. Cars and motorbikes tend to run at around 20-60 psi. Road bicycles run as high as 110psi again for performance reasons.
