Nexus is Shimano's name for their internally geared hubs and is also used for the supporting products such as cranksets, shifters, brake levers, hub brakes, hub dynamos and a CPU for automatically changing gears.
Shimano released the Nexus range as a clean and easy to use alternative to the derailleur systems in use on most modern bicycles and the weight of the Nexus is comparable to existing cassettes and rear derailleur. The gearing mechanism uses what is known as planetary gears and the hubs can be fitted with a rear cog sprocket from 16 to 23 teeth to change the gear range.
As of November 2006, The Nexus range comes in several ranges (INTER-3, INTER-7 and INTER-8) providing 3, 7 and 8 speed models respectively. The AUTO-3 range sports the 3 speed automatic gear system, which utilizes a front hub dynamo to power a CPU that automatically changes the three speed internally geared hub. There was also the INTER-4 which provides four gears and is now discontinued.
The Nexus is not made to withstand the rigours of off-road biking and is therefore not suitable for mountain bikes. It is aimed primarily at the "comfort" market such as commuters and tourers. Shimano offers a more sporty hub-gear under the Alfine brand.