Gear systems: derailleurs, hub gears and electronic shifting

The gear must do one thing: give the rider the right resistance to push against, whether going uphill or with the wind at their back. How that task is solved mechanically, however, there are several answers to, and as a mechanic you encounter all three main types — often in the same week.

The classic derailleur gear works on a simple, almost crude principle: a spring-loaded switching system pushes the chain sideways until it falls over the neighboring sprocket while spring-loaded pulley wheels in the rear derailleur continuously take up the chain slack that arises when the chain moves between different sprocket sizes. This gives a large gear range and low weight but simultaneously exposes chain and shifters to dirt and weather and requires continuous adjustment of cable tension and shift stops.

In a hub gear, the gearing is hidden inside the rear wheel's hub as a planetary gear set: a sun gear on the axle, planet gears that run around it, and an outer, internally toothed gear ring. By locking or freeing different parts of the planet set, the hub shifts between gears without the chain ever moving sideways. This means the chain can run in a completely enclosed chain case, which greatly extends its lifespan and makes the hub almost maintenance-free — an ideal solution for commuter bikes and cargo bikes that need to work in all kinds of weather with minimal care.

• 01Hub gear can be shifted while stationary — practical at traffic lights and in city traffic
• 02Known systems include, for example, Shimano's Nexus and Alfine as well as Rohloff's Speedhub, which has an unusually wide gear range collected in one closed hub
• 03The disadvantage is typically higher weight and slightly lower mechanical efficiency than a well-maintained chain drive.

On racing and gravel bikes at the higher end, mechanical cables are increasingly replaced by electronic shifting. Shimano Di2 is cable-based: one central battery powers motorized shifters controlled by electrical signals from the handlebar buttons. SRAM's eTap/AXS system takes it a step further and is fully wireless — each shifter has its own battery and communicates via the manufacturer's own radio protocol, separate from standard Bluetooth, precisely to avoid interference from other wireless devices nearby.

For the mechanic electronic shifting means a new type of troubleshooting: instead of tracing a sluggish wire or a bent shift stop you must be able to diagnose battery status component pairing and firmware — often via an app or diagnostic tool from the manufacturer. The basic mechanics of the shift itself (the chain being moved onto a different sprocket) is however still the same as with a mechanical chain drive.

There is no one system that is 'best' — it depends on use. A commuter who cycles in all kinds of weather and wants to spend the least time on maintenance is well served by a hub gear in a closed chain box. A competitive cyclist will typically choose a light efficient chain drive — perhaps with electronic shifting for precision. Your task as a mechanic is to understand what the customer actually needs to use the bike for and advise accordingly.