Transmission & Gears

Transmissions and gears
After the wheels, it's transmission system is arguably the most significant defining feature of a pedal cycle.

Bikes are often defined by the number of gears or 'speeds' that they have, with a 20-speed bike assumed better than a mere 18-speed. That may often be so, but more important than the number of gears is their size. The fastest Olympic cycling events are after all, undertaken on bikes with one single fixed gear! But for most purposes it helps to be able to vary the gear, and for that purpose a bike may have derailleur gears or hub gears - or even both.

It all starts with cranks and pedals (or handles) to push them round, then something: a chain a belt, maybe a shaft, to connect that motion to the driven wheel. The earliest pedal cycles didn't have anything in-between: the cranks were attached directly to the wheel like on a child's pedal toy. Except they were not toys. That wheel was big and got bigger as bikes developed, for that was the only way to cover more ground with each turn of the pedals. This is how the bike we now call a penny farthing evolved and these bikes were rated in inches, by the size of their wheel.

In their day these high wheelers were called ordinary bicycles, to distinguish them from some new thing called a safety bike. This had a chain drive to the rear wheel (sounds familiar?) that could 'gear up' the transmission by having more teeth at the cranks than the wheel. Three times as many teeth on the cranks, a 48 tooth chainwheel and 16 tooth rear sprocket for example, makes a 26in wheel equivalent to a 78 inch ordinary. It would be some tall guy who could straddle a wheel that size!

In This Section:

Bicycle pedals
Components 101: Learn the basics about different types of pedals before we go all technical
Bicycle bottom bracket
Components 101: Learn the basics about bottom-brackets before we go all technical
Cycle chain
Components 101: Learn the basics about chain and belt-drives before we go all technical
Internal hub gear
The common sort of internal gear is a hub-gear: quite simply a gearbox inside the rear hub.
Image of derailleur gear transmission
Making the chain hop from one sprocket to another looks like a crude way of altering the transmission ratio, but derailleur gearing can nevertheless work very reliably and most efficiently.
gear cable adjust
Gear cable / accessory
chain guard
gear inch chart
Gears are still measured in inches, and whilst it may seem antiquated to refer all gearing arrangements (including hub gears) back to the penny farthing's directly driven wheel, it's a very simple calculation that gives us an easy way to compare all sorts of pedal cycles.
Cycle components - Rear shifting
The main job of a rear derailleur, or mech, is to convert cable motion into sideways displacement ie make the mech move the chain to a different sprocket.
Front shifting - close up of bicycle front derailleur
Key to the shifting process is the front gear change mechanism called a derailleur or mech. It's a fairly simple process: a cage comprising two guide plates simply pushes the chain one way or the other.

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