Bike Gears: How Do They Work

Bike Gears
Re-cog-nise this? (arf arf)

Gears are a pretty fundamental part of the modern bicycle.

In fact, as the handlebar-moustache-sporting, Victorian gents amongst you will know, the introduction of gear rings and chains spelt the end for the Hi-wheel (the Penny Farthing to you and I) and other direct drive bicycles.

By varying the size of the chainring at the front and the sprockets (or cogs) at the back, cyclists could generate greater speeder without having to spin the pedals at ever-increasing RPMs (or balance precariously above gigantic front wheels).

In this post, I’m going to give an introduction to bike gears.

Since this is a blog about road cycling, I’ll stick to talking about derailleur gears (i.e. the ones seen on road bikes). Conveniently, this means I can avoid having to admit that I really don’t understand how hub gears work (what? ah…).

Useful terminology

The key bits of a bicycle gearing system are:

The chainrings – these attach to the pedals and can be found…. wherever you keep your pedals (assuming they’re attached to the bike). I think (i.e. someone can correct me if I’m wrong), that the combination of the chainrings, the pedal cranks and the bottom bracket assembly is referred to as a chainset.

The sprockets / cogs – these are attached to the hub (the bit in the middle) on one side of the rear wheel (generally the right). You may have between 8 and 11 cogs back there, collectively known as a cassette.

The chain – pretty self explanatory. The bit that attaches between the chainrings and the chosen cog within your cassette. It’s chain-shaped. It’s a chain.

Derailleurs – these are the things that do the physical moving of the chain from one ring to another, be it between the chain rings (the front derailleur) or the between the cogs of the cassette (the rear derailleur).

  • The front derailleur features a straightforward chain guide that positions the chain onto the selected chain ring
  • The rear derailleur has a dual purpose: as well as positioning the chain onto the chosen cog, the rear derailleur is fitted with a spring that maintains the right level of the tension in the chain as you move between gears

Gear shifters – these are the controls that you use to change gears. They are generally found on the handlebars, unless you’re going retro, in which case they might be attached to either side of the frame’s down tube. The gear shifters are attached to the derailleurs by gear cables (or electric wires in these new-fangled electronic systems).

How do gears work?

In the mechanical sense, derailleur gears ‘work’ by the rider pulling or pushing the gear shifter, which shortens or lengthens the relevant gear cable. This, in turn, moves the desired derailleur to its new position, guiding the chain to the chosen chain ring or sprocket.

Electronic gears are similar, only the derailleurs are moved by small electric motors. The cyclist clicks a button to select a gear, the signal is sent electronically to the relevant derailleur.

Yes, but what about gear ratios and all that guff?

Hmm, now we’re going to test my knowledge and explanatory skills (or rather my ability to Google for information).

So, you have a chain that is looped around one chain ring at the front and one sprocket at the back. It is the relative size of those two rings, as defined by the number of teeth they each have, that determines the gear ratio.

If the chain ring is large and the rear cog is small, the gear will be high. The pedals will be hard to turn from a standing start, but when moving, you’ll be able to go fast without your pedal cranks rotating at warp speed 10.

This makes intuitive sense. As you turn the pedal cranks, you’re pulling the chain across many teeth on the large front ring. That same chain moves around the teeth of the smaller cog at the rear. For every full turn of your 53-tooth front chainring, the rear 17-tooth cog (say)would need to turn just over three times, as would the rear wheel that it’s attached to.

Conversely, employing the smallest ring at the front and the largest sprocket at the back puts you in your lowest gear. You’ll find it easier to cycle up hill but on the flat you’ll soon find your speed is limited by the rate at which you can spin the pedals.

Show me the maths!

The gear ratio is generally denoted as “[Number of chainring teeth] / [Number of sprocket teeth]”, such as 53/13.

Curiously (to me at least), this is rarely translated into a decimal number, which would then allow you to rank your bike gears in order. Instead, we are left to wonder whether moving from the 53/19 gear to 39/14 is going to make our life easier on this climb*.

(* It won’t. In fact it won’t make any difference – 53/19 and 39/14 are essentially the same gear)

The following table sets out the gear ratios (shown as decimals) for someone with 53- and 39-teeth chain rings and a rear cassette with sprockets of between 11 and 28 teeth:


As well as showing that 53/19 and 39/14 produce the same gear, the table demonstrates that to move up the gears sequentially, you would have to dance between the large and small chainrings. I can happily confirm that I do not do this (or rather I do, but more as a desperate, random stabbing of my gear shifter rather than a consciously-smooth progression through the ratios).

If you have the details of your chainrings and cassette to hand, and you want to know the gear ratios sequence, you can plug the numbers into this handy calculator.

I’m so dizzy! My head is spinning…

I think we’ll leave it there.

I had designs on talking about double versus triple versus compact chainrings. I was going to wax lyrical about the difference between an 11-23 cassette and a 13-28. I have a poem drafted on the virtues of 9-speed cassettes versus 11-speed. But those will all have to wait for another post.

“Shame!”, cries the crowd, “Encore!”

If you would like to learn more about bike gears, and chainsets in particular (e.g. what is compact gearing?), then take a look at my other post on the subject: More About Bike Gears: A Short Treatise on Chain Rings.

(Photo credit:[email protected]/with/5794804491/)

Monty - Sportive Cyclist
Monty is an enthusiastic road cyclist with only moderate talent. He started Sportive Cyclist in 2013 to record the journey to his first 100 mile ride, the RideLondon 100. Over time the blog has expanded to include training advice, gear reviews and road cycling tales, all from the perspective of a not-very-fit MAMIL. Since you're here, Monty would also like you to check out his YouTube channel. Also, Monty really needs to stop referring to himself in the third person.

17 thoughts on “Bike Gears: How Do They Work”

Leave a comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.