Bike Gears: How Do They Work?

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 Bicycle 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:

 11121314151719212428
534.824.424.083.793.533.122.792.522.211.89
393.553.253.002.792.602.292.051.861.631.39

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.

More About Chainrings

The chain rings sit at the front of the drivetrain (i.e. where the pedals are). They attach to the axle (or bottom bracket spindle) that runs through the bottom bracket (the tube section to which the down tube, seat tube and chain stays are all joined, as set out in this handy diagram). The pedals attach via cranks to the chain rings on one side, and the axle on the other.

As mentioned in my previous post on gears, the size of the chain ring, along with the size of the cog you’re using on your rear cassette, determines the gear ratio in which you ride.

If the chain is on a larger ring at the front, you are in a higher gear and the bike will travel further for each rotation of the pedals. As a result, the pedals will be harder to turn and you will go faster (all else being equal).

Conversely, using a smaller ring at the front puts you in a lower gear, making it easier to pedal, but propelling you a shorter distance for each pedal turn.

Chainset versus Crankset

They’re the same thing. Both terms refer to the chain rings and the cranks that attach to them (to the other end of which we attach the pedals). It seems that we use ‘chainset’ in the UK, whilst ‘crankset’ is the equivalent term in the US.

Since I don’t plan on doing a post just about the cranks (there is a limit), I thought I’d cover the ‘chainset/crankset’ question (issue, debate, thingy?) here.

Double versus Triple Chainsets

In our world (yes, our world of cycling), ‘double’ and ‘triple’ are used to describe how many chain rings you have at the front of your bike. The more astute readers amongst you will not be surprised to discover that a double means that there are two rings; a triple means three.

Using three chain rings means a greater number of available gears, particularly at the lower end of the range. Adding another chain ring does not simply give you 9–11 more gears (depending on the size of your rear cassette) than a double, as some of those gears will be duplicates of ratios achievable using existing front/rear cog combinations, but it will give more usable gears to suit a greater range of circumstances.

The ‘extra’ ring in a triple chainset is generally the smallest one. Triples are therefore used to introduce more gears at the lower end of the range, to help you get up steeper climbs.

Yeah, well, big strong men sometimes need wee frail women to give them a helping hand Supergran (Supergran and the Day at the Sea, 1985)

Cycling dogma dictates that this additional ring be referred to as a ‘granny ring’ and anyone using a triple be heaped with derision. Grimpeur Heureux is an inclusive community which does not tolerate gearism, so I shall be making no such comments. Not least because I am at least considering a triple for my much-vaunted, and for the time being very hypothetical, new bike.

Whilst the large cogs are generally the same size on both setups, the middle ring on a triple is sometimes a bit larger than the smaller ring on a double (see the section below for the numbers). The difference in size between the three rings of a triple can therefore be smaller than that between the two rings of a double.

When riding a double, shifting to the big ring can feel like too extreme a change up in gear. As a result, the rider may have to shift down a cog at the back at the same time, in order to make a smoother progression through the gears. On a triple, with a smaller difference in size between the chain rings, changing up at the front does not increase the gear ratio to the same extent. The easier progression may mean that the cyclist doesn’t need to make a simultaneous shift down on the rear cassette, reducing wear and minimising the chance of a dropped chain.

What Are The Sizes Of ‘Normal’ Chain Rings?

Standard road double chainsets generally come in combinations of a 39/52 or 39/53. That is, 39 teeth on the smaller ring, 52 or 53 teeth on the larger one.

A triple might have 52 or 53 teeth on the largest ring (as on the double), 39–42 teeth on the middle ring (i.e. the same or slightly larger than the double’s smaller ring) and 30 teeth on the grann… er smallest ring (sorry).

And What Is A Compact Chainset/Crankset?

A compact crankset (chainset, whatever) has two chain rings but each of these rings is smaller (has fewer teeth) than the equivalent ring in a double setup. Where you might have a 39/52 double crankset, a compact would be something like 34/48 or 36/50.

If you’ve been paying attention, you would surmise that having smaller rings at the front means that there will be a nice range of lower gears on offer. You’d be right.

In fact, with the right cassette on the back (i.e. one with cogs in the high 20s), you can achieve many of the lower gear ratios offered by a triple. And no mention of a granny ring anywhere.

(It’s probably worth mentioning that it’s not really the number of teeth that determines the gear ratio per se. The number of teeth drives (or is driven by, depending which way you look at it) the size of the diameter of the chain ring, which is really what determines the gear ratio. I assume teeth size is standardised, at least within each product range, and thus becomes an easier shorthand description for size).

Non-circular Chain Rings

Some component manufacturers have produced chain rings that are not circular, in an attempt to improve performance (of the cyclist).

The idea is that by varying the shape of the chain ring slightly (making it oval- or egg-shaped), it becomes better suited to the different levels of power applied by the cyclist at different points of the pedal stroke.

The ‘non-round’ shape varies the distance between the engaged chain and the axle at each point around the circumference of the chain ring. The rider’s effective gear therefore changes as he or she rotates the pedals.

Non-circular rings tend mainly to be used by professionals (and aspiring professionals) in time trials on the flat, where every last watt of power needs to be transferred into optimal forward motion. Since the Grimpeur is neither professional nor a regular participant in time trials, we can probably leave this topic there.

Fin

So that is the subject of chain rings covered, at least for now…

See you next time!

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.

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