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More About Bike Gears: A Short Treatise on Chain Rings

chain rings

A cornucopia of chains and cogs

If you read my post about how bike gears work (if you didn’t, you can find it here), you may recall that I discovered that there was more to the subject than could be covered in a single outing.

It turns out (because, yes, sometimes I write these introductory sections AFTER I’ve written the body of a post), that there are many interesting things* that you can say about each of the components that form the drive train of a road bike.

(*Some interesting things. The occasional interesting thing.)

So today I lift the lid on the many and varied charms of chain rings. Enjoy…

What are these chain rings of which you speak?

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

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 a compact chinset/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…

If you want to learn more about bike gears, then I heartily recommend that you check out my other article: Bike Gears: How Do They Work.

See you next time!

Andrew

Photo credit: http://www.flickr.com/photos/restlessglobetrotter/

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{ 6 comments… add one }
  • I learnt a few things. Didn’t km now that a compact chain set was just a smaller / hidden granny gear that would be a lot slower.
    Also like the nested parentheses – very nice 🙂
    Keep up the good work.

  • daniel says:

    i just had the 2 front rings, the rear sprocket cluster and the chain replaced on my commuting roadster. i noticed that the teeth on the front large chain ring were not all identical, with most being kind of shark shaped but some being slightly flattened on the tooth tip. i’d like to know if this is normal. the shop showed me another one still packaged and it was the same… 🙁

  • […] sportive end of the market tend to have compact chainsets (the big gears near the pedals – read my post here if you’re unsure) with a cassette that ranges up into the high 20s (teeth) for the bigger sprockets (this post talks […]

  • […] Now, if you’d 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. […]

  • Giles Roadnight June 21, 2013, 5:45 am

    I learnt a few things. Didn’t km now that a compact chain set was just a smaller / hidden granny gear that would be a lot slower.
    Also like the nested parentheses – very nice 🙂
    Keep up the good work.

  • daniel September 3, 2013, 9:28 am

    i just had the 2 front rings, the rear sprocket cluster and the chain replaced on my commuting roadster. i noticed that the teeth on the front large chain ring were not all identical, with most being kind of shark shaped but some being slightly flattened on the tooth tip. i’d like to know if this is normal. the shop showed me another one still packaged and it was the same… 🙁

    • Matthew September 6, 2013, 8:10 am

      @Daniel – this is normal and is done to save weight (albeit a very small amount).

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