The derailleur rear wheel has a cluster of sprockets at the hub and a sprung mechanism to keep the chain tight. A derailleur system is easy to service and it can be readily modified to alter the gear range. In addition, it is relatively simple to remove a derailleur wheel to fix a puncture, as long as you remember to shift to the smallest rear sprocket first. However, the system requires regular maintenance and, due the fact that the mechanism is exposed, a derailleur system is more fragile than single speed or hub gear.
How many gears at the back?
The design of derailleur gearing has been subject to numerous incremental changes since the 1980s, and there are now lots of derailleur types on sale. For instance, it is possible to buy a bike with anything between five and eleven sprockets on the rear wheel. Why? As time has gone on, the standard number of gears has increased. So whereas, in the 1980s, new bikes had six gears on the rear wheel, it’s normal to find ten on a bike in 2016.
However, in order to achieve this progress, the engineers sacrificed durability. To fit in those extra sprockets, they made each sprocket slimmer. You can see the difference in sprocket thickness in the table compiled by Sheldon Brown here. And, of course, a thin sprocket wears faster than a thick one made from the same material.
They went on adding sprockets, making the sprockets slimmer and placing them nearer to each other, until they were so close together the system needed a slimmer chain. So now when replacing a derailleur chain, you need to choose one that’s compatible with the cassette you have.
And when the designers got to ten sprockets, they changed the geometry of the rear derailleur, which means 10-speed derailleurs won’t necessarily work with nine-speed shifters (or eight-speed shifters, etc.).
So how many gears is enough for transport cycling? In my experience, between six and eight sprockets on the rear wheel is plenty. Since the industry has moved on to nine and more, there is a perception that six-speed, seven-speed and eight-speed parts are difficult to find. (This is a view you might see expressed on cycle forums.) However, in practice, these parts are still stocked by many bike shops, and they are readily available online.
Bicycle chain – external widths
Here are the external widths for the derailleur chains you’re likely to find on transport bikes. Manufacturing tolerances can be one tenth of a millimetre.
5 speed: 7.8mm
6 speed: 7.8mm
7 speed: 7.3mm
8 speed: 7.1mm
9 speed: 6.8mm
At ten-speed, things get complicated because different manufacturers used different external widths, but they’re all thinner than 6.8mm.
Nine-speed chains are generally more expensive than eight-speed ones. Ten-speed chains are more expensive again.
Eight-speed chains are backwards compatible (they work with five-speed, six-speed, seven-speed and eight-speed cassettes).
‘Indexing’ is a system where the shifter clicks into preset positions corresponding with individual gears, to make changing gear easier. Almost all new bikes have it. Old gear shifters simply relied on friction. Indexed rear derailleurs have a different action than friction ones, so if you want indexed shifting, you need an indexed derailleur. However, friction shifters are compatible with indexed derailleurs, and certain types of shifter have both index and friction modes. Read more about index and friction.
Which brand to buy
There are two historically significant manufacturers of indexed derailleur system, Shimano and Campagnolo, and they are generally not compatible. This means if you have a Shimano multi-speed wheel and you want index shifting, you need a Shimano derailleur and that means you need a Shimano gear shifter (or a Shimano-compatible one). There is more information about Shimano and Campagnolo my Shimano and Campagnolo page. In recent years a third manaufacturer, SRAM, has come to prominence. Some of their gears systems are compatible with Shimano while others use different standards.
The importance of the front derailleur
There’s a good reason to have a choice of gears under the front derailleur too. If you have to stop suddenly when in a high gear, it is quicker to change down to an easier gear with the front derailleur than the rear one, to get going again.
But the front derailleur does another important job: it makes the chain less likely to fall off on bumpy ground. So, if a bike has a rear derailleur, it may as well have a front one too, unless the bike is fitted with a chainguard.
How many gears at the front?
Chainsets come in single, double or triple configuration, giving one, two or three chainrings. Mountain bikes traditionally had three but there is a now a trend for two, for simpler shifting. Road bikes traditionally had two rings, but new ones can be specced with either two or three. Regardless of the number of rings, road bikes tend to have larger chainrings than MTBs.
A triple makes for a relatively complex system, and getting the chain to sit properly on the middle ring of a triple can sometimes be tricky, even if the left gear shifter is indexed.
A double gives a simpler system that is far less prone to shifting problems, and should provide a range wide enough for almost all utility cycling. A compact double has a relatively large difference between chainring sizes, (up to 16 teeth difference) giving a gear range almost as wide as a triple with rings slightly smaller than you’ll find on a conventional double. Another way of getting a double with small rings is to use two of three, i.e. a mountain bike triple with one of the the outermost rings removed.
I find that a small ring with around 30 teeth and another with about 42 gives me a range suitable for a 25-mile round-trip rides on moderately hilly urban roads. If I needed a chainring bigger than a 42 I’d be travelling at more than 25mph, at which speed wind resistance has such a large effect that pedalling is not a good use of energy, and I’d rather coast.
Front derailleurs are designed to work with certain chainring sizes (or rather with a certain range of sizes) – the variables are the curvature of the front plate and the presence of ramps on the rear plate to aid shifting from small to middle ring on a triple. There is an excellent article about front derailleurs at Sheldon Brown’s site with plenty of detail on compatibility and work-arounds.
Which shifters are best for everyday cycling?
Certain types of gear shifter are more resilient. Some designs allow you to move through the entire gear range in one shift, which can be useful where you might need to stop unexpectedly. Some shifters allow indexing to be switched off, making them far more versatile. Read about the different types of gear shifter.
To identify a rear derailleur, or to check the specification of a particular model, use the Disraeli Gears website, a photographic catalogue of derailleurs.