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Bike Jargon Buster... Bike Gears Explained

The gears on a bike confuse a great many people, so if you are one of them, fear not, as you are far from alone!
Bicycles will almost always have one of four types of gearing system;
  • Single Speed
  • Fixed Gears
  • Hub Gears
  • Derailleur Gears
The first three types above take very little explanation but Derailleur Gears seem to leave the majority of people a little non-plussed and the majority of this page will deal with them.

Almost all bicycle gear systems consist of:
  • Pedals, connected via "Crank Arms" to one or more toothed rings known as a "chainring". The two crank arms are connected together through the frame by a metal shaft known as the "bottom bracket axle"
  • Chain. Bicycle chains are 1/2 inch pitch (the distance between the links) and vary in width according to the type of gear system being driven.
  • One or more toothed rings (known as "sprockets") attached to the rear wheel
Some gear systems replace the chain with a belt but these are not particularly common. Also there have been "shaft drive" bikes in the past.

Single Speed

In a Single Speed system, the bicycle has only a single gear ratio: One size of chainring at the front and a second (typically smaller) size sprocket at the back. The sprocket at the back has the ability to turn independently in reverse; this is called a "freewheel". With a freewheel, the wheel can continue to rotate without the pedals needing to and you can "back pedal". Some single speed bikes may also be "Fixed Gear" or "Fixies" (see below).

Fixed Gears

In a Fixed Gear system, the bicycle has only a single gear ratio (single speed [see above]) AND the pedals will continue to rotate (even when you stop pedalling) whilstever the bike is moving.

Hub Gears

In a Hub Gear system, the gears are enclosed within the bicycle's rear hub. When "3 Speed" or "8 Speed" is mentioned in relation to a hub gear, it means that the hub has 3 (or 8) distinct gear ratios.

There is a seperate type of hub gear system which uses CVT (Continuously Variable Transmission) principles in which there are no distinct fixed ratios; the gear ratio is variable anywhere between a lowest and highest ratio. At the time of writing, there is only one company manufacturing such a gear system (NuVinci) and they are not particularly common.

Derailleur Gears

Derailleur Gears are the most common and the ones which cause the greatest confusion.

If you haven't got a hub gear but want a bike with a variety of different gear ratios, derailleurs are the commonest solution.

In a derailleur system, the gear ratio is altered by increasing or decreasing the size of either the front chainring or the rear sprocket. It is the derailleur which performs this action, pushing the chain off one sprocket/chainring and on to another - hense the name... the chain is derailed.
In a derailleur system, there are either one or two derailleurs added into the transmission line.

Rear Derailleur

The rear derailleur mounts to the bicycle frame, near the back wheel axle. The chain routes through two small semi-toothed wheels (jockey wheels) before being fed onto the underside of the rear sprockets.
The derailleur itself can move the relative position of the jockey wheels sideways compared to the fixed mounting point on the frame by means of a control cable operated by a gear level on the frame or handlebars.
This movement allows the chain to be pushed/pulled off one sprocket onto the next.
The rear derailleur is sprung loaded to take up the slack in the chain.

Front Derailleur

The front derailleur mounts to the bicycle frame, near the pedals. The chain routes through a cage before being fed onto the top side of the chainrings.
The derailleur itself can move the relative position of the cage sideways compared to the fixed mounting point on the frame by means of a control cable operated by a gear level on the frame or handlebars.
This movement allows the chain to be pushed/pulled off one chainring onto the next.
A bicycle cannot have just a front derailleur unless there is some other mechanism to pick up the chain slack.
On a bicycle with BOTH front and rear derailleurs we have:
  • 2 or 3 different sized chainrings on the chainset, controlled by the front derailleur
  • 3, 5, 6, 7, 8, 9, 10 or 11 different sized sprockets on the back wheel, controlled by the rear derailleur
Each combination of chainring and sprocket gives us a different gear ratio.

10/18/21/27 etc "Speed" Bike

This is the area which most people don't understand.

In a derailleur geared system, bikes are often referred to as "21 Speed" or suchlike. We'll use 21 speed ongoing as it was very very common for quite a long time.

If our bike has three chainrings and seven sprockets, we have 3 x 7 = 21 gear ratios. This is what is referred to as 21 speed.

BUT to view these as 21 independent gears, numbered 1 to 21 would be wrong. To understand why, we first need to understand gear ratios.

Gear Ratios

In simple terms, a gear ratio on a bike refers to how many times the back wheel will rotate for each full turn of the crank arms (pedals).
If we have a chainring with 30 teeth and a sprocket with 30 teeth, there is no difference between the two; one full revolution of the pedals will result in one full revolution of the wheel. As a ratio, this is 1 to 1 (1:1).
If we have a chainring with 30 teeth and a sprocket with 10 teeth, the chainring is 3 times bigger than the sprocket; one full revolution of the pedals will result in THREE full revolutions of the wheel. As a ratio, this is 3 to 1 (3:1).
If we have a chainring with 20 teeth and a sprocket with 30 teeth, the chainring is only 2/3 the size of the sprocket; one full revolution of the pedals will result in TWO THIRDS of a revolution of the wheel. As a ratio, this is 2 to 3 (2:3).

Gear Inches
In the UK, bicycle gearing is expressed typically as gear inches (or gear centimetres for the metric among us).
If we imagine that our pedals were attached directly to the driving wheel we can use the gear ratio to determine the effective size of the imaginary driving wheel. Think of a Penny Farthing with its huge front wheel.
If our actual wheel is 26" in diameter and we have a gear ratio of 3:1, our imaginary wheel is 3/1 x 26" = 78" in diameter.
If our actual wheel is 26" in diameter and we have a gear ratio of 2:3, our imaginary wheel is 2/3 x 26" = 17.3" in diameter.

Progression Metres
Through most of Europe, bicycle gearing is typically expressed as progression metres.
Again, imagining our pedals attached directly to the driving wheel, Progression Metres refers to the distance (in metres) that we will travel for one full revolution of the pedals.

Gear Ratios in relation to 10/18/21/27 etc "Speed" Bike

Now we have an idea of what gear ratios, gear inches and progression metres are, we can start to look back at our "21 Speed" bike and see what happens when we chart the gear ratios for real. You can do this for yourself for the gears on your own bike using our gear calculator.

This example has three chainrings: 28/38/48 and a 7 speed sprocket set 12/13/15/17/20/24/28 and is described with a wheel diameter of 622mm.

622mm 12t 13t 15t 17t 20t 24t 28t
28t 1451.3mm
(2.33:1)
(PM:4.56m)
1339.7mm
(2.15:1)
(PM:4.21m)
1161.1mm
(1.87:1)
(PM:3.65m)
1024.5mm
(1.65:1)
(PM:3.22m)
870.8mm
(1.40:1)
(PM:2.74m)
725.7mm
(1.17:1)
(PM:2.28m)
622.0mm
(1.00:1)
(PM:1.95m)
38t 1969.7mm
(3.17:1)
(PM:6.19m)
1818.2mm
(2.92:1)
(PM:5.71m)
1575.7mm
(2.53:1)
(PM:4.95m)
1390.4mm
(2.24:1)
(PM:4.37m)
1181.8mm
(1.90:1)
(PM:3.71m)
984.8mm
(1.58:1)
(PM:3.09m)
844.1mm
(1.36:1)
(PM:2.65m)
48t 2488.0mm
(4.00:1)
(PM:7.82m)
2296.6mm
(3.69:1)
(PM:7.21m)
1990.4mm
(3.20:1)
(PM:6.25m)
1756.2mm
(2.82:1)
(PM:5.52m)
1492.8mm
(2.40:1)
(PM:4.69m)
1244.0mm
(2.00:1)
(PM:3.91m)
1066.3mm
(1.71:1)
(PM:3.35m)

The colour coding is intended to show the gears getting higher/harder in a visual sense. The more green, the easier/lower the gear is, the redder it is, the harder/higher the gear is.

Look carefully at the values for a 28t chainring with a 28t sprocket and you'll see it has a ratio of 1:1. For a 48t chainring with a 12t sprocket it has a ratio of 4:1.
The first row of the chart is the 7 gear ratios we have with the smallest (28t) chainring selected. The highest gear ratio is 28t with 12t; a ratio of 2.33:1.
The result is that, for the sprocket sizes above we have a set of 21 gears ratios which cover a range between 1:1 and 4:1; The highest/hardest gear will result in us travelling four times the distance of the lowest/easiest gear for each pedal revolution.
Now look on the second row where we have the middle 38t chainring selected. Our 28t+12t ratio sits somewhere between the ratio for our 15t and 17t sprockets when the middle chainring is being used. ie, the highest gear gear ratio on the smallest chainring is higher than the lowest gear ratio for the middle chainring.
Similarly 38t+12t has a ratio of 3.17:1 which again sits between the ratios for our 15t and 17t sprockets when the biggest chainring is being used.

What this results in is that rather than there being 21 "speeds", there are three ranges of seven gears. Each of our three gear ranges overlaps onto the gear ratios of its neighbour.
In fact, if you look closely, you will notice that almost every gear ratio when the middle 38t chainring is selected is almost duplicated by ratios on the 28t and 48t chainrings.

Why have gears that are (nearly) duplicated?

The answer is one of usability.

If you were to take out each gear ratio that was closely duplicated by another gear, we would be left with just 11 distinct gear ratios:
  • 28t + 28t
  • 28t + 24t
  • 38t + 28t
  • 38t + 24t
  • 38t + 20t
  • 38t + 17t
  • 38t + 15t
  • 38t + 13t
  • 38t + 12t
  • 48t + 13t
  • 48t + 12t
If we were to remove the duplicated gear ratios completely, ie make them unusable, we would need to change both the front and rear derailleur settings at the same time on occasion for what could be just a single degree of difficulty harder or easier. By leaving the duplications in the mix, we can get to that next easier or harder gear by only changing the rear derailleur by a single step.

Consider them as three gear ranges; in the case of a 3x7 setup, a low gear range with 5 different gears. A mid gear range with 6 or 7 different gears and finally, a high gear range with 5-6 different gears.

So, if you are in steep ascending landscape, you select your low gear range (smallest chainring)
If you are in steep or moderate descending landscape, you select your high gear range (biggest chainring)
For most other situations, moderate ascending and descending, you select your mid gear range (middle chainring)
If on tarmac, you possibly alternate between mid and high ranges.

Gear Ratios

By having three gear ranges which overlap significantly, you will find that you can minimise the changes of front chainring selection, making your gear transitions smoother.

Racing bikes (ie the old drop handlebar "I knew I was right about that saddle" type bikes) rarely encounter such steep or difficult ascents that the lowest range offered on mountain bikes is required (where the terrain makes the effort required increase). In more recent times, an increasing number of race bikes are now being fitted with triple chainrings. This serves two purposes; it allows the gear ratios within each of the three ranges to be tighter together giving more finite control AND it allows the makers to extend the number of sprockets fitted at the back. Pick any two of the ranges in the diagram and you'll see that at least one sprocket/chainring combination is inadvisable. With three, all sprockets and all chainrings become usable, though not every combination.

The shop told me to avoid certain gears. Why?

The issue is the relatively crude way in which derailleur systems work.

You have two sets of cogs, connected by a chain.
The front set of cogs (typically 3 nowadays) is known as the chainset/chainrings.
The back set of cogs (anywhere from 5 to 11) is the freewheel or "sprockets".

The bicycle chain is pinned together to allow the links to move (so they can go round the sprockets and chainrings) and whilst there is a small amount of lateral movement allowed, the chain essentially wants to travel in a straight line.

Now consider that both the front chainrings and rear sprockets form straight lines but there are only a few combinations of those front and rear cogs that have a straight line joining them.

The front set of chainrings are typically offset slightly in favour of higher gears, ie the middle chainring does not form a direct straight line to the middle rear sprocket.

The "grinding" noises you will sometimes experience are the direct result of contact between the chain sides and one or other of: the inner faces of the front derailleur, the inner face of a larger chainring than the selected one or the chain riding over the jockey wheels in the rear derailleur (those two small plastic toothed wheels in the rear derailleur that take up the slack in the chain). Most likely are the first two.

Gear CrossoversThe illustration to the right hopefully demonstrates the lineups of the various gears (in a 3x7 system).

In the left hand illustration, the smallest chainring is selected. As you can see, it lines up very well with the third sprocket and pretty well with sprockets 2 and 4. Even 1 and 5 don't show too much deviation from a straight line, but once we start to look at sprockets 6 and 7, the deviation from straight is becoming quite extreme. Indeed, on 7th sprocket there is a fair chance that the chain will begin to grind on the middle chainring's inner face. Pretty much all shops/stores will tell you to avoid using the smallest chainring with the smallest sprocket simply because the off-line aspect is so great that it will cause premature wear of your chain. It additionally is the point where there is the least "wrap" of the chain around sprockets making it at its slackest.

In the middle illustration, the middle chainring is selected. As you can see, it lines up pretty well with all of the rear sprockets with the exception of the biggest (1) sprocket. The middle chainring to largest sprocket is advisable not to be used though shouldn't cause excessive damage to the chain. The combination will, however, mean that the chain is almost definite to grind on the inner face of your front derailleur mechanism. It is also possible for the chain to grind on the inner face of the front derailleur in middle -> smallest. For this reason, many better quality front gear shifters may have two settings for the front derailleur position, to alleviate this grinding. Back in the days before "indexed" gears, cyclists would regularly adjust the front derailleur position to alleviate any grinding, but with modern index systems, this isn't so easy.

In the right hand illustration, the largest chainring is selected. Again, you can see that this lines up pretty well with the smallest four sprockets, however the biggest three get progressively more out of line... infact moreso than the combinations on the smallest chainring... because the chainset is offset to favour higher gears. On the biggest chainring, it is never wise to use the two biggest rear sprockets and the third biggest will, as above, also leave the chain grinding on the inner face of the front derailleur.

In short;
on the smallest chainring, do not use the smallest 2 sprockets.
on the middle chainring, you can use all rear sprockets but it is advisable to avoid using the very biggest
on the biggest chainring, do not use the biggest 2 sprockets and it is advisable to avoid using the third biggest aswell, but this depends upon your specific gear setup.

Using these poor combinations leads to excessive and premature wear of your chain, chainrings and sprockets AND means you are almost certain to experience grinding noises.

Further, more technical detail

The details given above are given in what we hope is laymans terms, as easy to understand as we can make it.

For a more detailed and thorough explanation (though quite technical in places) refer to WikiPedia's Bicycle Gearing article.
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