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How do bike gears work?

Shimano cassette gear sprockets

What determines the number of gears on a bike?

Quite simply, it is the number of sprockets (cogs) that you have on your rear cassette, multiplied by the number of chainrings at the front.

Eg. if you have 9 sprockets on the rear cassette and 3 chainrings at the front, you have a 27-speed bike.

What are a bike's gears used for?

A bike's gears are there so that a steady pedaling speed can be maintained, regardless of terrain or speed of travel.

Pedaling speed is also known as cadence and is the number of times the pedal rotates in a minute.

Cyclists typically have a cadence that they are comfortable with, regardless of the speed they are cycling at.

To maintain this comfortable cadence, across a variety of terrains and cycling speeds requires gearing.

If a slow cycling speed is required, when just setting off or when climbing a steep gradient, then the lowest gear might be selected by shifting to the smallest (inside) chainring at the front and shifting to the largest sprocket (inside) on the rear cassette.

This will allow a sensible pedal rotation of around 60-80 rpm while getting/keeping the bike moving.

At the other end of the spectrum, if a fast cycling speed is required when cycling downhill or when cycling at very fast speeds, the opposite gearing is required.

The highest gear, sometimes known as ‘big gear’, is selected by shifting to the largest (outside) chainring at the front, and shifting to the smallest sprocket (outside) on the rear cassette.

Do more gears mean faster cycling?

No. The gears are there to facilitate a comfortable ride, from the lowest setting to the highest. It is ultimately the ‘big gear’ (largest chainring and smallest sprocket) that determines the top speed and is the limiting factor.

The lower ones building up to this are there to allow a comfortable cadence at slower speeds, or inclines.

How are the gears changed?

On most bikes, the left-hand shifter operates the front gears and the right does the rear.

A simple way to remember is Right = Rear.

When the shifters are operated, the gear ratio is changed by shifting up or down on the front chainring or rear cassette.

The chain is moved over onto the next set of cogs by what’s known as a derailleur.

What is a derailleur?

With a derailleur gear system, the gear ratio is altered by changing the chainring or rear sprocket.

This is achieved via the front or rear derailleurs.

Front derailleur

The front derailleur is mounted to the bike frame adjacent to the pedals. The bicycle chain passes through the derailleurs cage section, which feeds the chain on to the top of the front chainrings.

The derailleur moves sideways, in either direction, so that it can deliver the chain to any one of the front chainrings.

It moves when the cable connected to it, is pushed or pulled via the left-hand gear shifter, which is attached to the other end of this cable.

When the derailleur moves, it pushes the chain off the current chainring and onto the next one along (assuming the gears are set up correctly).

This will alter the gear ratio and will have an effect on the cadence, depending on whether the gear ratio is increasing or decreasing.

When the derailleur reaches its outer or inner limit, it will not shift any further. This is achieved by limit screws that stop the derailleur from shifting too far either way.

This stops the chain from being pushed off the front chainrings, causing potential damage to your bike, it’s chain and maybe you!

Rear derailleur

The rear derailleur operates in a very similar way to the front, except it has the addition of two small toothed jockey wheels.

After the chain has been delivered from the front chainset, it travels in a clockwise direction to the rear of the bike. The chain is delivered to the rear derailleur and firstly feeds through its two jockey wheels.

Once the chain passes through these, it is delivered to the underside of the cassette and on to one of the sprockets.

As with the front derailleur, the rear one is moved when the right-hand gear shifter is operated, causing its cable to push or pull the derailleur, causing it to move sideways in either direction.

This will push the chain off the original sprocket and deliver the chain to the next one along, whether this be up or down.

The other purpose of the rear derailleur is to keep the chain tensioned via its spring loaded jockey wheels. In fact, the front derailleur would not work at all if it were not for this tension on the chain from the rear.

Gear ratios and crossover

In simple terms, one full revolution of the rear cassette will turn the rear wheel by one full revolution.

Therefore if your chain was currently on the largest sprocket of a 12-36 cassette (smallest cog 12 tooth, largest cog 36 tooth) and at the front end, the chain was on a 36 tooth chainring, for every full rotation of the pedals, the rear wheel would turn 1 full rotation.

The ratio here is 1:1

1 full pedal rotation > 1 full 36 tooth chainring rotation at the front > 1 full 36 tooth sprocket rotation at the rear = 1 full rear wheel rotation.

Alternatively -

If you were using a 36 tooth chainring at the front and a 12 toothed sprocket at the rear, for every full rotation of the pedal, the rear wheel would turn 3 times.

The ratio here is 3:1

1 full pedal rotation > 1 full 36 toothed chainring rotation at the front > 1 full 12 toothed sprocket rotation at the rear = 3 full rear wheel rotations.

Crossover

A 30 gear bike does not necessarily have 30 different gear ratios. Why? Because the ratios will likely be duplicated, or very closely duplicated, across the range of gears, depending on which chainring and sprocket combo you select.

Example - you may have a bike which has a crankset with 3 chainrings and a cassette with 10 sprockets. The table below shows the ratios between the crankset’s chainrings on the left-hand column and the sprockets on the cassette on the top row. The numbers with a 't' indicate the number of teeth the chainring or sprocket has.
 

As you can see, there are some direct ratio matches across the range.

If the smallest 22t chainring is selected and the smallest 11t sprocket is selected, the ratio is 2:1. This means that the rear wheel will rotate 2 times, for every full rotation of the pedal.

If you shifted up once to the middle chainring on the crankset and shifted up two cogs on the cassette at the rear, you would achieve exactly the same gear ratio of 2:1. So again, for every full rotation of the pedal, the rear wheel will rotate 2 times.

You can now see how some of the gears are replicated across their range, depending on what combination you select. Some are also extremely close in ratio to others, so much so that you would not really know the difference.

Example - See combination 40t - 19t, you will notice that the ratio of 2.11:1 is extremely close to the green highlighted 2:1 ratios. In practice, you would not be able to tell the difference here.

Chain crossing

To extend the life of your drivetrain (the collective term for the whole chain system) it is recommended that you do not cross over the gears.

What does this mean?

If you were to select the largest toothed chainring on the crankset (outside chainring), then select the largest toothed sprocket on the cassette at the rear (inside sprocket), the chain would be at an angle. This causes wear, as the chain will not be sitting squarely on the teeth of the chainring or sprocket.

The outside of the chain is also likely to rub on the inner faces of the front derailleur, or a larger chainring.

This will likely cause the grinding sound that you have probably experienced at some point.

To combat this, the gears should be selected so that the chain remains as straight as possible.

The general rule is that if the largest chainring is selected, then the smaller of the sprockets on the cassette should be used.

If the smallest chainring is selected, then the larger of the cassette’s sprockets should be used.

So to recap -

Large on the front, small on the rear.

Small on the front, large on the rear.

Or

Keep the chain as parallel to the bike as possible.

Further reading

We try to keep things as simple as possible here at Goose Systems, with the aim to get the point across without getting bogged down in too much detail.

However, if you would like to go a bit deeper into gearing, then the page at WhyCycle? is a good read and goes into the subject in more detail.

And if you really want to dig in, then another even more detailed one is Wikipedia's bicycle gearing page.

Be warned, its pretty heavy :)



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