How bike gears work

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Bicycles have two sets of gears, one attached to the pedal axle and known as the 'chain wheel', the other to the rear wheel and referred to as the 'freewheel', connected to one another by the chain. The majority of bikes have varying sizes of gears at both of these locations - three at the front and six at the rear in the case of '18 speed bicycles', one at the front and three at the rear in the case of '3 speed bicycles' and so on and so forth. The freewheel spins freely in one direction, locking in the other using a ratchet mechanism and therefore allowing the rider to choose not to pedal and simply coast.

Bikes also have a front and rear derailleur. The rear one has two cogs, one at the base of a long arm, the other nearer to the freewheel. The arm and lower cog are there to tension the chain and, when you change gears, you will see it moving to take up or let out slack. The upper cog's job is to shift when you change gears and carry the chain with it to its assigned location. The front derailleur works on a similar, but somewhat simpler, mechanism, guiding the chain between the different chain wheel cogs.

Ratios

Gears in every type of mechanical application are all about ratios. Bicycle gears have a number of teeth proportional to their size - and it is the ratio between the currently selected front and rear gears that make the difference. If the selected gears on both the chain wheel and the freewheel have twenty teeth, for example, then the ratio is 1:1. In this case, for every pedal stroke, the rear wheel turns once.

Speeds and Gears

A normal bicycle with 26" wheels at this gear setting would move forward about 82 inches (7.4 kph or 4.6 mph at a 60 rpm pedalling rate). The highest gear ratio achievable, the selected chain wheel gear having 44 teeth and the one on the freewheel having 11 (in other words 4:1), would give you a higher maximum speed - at 60rpm it would be 30kph and at 120rpm would be 60kph. One of the lowest, with the chain wheel hear having 22 teeth and the freewheel 30 (giving 0.73:1), gives a speed at 60rpm of 5.4kph. The range of potential speeds, particularly when considering competitive bikes on good ground, is frankly quite amazing. The current world speed record for unassisted cycling sits at 132kph (82mph) just to give you an idea as what a bike can theoretically do.

What all of this boils down to is that the lower the ratio, the lower the gear number - the gear with the 0.73:1 ratio could, for example, be gear number 1 (as in 1 on the left and right shifters) on a bicycle. It makes it very easy to pedal the bike, but doesn't let you travel very far for each rotation of the pedals. The higher ratios (such as 4:1) correspond to the higher gears (as in 18 - or 3 on the left shifter and 6 on the right). A bicycle set to a higher gear is much harder to pedal, but the maximum speed and the distance travelled with each rotation of the pedals is greatly increased. The majority of bikers therefore shift down to lower gears when tired or travelling up a steep hill and shift up to higher ones when wanting a good work out or speeding down the other side of that hill.

Rhythm

All bikers have a rhythm, much like runners or swimmers, an optimum cadence at which his or her legs produce power most efficiently. What bicycle racers try to do is find this speed and then adjust their gears so they are always moving at this rate. The other thing they try to do is to keep the chain straight - in other words, using an 18 speed bike as an example, they do not have the left shifter on 1 and the right shifter on 6. At this setting, the chain is at a diagonal and therefore under increased stress - which can lead to stretching (requiring replacement) of the chain and excessive wear to the gears as a whole. A good rule of thumb is as follows: when the left shifter is on 1, the right shifter should be no more than half way up (1-3 on an 18 speed bike); when the left shifter is on 2, the right shifter should be in the middle (2-4 on an 18 speed bike); when the left shifter is on 3, the right shifter should be above the half way point (4-6 on an 18 speed bike).

For everyday bike riding, however, finding whichever gear is comfortable at that particular moment is usually more than enough.

How bike gears work

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Ratios

Speeds and Gears

Rhythm

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