There is nothing as likely to bring a customer back to the bike store as a problem with their bicycle gears.
Most modern bicycles use variations of a design called the
dérailleur; literally, the
de-railer, a device which forces the chain from one cog to the next in a very unsophisticated and crude manner. In engineering terms, it really shouldn't work ... but it does.
Modern dérailleurs use all manner of tooth profiles and chain design to enable this to happen as smoothly as possible. But, just as changing gear on a manual gearbox car needs finesse and understanding of the principles involved, compared to say, an automatic gearbox, changing gear using dérailleurs on a bicycle requires a degree of involvement from the rider, more than just pushing the button and crunching on regardless.
Some modern bikes have up to
30 possible gear combinations -
3 at the front X
10 at the rear - but not all permutations are useful either because some combinations of front and rear cogs produce gear ratios which are very close to another or even identical, or are mechanically compromised. More about that later ...
Changing Gear:
The principle of the dérailleur depends on the chain moving forward through the gear change, so when changing gear, continue to pedal forward. However, it's really helpful to the change if pressure is taken off the pedals so that for the duration of the procedure the feet just spin until you sense the gear has changed and take up the effort again.
There are occasions when this isn't possible, but just assessing your gear changing needs ahead of the point where you have to change helps. This particularly applies when you're changing up to a higher gear, for example, on a hill, or changing to an easier gear just before coming to a halt.
People often ask,
"How do I know what gear I'm in." The fact is, you don't really need to know as long as you feel comfortable and can maintain a good pedal cadence and the drive sounds quiet. But there are some gear combinations to avoid.
Cross-Chaining:
The diagram shows the top view of a typical set up. I've indicated the chain line from the extremes of the chainwheel to the cassette. Although exaggerated, it demonstrates the degree of deformation the chain has to cope with in those gears. This tends to cause the chain to track badly, run noisily and the dérailleur mechanisms to have to contend with excessive chain wrap, extension and tension.
In practice, restrict your gear choices as in the diagram above; large chainwheel to outer selection of sprockets, small chainwheel to inner sprockets.
This picture illustrates a rear mechanism coping with chain wrap. This would be more excessive with a triple-chainset.
Modern gear indexing systems control the movement of the dérailleur, often to a tolerance of 0.1mm, less than 1/100th inch. One of the prime reasons for gears to go out of adjustment is cable stretch, particularly with new cables, so if you've recently bought a new bike, or installed a new cable, return to your LBS to have the adjustment done if you can't do it yourself.
Another frequent cause of poor shifting can be a bent dérailleur hanger, the component which connects the rear mechanism to the frame. If you suspect your hanger is bent you will need to visit your LBS where they will have an alignment device which can check and adjust the hanger. The hanger will need to be adjusted in three planes so it's not really a job you can do at home.
But adjustment can be affected by using excessive force, either through the gear changer or through the pedals while changing gear causing elements of the drive to distort or just go out of line, so learn to coordinate your changing/pedaling skills as outlined above.
A well adjusted gear mechanism will produce easy and smooth changes. However, it does need some input in terms of timing, sensitivity and skill from you, the rider.
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