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THis question might help out some newbies.....and even me.
I know that on a 21 speed bike, you have three gears in front and 7 in the back
So, on my road bike i have 2 in the front and 9 in the back, which would make it a 18 speed. Yet if i go to get a new deraileur, i need to get a nine speed one.
How do you determine the speed, or is it different for different circumstances?

i hope that made sense
 

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on a road bike, generally, the speed is what you described it to be, 21 for a 3front 7rear bike, or 18 for a 2front 9rear bike

derailers are based of off which part they are accompagnying. the rear cassette has 9 gears, so you need a derailer that handles 9 gears. on the 21 speed bike (3f 7r) you would need a 7speed derailer. thats for rear derailers. front derailers are different in that they are designed for triples, doubles, or compacts. most components are listed as part of an X speed group. ex. the new shimano 105 10 speed groupset. (with a double that is a 20 speed bike, a triple = 30 speed). yet the rear derailer is a 10 speed part.

hope that helps?
 

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In modern road cycling parlance, 9 speed, 10 speed, etc. refer to the number of gears in the rear cluster, not the overall number of gearing combinations. For instance, you might hear references to Shimano 9 speed vs. Shimano 10 speed. So, you would need to know whether your bike is set up for either Shimano or Campy 9 or 10 speed for most drivetrain parts. The number of chainrings you have, whether it be 2 or 3, has no impact on the type of chain, rear derailleur or cogs you need.
 

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The rear der. is affected by how many chainrings you have. With the triples you need a rear derailler with a long cage so that it can take up more slack in the chain when you are in the smallest chainring.
 

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The older chains that were made for 7 or 8 speeds would be too wide for a 9 or 10 speed cassette. Because the additional cogs in a 9 or 10 cassette requires a narrower chain. Therefore you need to get a chain that is specified as 9spd or 10spd. However a chain for 10spd will be compatible on a 9spd cassette.
 
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