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I'm in the process of installing my Shimano Tiagra left shifter for the front derailleur on my double crank and I'm a bit confused, the indicator shows it has 5 positions for a crank with only 2 chainrings (double), when using common sense says it should only have 3.

http://www.ebay.ca/itm/Shimano-Tiagra-2x9-STI-shifter-brake-levers-/300924155393?pt=US_Shifters&hash=item46107a3601

Depressing the large lever one click from the inner ring should put the front derailleur into the trim position, and depressing the large lever a full two click puts the chain onto the outer ring. Shouldn't that be it?

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Yes it should, your last para is correct. Sounds suspiciously like you have got yourself a shifter for a triple chainring. Do you have any packaging/part numbers? If the shifters are made for a double chainring, the part number should end in a 0 (e.g. ST-4600), if triple should end in 3 (ST-4603). –  PeteH Jun 25 '13 at 8:16
    
I'll have that checked when I get home, I think you may be right –  Locke McDonnell Jun 25 '13 at 22:25
    
@LockeMcDonnell, Any update on the part numbers? –  amcnabb Jul 30 '13 at 15:57
    
Common sense would be 2 positions for 2 chainrings. I don't know what you mean by trim position? –  Blam Jun 20 at 18:45
    
@Blam trim position is a smaller adjustment while still remaining on the same chainring to allow for cross chaining. –  Mac Dec 16 at 3:14

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