Triple to Double Crankset what I need to know
 

Hi,

My brother's bike a 9 speed Triple Chainring with 105 shifter and FD. If we change the crankset to Double - Do I need to replace the shifter and the FD ? Thank you

No, 105 shifters work either way, double or triple.

OK . I guess FD Crank and BB and RD also since it's a lon cage ?

From triple to double you don't have to change the derailleurs, that's up to you.

I could still use both derailleurs and just change the Crank and BB ?

Depending on your objective:

You could just remove the inner ring and change the BB to a slightly shorter spindle.

Or just take off the inner ring and set the low limit screw on the FD.

Or, just pretend the inner ring isn't there.

So yeah... The derailleurs and shifters will work fine.

Thank You!