I have a bike that has only one chainring and no front derailleur (obviously).
Could I take the chainring set, front derailleur and shifter from another bike and put it on my bike?
My bike never had a front derailleur and is 7 speed currently.
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Here are the issues:
Shape of the seat tube. If the seat tube is not cylindrical or too wide near the chainrings you wont be able to mount a band clamp derailleur.
You have a 3 piece crank with crank arms separate from the bottom bracket axle, with a square taper type crank-axle interface. If the multi-chainring cranks you want to mount are the same you can mount them also.
Getting the right chainline. Different cranks have the rings at different offsets from the square taper hole, and require different length axles to position the rings correctly. If the chainline is the same on both bikes and the axles are the same length then you can make a direct swap, otherwise you'll need to calculate what length axle you need and get a new bottom bracket.
Chainstay clearance. The single front ring bike may not have enough clearance for a multiple chainring crank.
Front derailleur cable housing mounts. If your frame does not have `em, you'll need an alternative. I believe there are ones you can clamp to you frame available.
You'll need a new chain as the required chain length will be longer (assuming you are adding a bigger chainring).
You'll need to check that you rear derailleur has a great enough total capacity Otherwise it will not be able to handle the increased variation in chain slack. Capacity needed is (difference in size smallest-largest sprockets) + (difference in size smallest-largest chainrings)