Hi All,

I am trying to put a compact double (50T/34T) road crank onto my single speed langster frame to make it a 2-speed.
My stock BB is 103mm, and the recommended BB for the compact double is 115mm.

The langster with the 103mm BB has a 42mm chain line, and the reported chain line of the compact double with a 115mm BB is 43.5mm. I do not understand how increasing my BB by 12mm will only change the chain line 1.5mm. Can someone help me understand this?

Also, assuming that the chain line will only increase to 43.5 (and assuming 5mm chain rings spacing it would give 41mm inner ring and 46mm outer ring spacing), does anyone think that this chain line increase will be too much?

Thanks for the advice, it is much appreciated.
Dave

Spider offset. If you were to mount your double crankset on your stock BB you would have a much greater difference in the chain line than 1.5mm

Why not just put a 16T sprocket on one side of the hub, and a 20T on the other side?

Much simpler, much easier, chainline stays exactly the same.

I want to have a 2-speed that I can shift while riding. I have a 16T in back, so the 34/16 would be good for hills and the 50/16 would be good for flat/downhill.
I don't want to use an internal hub since I just recently got new wheels.

I'm using a derailleur hanger, melvin chain tensioner, 3/32 chain, compact double crank, FSA compact double front derailleur

Note that with a single ring crankset chainline is measured to the centre of the chainring teeth. And with a double chainring crankset it is measured to the mid-point between the two chainrings.

With only one sprocket at the back chainline is not a life or death issue. The chain will not run at any significant angle like it would if you had, say, ten sprockets at the back, so there isn't much to worry about. Most front mechs these days allow plenty of adjustment between travel ranges.

But if your new set-up doesn't work you get what you deserve for buying a bike with only one gear.