I want to convert a 1-piece crank triple chainring to a double.
- Do I need to worry about the chain line?
- Do I need a spacer of some sort?
- Is this a simple swap of the chainrings?
- If not, what size (inner/outer diameter, width) spacer would I need?
- Yes, you need to set up your chain rings for the best possible chain link
- You might need a spacer
- It depends - details below
- Any steel washer with the right sized hole and thickness will work. "Right sized thickness" is determined by your situation.
Things to keep in mind
- As Argenti and mikes point out, you will need to worry about chain stay (frame) clearance.
- As Criggie points out you will need to worry about front derailleur height to clear the larger chain ring.
- You may need to adjust chain length
There are several ways triple chain rings for steel cranks are made.
Most of them do not have inter-changeable chain rings. You have to swap out the whole chain ring set as you are planning.
Chances are you have a chain ring that looks like this:
Chain Ring 1
Double chain ring with the a small chain ring riveted on the inside.
Or like this:
Chain Ring 2
Single chain ring with two chain bolted to the outside.
Drawing a picture of what it would look like to swap in a double chain ring in both situations.
Looking at the drawing, either way your chain line won't be any worse than it is today. If you don't have any chain line issues today you won't need a spacer.
With chain ring 1:
This design is essentially a double chain ring with a smaller chain ring riveted on the inside. Swapping for a double would be like removing the small chain ring.
You are less likely to have frame clearance problems with chain ring 1.
With chain ring 2:
This design is a single chain ring (the smallest one) with two chain rings bolted to the single chain ring. Swapping it for a double effectively moves the two outer chain rings in toward the frame one chain ring.
It's more likely you'll have frame clearance problems with chain ring 2. If your current frame clearance with the chain rings you have is very close you may have a problem.
Moving the chain ring toward the frame
The distance between the chain ring and the frame is determined by the "Stationary cone" - as seen in the Schwinn service manual picture below.
If you put a spacer between the crank shoulder and the chain ring you move the crank to the right in the bottom bracket but the chain ring won't move. You'd need a thinner stationary cone to move the chain ring in toward the frame.
Moving the chain ring away from the frame
You can put a spacer between the chain ring and the stationary cone to move the chain ring away from the frame. This has the effect of making the stationary cone thicker. But, you only have so many threads on the crank. Depending on the crank you may not have much wiggle room.
Best case scenario, this is a simple swap with no spacers.
Worst case scenario, a larger chain ring won't work on your bike at all.