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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?

Reasons, or what I'm trying to accomplish: I want a higher gear ratio. I recently converted an old junker "mountain-style" bike to an ebike with a conversion kit. I find myself "running out of gears." I counted the big ring at 42 teeth. A quick perusal of ebay shows 52/40t double chainrings for 1-piece cranks. So I want to swap out my current 42/x/x triple to a 52/40 double. The ebike is driven by a hub motor, so I don't need to worry about the higher gears putting more strain on the drivetrain.

Alternatives: The most straightforward option would be to swap in a 48t triple. This is the fallback option. I'd prefer the higher top-end of a 52t if possible. Another option is to replace the 6-speed freewheel, but I haven't found one that is geared "bigger" than currently equipped 14t. I've read that 7-speed freewheels are an easy upgrade, but I have an indexed twist shifter, so that route also involves a shifter change.

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  • Have you considered installkng a square taper BB (they're quite cheap, approx 5eu if bought form the right store for a budget one.. approx 10-20eu for one made by Shimano) and installing a triple crank with 52 teeth on biggest chainring? Iirc the standard bsa square taper BB should fit your frame. There are different lengths available so that gives you an opportunity to fine-tune/alter the chainline if needed
    – Maarten -Monica for president
    Commented Feb 22, 2020 at 6:59
  • I considered it, but that would require a lot of parts: adapter bushings (not a straight swap according to link), new BB, new chainrings, new cranks, new pedals. It's down to the 52/40 or the 48 triple. It's just simpler and more cost-effective.
    – warthogism
    Commented Feb 22, 2020 at 8:56
  • Sounds like you have a freewheel, where 14 tooth is the smallest common cog. You could get a freehub-based wheel, which takes a casssette, able to go to 11 tooth smallest, or for big money there are more unusual hubs that can go to 10 tooth or possibly even 9 tooth, like "Shimano Capreo"
    – Criggie ♦
    Commented Feb 22, 2020 at 13:24

2 Answers 2

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2

I want to convert a 1-piece crank triple chainring to a double.

  1. Do I need to worry about the chain line?
  2. Do I need a spacer of some sort?
  3. Is this a simple swap of the chainrings?
  4. If not, what size (inner/outer diameter, width) spacer would I need?
  1. Yes, you need to set up your chain rings for the best possible chain link
  2. You might need a spacer
  3. It depends - details below
  4. 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

enter image description here
Double chain ring with the a small chain ring riveted on the inside.

Or like this:
Chain Ring 2 enter image description here
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.
enter image description here

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.

About spacers.
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.

enter image description here

enter image description here

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.

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  • Thanks so much. Your answer is well fomatted and contains a LOT of information. I never even considered that there would be different types of triple chainrings. The section about spacers and where to put them is especially helpful. If it doesn't line up, I'll now know how to remedy that. I just checked my bike, and have one that looks like chain 1 from your examples. Hopefully it'll be an easy swap. Thanks again!
    – warthogism
    Commented Feb 23, 2020 at 2:11
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Yes, you need to consider chain line, but that does not mean you cant swap in larger rings on the middle and outer positions.

Sounds like right now you are are using the middle and outer chainrings exclusively on your triple crank. If you put two larger rings on the middle and outer positions, there's no actual difference in how you are treating the chain.

Obviously it would be better to use a double crank as this will put less strain and wear on the chain, but using the two outer triple rings may be acceptable, as it's the lowest cost option, especially if you are careful to always avoid cross-chaining from the large ring.

BTW, you'll want to make sure that there is clearance between the new large rings and the chainstay.

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  • ...and that the front mech can reah high enough to deal with the new big chainring.
    – Criggie ♦
    Commented Feb 22, 2020 at 13:25
  • ..and some mountain bike frames will not accept a 52 tooth. In some cases it will hit the chainstay.
    – mikes
    Commented Feb 22, 2020 at 14:24
  • The triple chainring is stamped/riveted, so have to swap out the whole thing. I forgot to consider chainstay clearance. Thanks for pointing that out.
    – warthogism
    Commented Feb 23, 2020 at 2:06

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