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When converting a geared bike to fg/ss, at some point there will be adjustments to be made regarding chainline. Without getting into details about what could be made to make it better (bb axle width, hub spacers, different cogs, whatever), how much disalignment can we have between chainring and rear cog?

Illustrating what bad and good chainlines are.

Currently, I am building a fixed gear out of 90s MTB frame, track rear wheel and road crankset, and I am sure it will work eventually, because I've done this before and I don't care about the hassle. What I hope is someone could give this question a final answer, to enable (or not) people who are into that "DIY fixed gear spirit" to have some confidence (or to know the risks with more certainty) before starting a new conversion project instead of simply buying new stuff.

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    There is no specification for being outside the specification. A bad chain line will either worn or throw a chain depending on a number of factors.
    – Rider_X
    Jun 30, 2018 at 20:56
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    Using a proper SS chain, either 1/8 or 3/32, I agree with ojs's numbers below. However its worth pointing out that if circumstances force you towards bad chainline, you can push it way further by using a derailer chain and a 3/32 cog and ring. Not saying one 'should' do this, especially not permanently.
    – Nathan Knutson
    Jul 1, 2018 at 6:25
  • As little as possible, the straighter the better.
    – Carel
    Jul 1, 2018 at 14:16

2 Answers 2

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There is no hard and fast limit. The farther off the cogs are, the worse vibration and wear you will get. My personal experience is that a couple of millimeters off is still smooth and at 5 mm the chain will be noisy but still work.

On a singlespeed it's virtually impossible to throw a properly tensioned chain without breaking cog teeth, but bad enough chainline will wear them out and make that easier.

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Freehub body spacer kits for positioning the rear sprocket include spacers of many different widths. The possible combinations of spacers on the inboard and outboard side of the sprocket to be adjusted to within a few millimeters, which is good enough to get good chainline.

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  • The question was primarily about fixed though.
    – Nathan Knutson
    Jul 1, 2018 at 18:14
  • @NathanKnutson I was assuming the OP is converting a freehub equipped wheel to a single speed sprocket, using a spacer kit, as that's what's portrayed in the images.
    – Argenti Apparatus
    Jul 1, 2018 at 18:17
  • First rule of writing: pictures do not have to be related
    – ojs
    Jul 2, 2018 at 8:46

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