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Although I have read Sheldon Brown's All About Bicycle Chainline article I'm still struggling to understand a key point.

If you are running a different chainline to the manufacturers (new crankset) can this be adjusted for in spacing of the rear axle? Essentially the question is, does the frame drive the chainline or the components and set up?

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To answer the question you haven't asked but looks as if it might be the underlying question - if changing your crankset has changed the chainline, buying a new bottom bracket will fix it a lot more easily than trying to compensate with axle spacers and redishing the wheel. (But, as the people giving actual answers have said, the frame imposes limits - if a shorter bottom bracket spindle would mean the new chainrings hit the chainstay, obviously you can't go that far.) –  armb Mar 7 '13 at 13:20

2 Answers 2

There's a number of things that affect and can be tuned to adjust chainline.

  • Bottom bracket shell width. If the bottom bracket is wider, the chainline is going to be further out.
  • Bottom bracket spindle length. A longer spindle can be used in a narrower shell to give the same chainline as a wider shell.
  • Crank sizing. Cranks are usually designed towards a certain chainline and mixing and matching these can alter it.
  • Chainring positioning. You can space out your chainrings to move them further from the crank spider.
  • Frame spacing, hub size & dishing. Using these you can build a hub that is farther to one side than the other and dish the wheel back to the center line.

Most of these adjustments are time consuming and a hassel, or need you yo carefully buy new parts. If this is bike has a derailler then your chainline is never going to be correct anyway and I wouldn't worry about it.

So, to answer your question, the frame tends to set a lower limit to the chainline in the front and an upper limit in the rear. With careful component choice you can always make a good chainline (most framebuiders are unlikely to stick a 73mm BB on a frame with 120mm rear spaceing, I would guess).

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Neither the frame nor the components drive the chainline individually. Rather it's a combination of the two.

Let's assume for one second that there was only one type of frame. That is: bottom bracket shells are all the same and rear triangle spacing is all the same. That's not true at all, but let's pretend for just one second...

If that were the case, a crankset that were slightly wider than other cranksets would pull the chaineline outward in the front and a narrower crankset would pull the chainline inward in the front. (Essentially the width of a cranskset is a function of the bottom bracket and the spindle, but when you throw in different manfuacturers it gets messy.) The same would be true of the rear hub. A narrower hub would pull the chainline inward in the rear while a wider hub would push it outward.

Now, as I indicated previously, frames are not all the same. On the flipside of that, let's forget everything I previously said and pretend for a minute that all frames are the same and all hubs and all bottom brackets are different.

Conversely to our previous make-believe world, a wider bottom bracket shell would pull the chainline outward in the front and a wider rear triangle spacing would do so in the rear. It should be obvious at this point that either one of these things would have to be compensated for by wider corresponding componentry.

So, to reiterate the answer to your specific question: Neither the frame nor the components drive the chainline. Rather, it is a combination of the two.

Whether or not proper chaineline can be achieved through spacing out the rear axle depends at least partly on the frame material. I.e, steel can be spaced out more than aluminum and carbon. More specifically, carbon can't be spaced out at all and spacing out aluminum isn't recommended (although I've heard second and third hand of people making it work, no personal experience though). However, if your frame spacing is too wide, you can always add spacers to the hub/axle.

To answer the question that I think you mean to ask: you can almost always (especially on a geared bike) achieve proper (or at least acceptable) chainline, either through spacers in the crankset/bottom bracket or spacers in the hub/cassette.

Where you decide to put the spacers is ultimately a product of your specific frame/component combination.

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