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I'm considering to replace my bottom bracket. My current one is a cheap 68x115mm squared. As I'm obsessed by bike weight, there is a titanium product I'd really love to install. However, the closest available sizes are 113 and 119mm. My chainring is already close to the frame, and I'm also considering to install an oval (so sometimes larger) chainring, so I cannot install the 113mm.

So, if I install the 119mm, this will theoretically make a chainline difference of 2mm. Is this acceptable?

My transmission is 1x10. I do most of my riding on the high speeds (small sprockets), which is great for the case here (chainline closer to these sprockets). Still, I need all the 10 speeds to work ;)

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    Side question - since you're already looking at a new BB, this might be a good time to upgrade cranks to get something lighter than square taper. Not that square taper is bad, but there's a lot of gravity inside those things.
    – Criggie
    Feb 3, 2020 at 3:43
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    I had thought about it (e.g. switching to octalink), but I'm looking for a cheap titanium BB, which is available only squared. Also it would require additional purchases: new crank arms, and yet another tool. Feb 3, 2020 at 5:22
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    Octalink isn't that much an improvement over square taper, if you want lightweight two piece external cup system is it.
    – ojs
    Feb 3, 2020 at 7:42
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    Titanium bottom bracket is not slight improvement but far into diminishing returns high end region. There are easier places to save weight.
    – ojs
    Feb 3, 2020 at 20:48
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    Be warned titanium square tapers can flex a fair bit giving the sensation of wind-up. This can make the bike feel less responsive.
    – Rider_X
    Feb 3, 2020 at 23:55

3 Answers 3

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Since I crosschain my bikes (yeah that's how I roll and I even do this on triple cranksets) so I'm often in the two big rings and almost never have I had a chain come off. So I wouldn't expect a chainline issue by moving the chainline 2mm (I'm assuming the 4mm increase is split 2mm longer on each side).

BUT this assumption may not be correct. Some cranksets were designed with a larger BB axle offset for the chainrings on the right. In this case you could end up with crankarms which are 4mm closer to the frame on one side than the other and it could mean the chainline will change by 4mm instead of 2mm. I could only check this by measuring the two BB's side by side.

If this is a double crankset converted to single, and if the chainring currently is mounted on the outside position, and there is a problem with the chain coming off, you could also consider moving the chainring to the inside position.

BTW, make sure to use an anti-sieze compound to keep the crankarm bolts from permanently attaching inside the titanium BB axle. Ti likes to do that.

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I think this is fine, especially if you are using the outer sprockets most of the time.

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  • OP may need to look at chain retention on the chain ring - either a narrow-wide chain+chainringring, or a derailleur locked off, which won't help the weight much though there are plastic chain guides.
    – Criggie
    Feb 3, 2020 at 3:40
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    I ride like 95% on road and 5% on easy dirt paths. Thus, I virtually never derail :) Feb 3, 2020 at 5:27
  • It's not necessarily fine. None of the information that determines whether it's fine is present. Feb 4, 2020 at 7:49
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Not answerable in the abstract. You need to measure the chainline that your existing spindle is giving you. Frame alignment and especially chainstay length are also factors. Only once you have that information can you make a reasonable guess what is and isn't likely to cause issues.

Most 1x bikes come with the chainline out in front to begin with, for clearance reasons as well as to prevent unwanted catching on the small end of the cassette. It's likely that if you did what you're proposing, it would push it too far and cause issues in your big cog.

Also, your bike needs the right length spindle way more than it needs a titanium one. If you want both just get a Phil, which come in all sizes, last forever, and have some chainline tuneability.

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