Cog Switch Changing Chainline

Browsing through I see there are a lot of threads talking about chainline, it’s always something I try to get as good as possible but I have a small mystery.

In peoples experience here, how much, if at all does swapping a cog affect your chainline? I would have expected little if any variation and that most cogs would be lined up the same and that just hub and BB would be the real significant factors (assuming matching taper etc).

I recently swapped out a generic cog on my bike for a Phil one (and went from 15t to 14t in the process) and noticed the drive chain was quite a bit noisier afterwards. Sometimes I have just found that this goes away quickly as everything works itself together but not this time.

Finally decided I needed to look at it in more detail when I threw a chain while riding the other day (nothing to make you feel like a complete muppet like throwing a chain). Tension was a little loose but not so that I would expect it to throw… unless the chainlink was out.

Measuring it out it looks like the rear is about 40mm and the front was more like 44-45mm, watching, the links come into the rear cog I could see they were coming on and grazing rather than cleanly. As an experiment I put the chainring on the inside of my crank (hate doing this, feels like such a kludge) and in general everything felt more ‘liquid’ on a quick test ride.

So, has anyone experienced a cog change making such a strong chainline change? Guessing I will have to put back on the old cog to really see what is going on (pain because it made it’s way onto another bike in the meantime). I tried to take some pictures but it’s not easy to make the chainline obvious:

I guess I may have also bent the chainring at some point, the difference between the slack and taught part of the chain position does seem a bit large to me.

Drive chain still feels a bit ‘crunchy’ when applying back pressure, thinking I might need to give the chain a good clean or get a new one, current one is a KMC, not sure what they are like as I usually run Izumi.

Any pearls of wisdom would be greatly appreciated.

take phil cog off, put it on level bench. retrieve previously used cog, put on said level bench.

phil do stupid shit all the time, so it’s probably just different for no other reason than MERICA.

i hate kmc chains with a passion, so kill that too.

Got both cogs off and took a good look, seems they line up pretty well, perhaps the Phil is 1mm or so further out but nothing significant. I can now only assume that the chainline wasn’t great to start with and I just didn’t notice it until I switched the cog and the crappy KMC chain started getting louder.

Got a Miche track chain, cleaned everything up and popped the new chain on, runs quieter now and doesn’t get that binding feeling on back pressure now. The slack and taught area of the chain in the rotation still seems very exaggerated compared to my other bikes though.

Still got the chainring on the inside, line isn’t perfect but better than the outside which is out by nearly 5mm by the looks of things (when measured). Will see how it runs for a while. I am thinking about new cranks eventually, though looks like I will need to see how I go with clearance if I want to line things up better.

Regarding your tight/loose chain symptom, either the BCD in the crank isn’t concentric with the taper or the teeth aren’t cut concentric with the BCD in the chainring(more likely). Sheldon has a write up about this with a fix but basically it goes; turn cranks to position with tightest chain, loosen bolts, tap chainring back toward rear wheel and re-tighten bolts. Same goes for the rear cog, teeth may not be cut concentric with the thread but Phil cogs should be pretty good.


^ Cheers, I will give that a go tonight I think. I don’t think the cranks are particularly good (or the chainring for that matter), so might well not be engineered to perfection.

I actually want to drop some Paul cranks on it but they have a 44mm chainline with the recommended BB and my rear hub looks to be 40-42mm. I have plenty of frame clearance though so might be able to play with the spindle length to help the chainline, saying that, I could of course try that even with my current cranks.

Made the chainring adjustment to balance out the chain and it has certainly reduced the difference between the slack and tight parts now. Did about 18km today on commute, feels pretty good with the chainring on the inside and things tweaked up.

Got a new BB on order though and will throw some spare Miche Primato cranks I have on to move to a proper 42mm chainline, fingers crossed that will bring everything into line.