Uneven chain tension

[howdee]

Recently got a new single speed bike which I'm enjoying riding fixed, but still very much learning.
Is it normal for there to be a degree of eneven chain tension when turning the pedals? There are spots on each chainring revolution where the tension is tight making it difficult to adequate tension across the whole chainring. I think it is excessive, perhaps around 2-3cm difference in vertical chain movement between the low/high spots if I wiggle the chain up and down with my fingers.

It has a new Stronglight chainring but had the same issue with the previous chainring. Wondering if the spider might not be perfectly centered. Crankset is a spa TD2.

[matthew]

Howdee, as you surmised this is typically caused by either an un-round chainring or a chainring that isn't aligned on the spider. Can you identify where in the pedal stroke the chain is tightest? If so it may be as simple as slackening the bolts on the spider to allow you to center the chainring on the spider.

[Pickled Onion]

Recently got a new single speed bike which I'm enjoying riding fixed, but still very much learning.
Is it normal for there to be a degree of eneven chain tension when turning the pedals? There are spots on each chainring revolution where the tension is tight making it difficult to adequate tension across the whole chainring. I think it is excessive, perhaps around 2-3cm difference in vertical chain movement between the low/high spots if I wiggle the chain up and down with my fingers.

Be careful!

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As matthew says, put the bike on a work stand (or upside down) and slacken the chainring bolts just a small amount, so they're still holding but not fully gripping. Tighten the chain a little and turn the pedals a few revolutions. Then repeat. You need to do it gradually or you'll just be pulling the chainring from one side to the other. When you're done tighten up the bolts fully. Then loosen the chain off a bit, you don't want it too tight, a few mm movement at the pedals when the wheel is held still is fine and won't be noticeable as slop when riding.

[howdee]

Howdee, as you surmised this is typically caused by either an un-round chainring or a chainring that isn't aligned on the spider. Can you identify where in the pedal stroke the chain is tightest? If so it may be as simple as slackening the bolts on the spider to allow you to center the chainring on the spider.

The chain is tightest at the 9 o'clock position, and slackest at the 1 o'clock position.

As matthew says, put the bike on a work stand (or upside down) and slacken the chainring bolts just a small amount, so they're still holding but not fully gripping. Tighten the chain a little and turn the pedals a few revolutions. Then repeat. You need to do it gradually or you'll just be pulling the chainring from one side to the other. When you're done tighten up the bolts fully. Then loosen the chain off a bit, you don't want it too tight, a few mm movement at the pedals when the wheel is held still is fine and won't be noticeable as slop when riding.

Just tried that and it does seem to have improved things slightly. I'll play with it a bit more. Thanks.

[JonB]

Uneven chain tension has been a longstanding problem for me since I started riding fixed, I think if you go down the Velo Fixie posts a couple of years you'll find a similar thread from me. I'm not sure I've ever really cracked it but I've never had good results with Stronglight cranks or rings. I've never found the loosening of chainring bolts to be an effective solution partly because on all of the single speed cranks I've owned there has not been any movement between the chainring and the crank and there is no slack to work with. One solution is to file part of the chainring to create some movement but I've never gone down this route. I have gone long periods where I've stopped worrying about it and just tensioned it on rides to stop it getting dangerously slack but I think the problem only gets worse with chain ring wear. I've recently switched to a Miche crank which has given a more consistent (but not perfect) tension. If you feel confident with filing then maybe worth giving that a go in tandem with the method suggested previously.

[Timb]

I don’t think you ever quite get it perfect.  I found on my most recent chain ring that a couple of the attachments to the spider were slightly too long and preventing it from evening out.  I did file it but just be wary as it didn’t need very much off at all.  Good luck and enjoy that fixed riding.

[rogerzilla]

More expensive cranks, rings, and cogs tend to be more concentric.  Medium-priced stuff is usually ok if there is enough play between the ring and spider to adjust the centreing.  The cheapest Chinese dross can't be made to work well - I tried some Litepro cranks and they weren't even good enough for a hub gear.