I have a new single-speed bike (after purchase this year, it was unused in a garage for a few months), but as I turn the pedals, the tension in the chain varies noticeably.

What should I check, one-by-one, to find the source (and fix it)?

If not mitigated (I'm guessing it'll never be perfect), can it wear down the chain/sprokets faster? Cause any other issues?

More information, and bonus question:

I'm also having an issue that twice per a full pedal rotation, there is a click sound that seems to come from the bottom bracket. Not sure if related?

  • 1
    What do you mean by "tension in the chain varies"? The tension in the top segment of chain is directly related to how much force you put on the petals. The tension in the bottom segment is basically controlled by the rear derailer spring and which set of cogs you use. The only reasons for this to vary much as you pedal would be 1) severely worn cogs or "sticky" chain, or 2) you happen to have purchased a bike with an elliptical or "Biopace" chainring (both of which have a varying radius). Apr 1, 2014 at 20:23
  • 2
    But clicking from the bottom bracket needs to be investigated -- you should take the bike to a bike shop if you don't know much about bike maintenance. Apr 1, 2014 at 20:24
  • I'm going to guess that there is no rear derailleur in this case given its a single speed. Maybe you can post a video of the situation?
    – Batman
    Apr 1, 2014 at 22:31
  • Ah, I missed that it was single speed. In that case likely the front cog is off-axis for some reason. Apr 2, 2014 at 0:56

2 Answers 2


Possible causes:

  1. There is "play" in the bottom bracket bearings, this could also explain the clicks. Usually this is quite noticeable, and you can check it by grabbing the crank-arm and trying to move it sideways. Usually this is not the cause for variable chain tension on singles;
  2. The chainring is "eccentric", either because of haveing been tightened off-center, being manufactured off-center (some defect or poor quality), or damaged. The best way to check is to suspend the bike or turn it upside-down, then slowly spin the cranks do see if it looks off-center. If it is badly assembled, you can try to loosen the bolts and recenter it manually;
  3. The sprocket might be off-center. This can be very subtle, but you can see the difference between the sprocket or the chainring being off-center depending on how many times per turn the chain reaches its tight spot, since the sprocket has a much smaller diameter;
  4. The chain itself can be unevenly worn out, or be uneven from factory. This can happen with very cheap chains, and can cause even a combination of slight unevenness of chain, sprocket and chainring to create a somewhat "random" pattern of tightening and loosening of the chain as the bike rolls.

As for the consequences, I would say that the main problem would be to adjust the rear axle too tight, as this would cause excessive mechanical stress on the drivetrain, or to assemble it so loose that it falls off. Except if you ride fixed or coaster brake (when the lag while pedalling backwards would become a bit annoying), there is not much problem in letting it a bit more on the loose side (enough not to reach an overtight position).

Also, if you use good quality chains and replace the chain often, the rest of the drivetrain tends to "age well" while wearing, because the teeth tend to "even out" in the process. But if you use crappy chains, even good quality gears will suffer a lot more...

Hope this helps!

  • Nailed it! I will check those out (I tried #1 without luck)
    – Baumr
    Jul 1, 2013 at 16:07

I would suggest taking it in to a small bike shop. Most of the employees that I have seen at bike shops are friendly and willing to help. If you show them what is wrong they might be able to tell you how to fix it. I have done it that way in the past and it has worked for me.


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