Off round chainring

I noticed today while lubing my chain that as I the crank turns the chain gets tighter and slacker. It does this at the same point of the pedal rotation each time which indicates that the chainring is the cause (ie. its not in time with the rear wheel rotation which would suggest sprocket problem or at random intervals which would suggest tight point in the chain). No matter how carefully I look at the chainring as it rotates I can't see it bobbing up and down so it is obviously a very small off roundness maybe .5mm but does make the chain go noticably slack then tight. The chainring seems to be centered on the crank arm ok and the crank isn't crooked on the BB. I should add that these are cheap no name cranks I got off ebay.

Being new to fixed gear is this something I need to sort out or is it just something I have to live with using cheap components with poor tolerances.

This is a common problem.
Uneven chain tension caused by off-center/elliptical chainring.
There is a solution:
https://www.londonfgss.com/thread907.html - (back in the day EVERYONE knew about sheldon.)
https://www.londonfgss.com/thread5751.html - (modern hipster thread.)

The other possibility is the BB itself. A small difference in the centre can make a noticeable difference at the edge when under pressure.

i had a similar problem when i had my first bike but later found out that the chainring wasnt an even diameter all round, try measuring the diameter of the chainring all around and see whether its the same before you do anything else, it might save you unnecessary money spending and anguish later

is it a bio pace chain ring ?

I had this problem. I just chucked the chainring and bought an expensive one
You only get this with cheap crap. Not with Campag record, DA or Sugino Zen. Worth paying the extra for IMO

I had this problem. I just chucked the chainring and bought an expensive one
You only get this with cheap crap. Not with Campag record, DA or Sugino Zen. Worth paying the extra for IMO

And I remember the exact moment when you came to this conclusion. Oh, the fun we had :-)

Sorry to raise a thread from the dead, but i noticed this today, i have a BLB brand chainring, they are quite expensive, probs just overpriced crap, but is it a problem? i dont have money or means to buy a new chain ring...

erm, its not a problem.

just make sure it's centred correctly.

ye thanks.
I will try the sheldon brown thing..

Ok, can't seem to find this specific topic being covered...

I'm swapping a 48t Miche chainring for a 46t TA chainring (both 135 bcd). The problem is that the TA chainring is slightly too 'tight' to fit on the crank arm. To get it on I had to place a long screwdriver in there and kind of lever it on, but this resulted in the chainring being pulled out of round. I don't think I've permanently altered the shape of the chainring, but can't get it on at the moment without altering it such that the chain tension varies wildly from too slack to too tight. Is there some way of getting it on, by maybe filing some material away, or are different chainrings/cranks from different makes just not compatible?

Cheap(er) chain rings you have about a 50/50 shot of getting a round one.

That's why people buy more expensive rings.

don't think it's that the chainring is out of round, just that it doesn't fit on the spider (?) because the raised bits of the spider go out too far by a fraction of a mm

cheap shit chainring do this alot. also bent bb axle but youd feel that at the pedal. most likely out of true chainring.
with a bit of filing you will get it to fit, just dont go mad. if its a tiny bit then fine, if its loads (more than a mm) then leave it. get the chainring that goes with the crank arm.

.

File your crank profile.
Out of interest, do they fit on the other side? Oh, and TA are not exactly cheap.

File your crank profile.
Out of interest, do they fit on the other side? Oh, and TA are not exactly cheap.
Thanks, 'crank profile' was the phrase i was looking for. Not tried the other side as the chain line would be out, but I suppose if it fits there it might rule out the idea that the chainring is not round? yes, the TA was £40 but I got a discount at Condor, so not cheap.

That was exactly my thinking on trying the other side.
You can always go back to Condor, explain the problem to them (although I wouldn't do it on a Saturday) and see what they recommend. If you involve them and their opinions in the process they are more likely to help you if something is genuinly wrong/a warrenty issue.

Double check it's not your spider. I had two chain rings in a row that were "off centre". Both in the same place, which was suspicious. So I filed the spider back a bit towards the centre of the crankset where the chain ring sits and now I'm able to centre the chain ring again properly. It was a slight defect on the spider.

Just bought a new crankset and found that the chainring is slightly off centre. Looking at the chainring the out of trueness is barely visible, probably no more than 1mm but where it is obvious is in the chain tension. The difference between the loose and tight point adds around 20mm to the play in the chain. I cannot use the Sheldon method to adjust the chainring on the spider so it's where it's at.
I understand it's not unusual to have some out of roundness but the question is just how much is acceptable. The offending article is a Stronglight Track 2000 so not the most expensive but not too cheap either.