London Fixed-gear and Single-speed SJS Cycles

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:
http://www.lfgss.com/thread907.html - (back in the day EVERYONE knew about sheldon.)
http://www.lfgss.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

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..