Dulwich Hill Bicycle Club

When adjusting chain tension on a singlespeed/fixie, you typically see tight and loose spots. So at one point in the revolution you get the tension just right, but half a crank rotation later it'll be quite different - either too tight or too loose. I'm seeing this on my commuter and I believe it's getting worse - there's a significant difference in chain tension between the tightest and loosest spots.

Is it possible the square taper bottom bracket is warped, or is it more likely a poorly aligned bottom bracket shell or out of round chainring (stronglight)?

I would think that the chainring isn't exactly round.
If your bottom bracket was warped you would notice it in your pedal stroke feeling "wonky"
and if your chainring was perfectly round but the bottom bracket was poorly aligned the ring would still run perfectly round but with a poor chain line (like being in the wrong gear on a geared bike i.e. big ring big cog or small/small)

What about the chain?

Cheap chainrings and cheap cranks are often poorly centered. It's not a problem if you have a derailleur or chain tensioner, but it's a problem on a fixed-gear bike. One of the main reasons you pay more for a good track crank and chainrings is the high-tolerance precision machining of the parts to ensure everything stays centered.

Your chainring will be toast for fixed-gear riding. My suggestion would be to buy a new chainring - preferably designed for track use. There may be a matching "stretch" in your chain too (probably just in some of the links) as a result of the wear pattern. If you are going to change the chainring, it might be prudent to change the chain too.

Should last almost forever if it is true to begin with...

It's a Stronglight crankset and chainring so not top of the range but not totally cheap and nasty either, however I will go for a new chainring.

Thanks for the tip on the chain - hadn't thought of that being stretched in certain areas.

In terms of chainrings, fyxomatosis seems to have a reasonable name. Any other suggestions?

What is the BCD

144 - 48t.

I used to have a similar problem on my old fuji, but there is something you should try before you spend money on new bb/ring etc.
If you have the bike in a work stand, set it up as you normally would with regard to wheel position and desired chain tension. Next, loosen all 5 of the chainring bolts a little bit, but not a lot. Slowly rotate the cranks and push down on the top length of chain in the middle of the chainstay. Do this 5 times, one for each loosened bolt as you slowly turn it. This will get the chainring centred on the crankarm spider. Do each bolt up a little at a time and this should hopefully fix the problem.

I have a quality 144 - 48t if you want to try before you buy

I would like to fit x 2 chain tensioners on the frame so it can pull back the wheel evenly for a good chain tension - just can't seem to get a good go at tightening the wheel whilst maintaining chain tension

Thanks I'll try loosening the chainring bolts - if that doesn't work I might try your loaner chainring Mike.

Mick I can recommend the Surly Tuggnut http://www.63xc.com/rickc/tuggnuts.jpg though I had to grind off the bottle opener so it would fit the Felt dropouts. Despite my current chain tension issues it (and you really only need one) provided instant relief from constantly having to readjust tension and just about stripping the axle thread so it wouldn't move.

Has anyone had any experience with bottom brackets with plastic cups?

The Stronglight square taper BB's from Ribble now have plastic cups - If they hold up ok maybe plastic will help with those inevitable BB creaks on a singlespeed?

On a related note, motorbike chains do this all the time due to uneven stretch in the chain due to power delivery changes when you twist the throttle. I assume this would translate to a fixie as well.

Has anyone had any experience with bottom brackets with plastic cups?

The Stronglight square taper BB's from Ribble now have plastic cups - If they hold up ok maybe plastic will help with those inevitable BB creaks on a singlespeed?

Yep, I have plastic cups on my Shimano BB on my singlespeed. I can't recall the model number but it is relatively high end, so don't let it be assumed that plastic is not fantastic.

I haven't ever had a squeak or creak with it and I ride the machine daily for the commute; rain, hail or shine.

Thanks for the feedback on the plastic cups. I'll give them a go.

I don't think my legs are capable of stretching a 1/8" chain but you never know.

Anyone can stretch a 1/8" chain.

The term "stretch" is not technically correct, as it is wear rather than the size of the load under tension that elongates the chain. Dirt and grit gets inside the wheels that spin on the pins, and the dirt acts as an abrasive that cuts a groove into the pins. Over time, as these grooves get deeper, they act to elongate the overall chain length. As the chain gets longer, it gives the appearance of being "stretched".

Strength is not a key factor in chain stretch. Rather, it is chain maintenence, the conditions in which the bike is ridden, and, to a lesser degree, the choice of cogs used.