Problem: Eccentric Bottom Bracket with Constantly Changing Chain Tension
 

I've got a sweet bike with a Rohloff hub and an eccentric bottom bracket. However, as the crank goes around the BB, the tension of the chain itself changes.

I can't for the life of me figure out why it's happening. Is the chainring oblong? Is something moving that I'm not seeing? Is this perfectly normal?

Anybody have an explanation.

Here's a video of it in action. When the crank arm is fully forward, the chain is tight. When it's fully backward the chain is very slack.

There's a bit of a catch / pop at times when I'm pedaling, though I'm not sure if it's from the chain tension.

Thanks a bunch.

It's fairly common for the chain rings not to end up centered on the crank arm. SHeldon Brown has a nice write-up about how to center them. Probably in the fixie section.

Centering crank arm by Sheldon Brown. I'm going to try it now.

+1 The chainring is off center, and acting like a cam. Possibly this is in the crank and spindle interface or possibly the chainring isn't mounted properly on the arms.

If, as it appears, the chainring is supported on shoulders on the crank arm spider, there probably isn't any float to recenter the ring on the arms, but if it's supported by the chainring bolts alone you might be able to float it closer to center by loosening the bolts, then tightening them firm and tapping the ring in the appropriate direction until it runs true. You can do this with the chain on using the tension changes as a guide. When you've achieved the best position tighten the bolts .

If there isn't enough float to get the ring centered, or if it's supported on shoulders, then either the chainring, crank or spindle is defective and needs replacing.

The chain ring can be both out of round and off center. Center it and see if chain tension becomes more consistent.

Start by centering the chain ring as per Sheldon Brown's method and see how that improves the overall chain tension... if the problem cannot be resolved in this manner I'd look at replacing the chain ring to see if a different one resolves the issue.

I have come across a few bikes where no amount of adjusting could correct the variances caused by an out of round ring... the tolerances on some parts are not all they could be and if they are designed to be used with a derailer equipped bike would not be as noticeable.

Better quality ss and fg parts have closer tolerances and I have always been really pleased with Sugino chain rings as their QC is very high.

I wish everyone could provide videos like this.

It's a Sugino crank with Salsa rings.

I centered it in about 10 minutes without any trouble. The stack bolts were a little rusty. Put on some Boeshield T-9.

I think the chainring was knocked loose in shipping and then I retightened it without centering. Never run a singlespeed before and had no idea how to center it (or that I needed to).

No problems at all now, though I need to move the EBB a bit to get it nice and tight all around.

Thanks everybody. Cheers!

:beer::beer::beer:

I've got a few IGH bikes with Salsa rings, they fit kinda loosely on the spyder, I've had to center the Salsa ring everytime.

Dont overlook the fact that the bottom bracket is oval. My son bought a new high end Schwinn frame, and the bottom bracket kept comming loose. The bike shop kept tightening it but it always came loose. Finally I looked at it my self and found out the frame had an oval bottom bracket. The Schwinn rep came to town and agreeded with me and gave my son a new frame. He was amazed that I was able to diag the problem myself.