... a chain to be taut for 1/2 a crank rotation and loose the other half? Any other possibility?

It's my buddies Surly Steamroller. I swapped out his 19t cog for a 16t and noticed while tensioning the back wheel, the chain seemed taut then all the sudden it was loose. I rotated the crank and the chain goes from taut to loose each half rotation. The back wheel is straight and true. The cog is on properly. I know a bent chain ring can cause a symptom like this but could it be anything else? I suppose I should take off the chain and eyeball the crank and chain ring to check but any other possibility?

-R

Is there any chance he's using one of those Biopace elliptical chainrings? I guess those aren't supposed to change the chain tension THAT much through a revolution, but maybe that's what you're seeing?

by bent do you mean ovalized or tacoed?

You will always have this, unless your drivetrain is perfect (all high end components, even then not guaranteed). The only thing that really matters is whether or not the variation is big enough that you have a chain that is loose enough at one point of the rotation for it to actually derail. If that's the case, you are screwed unless you start replacing components.

Sheldon details a way of "centering" the chainring on the crank spider. I find this usually doesn't improve things dramatically, and it didn't need it to begin with anyways.

And consider post #2 as well. Bent chainrings are usually bent laterally which will be extremely obvious upon inspection. There's no other way for the chainring to be bent.

Its the stock Sugino RD that came on it. Its kinda weird... I didnt notice it at all. Now I started looking at my bike with a Sugino Track ZEN 49t on my Sugino 75 crank and its almost the same... odd. Maybe I'm just anal.

Taut for 1/2 the crank rotation, slightly loose for the 2nd 1/2 rotation.

Again, normal.

My dura ace/bb/crank/chainring/phil cog combo does not have any noticeable variation, which is fairly rare.

Set the rear axle so that the chain pulls taut at the tightest part of the cranks' rotation. One at a time, loosen up each of the stack bolts, and tighten it back just finger tight. Spin the crank slowly and watch for the chain to get to its tightest point. Strike the taut chain lightly with a convenient tool to make the chain ring move a bit on its spider. Then rotate the crank some more, finding the new tightest spot, and repeat as necessary.

https://sheldonbrown.com/fixed.html

Ah I didnt even think to adjust the stack bolts... That'll probably correct it. I just assumed the chainring may have been ovalized, not so much bent. Thanks mang.

You could also loosen your chain. No reason for having it that tight.

But that should solve the problem and loosening it would make it good at the tight part and SUPER slack on the loose side.

Depends on your definition of super slack. Assuming things were assembled correctly, there shouldn't really be an issue. Hell, I've run biopace on fixed gears and single speeds without dropping the chain. Just make sure you have a good chainline and you're set.

I only knew what he was talking about cause I had it happen to me. It's annoying but an easy fix, no point in leaving it it's fixable.

Yeah, its not that big a deal but it was bothering me. Taut, the chain has maybe 1cm play total. When it loosened up, it was about 1.5cm. Even at the loosest point, it was what most people consider to be reasonable chain tension. I just prefer it to have no noticeable sag. I'll F with it tomorrow. It rides fine.

It could also be uneven chainring wear. Always skid and trackstand with the same foot forward?

Sorry this is nonsense. The only thing this would cause uneven wear to is the rear tire, depending on the skidpatches. And again, the lower limit of chain tension is one where you can push the chain off the chainring/cog. If it is that lose you can be guaranteed to drop the chain on bumps.

It was exactly an off-center chainring. I should have checked it first. I replaced the standard Sugino 75 48t chainring a few months ago with a ZEN Track 49t. 2 of the chainring bolts were very loose and the others werent tight LOL. I loosened em up, snugged em up slowly as I rotated the crank and all is symmetrical in the drivetrain once again. PARK chainring bolt tool to the rescue!

This always happens to me when one of my chainring bolts looses up/