It seems to not be a perfect circle. I'm aware some are designed this way, but I don't know if this is intentional. It's a Monster 42t, and the chain slackens and tightens with every spin of the cranks a little bit. If you spin it freely without the chain on, you can see that it varies about 1 mm from the center (pulling the chain tighter when the right crank is forward).

Have you tried to adjust it by loosening the chainring bolts?

yeah, it could just be seated slightly off-center. loosen the bolts, make sure it's centered, and then re-tighten.

The bolts that keep it secured to the crank? Yes I tried that. It fits snugly around the crank, no room to push it towards the side that's 'larger.'

You ned to learn how to centre your chainring correctly.

The obligatory Sheldon Brown reference.

From https://sheldonbrown.com/fixed.html

"Chain Tension

he chain tension on a fixed gear is quite critical, and is regulated by moving the rear axle back and forth in the fork ends. If the chain is too tight, the drive train will bind, perhaps only at one angle of the pedals (chainwheels are not usually perfectly concentric). It should be tight as it can be without binding. If the chain is too loose, it can fall off, which is quite dangerous on a fixed gear.

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.

This takes a little bit of your hands learning how hard to hit the chain, and how loose to set the stack bolts, but it is really quite easy to learn.

Tighten up the stack bolts a bit and re-check. Tighten the stack bolts in a regular pattern, like the lug nuts on a car wheel. My standard pattern is to start by tightening the bolt opposite the crank, then move clockwise 2 bolts (144 degrees), tighten that one, clockwise 2 more, and so on. Never tighten two neighboring bolts in a row. You may prefer to go counterclockwise, but try to get in the habit of always starting at the same place and always going the same way. This reduces the chances of accidentally missing a bolt.

Once you have the chainrings centered and secured, adjust the position of the rear axle to make the chain as nearly tight as possible without binding. Notice how freely the drive train turns when the chain is too loose. That is how freely it should turn when you are done, but with as little chain droop as possible."

You might want to read the rest too or else you'll risk getting burnt to death if you ask anymore questions like that.

if there is *absolutely* no room to adjust it, you could make a tiny bit of space by taking a file to the interface point on the chainring.

whenaminotme- thanks but what if there are no stack bolts? i guess i do the same thing but just loosen the chainring bolts?

xxguitarist- i hope it doesn't come to that, i'll be fiddling tonight when i get home


VVVVVVVV yeah this is a 1/8" but i think it's a bmx ring, not track so maybe that's why it's not perfect. is my bike much less efficient due to the 'slop'? VVVVVVVV

Most rings are slightly out of round. True track rings (1/8") tend to be held to closer tolerances than generic rings and are priced accordingly. Do as above to minimize the effects of the eccentricity and then set the chain tension at the tightest point. Expect some slop from the eccentricity.

Stack bolts are chainring bolts are they not? In this case anyway.

I'm guessing your chain is 3/32". If so it'll handle a little more movement than a 1/8". One of their advantages.

apparently stack bolts are the things that connect a spider to the ring. they hold together multiple 'stacked' chainrings (or just one to the crank). mine's a solid piece, no spider.

Ahh I see. That's beyond my mechanical skill though I'd guess it would need some type of cold forging (bending) if it's able to withstand it (not familiar with Monsters), though I'd tend to doubt it's advisable or even possible on that part of one's steed.
Can you not run a slightly slacker chain? At least as tight as possible at the tightest point. 1mm shouldn't leave your chain jumping off.
There's a tendency amongst some to run a looser chain than most would though I've not read up why.

No, the chain doesn't jump off, or even come close to doing so. It just rattles and doesn't spin completely smoothly and I feel like maybe it'll slow down the bike.

At 42t & without a chainring, you probably have an old BMX set up. It's not a on-piece crank is it?

it is one piece.

Okay, less to adjust but no chance to change your chainrings. You can change your cogs though plus they're cheaper, but 42 is still quite low.

Either way the only thing that'll slow the bike down is your legs, IMO.

If 1mm of slop matters to you, then you're running your chain way too tight.

i actually did this and it did help center it a bit more. didn't make it perfect but maybe half better, which is enough that it spins more smoothly. i also felt like a badass using a file on my bike. thanks.

And for future reference. Your problem is not with chain line.

Chain line is getting the chain ring and the rear cog perfectly in line with each other.

The misallignment you experience in your chainring is indicative of cheap chain rings. You having a single piece crank setup removes any opportunity there is of adjustment. It is also the lowest quality and cheapest of the cranksets. Just try to get the chain tightness correct for it at it's tightest point. As long as it is not getting too loose or falling off then you will be fine. The problem you experience occurs when the chain is pulling too tight as the chainring rotates about it's centre of rotation.

1/8" Track Rings are usually rounder.