All my single speed/fixed gear setups have this problem to some degree. Some have it more than others.

As I turn the chainring one full revolution, the chain tension will go up and down rather than staying steady. It is as though the chainring is egg-shaped.

Is this unavoidable? Is it a product of low quality parts? Improper installation?

Sheldon Brown
Fixed Gear Bicycles for the Road

Yep, chainring is out of center

Par for the course. Only the most expensive cranksets are round and turn on center. All the rest are cams and the chain will tighten and loosen as they go around.

As noted above, SB offers some ideas on improving concentricity, which works on some, but not all cranksets. Otherwise live with it, but be sure to adjust chain tension (slack) so there's at lease vestigial slack at the tightest position. If not, you'll be back posting about hearing Rice Krispies in your drivetrain.

Pretty much. Higher price = more perfect round spin, among other things. There are always somethings a person can do on the user end, like proper tension and chain ring centering. Depends on what your "good enough" is.

here

What you, OP, are describing is normal. Even on some higher priced chainrings I've noticed this slight variation to some degree. It's nothing to worry about. Either follow Sheldon's method of "centering the chainring," or learn how to adjust the chain tension where the looser spots aren't too loose, and the tighter spots aren't too tight. Obviously there will be some compromising, but that's the secret to life...right?

my Sugino 75 cranks are no more "round" than my Andel cranks. I locate the tightest spot and adjust from there. It works fine.

Here is something I always found to be really weird. This experience has been 100% consistent in my decades of working on bikes.

Install brand new chain. When adjusting it, I find no inconsistencies in slack/tension as I rotate the crank. After the first ride there is a lot more slack and there are BIG differences between loose and tight spots.

1. I aint strong enough to stretch a metal chain.
2. The axle didn't move in the dropouts. I checked.
3. WTF?

The loose/tight spots have never bothered me, but the mystery has always caused me to scratch my head.

I have this issue too and the parts are new-ish, not more than 200 miles. I noticed this after I switched my cog from 18 to 17t.

Rear sprockets aren't as eccentric as cranksets, but they're not perfect either, especially low end Single freewheels. Other than cosmetic concerns over droopy chains, I've never seen a SS bike that was so bad it was a mechanical problem. Just remember to leave a bit of slack at the tightest point.

The chainrings are not out of round. The holes in the crank and the holes on the chainring have a tolerance. Just like anything else in the world manufactured. In other words the holes in the rings and the cranks have to be large enough
to let the parts all match up. The holes are a little oversize to get everything to match up. This means that the rings will be a little off center. I have used shim stock to center my chain rings on the crank. This reduces the off center position of the rings and helps with tension variations. Sheldon Brown has another way to center the
rings.

The bottom line is that there will be some tension variations in your chain. So just make sure it does not get too tight at any point. This is normal.

Not always true. Poorly machined chainring and or freewheels/fixie cogs are often slightly ovalized which causes the chain to be tight in some spots and loose in others.

^ It's a combination of the chain ring bolt holes and chain ring itself. If you think poor machining results in chain rings being produced out of round what makes you think the bolt holes would be any more perfect? Just sayin' it's a combination... also inconsistent tension is only a problem for people with OCD.

I have FOUR operational ss/fixed bikes and the gearing is as follows. 52:18, 48:19, 40:20 and 38:13.

The 52:18 and 48:19 have VERY little inconsistency in chain tension where as the 40:20 and 38:13 have a great deal more.

Is it coincidence that the smaller chainrings are off by more than the bigger ones or is this the nature of smaller chairings?

It's usually not the chainrings per se. The rings tend to be fairly round, with the bolt hole circle pretty concentric. OTOH, if you look at the cranks, there are two sources of error, one - where the crank mounts to the spindle, and the spider circle with respect to the spindle bolt. Those tolerances are harder to maintain, especially on cranks where the spider is a separate part.

Lastly is how the chainring is mounted. Better cranks have a fairly precise turned shoulder on the chainring spider, and the rings likewise have a turned ID. This arrangement ensures good concentricity when mounted (subject to the other errors).

Lower quality designs eliminate the extra work of making close fitting shoulders, and locate the ring only by the mounting bolts, which is a far sloppier arrangement, which can lead to eccentricity. (some of these may appear to have locating shoulders but really don't, as in this case, note the gaps).

These sloppy rings can sometimes be corrected by tightening firmly but not rock tight, and tapping the ring toward center iteratively using the changing chain tension as a guide, then tightening. Unfortunately, they lack rigid support and sometimes drift back off center.

So, it's not the ring size, it's the crank and ring quality.

My experience is that chain issues got a lot better after I started using a Sugino 75 crankset and either Eur-Asian Imports or Dura-Ace cogs. Not prefect, but far better. Good enough that I rarely check the chain tension and I change cogs a lot, often mid-ride. (A trick to setting chain tension that puts me in the ballpark virtually every time is to pull the wheel back with my left hand and over to the left chainstay, both firmly, while standing behind the bike. I tighten the right nut, center the wheel and tighten the left nut. Done. Proper slack is nearly automatic when I do that, Now this might have to be modified a little for different chainstay geometries and tire widths.)

Ben

Yes, this method works decently if the ring is fairly concentric. Tightening the right first, then centering the wheel has the left side move forward a bit producing the needed slack (though not always enough if the rings are off).

Unfortunately most people do the exact opposite, tightening the left nut first, then levering the right side back to center, thereby ADDING chain tension, which is probably one reason we have so many newbie posts on the subject.

IMO- it would be easier and clearer if we simply stopped saying chain tension, since there isn't supposed to be any. If we just said minimum chain SLACK, or take up excess SLACK, it would remind people that slack is the desired condition. Likewise for those infernal "chain tensioners" newbies install on the rear axle. They're not made to add tension like a screw jack, but to restrain the wheel from slipping under load.