krets

Last week I put some new phils on my commuter. I decided to continue the splurge with some sugino 75's and a Zen chainring. I got everything together and I am now noticing that my chain purrs louder at one position in my stroke. With further inspection I notice that my chain tightness varies through a full stroke.

Just this morning I rotated my chainring to try to verify that it was the chainring. After my morning commute of 6 miles, I still think it's the chainring (it was louder at a different and anticipated part of my stroke). I was riding hard and may not have spared enough attention to it, so I am going to take a cruise at lunch to be sure.

Is it common for these chainrings to be a slightly untrue? If so, why spend the extra money?

Thanks

Retem

almost all chainrings are not perfectly round

piratelove

You may have just not centered it when you put it on.

I bought 75's with the stock chainring, and they were perfect, but recently I took the chainring off to clean it, and when I put it back on I have tight/loose spots in my chain. Why? Because I didn't pay much attention to getting it centered when I put it back on..

Read Sheldon's article/method for centering chainrings.. as soon as I get a chainring bolt tool I'm going to fix mine!

lyledriver

chainring bolt tools kick ass. Mine has a 10mm wrench on it too!

krets

Wow, being that I did not know the chainring could be centered, this must be my problem. I found this tandem article that mentions adjusting the chainring:
https://www.sheldonbrown.com/synchain.html
[edit]to feel better you can read the same text on a fixed gear page
https://sheldonbrown.com/fixed.html#tension
[/edit]

It looks like I have a lot to learn about my simple fixed gear bicycle.

Thanks pirate. I will report back with what I find.

mihlbach

You may find that you can never get it perfect. My Zen chainring is close to perfect, but there is a slight tight spot. But if, as you note, your can hear the chain more when you pedal it through the tight spot, you're chain is too tight...give it little bit of slack, so that it has just a little play at the tight spot, but not excessive play at the loose spot. Given the approriate chain tension, a slighly off-round chainring is not a problem.

Telix

Yes, with high-end cranks and chainrings you should be able to have almost perfect roundness. On my cheap TruVativ crank/chainring I can't get great roundness no matter what I do, simply because the tolerances aren't so great. I'll live.

krets

I tried playing with my stack bolts last night, but on my sugino 75s they seem to be joined to the cranks. I can't get any of them to move at all. The tension is something that I have played with, but I really hate having it so that there is a lot of play when I track stand.

Has anyone adjusted stack bolts on a 75 spider?

queerpunk

it's not adjusting the bolts, it's adjusting (in pretty tiny measurements) how the chainring is centered on the spider. even if the spider is really really really close to round, and the chainring is really really really close to round, unless they share the same centerpoint, you're going to have uneven tension. centering the chainring on the spider isn't about moving the bolts around.

krets

Ahh, that is what I did not understand from sheldon's page.

I am going to spend some time with my bicycle again.

thank you.

11.4

Huh? That Sheldon Brown article seems a bit confused. If you look at your crankset, you'll notice that there is a shelf that the inside edge of the chainring spider rests against. There's no way that you'll have a chainring collapse inwards to create a low spot, and no way it'll get pushed outwards either (in either case, the chainring bolts wouldn't fit because the holes in the chainring and crankarm spider wouldn't match up properly). This isn't to say that you shouldn't alternate bolts when you're tightening, because this can help make sure that the chainring doesn't warp (sideways), but even then the crankarm will limit any warpage to something very small.

I haven't found a high-end or low-end crankarm or chainring that doesn't have roundness problems from time to time. I've seen $300 Sugino Gigas chainrings that aren't round; a couple of my Dura Ace rings are pretty bad; and so on. And the problem is often with the crankarm as well as with the chainring. If the crank is perfectly milled and drilled, then you can't rotate a chainring on the crankarm to find a position where chainring out-of-roundness is minimized -- you'd always have the same issue, just at a different place in the pedal stroke. So where does the problem come from? The milling of both chainrings and crankarms from a forged or stamped laser-cut blank has to be done in multiple steps. On a crankarm, for example, one machine setup has to mill the spline where it connects to the bottom bracket, and another setup has to drill the holes for the chainring bolts. A third setup does the flats where the chainring and crankarm mate. And so on. I find it amazing how many times the spline is milled off-center, which puts the whole crankarm out of round. That also means that your pedal stroke is slightly eccentric as well (i.e., your crank arm is effectively a couple millimeters longer at one point than at the opposite point in the revolution). The milling is only as good as the setup and since forged crankarms in particular have a certain amount of natural variance in dimension (it goes with the forging process), every setup is slightly different. It's actually very hard to make everything line up perfectly. The same problem occurs with chainrings as well (it's actually easier with chainrings that are simply milled and have no other processing going on, including cutting holes -- that's why Sugino Track Zens are a bit better than most -- it's one setup and the machining is all keyed off the initial set of holes drilled for chainring bolts).

Since it only takes 2-3 millimeters of chainlength to make the difference between slack and too tight, and since that chain length can be taken up over about 2/3 of the circumference of a chainring (the amount wrapped by the chain), you most likely won't be able to measure the size difference of a chainring with a ruler. I always check new chainrings (and cogs, by the way, because they are sometimes incredibly bad as well) to make sure I don't have a dud. If I do, it goes back right away for an exchange.

Since the original question was about the Zen's, I'd say that I've seen quite a bit of sloppy machining on the so-called "messenger" Zen's but very good machining on the true track ones (the track Zen's are about twice the price of the "messenger" ones, and there's a reason why). That's not to say that the "messenger" Zen's aren't good if you find the right one, but they are made from a lower grade alloy, the finishing isn't quite as good, and there isn't quite as much milling (especially on the teeth) and what's done isn't quite as good. However, it's only a chainring and on the street all you need is one that's round. Any fine shaping on the teeth will be moot after several hundred miles anyway -- on the track it takes a while to rack up that much mileage, but on the road you do it quite quickly.

mander

Thanks for this helpful explanation 11.4!

krets

11.4, your explanation seems to fit my experiences in the past couple days. There is minimal if any play when trying to shift my track Zen around my Sugino 75 spider. I have been spending my stand time trying to center the chainring on the spider, but it does not seem that I can get it any better. I need to determine if it is the crank arm or the chainring that is eccentric.
This is the first time I've spent more than $80 on a crankset, so I might be overly sensitive to play in the chain. It can bind with one foot forward and wobble with the other. It does seem worse then my cheaper cranksets were out of the box.

na975

now that i've read this, i wont buy a zen chainring waist O $.......