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Old 01-19-07, 09:48 AM   #1
fenester
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Adjusting chainring / imperfect chainring

Greetings All,

My question relates to my SS/FG bicycle. I noticed that at one point in the crank/chainring rotation the chain is much tighter (when the drive-side crank is at 3 o'clock when viewed from drive-side). Result is that when I adjust the rear-wheel accordingly, the chain is really (dangerously) loose when the crank is rotated 180 from that position.

[Sugino cranks, Rocket Ring 46t, Surly freewheel 18t]

I tried SB's suggestion/method of loosening all the chainring bolts and tapping/bouncing tool (15mm wrench) on chain to nudge it into position to compensate for the imperfection in the chainring. So I tried really squeezing the chain together, thinking the chainring might just be a little stuck. Still didn't help, the chain remained very tight/very loose.

So, my question(s):

Did I miss something? Any suggestions before I buy a new chainring? (the current chainring and whole bike only have about 200 miles on them)

I've gleened that chainrings often aren't truly circular, but this seems extreme to me. [edit: I'm reasonably sure it's not the freewheel because the tightening/loosening of the chain occurs at the same point on every rotation of the cranks...but feel free to correct me on that.]

Thanks for your suggestions and comments!

Last edited by fenester; 01-19-07 at 09:56 AM.
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Old 01-19-07, 10:23 AM   #2
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Sounds like you've got a Biopace(ovoid) ring there--no good for a fixie. A standard ring will solve all your problems (well, the chain related ones anyway).
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Old 01-19-07, 12:04 PM   #3
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Hi, thanks for the reply.
I'm reasonably sure it's not biopace. As far as I know Rocket Rings are made for BMX/singlespeed and supposed to be more or less circular.
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Old 01-20-07, 12:10 AM   #4
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tight spot in the chain

You might try changing the orientation of the ring in relation to the cranks... i.e. shift over a bolthole or two.

Does the ring wobble if you look at its position re: the chainstay? This would suggest a bent ring, a bent crankarm spider, or a bent BB spindle. It's a long shot but I've seen a bent spindle create a tight spot, and flipping the arms on the spindle by 180 degrees solved it. (This was with a used arm & spindle that took a hit together.)

Good luck.
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Old 01-21-07, 07:19 PM   #5
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Sure it's not the chain itself? A tight spot in the chain is often a symptom of a worn chain or a new chain with a couple of tight sideplates (can be loosened by flexing the chain side to side at that point).

Richard
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Old 01-21-07, 07:22 PM   #6
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Quote:
Originally Posted by xlrogue
Sounds like you've got a Biopace(ovoid) ring there--no good for a fixie. A standard ring will solve all your problems (well, the chain related ones anyway).
Fear mongering. Biopace works fine on fixed gears.
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Old 01-22-07, 09:18 AM   #7
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Ok,
I checked for horizontal "wobble" on the chainring by sighting down it from above, it looks straight. Also, it's not just that the chain binds a little in the "tight" position, the chain tension gets tight at that point (when checked by pressing vertically on the chain between chainring and cog), so I don't think it's the links.

Unfortunately I don't have a spare chainring to troubleshoot with, but I'm going to my LBS tonight so I'll see if they have one I can throw on quick just to check. I'm with you, better to check for the cheap fix first. (I haven't had a chance yet to rotate the chainring on the cranks yet, but will check it before buying new one)

I'll post back with results when I get it fixed

Thanks again,
fen.
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Old 01-22-07, 11:55 PM   #8
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Although unlikely it could be due to uneven wear on the spindle. However it would display horizontal deflection.I would move the chainring to different bolt holes and see what happens.
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